Qt WebChannel JavaScript API | Qt WebChannel 5.4

- Qt 5.4
- Qt WebChannel
- Qt WebChannel JavaScript API
- Qt 5.4.2 Reference Documentation

# Qt WebChannel JavaScript API

## Setup

To communicate with a QWebChannel or WebChannel, any HTML client must use and setup the JavaScript API provided by `qwebchannel.js`. For HTML clients run inside Qt WebKit, you can load the file via `qrc:///qtwebchannel/qwebchannel.js`. For external clients you will need to copy the file to your webserver. Then instantiate a QWebChannel object and pass it a transport object and a callback function, which will be invoked once the initialization of the channel finished and published objects become available.

The transport object implements a minimal message passing interface. It should be an object with a `send()` function, which takes a stringified JSON message and transmits it to the server-side QWebChannelAbstractTransport object. Furthermore, its `onmessage` property should be called when a message from the server was received. This interface is implemented internally by the Qt WebKit navigator.qtWebChannelTransport object. Alternatively, you can also use a WebSocket, which also implements this interface.

Note that the JavaScript QWebChannel object should be constructed once the transport object is fully operational. In case of a WebSocket, that means you should create the QWebChannel in the socket's `onopen` handler. Take a look at the Qt WebChannel Standalone Example to see how this is done.

## Interacting with QObjects

Once the callback passed to the QWebChannel object is invoked, the channel has finished initialization and all published objects are accessible to the HTML client via the `channel.objects` property. Thus, assuming an object was published with the identifier "foo", then we can interact with it as shown in the example below. Note that all communication between the HTML client and the QML/C++ server is asynchronous. Properties are cached on the HTML side. Furthermore keep in mind that only QML/C++ data types which can be converted to JSON will be (de-)serialized properly and thus accessible to HTML clients.

    new QWebChannel(yourTransport, function(channel) {

        // Connect to a signal:
        channel.objects.foo.mySignal.connect(function() {
            // This callback will be invoked whenever the signal is emitted on the C++/QML side.
            console.log(arguments);
        });

        // To make the object known globally, assign it to the window object, i.e.:
        window.foo = channel.objects.foo;

        // Invoke a method:
        foo.myMethod(arg1, arg2, function(returnValue) {
            // This callback will be invoked when myMethod has a return value. Keep in mind that
            // the communication is asynchronous, hence the need for this callback.
            console.log(returnValue);
        });

        // Read a property value, which is cached on the client side:
        console.log(foo.myProperty);

        // Writing a property will instantly update the client side cache.
        // The remote end will be notified about the change asynchronously
        foo.myProperty = "Hello World!";

        // To get notified about remote property changes,
        // simply connect to the corresponding notify signal:
        foo.onMyPropertyChanged.connect(function(newValue) {
            console.log(newValue);
        });

        // One can also access enums that are marked with Q_ENUM:
        console.log(foo.MyEnum.MyEnumerator);
    });