qt-everywhere-src-5.14.1/qtdoc/doc/src/qmlapp/debugging.qdoc

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/*!
\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

\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 2.3

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:

\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.

\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_from>[,port_to][,host:<ip address>][,block][,file:<local socket>][,services:<comma-separated list of services to enable>]}

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 <port_number>}
or
\c {QML Debugger: Connecting to socket at <file>"}

\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 <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
JavaScript, inspecting the object tree, and profiling the activities of a QML engine. For more
information, see \l{Qt Creator: Debugging Qt Quick Projects}.

*/