[profile/ivi/qtdeclarative.git] / src / quick / doc / src / appdevguide / performance.qdoc

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/*!
\page qtquick-performance.html
\title Performance Considerations And Suggestions
\brief Discussion of performance-related tradeoffs and best-practices

\section1 Timing Considerations

As an application developer, you must strive to allow the rendering engine
to achieve a consistent 60 frames-per-second refresh rate.  60 FPS means
that there is approximately 16 milliseconds between each frame in which
processing can be done, which includes the processing required to upload
the draw primitives to the graphics hardware.

In practice, this means that the application developer should use asynchronous,
event driven programming wherever possible, should use worker threads to do
significant processing, should never manually spin the event loop, and should
never spend more than a couple of milliseconds per frame within blocking functions.
Failure to do so will result in skipped frames, which has a drastic effect on the
user experience.

\note A pattern which is tempting, but should \e never be used, is creating your
own QEventLoop or calling QCoreApplication::processEvents() in order to avoid
blocking within a C++ code block invoked from QML. This is dangerous because
when an event loop is entered in a signal handler or binding, the QML engine
continues to run other bindings, animations, transitions, etc. Those bindings
can then cause side effects which, for example, destroy the hierarchy containing
your event loop.

\section1 Profiling

The most important tip is: use the QML profiler included with Qt Creator.  Knowing
where time is spent in an application will allow you to focus on problem areas which
actually exist, rather than problem areas which potentially exist.  See the Qt Creator
manual for more information on how to use the QML profiling tool.

Determining which bindings are being run the most often, or which functions your
application is spending the most time in, will allow you to decide whether you need
to optimize the problem areas, or redesign some implementation details of your
application so that the performance is improved.  Attempting to optimize code without
profiling is likely to result in very minor rather than significant performance
improvements.

\section1 JavaScript

Most QML applications will have a large amount of JavaScript code in them, in the
form of dynamic functions, signal handlers, and property binding expressions.
This is not generally a problem (in fact, due to some optimizations in the QML engine
(bindings compiler, etc) it can for some use-cases be faster than calling a C++ function)
however care must be taken to ensure that unnecessary processing isn't triggered
accidentally.

\section2 Bindings

There are two types of bindings in QML: optimized and non-optimized bindings.
It is a good idea to keep binding expressions as simple as possible, since the
QML engine makes use of an optimized binding expression evaluator which can
evaluate simple binding expressions without needing to switch into a full
JavaScript execution environment.  These optimized bindings are evaluated far
more efficiently than more complex (non-optimized) bindings.

Things to avoid in binding expressions to maximize optimizability:
\list
  \li declaring intermediate JavaScript variables
  \li calling JavaScript functions
  \li constructing closures or defining functions within the binding expression
  \li accessing properties outside of the immediate context (generally, this means outside the component)
  \li writing to other properties as side effects
\endlist

The QML_COMPILER_STATS environment variable may be set when running a QML application
to print statistics about how many bindings were able to be optimized.

Bindings are quickest when they know the type of objects and properties they are working
with.  This means that non-final property lookup in a binding expression can be slower
in some cases, where it is possible that the type of the property being looked up has
been changed (for example, by a derived type).

Note that if a binding cannot be optimized by the QML engine's optimized binding
expression evaluator, and thus must be evaluated by the full JavaScript environment,
some of the tips listed above will no longer apply.  For example, it can sometimes be
beneficial to cache the result of property resolution in an intermediate JavaScript
variable, in a very complex binding.  Upcoming sections have more information on these
sorts of optimizations.

\section2 Type-Conversion

One major cost of using JavaScript is that in most cases when a property from a QML
element is accessed, a JavaScript object with an external resource containing the
underlying C++ data (or a reference to it) is created.  In most cases, this is fairly
inexpensive, but in others it can be quite expensive.  One example of where it is
expensive is assigning a C++ QVariantMap Q_PROPERTY to a QML "variant" property.
Lists can also be expensive, although sequences of specific types (QList of int,
qreal, bool, QString, and QUrl) should be inexpensive; other list types involve an
expensive conversion cost (creating a new JavaScript Array, and adding new elements
one by one, with per-element conversion from C++ type instance to JavaScript value).

Converting between some basic property types (such as "string" and "url" properties)
can also be expensive.  Using the closest matching property type will avoid unnecessary
conversion.

If you must expose a QVariantMap to QML, use a "var" property rather than a "variant"
property.  In general, "property var" should be considered to be superior to
"property variant" for every use-case in QtQuick 2.0 (note that "property variant"
is marked as obsolete in the QtQuick 2.0 documentation), as it allows a true JavaScript
reference to be stored (which can reduce the number of conversions required in certain
expressions).

\section2 Resolving Properties

Property resolution takes time.  While in some cases the result of a lookup can be
cached and reused, it is always best to avoid doing unnecessary work altogether, if
possible.

