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Debugging QML Applications

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宝剑锋从磨砺出，梅花香自苦寒来。这篇文章主要讲述Debugging QML Applications相关的知识，希望能为你提供帮助。
Debugging QML Applications 
Console API Log
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.
Assert
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"); }

Timer
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() { console.time("wholeFunction"); console.time("firstPart"); // first part console.timeEnd("firstPart"); // second part console.timeEnd("wholeFunction"); }

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

Exception
console.exception  prints an error message together with the stack trace of JavaScript execution at the point where it is called.
Debugging Module ImportsThe  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 2.3Rectangle { 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 InfrastructureThe  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 1:  QT_DECLARATIVE_DEBUG
  • 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 CreatorQt 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.
 
参考链接：
http://doc.qt.io/qt-5/qtquick-debugging.html
http://doc.qt.io/qtcreator/creator-debugging-helpers.html
【Debugging QML Applications】 

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