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29 \page qtqml-javascript-dynamicobjectcreation.html
30 \title Dynamic QML Object Creation from JavaScript
31 \brief instantiating and managing QML objects from JavaScript
33 QML supports the dynamic creation of objects from within JavaScript. This is
34 useful to delay instantiation of objects until necessary, thereby improving
35 application startup time. It also allows visual objects to be dynamically
36 created and added to the scene in reaction to user input or other events.
38 See the \l {declarative/toys/dynamicscene}{Dynamic Scene example} for a
39 demonstration of the concepts discussed on this page.
42 \section1 Creating Objects Dynamically
44 There are two ways to create objects dynamically from JavaScript. You can
45 either call \l {QML:Qt::createComponent()}{Qt.createComponent()} to
46 dynamically create a \l Component object, or use \l{QML:Qt::createQmlObject()}
47 {Qt.createQmlObject()} to create an object from a string of QML. Creating a
48 component is better if you have an existing component defined in a QML document
49 and you want to dynamically create instances of that component. Otherwise,
50 creating an object from a string of QML is useful when the object QML itself is
54 \section2 Creating a Component Dynamically
56 To dynamically load a component defined in a QML file, call the
57 \l {QML:Qt::createComponent()}{Qt.createComponent()} function in the
58 \l {QmlGlobalQtObject}{Qt object}.
59 This function takes the URL of the QML file as its only argument and creates
60 a \l Component object from this URL.
62 Once you have a \l Component, you can call its \l {Component::createObject()}
63 {createObject()} method to create an instance of the component. This function
64 can take one or two arguments:
66 \li The first is the parent for the new object. The parent can be a graphical
67 object (QtQuick item) or non-graphical object (QtQml QtObject or C++
68 QObject). Only graphical objects with graphical parent objects will be
69 rendered to the QtQuick visual canvas. If you wish to set the parent later
70 you can safely pass \c null to this function.
71 \li The second is optional and is a map of property-value pairs that define
72 initial any property values for the object. Property values specified by
73 this argument are applied to the object before its creation is finalized,
74 avoiding binding errors that may occur if particular properties must be
75 initialized to enable other property bindings. Additionally, there are
76 small performance benefits when compared to defining property values and
77 bindings after the object is created.
80 Here is an example. First there is \c Sprite.qml, which defines a simple QML component:
82 \snippet qml/Sprite.qml 0
84 Our main application file, \c main.qml, imports a \c componentCreation.js
85 JavaScript file that will create \c Sprite objects:
87 \snippet qml/createComponent.qml 0
89 Here is \c componentCreation.js. Notice it checks whether the component
90 \l{Component::status}{status} is \c Component.Ready before calling
91 \l {Component::createObject()}{createObject()} in case the QML file is loaded
92 over a network and thus is not ready immediately.
94 \snippet qml/componentCreation.js vars
96 \snippet qml/componentCreation.js func
97 \snippet qml/componentCreation.js remote
98 \snippet qml/componentCreation.js func-end
100 \snippet qml/componentCreation.js finishCreation
102 If you are certain the QML file to be loaded is a local file, you could omit
103 the \c finishCreation() function and call \l {Component::createObject()}
104 {createObject()} immediately:
106 \snippet qml/componentCreation.js func
107 \snippet qml/componentCreation.js local
108 \snippet qml/componentCreation.js func-end
110 Notice in both instances, \l {Component::createObject()}{createObject()} is
111 called with \c appWindow passed as the parent argument, since the dynamically
112 created object is a visual (QtQuick) object. The created object will become a
113 child of the \c appWindow object in \c main.qml, and appear in the scene.
115 When using files with relative paths, the path should
116 be relative to the file where \l {QML:Qt::createComponent()}
117 {Qt.createComponent()} is executed.
119 To connect signals to (or receive signals from) dynamically created objects,
120 use the signal \c connect() method. See
121 \l{Signal and Handler Event System#Connecting Signals to Methods and Signals}
122 {Connecting Signals to Methods and Signals} for more information.
124 It is also possible to instantiate components without blocking via the
125 \l {Component::incubateObject()}{incubateObject()} function.
127 \section2 Creating an Object from a String of QML
129 If the QML is not defined until runtime, you can create a QML object from
130 a string of QML using the \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()}
131 function, as in the following example:
133 \snippet qml/createQmlObject.qml 0
135 The first argument is the string of QML to create. Just like in a new file,
136 you will need to import any types you wish to use. The second argument is the
137 parent object for the new object, and the parent argument semantics which apply
138 to components are similarly applicable for \c createQmlObject().
139 The third argument is the file path to associate with the new object; this is
140 used for error reporting.
142 If the string of QML imports files using relative paths, the path should be
143 relative to the file in which the parent object (the second argument to the
147 \section1 Maintaining Dynamically Created Objects
149 When managing dynamically created objects, you must ensure the creation context
150 outlives the created object. Otherwise, if the creation context is destroyed
151 first, the bindings in the dynamic object will no longer work.
153 The actual creation context depends on how an object is created:
156 \li If \l {QML:Qt::createComponent()}{Qt.createComponent()} is used, the
157 creation context is the QQmlContext in which this method is called
158 \li If \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()} is called, the
159 creation context is the context of the parent object passed to this method
160 \li If a \c {Component{}} object is defined and \l {Component::createObject()}
161 {createObject()} or \l {Component::incubateObject()}{incubateObject()} is
162 called on that object, the creation context is the context in which the
163 \c Component is defined
166 Also, note that while dynamically created objects may be used the same as other
167 objects, they do not have an id in QML.
170 \section1 Deleting Objects Dynamically
172 In many user interfaces, it is sufficient to set a visual object's opacity to 0
173 or to move the visual object off the screen instead of deleting it. If you have
174 lots of dynamically created objects, however, you may receive a worthwhile
175 performance benefit if unused objects are deleted.
177 Note that you should never manually delete objects that were dynamically
178 created by convenience QML object factories (such as \l Loader and
179 \l Repeater). Also, you should avoid deleting objects that you did not
180 dynamically create yourself.
182 Items can be deleted using the \c destroy() method. This method has an optional
183 argument (which defaults to 0) that specifies the approximate delay in
184 milliseconds before the object is to be destroyed.
186 Here is an example. The \c application.qml creates five instances of the
187 \c SelfDestroyingRect.qml component. Each instance runs a NumberAnimation,
188 and when the animation has finished, calls \c destroy() on its root object to
193 \li \c application.qml
194 \li \c SelfDestroyingRect.qml
197 \li \snippet qml/dynamicObjects-destroy.qml 0
198 \li \snippet qml/SelfDestroyingRect.qml 0
202 Alternatively, the \c application.qml could have destroyed the created object
203 by calling \c object.destroy().
205 Note that it is safe to call destroy() on an object within that object. Objects
206 are not destroyed the instant destroy() is called, but are cleaned up sometime
207 between the end of that script block and the next frame (unless you specified a
210 Note also that if a \c SelfDestroyingRect instance was created statically like
221 This would result in an error, since objects can only be dynamically
222 destroyed if they were dynamically created.
224 Objects created with \l{QML:Qt::createQmlObject()}{Qt.createQmlObject()}
225 can similarly be destroyed using \c destroy():
227 \snippet qml/createQmlObject.qml 0
228 \snippet qml/createQmlObject.qml destroy