In the following example, we have a block of code which is run often (in this case, it
is the contents of an explicit loop; but it could be a commonly-evaluated binding expression,
for example) and in it, we resolve the element with the "rect" id and its "color" property
multiple times:

\qml
// bad.qml
import QtQuick 2.0

Item {
    width: 400
    height: 200
    Rectangle {
        id: rect
        anchors.fill: parent
        color: "blue"
    }

    function printValue(which, value) {
        console.log(which + " = " + value);
    }

    Component.onCompleted: {
        var t0 = new Date();
        for (var i = 0; i < 1000; ++i) {
            printValue("red", rect.color.r);
            printValue("green", rect.color.g);
            printValue("blue", rect.color.b);
            printValue("alpha", rect.color.a);
        }
        var t1 = new Date();
        console.log("Took: " + (t1.valueOf() - t0.valueOf()) + " milliseconds for 1000 iterations");
    }
}
\endqml

We could instead resolve the common base just once in the block:

\qml
// good.qml
import QtQuick 2.0

Item {
    width: 400
    height: 200
    Rectangle {
        id: rect
        anchors.fill: parent
        color: "blue"
    }

    function printValue(which, value) {
        console.log(which + " = " + value);
    }

    Component.onCompleted: {
        var t0 = new Date();
        for (var i = 0; i < 1000; ++i) {
            var rectColor = rect.color; // resolve the common base.
            printValue("red", rectColor.r);
            printValue("green", rectColor.g);
            printValue("blue", rectColor.b);
            printValue("alpha", rectColor.a);
        }
        var t1 = new Date();
        console.log("Took: " + (t1.valueOf() - t0.valueOf()) + " milliseconds for 1000 iterations");
    }
}
\endqml

Just this simple change results in a significant performance improvement.
Note that the code above can be improved even further (since the property
being looked up never changes during the loop processing), by hoisting the
property resolution out of the loop, as follows:

\qml
// better.qml
import QtQuick 2.0

Item {
    width: 400
    height: 200
    Rectangle {
        id: rect
        anchors.fill: parent
        color: "blue"
    }

    function printValue(which, value) {
        console.log(which + " = " + value);
    }

    Component.onCompleted: {
        var t0 = new Date();
        var rectColor = rect.color; // resolve the common base outside the tight loop.
        for (var i = 0; i < 1000; ++i) {
            printValue("red", rectColor.r);
            printValue("green", rectColor.g);
            printValue("blue", rectColor.b);
            printValue("alpha", rectColor.a);
        }
        var t1 = new Date();
        console.log("Took: " + (t1.valueOf() - t0.valueOf()) + " milliseconds for 1000 iterations");
    }
}
\endqml

\section2 Property Bindings

A property binding expression will be re-evaluated if any of the properties
it references are changed.  As such, binding expressions should be kept as
simple as possible.

If you have a loop where you do some processing, but only the final result
of the processing is important, it is often better to update a temporary
accumulator which you afterwards assign to the property you need to update,
rather than incrementally updating the property itself, in order to avoid
triggering re-evaluation of binding expressions during the intermediate
stages of accumulation.

The following contrived example illustrates this point:

\qml
// bad.qml
import QtQuick 2.0

Item {
    id: root
    width: 200
    height: 200
    property int accumulatedValue: 0

    Text {
        anchors.fill: parent
        text: root.accumulatedValue.toString()
        onTextChanged: console.log("text binding re-evaluated")
    }

    Component.onCompleted: {
        var someData = [ 1, 2, 3, 4, 5, 20 ];
        for (var i = 0; i < someData.length; ++i) {
            accumulatedValue = accumulatedValue + someData[i];
        }
    }
}
\endqml

The loop in the onCompleted handler causes the "text" property binding to
be re-evaluated six times (which then results in any other property bindings
which rely on the text value, as well as the onTextChanged signal handler,
to be re-evaluated each time, and lays out the text for display each time).
This is clearly unnecessary in this case, since we really only care about
the final value of the accumulation.

It could be rewritten as follows:

\qml
// good.qml
import QtQuick 2.0

Item {
    id: root
    width: 200
    height: 200
    property int accumulatedValue: 0

    Text {
        anchors.fill: parent
        text: root.accumulatedValue.toString()
        onTextChanged: console.log("text binding re-evaluated")
    }

    Component.onCompleted: {
        var someData = [ 1, 2, 3, 4, 5, 20 ];
        var temp = accumulatedValue;
        for (var i = 0; i < someData.length; ++i) {
            temp = temp + someData[i];
        }
        accumulatedValue = temp;
    }
}
\endqml

\section2 Sequence tips

As mentioned earlier, some sequence types are fast (eg, QList<int>, QList<qreal>,
QList<bool>, QList<QString>, QStringList and QList<QUrl>) while others will be
much slower.  Aside from using these types wherever possible instead of slower types,
there are some other performance-related semantics you need to be aware of to achieve
the best performance.

