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/*!
\page qtquick-debugging.html
\ingroup qtquick-tools
\title Debugging QML Applications
\brief debugging tools in QML

\section1 Console API

\section2 Log

\c console.log, console.debug, console.info, console.warn and console.error can be used to print
debugging information to the console. For example:

\code
function f(a, b) {
  console.log("a is ", a, "b is ", b);
}
\endcode

The output is generated using the qDebug, qWarning, qCritical methods in C++
(see also \l {Debugging Techniques}).

Setting the environment variable QML_CONSOLE_EXTENDED also prints the source
code location of the call.

\section2 Assert

\c console.assert tests that an expression is true. If not, it will write an optional message
to the console and print the stack trace.

\code
function f() {
  var x = 12
  console.assert(x == 12, "This will pass");
  console.assert(x > 12, "This will fail");
}
\endcode

\section2 Timer

\c console.time and console.timeEnd log the time (in milliseconds) that was spent between
the calls. Both take a string argument that identifies the measurement. For example:

\code
function f() {
    console.time("wholeFunction");
    console.time("firstPart");
    // first part
    console.timeEnd("firstPart");
    // second part
    console.timeEnd("wholeFunction");
}
\endcode

\section2 Trace

\c console.trace prints the stack trace of the JavaScript execution at the point where
it was called. The stack trace info contains the function name, file name, line number
and column number. The stack trace is limited to last 10 stack frames.

\section2 Count

\c console.count prints the current number of times a particular piece of code has been executed,
along with a message. That is,

\code
function f() {
  console.count("f called");
}
\endcode

will print \c{f called: 1}, \c{f called: 2} ... whenever \c{f()} is executed.

\section2 Profile

\c console.profile turns on the QML and JavaScript profilers. Nested calls are not
supported and a warning will be printed to the console.

\c console.profileEnd turns off the QML and JavaScript profilers. Calling this function
without a previous call to console.profile will print a warning to the console. A
profiling client should have been attached before this call to receive and store the
profiling data. For example:

\code
function f() {
    console.profile();
    //Call some function that needs to be profiled.
    //Ensure that a client is attached before ending
    //the profiling session.
    console.profileEnd();
}
\endcode

\section2 Exception

\c console.exception prints an error message together with the stack trace of JavaScript
execution at the point where it is called.

\section1 Debugging Module Imports

The \c QML_IMPORT_TRACE environment variable can be set to enable debug output
from QML's import loading mechanisms.

For example, for a simple QML file like this:

\qml
import QtQuick 2.0

Rectangle { width: 100; height: 100 }
\endqml

If you set \c {QML_IMPORT_TRACE=1} before running the \l{qtquick-qmlscene.html}
{QML Scene} (or your QML C++ application), you will see output similar to this:

\code
QQmlImportDatabase::addImportPath "/qt-sdk/imports"
QQmlImportDatabase::addImportPath "/qt-sdk/bin/QMLViewer.app/Contents/MacOS"
QQmlImportDatabase::addToImport 0x106237370 "." -1.-1 File as ""
QQmlImportDatabase::addToImport 0x106237370 "Qt" 4.7 Library as ""
QQmlImportDatabase::resolveType "Rectangle" = "QDeclarativeRectangle"
\endcode

\section1 QML Debugging Infrastructure

The \l{Qt QML} module provides services for debugging, inspecting, and profiling
applications via a TCP port.

\section2 Enabling the Infrastructure

You have to explicitly enable the debugging infrastructure when compiling your
application. If you use qmake, you can add the configuration parameters to
the project .pro file:

\list
    \li Qt Quick 1: \c {CONFIG+=declarative_debug}
    \li Qt Quick 2: \c {CONFIG+=qml_debug}
\endlist

If you use some other build system, you can pass the following defines to the
compiler:

\list
    \li Qt Quick 1: \c {QT_DECLARATIVE_DEBUG}
    \li Qt Quick 2: \c {QT_QML_DEBUG}
\endlist

\note Enabling the debugging infrastructure might compromise the integrity of
the application and system, and therefore, you should only enable it in a
controlled environment. When the infrastructure is enabled, the application
displays the following warning:

\c {QML debugging is enabled. Only use this in a safe environment.}

\section2 Starting Applications

Start the application with the following arguments:

\c {qmljsdebugger=port:<port_from>[,port_to][,host:<ip address>][,block]}

Where \c {port_from} (mandatory) specifies either the debugging port or the
start port of a range of ports when \c {port_to} is specified, \c {ip address}
(optional) specifies the IP address of the host where the application is
running, and \c block (optional) prevents the application from running until the
debug client connects to the server. This enables debugging from the start.

After the application has successfully started, it displays the following
message:

\c {QML Debugger: Waiting for connection on port <port_number>}

\section2 Connecting to Applications

When the application is running, an IDE or a tool that implements the binary
protocol can connect to the open port.

Qt provides a \c qmlprofiler command line tool to capture profiling data in a
file. To run the tool, enter the following command:

\c {qmlprofiler -p <port> -attach <ip address>}

\section1 Debugging with Qt Creator

Qt Creator uses the debugging infrastructure to debug, inspect
and profile Qt Quick applications on the desktop as well as on remote devices.
Qt Creator provides integrated clients for debugging JS, inspecting the object
tree, and profiling the activities of a QML engine. For more information, see
\l{Qt Creator: Debugging Qt Quick Projects}.
*/
