public async int call_async () {
    return 1;
}

call_async.begin ((obj, res) => {
    var ret = call_async.end (res);
});

To call an asynchronous functions from a synchronous context, use the begin method and pass a callback to receive the result. The two arguments are:

• obj is a GLib.Object if this call was defined in a class
• res is a GLib.AsyncResult holding the result of the asynchronous operation

The end method extract the result of the operation.

The GLib.Task provide low-level API for performing asynchronous operations.

var task = new GLib.Task (null, null, (obj, result) => {
    try {
        var ret = result.propagate_boolean ();
    } catch (Error err) {
        // handler err...
    }
});

Later in a thread or a callback:

task.return_boolean (true);

To use the GLib.Task internal thread pool:

task.run_in_thread (() => {
    task.return_boolean (true);
});

In order to chain asynchronous operations and avoid a callback hell, Vala supports the yield statement.

Used with an asynchronous invocation, it will pause the current coroutine until the call is completed and extract the result.

Used alone, yield pause the current coroutine until it's being woken up by invoking its source callback.

public async int foo_async () {
    yield; // pause the coroutine
    Timeout.add_seconds (5, bar_async.callback); // wakeup in 5 seconds
    return ret + 10;
}

public async int bar_async () {
    var ret = yield foo_async ();
}