Firstly, there are two different implementations for sequence types: one for where
the sequence is a Q_PROPERTY of a QObject (we'll call this a reference sequence),
and another for where the sequence is returned from a Q_INVOKABLE function of a
QObject (we'll call this a copy sequence).

A reference sequence is read and written via QMetaObject::property() and thus is read
and written as a QVariant.  This means that changing the value of any element in the
sequence from JavaScript will result in three steps occurring: the complete sequence
will be read from the QObject (as a QVariant, but then cast to a sequence of the correct
type); the element at the specified index will be changed in that sequence; and the
complete sequence will be written back to the QObject (as a QVariant).

A copy sequence is far simpler as the actual sequence is stored in the JavaScript
object's resource data, so no read/modify/write cycle occurs (instead, the resource
data is modified directly).

Therefore, writes to elements of a reference sequence will be much slower than writes
to elements of a copy sequence.  In fact, writing to a single element of an N-element
reference sequence is equivalent in cost to assigning a N-element copy sequence to that
reference sequence, so you're usually better off modifying a temporary copy sequence
and then assigning the result to a reference sequence, during computation.

Assume the existence (and prior registration into the "Qt.example 1.0" namespace) of the
following C++ type:

\code
class SequenceTypeExample : public QQuickItem
{
    Q_OBJECT
    Q_PROPERTY (QList<qreal> qrealListProperty READ qrealListProperty WRITE setQrealListProperty NOTIFY qrealListPropertyChanged)

public:
    SequenceTypeExample() : QQuickItem() { m_list << 1.1 << 2.2 << 3.3; }
    ~SequenceTypeExample() {}

    QList<qreal> qrealListProperty() const { return m_list; }
    void setQrealListProperty(const QList<qreal> &list) { m_list = list; emit qrealListPropertyChanged(); }

signals:
    void qrealListPropertyChanged();

private:
    QList<qreal> m_list;
};
\endcode

The following example writes to elements of a reference sequence in a
tight loop, resulting in bad performance:

\qml
// bad.qml
import QtQuick 2.0
import Qt.example 1.0

SequenceTypeExample {
    id: root
    width: 200
    height: 200

    Component.onCompleted: {
        var t0 = new Date();
        qrealListProperty.length = 100;
        for (var i = 0; i < 500; ++i) {
            for (var j = 0; j < 100; ++j) {
                qrealListProperty[j] = j;
            }
        }
        var t1 = new Date();
        console.log("elapsed: " + (t1.valueOf() - t0.valueOf()) + " milliseconds");
    }
}
\endqml

The QObject property read and write in the inner loop caused by the
\c{"qrealListProperty[j] = j"} expression makes this code very suboptimal.  Instead,
something functionally equivalent but much faster would be:

\qml
// good.qml
import QtQuick 2.0
import Qt.example 1.0

SequenceTypeExample {
    id: root
    width: 200
    height: 200

    Component.onCompleted: {
        var t0 = new Date();
        var someData = [1.1, 2.2, 3.3]
        someData.length = 100;
        for (var i = 0; i < 500; ++i) {
            for (var j = 0; j < 100; ++j) {
                someData[j] = j;
            }
            qrealListProperty = someData;
        }
        var t1 = new Date();
        console.log("elapsed: " + (t1.valueOf() - t0.valueOf()) + " milliseconds");
    }
}
\endqml

Secondly, a change signal for the property is emitted if any element in it changes.
If you have many bindings to a particular element in a sequence property, it is better
to create a dynamic property which is bound to that element, and use that dynamic
property as the symbol in the binding expressions instead of the sequence element,
as it will only cause re-evaluation of bindings if its value changes.

This is an unusual use-case which most clients should never hit, but is worth being
aware of, in case you find yourself doing something like this:

\qml
// bad.qml
import QtQuick 2.0
import Qt.example 1.0

SequenceTypeExample {
    id: root

    property int firstBinding: qrealListProperty[1] + 10;
    property int secondBinding: qrealListProperty[1] + 20;
    property int thirdBinding: qrealListProperty[1] + 30;

    Component.onCompleted: {
        var t0 = new Date();
        for (var i = 0; i < 1000; ++i) {
            qrealListProperty[2] = i;
        }
        var t1 = new Date();
        console.log("elapsed: " + (t1.valueOf() - t0.valueOf()) + " milliseconds");
    }
}
\endqml

Note that even though only the element at index 2 is modified in the loop, the three
bindings will all be re-evaluated since the granularity of the change signal is that
the entire property has changed.  As such, adding an intermediate binding can
sometimes be beneficial:

\qml
// good.qml
import QtQuick 2.0
import Qt.example 1.0

SequenceTypeExample {
    id: root

    property int intermediateBinding: qrealListProperty[1]
    property int firstBinding: intermediateBinding + 10;
    property int secondBinding: intermediateBinding + 20;
    property int thirdBinding: intermediateBinding + 30;

    Component.onCompleted: {
        var t0 = new Date();
        for (var i = 0; i < 1000; ++i) {
            qrealListProperty[2] = i;
        }
        var t1 = new Date();
        console.log("elapsed: " + (t1.valueOf() - t0.valueOf()) + " milliseconds");
    }
}
\endqml

In the above example, only the intermediate binding will be re-evaluated each time,
resulting in a significant performance increase.

