// Copyright (C) 2022 The Qt Company Ltd. // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only /*! \page qtquick-debugging.html \ingroup qtquick-tools \title Debugging QML Applications \brief Provides information on how to use QML's debugging tools. When you develop an application with QML, there are many ways to debug possible issues that you may face. The sections below describe the debugging tools available and how to use them. \section1 Console API \table \header \li Feature \li Description \row \keyword console log \li Log \li Use \c{console.log}, \c{console.debug}, \c{console.info}, \c{console.warn}, or \c{console.error} 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 qCDebug, qCWarning, or qCCritical methods in C++, with a category of \c qml or \c js, depending on the type of file doing the logging. See also \l {Debugging Techniques}. \row \keyword console assert \li Assert \li \c console.assert tests that an expression is true. If not, it writes an optional message to the console and prints the stack trace. For example: \code function f() { var x = 12 console.assert(x == 12, "This will pass"); console.assert(x > 12, "This will fail"); } \endcode \row \keyword console time \keyword console timeEnd \li Timer \li \c{console.time} and \c{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 \row \keyword console trace \li Trace \li \c console.trace prints the stack trace of the JavaScript execution at the point where it was called. This stack trace information contains the function name, file name, line number, and column number. The stack trace is limited to last 10 stack frames. \row \keyword console count \li Count \li \c console.count prints the current number of times a particular piece of code has run, along with a message. For example: \code function f() { console.count("f called"); } \endcode The code sample above prints \c{f called: 1}, \c{f called: 2} ... whenever \c{f()} is run. \row \keyword console profile \li Profile \li \c console.profile turns on the QML and JavaScript profilers. Nested calls are not supported and prints a warning to the console. \row \keyword console profileEnd \li ProfileEnd \li \c console.profileEnd turns off the QML and JavaScript profilers. Calling this function without a previous call to \c console.profile prints a warning to the console. A profiling client needs to be 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 \row \keyword console exception \li Exception \li \c console.exception prints an error message together with the stack trace of JavaScript execution at the point where it is called. \endtable Alternatively, a \l {QLoggingCategory}{logging category} can be passed as the first argument to any of these \c {console} functions. See \l [QML] LoggingCategory for more details. \section1 Debugging Module Imports Set the \c QML_IMPORT_TRACE environment variable to enable debug output from QML's import loading mechanisms. For example, for a simple QML file like this: \qml import QtQuick Rectangle { width: 100; height: 100 } \endqml If you set \c {QML_IMPORT_TRACE=1} before running the \l{qml_runtime_tool}{QML Runtime Tool} or your QML C++ application, you will see output similar to: \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 \keyword declarative_debug \keyword qml debug The \l{Qt Qml} module provides services for debugging, inspecting, and profiling applications via a TCP port or a local socket. \note The \c qmltooling plugins that are required for debugging and profiling QML applications on devices are automatically installed during Qt installation. They must be deployed to the devices for debugging and profiling to work. \section2 Enabling the Infrastructure When you compile your application, you must explicitly enable the debugging infrastructure. If you use qmake, you can add the configuration parameters to the project \c{.pro} file: \list \li Qt Quick 1: \c {CONFIG+=declarative_debug} \li Qt Quick 2: \c {CONFIG+=qml_debug} \endlist If you use another 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 may compromise the integrity of your 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 To enable debugging -- from the start or to attach a debugger later on -- start the application with the following arguments: \c {-qmljsdebugger=port:[,port_to][,host:][,block][,file:][,services:]} Where: \list \li the mandatory \c {port_from} specifies either the debugging port or the start port of a range of ports when \c {port_to} is specified \li the optional \c {ip address} specifies the IP address of the host where the application is running \li the optional \c block prevents the application from running until the debug client connects to the server \li the optional \c {file} specifies the local socket. \li the optional \c {services} specifies the services to enable; the default is all that are found. Note that the \c{v4 debug} service disables the JIT. \endlist After the application has successfully started, it displays the following message: \c {QML Debugger: Waiting for connection on port } or \c {QML Debugger: Connecting to socket at "} \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 this tool, enter the following command: \c {qmlprofiler -p -attach } \section1 Debugging with \QC \QC uses the debugging infrastructure to debug, inspect, and profile Qt Quick applications on the desktop as well as on remote devices. \QC provides integrated clients for debugging JavaScript, inspecting the object tree, and profiling the activities of a QML engine. For more information, see \l{\QC: Debugging Qt Quick Projects}. */