Qt Reference Documentation

# Modules

## QML Modules

A module is a set of QML content files that can be imported as a unit into a QML application. Modules can be used to organize QML content into independent units, and they can use a versioning mechanism that allows for independent upgradability of the modules.

While QML component files within the same directory are automatically accessible within the global namespace, components defined elsewhere must be imported explicitly using the `import` statement to import them as modules. For example, an `import` statement is required to use:

- A component defined in another QML file that is not in the same directory
- A component defined in a QML file located on a remote server
- A QML extension plugin library (unless the plugin is installed in the same directory)
- A JavaScript file (note this must be imported using named imports)

An `import` statement includes the module name, and possibly a version number. This can be seen in the snippet commonly found at the top of QML files:

     import QtQuick 1.0

This imports version 1.0 of the "QtQuick" module into the global namespace. (The QML library itself must be imported to use any of the QML Elements, as they are not included in the global namespace by default.)

The `Qt` module is an installed module; it is found in the import path. There are two types of QML modules: located modules (defined by a URL) and installed modules (defined by a URI).

## Located Modules

Located modules can reside on the local filesystem or a network resource, and are referred to by a quoted location URL that specifies the filesystem or network URL. They allow any directory with QML content to be imported as a module, whether the directory is on the local filesystem or a remote server.

For example, a QML project may have a separate directory for a set of custom UI components. These components can be accessed by importing the directory using a relative or absolute path, like this:

Directory structure | Contents of application.qml

     MyQMLProject
         |- MyComponents
             |- CheckBox.qml
             |- Slider.qml
             |- Window.qml
         |- Main
             |- application.qml

     import "../MyComponents"

     Window {
         Slider {
             // ...
         }
         CheckBox {
             // ...
         }
     }

Similarly, if the directory resided on a network source, it could be imported like this:

     import "http://www.my-server.com/MyQMLProject/MyComponents"
     import "http://www.my-server.com/MyQMLProject/MyComponents" 1.0

A located module can also be imported as a network resource if it has a qmldir file in the directory that specifies the QML files to be made available by the module. For example, if the `MyComponents` directory contained a `qmldir` file defined like this:

     Slider 1.0 Slider.qml
     CheckBox 1.0 CheckBox.qml
     Window 1.0 Window.qml

If the `MyComponents` directory was then hosted as a network resource, it could be imported as a module, like this:

     import "http://the-server-name.com/MyQMLProject/MyComponents"

     Window {
         Slider {
             // ...
         }
         CheckBox {
             // ...
         }
     }

with an optional "1.0" version specification. Notice the import would fail if a later version was used, as the `qmldir` file specifies that these elements are only available in the 1.0 version.

Note that modules imported as a network resource allow only access to components defined in QML files; components defined by C++ QML extension plugins are not available.

## Installed Modules

Installed modules are modules that are made available through the QML import path, as defined by QDeclarativeEngine::importPathList(), or modules defined within C++ application code. An installed module is referred to by a URI, which allows the module to be imported from QML code without specifying a complete filesystem path or network resource URL.

When importing an installed module, an un-quoted URI is used, with a mandatory version number:

     import QtQuick 1.0
     import com.nokia.qml.mymodule 1.0

When a module is imported, the QML engine searches the QML import path for a matching module. The root directory of the module must contain a qmldir file that defines the QML files and/or C++ QML extension plugins that are made available to the module.

Modules that are installed into the import path translate the URI into directory names. For example, the qmldir file of the module `com.nokia.qml.mymodule` must be located in the subpath `com/nokia/qml/mymodule/qmldir` somewhere in the QML import path. In addition it is possible to store different versions of the module in subdirectories of its own. For example, a version 2.1 of the module could be located under `com/nokia/qml/mymodule.2/qmldir` or `com/nokia/qml/mymodule.2.1/qmldir`. The engine will automatically load the module which matches best.

The import path, as returned by QDeclarativeEngine::importPathList(), defines the default locations to be searched by the QML engine for a matching module. By default, this list contains:

- The directory of the current file
- The location specified by QLibraryInfo::ImportsPath
- Paths specified by the `QML_IMPORT_PATH` environment variable

Additional import paths can be added through QDeclarativeEngine::addImportPath() or the `QML_IMPORT_PATH` environment variable. When running the QML Viewer, you can also use the `-I` option to add an import path.

### Creating Installed Modules

As an example, suppose the `MyQMLProject` directory in the previous example was located on the local filesystem at `C:\qml\projects\MyQMLProject`. The `MyComponents` subdirectory could be made available as an installed module by adding a qmldir file to the `MyComponents` directory that looked like this:

     Slider 1.0 Slider.qml
     CheckBox 1.0 CheckBox.qml
     Window 1.0 Window.qml

Providing the path `C:\qml` is added to the QML import path using any of the methods listed previously, a QML file located anywhere on the local filesystem can then import the module as shown below, without referring to the module's absolute filesystem location:

     import projects.MyQMLProject.MyComponents 1.0

     Window {
         Slider {
             // ...
         }
         CheckBox {
             // ...
         }
     }

Installed modules are also accessible as a network resource. If the `C:\qml` directory was hosted as `http://www.some-server.com/qml` and this URL was added to the QML import path, the above QML code would work just the same.