\section2 Value-Type tips

Value-type properties (font, color, vector3d, etc) have similar QObject property
and change notification semantics to sequence type properties.  As such, the tips
given above for sequences are also applicable for value-type properties.  While
they are usually less of a problem with value-types (since the number of
sub-properties of a value-type is usually far less than the number of elements
in a sequence), any increase in the number of bindings being re-evaluated needlessly
will have a negative impact on performance.

\section2 Other JavaScript Objects

Different JavaScript engines provide different optimizations.  The JavaScript engine
which QtQuick 2 uses is optimized for object instantiation and property lookup, but
the optimizations which it provides relies on certain criteria.  If your application
does not meet the criteria, the JavaScript engine falls back to a "slow-path" mode
with much worse performance.  As such, always try to ensure you meet the following
criteria:

\list
\li Avoid using eval() if at all possible
\li Do not delete properties of objects
\endlist

\section1 Common Interface Elements

\section2 Text Elements

Calculating text layouts can be a slow operation.  Consider using the \c PlainText
format instead of \c StyledText wherever possible, as this reduces the amount of work
required of the layout engine.  If you cannot use \c PlainText (as you need to embed
images, or use tags to specify ranges of characters to have certain formatting (bold,
italic, etc) as opposed to the entire text) then you should use \c StyledText.

You should only use \c AutoText if the text might be (but probably isn't)
\c StyledText as this mode will incur a parsing cost.  The \c RichText mode should
not be used, as \c StyledText provides almost all of its features at a fraction of
its cost.

\section2 Images

Images are a vital part of any user interface.  Unfortunately, they are also a big
source of problems due to the time it takes to load them, the amount of memory they
consume, and the way in which they are used.

\section3 Asynchronous Loading

Images are often quite large, and so it is wise to ensure that loading an image doesn't
block the UI thread.  Set the "asynchronous" property of the QML Image element to
\c true to enable asynchronous loading of images from the local file system (remote
images are always loaded asynchronously) where this would not result in a negative impact
upon the aesthetics of the user interface.

Image elements with the "asynchronous" property set to \c true will load images in
a low-priority worker thread.

\section3 Explicit Source Size

If your application loads a large image but displays it in a small-sized element, set
the "sourceSize" property to the size of the element being rendered to ensure that the
smaller-scaled version of the image is kept in memory, rather than the large one.

Beware that changing the sourceSize will cause the image to be reloaded.

\section3 Avoid Run-time Composition

Also remember that you can avoid doing composition work at run-time by providing the
pre-composed image resource with your application (e.g., providing elements with shadow
effects).

\section2 Position Elements With Anchors

It is more efficient to use anchors rather than bindings to position items
relative to each other.  Consider this use of bindings to position rect2
relative to rect1:

\code
Rectangle {
    id: rect1
    x: 20
    width: 200; height: 200
}
Rectangle {
    id: rect2
    x: rect1.x
    y: rect1.y + rect1.height
    width: rect1.width - 20
    height: 200
}
\endcode

This is achieved more efficiently using anchors:

\code
Rectangle {
    id: rect1
    x: 20
    width: 200; height: 200
}
Rectangle {
    id: rect2
    height: 200
    anchors.left: rect1.left
    anchors.top: rect1.bottom
    anchors.right: rect1.right
    anchors.rightMargin: 20
}
\endcode

Note that this is still not as efficient as specifying a static size, so you should still specify static sizes via
the x, y, width and height properties.

Item coordinates are always relative to their parent, so if you wanted to be a fixed offset from your parent's
0,0 coordinate you should not use anchors. For example the following items are in the same place, but the anchors
code is not as resource efficient as fixed positioning.

\code
Rectangle {
    width: 60
    height: 60
    Rectangle {
        x: 20
        y: 20
        width: 20
        height: 20
    }
    Rectangle {
        anchors.fill: parent
        anchors.margins: 20
    }
}
\endcode

\section1 Models and Views

Most applications will have at least one model feeding data to a view.  There are
some semantics which application developers need to be aware of, in order to achieve
maximal performance.

\section2 Custom C++ Models

It is often desirable to write your own custom model in C++ for use with a view in
QML.  While the optimal implementation of any such model will depend heavily on the
use-case it must fulfil, some general guidelines are as follows:

\list
\li Be as asynchronous as possible
\li Do all processing in a (low priority) worker thread
\li Batch up backend operations so that (potentially slow) I/O and IPC is minimized
\li Use a sliding slice window to cache results, whose parameters are determined with the help of profiling
\endlist

It is important to note that using a low-priority worker thread is recommended to
minimise the risk of starving the GUI thread (which could result in worse perceived
performance).  Also, remember that synchronization and locking mechanisms can be a
significant cause of slow performance, and so care should be taken to avoid
unnecessary locking.

