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:
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 Qt 4.7
This imports version 4.7 of the "Qt" 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: location modules (defined by a URL) and installed modules (defined by a URI).
Location 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
|- Slider.qml
|- CheckBox.qml
|- Main
|- application.qml
| import "../MyComponents"
Slider { ... }
CheckBox { ... }
|
Similarly, if the directory resided on a network source, it could be imported like this:
import "https://qml.nokia.com/qml/qmlcomponents" import "https://qml.nokia.com/qml/qmlcomponents" 1.0
Remote location modules must have a qmldir file in the same directory to specify which QML files should be made available. See the example below. The qmldir file is optional for modules on the local filesystem.
Installed modules are modules that are installed on the local filesystem within the QML import path, or modules defined in C++ application code. When importing an installed module, an un-quoted URI is used, with a mandatory version number:
import Qt 4.7
import com.nokia.qml.mymodule 1.0
Installed modules that are installed into the import path or created as a QML C++ plugin must define a qmldir file.
The QML engine will search the import path for a requested installed module. The default import path includes:
The import path can be queried using QDeclarativeEngine::importPathList() and modified using QDeclarativeEngine::addImportPath().
When running the QML Viewer, use the -I option to add paths to the import path.
C++ applications can dynamically define installed modules using qmlRegisterType().
For QML C++ plugins, the module URI is automatically passed to QDeclarativeExtensionPlugin::registerTypes(). The QDeclarativeExtensionPlugin documentation shows how to use this URI to call qmlRegisterType() to enable the plugin library to be built as an installed module. Once the plugin is built and installed, the module is importable in QML, like this:
import com.nokia.TimeExample 1.0
A QML C++ plugin also requires a qmldir file to make it available to the QML engine.
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 Qt 4.7 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 Qt 4.7 as Nokia
import Ovi 1.0 as Nokia
JavaScript files must always be imported with a named import:
import "somescript.js" as MyScript Item { //... Component.onCompleted: MyScript.doSomething() }
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 location 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>]
# <Commment> 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 (eg. 1.2) must precede earlier versions (eg. 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.
If the components in the MyComponents directory from the earlier example were to be made available as a network resource, the directory would need to contain a qmldir file similar to this:
ComponentA 1.0 ComponentA.qml ComponentB 1.0 ComponentB.qml
The MyComponents directory could then be imported as a module using:
import "http://the-server-name.com/MyComponents"
Slider { ... }
CheckBox { ... }
with an optional "1.0" version specification. Notice the import fails if a later version is used, as the qmldir file specifies that these elements are only available in the 1.0 version.
For examples of qmldir files for plugins, see the Plugins example and Tutorial: Writing QML extensions with C++.
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.