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Qml Compiler

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Qml Compiler

The Qml Compiler can be used to convert Qml source code files into precompiled Qml files. The precompiled Qml files are faster to load and do not expose the source code. Normally, the Qt either compiles the Qml files in the startup or interprets the Qml files runtime.

The Qml uses the JIT compiler shipped with Qt to create precompiled files. It works thus in same way, as Qt when it is using JIT. Only that the JIT compilation step is performed already when the program launches. The loader is used to load precompiled files and link them into the Qml Engine.

The compiler is created by creating a new version of the Qml compilation framework, that is builtin in the Qt. The class QQmlTypeCompiler, that is the core class handling the compilation, is taken as a base for the compiler. Some of its dependencies are exported on the Qt, some of the dependencies are rewritten, due to need for modifications.

The compiler, loader and examples are all based on Qt source code. They are licensed under LGPL 2.1 (with Digia Qt LGPL exception 1.1) or BSD license. See source code files for license. Collectively this component can be used under terms of LGPL 2.1 with Digia Qt LGPL exception 1.1. See LICENSE.LGPL and LGPL_EXCEPTION.txt for details.

Please read the whole document from top to bottom to get a good overview of how things work.

Working architectures

  • X86(32/64 bit)
  • ARM Thumb2

Building the Qml Compiler/Loader


At the moment to use the compiler one has to build it first. For this one first need to build the qt source distribution with the qtdeclarative directory changed. After that qmlc can be build.

When the building is completed one can compile Qml and Javascript files with the compiler and load and run with the compiled files.

Build requirements

Tested with Ubuntu 13.10 but will probably work fine with any Linux.


Building Qt and QtDeclarative

  • Download Qt source package,
  • Extract the archive and enter directory,
tar zxvf qt-everywhere-opensource-src-5.3.1.tar.gz
cd qt-everywhere-opensource-src-5.3.1/
  • Move QtWebkit things so it excluded from the build process. It takes very long to build, prone to errors and we don't need it:
mkdir bak
mv qtwebkit bak
mv qtwebkit-examples bak
  • Replace qtdeclarative directory,
mv qtdeclarative bak
git clone
  • Configure and build
./configure -developer-build -nomake tests -debug -opensource -confirm-license
make # use makes -j.. option for faster builds

Building qmlc

While still in the qt-everywhere-opensource-src-5.3.1 directory

  • Clone
git clone
  • Build
cd qmlc
source qmlc.env

Compiling and loading qml/js files

  • In order to use the compiler to compile qml/js files always source the qmlc.env of a compiled qmlc package.
source qmlc.env

Compiling and loading single qml/js files

  • Use qmc to compile qml/js files. A .qmc/.jsc file with the same name will be created. For example if test.qml have no dependencies on other qml/js files or c++ plugins.
qmc test.qml
  • Use qmcloader from the tools directory to load(run) the compiled qml file.
qmcloader test.qmc

Compiling more complex projects

When compiling a qml/js file which have imports and elements to other qml/js/c++ plugins all those have to be available during compiling just like if one were going to run the qml/js as normal through qmlscene or one's program.

One can look at the source and build files of tests/manual/multipleitems/ to see how to handle projects with multiple files, c++ plugins etc. via a .pri file.

For example with two qml files A.qml and B.qml in the same directory. A uses B but B doesn't use A. Both have to be compiled to be loaded. The order that one compiles them doesn't matter, only that the raw qml is available.

qmc A.qml
qmc B.qml
qmcloader A.qmc

If one is importing and using a c++ plugin extension that contains qml/js those have to be compiled first and the resulting .so, qmldir and qml/js files has to be available just as when those dependencies are installed.

For example with A.qml having an 'import Plugin 2.0' import with Plugin containing qml/js, there has to be a Plugin directory with the .so and a qmldir file as usual. Then the following should succeed.

# Plugin directory contains at least qmldir, If not using the
# resource system qml/js files have to be there as well.
qmc A.qml

To run/load an application that uses a plugin that contains qml/js one needs to add a qmldir_loader file to the plugins directory that is similar to the qmldir file with the qml/js extensions replaces with qmc/js. Note if a plugin doesn't contain qml/js it doesn't need any changes in order to be used from compiled files.

# Plugin directory contains at least qmldir_loader, and qmc/jsc
# files.  All other qml/js it depends on has to be available as compiled
# qmc/jsc files.
qmcloader A.qmc

Using the compiler from qmake

This explains how to change existing applications or plugins to use the compiler.

There is a qmlc.pri file in qmlc that can be included in .pro files to facilitate application and plugin extension qmlc/qmc building. It has comments at the top explaining possible variables one can set.

In tests/manual/multipleitems/ there is a plugin and an application that uses it that can be checked for example usage.

The application in tests/manual/multipleitems/ depends on the c++ plugin in tests/manual/multipleitems/plugin so we build the plugin first with tests/manual/multipleitems/plugin/


Example tests/manual/multipleitems/plugin/

Basic steps are,

  • Make a copy of qmldir and name it qmldirloader and in it change qml/js extensions to qmc/jsc extensions. This is used by the loader program to load the qmc/jsc files. Check for example the difference between tests/manual/multipleitems/plugin/qmldir and tests/manual/multipleitems/plugin/qmldirloader.

  • If one were already building an uncompiled version of the plugin one might have a file like tests/manual/multipleitems/plugin/ up to the 'qmc start' comment. To compile the qml/js files in of the plugin one would add something like between the 'qmc start' and 'qmc end' comments.


For an existing application with a tests/manual/multipleitems/main.cpp that contains the main() entry pint and a tests/manual/multipleitems/ file to build the application.

  • Create a mainloader.cpp file, it would be very similar to tests/manual/multipleitems/mainloader.cpp except the headers and qRegister.. calls will be different for different applications and loader.loadComponent will have the applications root qml file as argument.

  • We create a new .pro file that has the same content as the uncompiled programs .pro file with changes so the loader gets build. We also add QMLC.. variables and include qmlc.pri to compile it's qml/js. See, tests/manual/multipleitems/ and tests/manual/multipleitems/

Developing the compiler and loader further

  • In order to work on the compiler after it was build one has to source the qmlc.env file that setup the paths.
source qmlc.env
  • There are test cases in the compiletest directory. One can run it like this,
  • The multipleitems manual test is currently used to test complex application layouts with multiple files/c++ plugins etc.
cd tests/manual/multipleitems/

Known limitations

There are few limitations with the compiler. The compiler requires a modified version of the Qt. The modifications are available in the qtdeclarative repository. Other than that, the target is to remove these limitations. There might be also a way to no require modified Qt. The reason for the other limitations is the lack of implementation.

  • It is not possible to mix precompiled and source Qml files
  • It is not possible to load Qml from network
  • Few structures are still unsupported, for example composite singleton
  • The test cases are still not very comprehensive

List of current issues can be found at:

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