\section2 ListModel

QML provides a ListModel element which can be used to feed data to a ListView.
It should suffice for most use-cases and be relatively performant so long as
it is used correctly.

\section3 Populate Within A Worker Thread

ListModel elements can be populated in a (low priority) worker thread in JavaScript.  The
developer must explicitly call "sync()" on the ListModel from within the WorkerScript to
have the changes synchronized to the main thread.  See the WorkerScript documentation
for more information.

Please note that using a WorkerScript element will result in a separate JavaScript engine
being created (as the JavaScript engine is per-thread).  This will result in increased
memory usage.  Multiple WorkerScript elements will all use the same worker thread, however,
so the memory impact of using a second or third WorkerScript element is negligible once
an application already uses one.

\section3 Don't Use Dynamic Roles

The ListModel element in QtQuick 2.0 is much more performant than in QtQuick 1.0.  The
performance improvements mainly come from assumptions about the type of roles within each
element in a given model - if the type doesn't change, the caching performance improves
dramatically.  If the type can change dynamically from element to element, this optimization
becomes impossible, and the performance of the model will be an order of magnitude worse.

Therefore, dynamic typing is disabled by default; the developer must specifically set
the boolean "dynamicRoles" property of the model to enable dynamic typing (and suffer
the attendant performance degradation).  We recommend that you do not use dynamic typing
if it is possible to redesign your application to avoid it.

\section2 Views

View delegates should be kept as simple as possible.  Have just enough QML in the delegate
to display the necessary information.  Any additional functionality which is not immediately
required (e.g., if it displays more information when clicked) should not be created until
needed (see the upcoming section on lazy initialization).

The following list is a good summary of things to keep in mind when designing a delegate:
\list
\li The fewer elements that are in a delegate, the faster they can be created, and thus
   the faster the view can be scrolled.
\li Keep the number of bindings in a delegate to a minimum; in particular, use anchors
   rather than bindings for relative positioning within a delegate.
\li Avoid using ShaderEffect elements within delegates.
\li Never enable clipping on a delegate.
\endlist

You may set the \c cacheBuffer property of a view to allow asynchronous creation and
buffering of delegates outside of the visible area.  Utilizing a \c cacheBuffer is
recommended for view delegates that are non-trivial and unlikely to be created within a
single frame.

Be mindful that a \c cacheBuffer keeps additional delegates in-memory and therefore the
value derived from utilizing the \c cacheBuffer must be balanced against additional memory
usage.  Developers should use benchmarking to find the best value for their use-case, since
the increased memory pressure caused by utilizing a \c cacheBuffer can, in some rare cases,
cause reduced frame rate when scrolling.

\section1 Visual Effects

QtQuick 2 includes several features which allow developers and designers to create
exceptionally appealing user interfaces.  Fluidity and dynamic transitions as well
as visual effects can be used to great effect in an application, but some care must
be taken when using some of the features in QML as they can have performance implications.

\section2 Animations

In general, animating a property will cause any bindings which reference that property
to be re-evaluated.  Usually, this is what is desired but in other cases it may be better
to disable the binding prior to performing the animation, and then reassign the binding
once the animation has completed.

Avoid running JavaScript during animation.  For example, running a complex JavaScript
expression for each frame of an x property animation should be avoided.

Developers should be especially careful using script animations, as these are run in the main
thread (and therefore can cause frames to be skipped if they take too long to complete).

\section2 Particles

The QtQuick 2.0 Particles module allows beautiful particle effects to be integrated
seamlessly into user interfaces.  However every platform has different graphics hardware
capabilities, and the Particles module is unable to limit parameters to what your hardware
can gracefully support.  The more particles you attempt to render (and the larger they are),
the faster your graphics hardware will need to be in order to render at 60 FPS.  Affecting
more particles requires a faster CPU.  It is therefore important to test all
particle effects on your target platform carefully, to calibrate the number and size of
particles you can render at 60 FPS.

It should be noted that a particle system can be disabled when not in use
(e.g., on a non-visible element) to avoid doing unnecessary simulation.

See the \l{Particle System Performance Guide} for more in-depth information.

\section2 Shaders

Shaders written in GLSL allow for complex transformations and visual effects to be written,
however they should be used with care.  Using a ShaderEffectSource causes a scene to
prerendered into an FBO before it can be drawn.  This extra overhead is quite expensive.

A ShaderEffect element can imply a ShaderEffectSource (and the indirect rendering costs
associated with that) and also involves uploading a vertex and fragment shader program
(which is then compiled into a GLSL shader).  Each fragment shader runs once for every
pixel of the scene, and so these should be kept as simple as possible.

