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Debugging QML Applications

Console API


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

function f(a, b) {
  console.log("a is ", a, "b is ", b);

The output is generated using the qCDebug, qCWarning, qCCritical methods in C++, with a category of "qml" or "js", depending on the type of file doing the logging. See also Debugging Techniques.


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

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


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:

function f() {
    // first part
    // second part


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.


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

function f() {
  console.count("f called");

will print f called: 1, f called: 2 ... whenever f() is executed.


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

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:

function f() {
    //Call some function that needs to be profiled.
    //Ensure that a client is attached before ending
    //the profiling session.


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

Debugging Module Imports

The 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:

import QtQuick 2.3

Rectangle { width: 100; height: 100 }

If you set QML_IMPORT_TRACE=1 before running the QML Scene (or your QML C++ application), you will see output similar to this:

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"

QML Debugging Infrastructure

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

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:

  • Qt Quick 1: CONFIG+=declarative_debug
  • Qt Quick 2: CONFIG+=qml_debug

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

  • Qt Quick 2: QT_QML_DEBUG

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:

QML debugging is enabled. Only use this in a safe environment.

Starting Applications

Start the application with the following arguments:

-qmljsdebugger=port:<port_from>[,port_to][,host:<ip address>][,block]

Where port_from (mandatory) specifies either the debugging port or the start port of a range of ports when port_to is specified, ip address (optional) specifies the IP address of the host where the application is running, and 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:

QML Debugger: Waiting for connection on port <port_number>

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

qmlprofiler -p <port> -attach <ip address>

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 Qt Creator: Debugging Qt Quick Projects.

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