Note that modules imported as a network resource allow only access to components defined in QML files; components defined by C++ QML extension plugins are not available.

### Creating Installed Modules in C++

C++ applications can define installed modules directly within the application using qmlRegisterType(). For example, the Writing QML extensions with C++ tutorial defines a C++ class named `PieChart` and makes this type available to QML by calling qmlRegisterType():

     qmlRegisterType<PieChart>("Charts", 1, 0, "PieChart");

This allows the application's QML files to use the `PieChart` type by importing the declared `Charts` module:

     import Charts 1.0

For QML plugins, the module URI is automatically passed to QDeclarativeExtensionPlugin::registerTypes(). This method can be reimplemented by the developer to register the necessary types for the module. Below is the `registerTypes()` implementation from the QML plugins example:

     class QExampleQmlPlugin : public QDeclarativeExtensionPlugin
     {
         Q_OBJECT
     public:
         void registerTypes(const char *uri)
         {
             Q_ASSERT(uri == QLatin1String("com.nokia.TimeExample"));
             qmlRegisterType<TimeModel>(uri, 1, 0, "Time");
         }
     };

Once the plugin is built and installed, and includes a qmldir file, the module can be imported from QML, like this:

     import com.nokia.TimeExample 1.0

Unlike QML types defined by QML files, a QML type defined in a C++ extension plugin cannot be loaded by a module that is imported as a network resource.

## Namespaces: Using Named Imports

By default, when a module is imported, its contents are imported into the global namespace. You may choose to import the module into another namespace, either to allow identically-named types to be referenced, or purely for readability.

To import a module into a specific namespace, use the as keyword:

     import QtQuick 1.0 as QtLibrary
     import "../MyComponents" as MyComponents
     import com.nokia.qml.mymodule 1.0 as MyModule

Types from these modules can then only be used when qualified by the namespace:

     QtLibrary.Rectangle {
         // ...
     }

     MyComponents.Slider {
         // ...
     }

     MyModule.SomeComponent {
         // ...
     }

Multiple modules can be imported into the same namespace in the same way that multiple modules can be imported into the global namespace:

     import QtQuick 1.0 as Nokia
     import Ovi 1.0 as Nokia

### JavaScript Files

JavaScript files must always be imported with a named import:

     import "somescript.js" as MyScript

     Item {
         //...
         Component.onCompleted: MyScript.doSomething()
     }

The qualifier ("MyScript" in the above example) must be unique within the QML document. Unlike ordinary modules, multiple scripts cannot be imported into the same namespace.

## Writing a qmldir File

A `qmldir` file is a metadata file for a module that maps all type names in the module to versioned QML files. It is required for installed modules, and located modules that are loaded from a network source.

It is defined by a plain text file named "qmldir" that contains one or more lines of the form:

     # <Comment>
     <TypeName> [<InitialVersion>] <File>
     internal <TypeName> <File>
     plugin <Name> [<Path>]

# <Comment> lines are used for comments. They are ignored by the QML engine.

<TypeName> [<InitialVersion>] <File> lines are used to add QML files as types. <TypeName> is the type being made available, the optional <InitialVersion> is a version number, and <File> is the (relative) file name of the QML file defining the type.

Installed files do not need to import the module of which they are a part, as they can refer to the other QML files in the module as relative (local) files, but if the module is imported from a remote location, those files must nevertheless be listed in the `qmldir` file. Types which you do not wish to export to users of your module may be marked with the `internal` keyword: internal <TypeName> <File>.

The same type can be provided by different files in different versions, in which case later versions (e.g. 1.2) must precede earlier versions (e.g. 1.0), since the first name-version match is used and a request for a version of a type can be fulfilled by one defined in an earlier version of the module. If a user attempts to import a version earlier than the earliest provided or later than the latest provided, the import produces a runtime error, but if the user imports a version within the range of versions provided, even if no type is specific to that version, no error will occur.

A single module, in all versions, may only be provided in a single directory (and a single `qmldir` file). If multiple are provided, only the first in the search path will be used (regardless of whether other versions are provided by directories later in the search path).

The versioning system ensures that a given QML file will work regardless of the version of installed software, since a versioned import only imports types for that version, leaving other identifiers available, even if the actual installed version might otherwise provide those identifiers.

plugin <Name> [<Path>] lines are used to add QML C++ plugins to the module. <Name> is the name of the library. It is usually not the same as the file name of the plugin binary, which is platform dependent; e.g. the library `MyAppTypes` would produce `libMyAppTypes.so` on Linux and `MyAppTypes.dll` on Windows.

<Path> is an optional argument specifying either an absolute path to the directory containing the plugin file, or a relative path from the directory containing the `qmldir` file to the directory containing the plugin file. By default the engine searches for the plugin library in the directory that contains the `qmldir` file. The plugin search path can be queried with QDeclarativeEngine::pluginPathList() and modified using QDeclarativeEngine::addPluginPath(). When running the QML Viewer, use the `-P` option to add paths to the plugin search path.

## Debugging

The `QML_IMPORT_TRACE` environment variable can be useful for debugging when there are problems with finding and loading modules. See Debugging module imports for more information.