\section1 Controlling Element Lifetime

By partitioning an application into simple, modular components, each contained in a single
QML file, you can achieve faster application startup time and better control over memory
usage, and reduce the number of active-but-invisible elements in your application.

\section2 Lazy Initialization

The QML engine does some tricky things to try to ensure that loading and initialization of
components doesn't cause frames to be skipped, however there is no better way to reduce
startup time than to avoid doing work you don't need to do, and delaying the work until
it is necessary.  This may be achieved by using either \l Loader or creating components
\l {qtqml-javascript-dynamicobjects.html}{dynamically}.

\section3 Using Loader

The Loader is an element which allows dynamic loading and unloading of components.

\list
\li Using the "active" property of a Loader, initialization can be delayed until required.
\li Using the overloaded version of the "setSource()" function, initial property values can
   be supplied.
\li Setting the Loader \l {Loader::asynchronous}{asynchronous} property to true may also
   improve fluidity while a component is instantiated.
\endlist

\section3 Using Dynamic Creation

Developers can use the Qt.createComponent() function to create a component dynamically at
runtime from within JavaScript, and then call createObject() to instantiate it.  Depending
on the ownership semantics specified in the call, the developer may have to delete the
created object manually.  See \l{qtqml-javascript-dynamicobjects.html}
{Dynamic Object Management in QML} for more information.

\section2 Destroy Unused Elements

Elements which are invisible because they are a child of a non-visible element (e.g., the
second tab in a tab-widget, while the first tab is shown) should be initialized lazily in
most cases, and deleted when no longer in use, to avoid the ongoing cost of leaving them
active (e.g., rendering, animations, property binding evaluation, etc).

An item loaded with a Loader element may be released by resetting the "source" or
"sourceComponent" property of the Loader, while other items may be explicitly
released by calling destroy() on them.  In some cases, it may be necessary to
leave the item active, in which case it should be made invisible at the very least.

See the upcoming section on Rendering for more information on active but invisible elements.

\section1 Rendering

The scene graph used for rendering in QtQuick 2.0 allows highly dynamic, animated user
interfaces to be rendered fluidly at 60 FPS.  There are some things which can
dramatically decrease rendering performance, however, and developers should be careful
to avoid these pitfalls wherever possible.

\section2 Clipping

Clipping is disabled by default, and should only be enabled when required.

Clipping is a visual effect, NOT an optimization.  It increases (rather than reduces)
complexity for the renderer.  If clipping is enabled, an item will clip its own painting,
as well as the painting of its children, to its bounding rectangle.  This stops the renderer
from being able to reorder the drawing order of elements freely, resulting in a sub-optimal
best-case scene graph traversal.

Clipping inside a delegate is especially bad and should be avoided at all costs.

\section2 Over-drawing and Invisible Elements

If you have elements which are totally covered by other (opaque) elements, it is best to
set their "visible" property to \c false or they will be needlessly drawn.

Similarly, elements which are invisible (e.g., the second tab in a tab widget, while the
first tab is shown) but need to be initialized at startup time (e.g., if the cost of
instantiating the second tab takes too long to be able to do it only when the tab is
activated), should have their "visible" property set to \c false, in order to avoid the
cost of drawing them (although as previously explained, they will still incur the cost of
any animations or bindings evaluation since they are still active).

\section2 Manual Layouts

The scene graph renderer is able to batch up certain operations to minimise the number of
OpenGL state changes required.  However, this optimization is only possible for the
built-in layout elements provided by QtQuick 2.0, and cannot be applied to manual layouts.

Therefore, application developers should use the Row, Column, Grid, GridView and ListView
elements instead of manual layouts wherever possible.

\section1 Memory Allocation And Collection

The amount of memory which will be allocated by an application and the way in which that
memory will be allocated are very important considerations.  Aside from the obvious
concerns about out-of-memory conditions on memory-constrained devices, allocating memory
on the heap is a fairly computationally expensive operation, and certain allocation
strategies can result in increased fragmentation of data across pages.  JavaScript uses
a managed memory heap which is automatically garbage collected, and this provides some
advantages but also has some important implications.

An application written in QML uses memory from both the C++ heap and an automatically
managed JavaScript heap.  The application developer needs to be aware of the subtleties
of each in order to maximise performance.

\section2 Tips For QML Application Developers

The tips and suggestions contained in this section are guidelines only, and may not be
applicable in all circumstances.  Be sure to benchmark and analyse your application
carefully using empirical metrics, in order to make the best decisions possible.

\section3 Instantiate and initialize components lazily

If your application consists of multiple views (for example, multiple tabs) but only
one is required at any one time, you can use lazy instantiation to minimize the
amount of memory you need to have allocated at any given time.  See the prior section
on \l{Lazy Initialization} for more information.

\section3 Destroy unused objects

If you lazily instantiate components, or dynamically create objects during a JavaScript
expression, it is often better to manually \c{destroy()} them rather than waiting for
automatic garbage collection to do so.  See the prior section on
\l{Controlling Element Lifetime} for more information.

