Intents

Much like the intent concept used on mobile operating systems, the application manager's intent system is built around the idea of having a light weight inter-process communication (IPC) mechanisms between apps and the System UI without hard coded coupling or even compile time dependencies. The intent itself is a description of an action that the sender application would like to be performed by whichever part of the system is most capable of doing so. It is the application manager's responsibility to make sure that these intent requests are routed to the correct receivers. A very simple example would be a showImage intent, where a sending application could ask the system to display an image file: if the system has an image viewer application installed, that registered itself for handling the showImage intent, then any application could display image files without knowing any specifics of the image viewer application.

The Android documentation also has a nice introduction to this concept.

The application manager's approach to intents is much simpler: although it (still) lacks a few features compared to other solutions (e.g. support for binary data or system broadcasts), it also comes with a very simple and straight forward API.

Note: While mobile operating systems provide you with both the intent mechanism as well as a description of all the defined intent APIs, the application manager is only providing the mechanism. This is in line with the philosophy used in other modules within the application manager, where the System UI is responsible for defining the system's interface and behavior.

Here is some terminology to make it easier to understand the difference between all the IDs involved in an intent request:

  • the intentId: the name or class of an intent. In complex systems it makes sense to use reverse-DNS notation here, e.g. io.qt.openUrl.
  • the requestId: a UUID used to track an intent request throughout the system, as it crosses process boundaries between System UI and up to two applications.
  • the applicationId denotes the application that is responsible for handling the request. This directly maps to the ApplicationObject::id value. If the handling application is in fact the System UI, this id will have the special value :sysui:.
  • the requestingApplicationId does the same as the applicationId, but for the application sending the request. If the requesting application is in fact the System UI, this id will have the special value :sysui:.

Arbitrary data can be attached to an intent request (see IntentClient::sendIntentRequest) and to an intent request reply (see IntentRequest::sendReply). Please be aware that this data needs to be first serialized from JavaScript objects to the underlying IPC transport and then deserialized back to a JavaScript object on the receiving side. Restricting the types used within the parameters and result object to JSON types is strongly advised.

Registering Intents

Each application defines the intents it is capable of handling in its info.yaml manifest file. Please note that neither the packager tool nor the application manager are able to validate the intent ids in the manifest file to be valid for your specific project.

Also the System UI itself can register intents: in this case no separate manifest file is needed, but instead the QML handler object itself will take care of providing the data necessary for the registration; see the next section.

The application manager parses all those manifest files on startup (as well as when an application is installed at run time). Only the application manager itself and the System UI have access to the full list of registered intents defined by all the applications; for security and privacy reason sthe applications themselves can't access this list.

Handling Intents in QML

In order to handle intents from within your QML application, you have to instantiate IntentHandler items for all the intents you have defined in your manifest file. Make sure that these items are constructed together with the application QML root object and are never deleted afterwards.

Any incoming intent request is matched against all the previously registered intents in applications and the System UI. When the application manager determines that your application is responsible for handling such an intent, it will first start your application (if it was not already running) and will then emit the appropriate IntentHandler's requestReceived signal on the application side. Replying to those incoming requests can be done either synchronously or asynchronously.

As for handling intents within the System UI: instead of creating IntentHandler items, you have to instantiate IntentServerHandler items. The IntentServerHandler is actually derived from IntentHandler, so it works the same way as its application side counterpart: it only adds the required properties to define all the meta-data (e.g. names, icon, ...), whereas those are provided through the manifest file for applications.

Creating Intent Requests in QML

An application that wants to send out an intent request can do so by calling the factory function IntentClient::sendIntentRequest(). These requests cannot be created by other means. The request is sent to the system immediately and the returned IntentRequest object can then be used to track the completion and possible result value of this request.

Within the System UI you can use exactly the same mechanism to create and track intent requests.

Disambiguating Intent Request in the System UI

In case an incoming intent request could potentially be handled by more than one application, the application manager will reach out to the System UI to disambiguate the request and pick an application. The System UI can react on this by connecting to the IntentServer::disambiguationRequest() signal and then picking one of the possible applications via acknowledgeDisambiguationRequest. It could even reject the request via rejectDisambiguationRequest if the request cannot be disambiguated at all.

The IntentModel in the System UI

While the applications only know about their own intents, the System UI has full access to meta-data of all the registered intents via the IntentServer singleton and IntentModel classes. Just like with the ApplicationManager singleton and the ApplicationModel class, the IntentServer is the system-wide singleton holding all the data, while the IntentModel class can be used to have a convenient sorted and filtered model interface to the raw data.

By using these model interfaces, the classic launcher grid that normally operates on an ApplicationModel can be replaced with a version using an IntentModel, so that applications are implicitly launched via specific intent requests that could include parameters, instead of just calling the ApplicationObject::start() function.

Intents for Native Applications

The intents mechanism is using D-Bus as a transport protocol between the System UI and the applications' processes. The D-Bus interface is defined in the file src/dbus-lib/io.qt.applicationmanager.intentinterface.xml. The D-Bus API closely resembles the QML API. The intent, application and request ids are sent as strings, while the parameters are converted between JavaScript objects and D-Bus a{sv} variant dictionaries.

If you are using the application manager's convenience launcher-lib as a basic building block for your mixed native and QML application, then all the D-Bus handling is already implemented for you and you can use the IntentHandler items as described above.

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