In addition, it compiles some functions and expressions in QML files into C++ code, within limitations set by the nature of JavaScript. It generates C++ code for functions that can be exhaustively analyzed. The following flow chart explains the compilation workflow.
QML script compiler is available in two versions. One is qmlcachegen , which is a part of the Qt Quick 编译器 . Another one is qmlsc , which is a part of the commercial-only add-on Qt Quick Compiler Extensions .
The base functionality of the QML script compiler is available in both the
qmlsc
tool and the
qmlcachegen
工具。
qmlsc
can generate more efficient C++ code when run in
direct
mode. It is enabled by passing
--direct-calls
凭借
QT_QMLCACHEGEN_ARGUMENTS
to
qt_add_qml_module
. In direct mode
qmlsc
expects that all C++ types used in your QML code can be made available to the generated code by simple C++ include statements. This means you have to link to private Qt APIs by passing Qt::QuickPrivate, Qt::QmlPrivate, etc to link_libraries() in CMake.
qmlcachegen
does not implement direct mode and ignores the
–direct-calls
option. Furthermore,
qmlsc
accepts a
–static
option that you can also pass via
QT_QMLCACHEGEN_ARGUMENTS
in
qt_add_qml_module
。在
static
mode,
qmlsc
ignores shadowing of properties. This is dangerous if your code actually shadows properties, but allows for more JavaScript code to be compiled to C++.
qmlcachegen
ignores the
–static
选项。
Many JavaScript constructs cannot be efficiently represented in C++. The QML script compiler skips the C++ code generation for functions that contain such constructs and only generates byte code to be interpreted or run through the Just-in-time compiler. Most common QML expressions are rather simple: value lookups on QObjects, arithmetics, simple if/else or loop constructs. Those can easily be expressed in C++, and doing so makes your application run faster.