\section3 Don't manually invoke the garbage collector

In most cases, it is not wise to manually invoke the garbage collector, as it will block
the GUI thread for a substantial period of time.  This can result in skipped frames and
jerky animations, which should be avoided at all costs.

There are some cases where manually invoking the garbage collector is acceptable (and
this is explained in greater detail in an upcoming section), but in most cases, invoking
the garbage collector is unnecessary and counter-productive.

\section3 Avoid complex bindings

Aside from the reduced performance of complex bindings (for example, due to having to
enter the JavaScript execution context to perform evaluation), they also take up more
memory both on the C++ heap and the JavaScript heap than bindings which can be
evaluated by QML's optimized binding expression evaluator.

\section3 Avoid defining multiple identical implicit types

If a QML element has a custom property defined in QML, it becomes its own implicit type.
This is explained in greater detail in an upcoming section.  If multiple identical
implicit types are defined inline in a component, some memory will be wasted.  In that
situation it is usually better to explicitly define a new component which can then be
reused.

Defining a custom property can often be a beneficial performance optimization (for
example, to reduce the number of bindings which are required or re-evaluated), or it
can improve the modularity and maintainability of a component.  In those cases, using
custom properties is encouraged; however, the new type should, if it is used more than
once, be split into its own component (.qml file) in order to conserve memory.

\section3 Re-use existing components

If you are considering defining a new component, it's worth double checking that such a
component doesn't already exist in the component set for your platform.  Otherwise, you
will be forcing the QML engine to generate and store type-data for a type which is
essentially a duplicate of another pre-existing and potentially already loaded component.

\section3 Use module APIs instead of pragma library scripts

If you are using a pragma library script to store application-wide instance data,
consider using a QObject module API instead.  This should result in better performance,
and will result in less JavaScript heap memory being used.

\section2 Memory Allocation in a QML Application

The memory usage of a QML application may be split into two parts: its C++ heap usage,
and its JavaScript heap usage.  Some of the memory allocated in each will be unavoidable,
as it is allocated by the QML engine or the JavaScript engine, while the rest is
dependent upon decisions made by the application developer.

The C++ heap will contain:
\list
  \li the fixed and unavoidable overhead of the QML engine (implementation data
  structures, context information, and so on)
  \li per-component compiled data and type information, including per-type property
  metadata, which is generated by the QML engine depending on which modules are
  imported by the application and which components the application loads
  \li per-object C++ data (including property values) plus a per-element metaobject
  hierarchy, depending on which components the application instantiates
  \li any data which is allocated specifically by QML imports (libraries)
\endlist

The JavaScript heap will contain:
\list
  \li the fixed and unavoidable overhead of the JavaScript engine itself (including
  built-in JavaScript types)
  \li the fixed and unavoidable overhead of our JavaScript integration (constructor
  functions for loaded types, function templates, and so on)
  \li per-type layout information and other internal type-data generated by the JavaScript
  engine at runtime, for each type (see note below, regarding types)
  \li per-object JavaScript data ("var" properties, JavaScript functions and signal
  handlers, and non-optimized binding expressions)
  \li variables allocated during expression evaluation
\endlist

Furthermore, there will be one JavaScript heap allocated for use in the main thread, and
optionally one other JavaScript heap allocated for use in the WorkerScript thread.  If an
application does not use a WorkerScript element, that overhead will not be incurred.  The
JavaScript heap can be several megabytes in size, and so applications written for
memory-constrained devices may be best served to avoid using the WorkerScript element
despite its usefulness in populating list models asynchronously.

Note that both the QML engine and the JavaScript engine will automatically generate their
own caches of type-data about observed types.  Every component loaded by an application
is a distinct (explicit) type, and every element (component instance) which defines its
own custom properties in QML is an implicit type.  Any element (instance of a component)
which does not define any custom properties is considered by the JavaScript and QML engines
to be of the type explicitly defined by the component, rather than its own implicit type.

Consider the following example:
\qml
import QtQuick 2.0

Item {
    id: root

    Rectangle {
        id: r0
        color: "red"
    }

    Rectangle {
        id: r1
        color: "blue"
        width: 50
    }

    Rectangle {
        id: r2
        property int customProperty: 5
    }

    Rectangle {
        id: r3
        property string customProperty: "hello"
    }

    Rectangle {
        id: r4
        property string customProperty: "hello"
    }
}
\endqml

In the previous example, the rectangles \c r0 and \c r1 do not have any custom properties,
and thus the JavaScript and QML engines consider them both to be of the same type.  That
is, \c r0 and \c r1 are both considered to be of the explicitly defined \c Rectangle type.
The rectangles \c r2, \c r3 and \c r4 each have custom properties and are each considered
to be different (implicit) types.  Note that \c r3 and \c r4 are each considered to be of
different types, even though they have identical property information, simply because the
custom property was not declared in the component which they are instances of.

