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29 \page qdeclarativedebugging.html
30 \inqmlmodule QtQuick 2
37 \c console.log, console.debug, console.info, console.warn and console.error can be used to print
38 debugging information to the console. For example:
42 console.log("a is ", a, "b is ", b);
46 The output is generated using the qDebug, qWarning, qCritical methods in C++
47 (see also http://doc.qt.nokia.com/latest/debug.html#warning-and-debugging-messages).
49 \hint Setting the environment variable QML_CONSOLE_EXTENDED also prints the source
50 code location of the call.
54 \c console.assert tests that an expression is true. If not, it will write an optional message
55 to the console and print the stack trace.
60 console.assert(x == 12, "This will pass");
61 console.assert(x > 12, "This will fail");
67 \c console.time and console.timeEnd log the time (in milliseconds) that was spent between
68 the calls. Both take a string argument that identifies the measurement. For example:
72 console.time("wholeFunction");
73 console.time("firstPart");
75 console.timeEnd("firstPart");
77 console.timeEnd("wholeFunction");
83 \c console.trace prints the stack trace of JavaScript execution at the point where
84 it was called. The stack trace info contains function name, file name, line number
85 and column number. The stack trace is limited to last 10 stack frames.
89 \c console.count prints the current number of times a particular piece of code has been executed,
90 along with a message. That is,
94 console.count("f called");
98 will print \c{f called: 1}, \c{f called: 2} ... whenever \c{f()} is executed.
102 \c console.profile turns on the QML and JavaScript profilers. Nested calls are not
103 supported and a warning will be printed to the console.
105 \c console.profileEnd turns off the QML and JavaScript profilers. Calling this function
106 without a previous call to console.profile will print a warning to the console. A
107 profiling client should have been attached before this call to receive and store the
108 profiling data. For example:
113 //Call some function that needs to be profiled.
114 //Ensure that a client is attached before ending
115 //the profiling session.
116 console.profileEnd();
122 \c console.exception prints an error message together with the stack trace of JavaScript
123 execution at the point where it is called.
125 \section1 Debugging Transitions
127 When a transition doesn't look quite right, it can be helpful to view it in slow
128 motion to see what is happening more clearly. This functionality is supported
129 in the \l {QML Viewer} tool: to enable this,
130 click on the "Debugging" menu, then "Slow Down Animations".
133 \section1 Debugging module imports
135 The \c QML_IMPORT_TRACE environment variable can be set to enable debug output
136 from QML's import loading mechanisms.
138 For example, for a simple QML file like this:
143 Rectangle { width: 100; height: 100 }
146 If you set \c {QML_IMPORT_TRACE=1} before running the \l {QML Viewer}
147 (or your QML C++ application), you will see output similar to this:
150 QDeclarativeImportDatabase::addImportPath "/qt-sdk/imports"
151 QDeclarativeImportDatabase::addImportPath "/qt-sdk/bin/QMLViewer.app/Contents/MacOS"
152 QDeclarativeImportDatabase::addToImport 0x106237370 "." -1.-1 File as ""
153 QDeclarativeImportDatabase::addToImport 0x106237370 "Qt" 4.7 Library as ""
154 QDeclarativeImportDatabase::resolveType "Rectangle" = "QDeclarativeRectangle"
158 \section1 Debugging with Qt Creator
160 \l{http://qt.nokia.com/products/developer-tools}{Qt Creator} provides built-in
161 support for QML debugging. QML projects and standalone C++ applications that
162 utilize QML can be debugged on desktops as well as on remote devices.
163 For more information, see the Qt Creator Manual.