If \c r3 and \c r4 were both instances of a \c RectangleWithString component, and that
component definition included the declaration of a string property named \c customProperty,
then \c r3 and \c r4 would be considered to be the same type (that is, they would be
instances of the \c RectangleWithString type, rather than defining their own implicit type).

\section2 In-Depth Memory Allocation Considerations

Whenever making decisions regarding memory allocation or performance trade-offs, it is
important to keep in mind the impact of CPU-cache performance, operating system paging,
and JavaScript engine garbage collection.  Potential solutions should be benchmarked
carefully in order to ensure that the best one is selected.

No set of general guidelines can replace a solid understanding of the underlying
principles of computer science combined with a practical knowledge of the implementation
details of the platform for which the application developer is developing.  Furthermore,
no amount of theoretical calculation can replace a good set of benchmarks and analysis
tools when making trade-off decisions.

\section3 Fragmentation

Fragmentation is a C++ development issue.  If the application developer is not defining
any C++ types or plugins, they may safely ignore this section.

Over time, an application will allocate large portions of memory, write data to that
memory, and subsequently free some portions of that memory once it has finished using
some of the data.  This can result in "free" memory being located in non-contiguous
chunks, which cannot be returned to the operating system for other applications to use.
It also has an impact on the caching and access characteristics of the application, as
the "living" data may be spread across many different pages of physical memory.  This
in turn could force the operating system to swap which can cause filesystem I/O - which
is, comparatively speaking, an extremely slow operation.

Fragmentation can be avoided by utilizing pool allocators (and other contiguous memory
allocators), by reducing the amount of memory which is allocated at any one time by
carefully managing object lifetimes, by periodically cleansing and rebuilding caches,
or by utilizing a memory-managed runtime with garbage collection (such as JavaScript).

\section3 Garbage Collection

JavaScript provides garbage collection.  Memory which is allocated on the JavaScript
heap (as opposed to the C++ heap) is owned by the JavaScript engine.  The engine will
periodically collect all unreferenced data on the JavaScript heap, and if fragmentation
becomes an issue, it will compact its heap by moving all "living" data into a contiguous
region of memory (allowing the freed memory to be returned to the operating system).

\section4 Implications of Garbage Collection

Garbage collection has advantages and disadvantages.  It ensures that fragmentation is
less of an issue, and it means that manually managing object lifetime is less important.
However, it also means that a potentially long-lasting operation may be initiated by the
JavaScript engine at a time which is out of the application developer's control.  Unless
JavaScript heap usage is considered carefully by the application developer, the frequency
and duration of garbage collection may have a negative impact upon the application
experience.

\section4 Manually Invoking the Garbage Collector

An application written in QML will (most likely) require garbage collection to be
performed at some stage.  While garbage collection will be automatically triggered by
the JavaScript engine when the amount of available free memory is low, it is occasionally
better if the application developer makes decisions about when to invoke the garbage
collector manually (although usually this is not the case).

The application developer is likely to have the best understanding of when an application
is going to be idle for substantial periods of time.  If a QML application uses a lot
of JavaScript heap memory, causing regular and disruptive garbage collection cycles
during particularly performance-sensitive tasks (for example, list scrolling, animations,
and so forth), the application developer may be well served to manually invoke the
garbage collector during periods of zero activity.  Idle periods are ideal for performing
garbage collection since the user will not notice any degradation of user experience
(skipped frames, jerky animations, and so on) which would result from invoking the garbage
collector while activity is occurring.

The garbage collector may be invoked manually by calling \c{gc()} within JavaScript.
This will cause a comprehensive collection and compaction cycle to be performed, which
may take from between a few hundred to more than a thousand milliseconds to complete, and
so should be avoided if at all possible.

\section3 Memory vs Performance Trade-offs

In some situations, it is possible to trade-off increased memory usage for decreased
processing time.  For example, caching the result of a symbol lookup used in a tight loop
to a temporary variable in a JavaScript expression will result in a significant performance
improvement when evaluating that expression, but it involves allocating a temporary variable.
In some cases, these trade-offs are sensible (such as the case above, which is almost always
sensible), but in other cases it may be better to allow processing to take slightly longer
in order to avoid increasing the memory pressure on the system.

In some cases, the impact of increased memory pressure can be extreme.  In some situations,
trading off memory usage for an assumed performance gain can result in increased page-thrash
or cache-thrash, causing a huge reduction in performance. It is always necessary to benchmark
the impact of trade-offs carefully in order to determine which solution is best in a given
situation.

For in-depth information on cache performance and memory-time trade-offs, please see
Ulrich Drepper's excellent article "What Every Programmer Should Know About Memory"
(available at http://ftp.linux.org.ua/pub/docs/developer/general/cpumemory.pdf as at 18th
April 2012), and for information on C++-specific optimizations, please see Agner Fog's
excellent manuals on optimizing C++ applications (available at
http://www.agner.org/optimize/ as at 18th April 2012).

*/