4.16. Dynamic Objects

Vala normally checks every property, method and signal you use against the bindings (VAPI files) of the library at compile time. Sometimes a GObject-based library is only partly described by its bindings, so members that exist at run time can't be found by the compiler. With the dynamic keyword, an object can be treated as if it did not have all of its properties and signals specified.

Without dynamic, using a member the compiler doesn't know about is an error:

Object action = new SimpleAction ("greet", null);
action.enabled = false;   // error: The name `enabled' does not exist in the context of `GLib.Object?'

Declaring the variable as dynamic defers the lookup to run time:

void on_activate (Object source, Variant? parameter) {
    print ("activated\n");
}

void main () {
    dynamic Object action = new SimpleAction ("greet", null);

    // Read and write a property
    print ("name: %s\n", (string) action.name);
    action.enabled = true;

    // Connect to a signal and invoke it
    action.activate.connect (on_activate);
    action.activate.emit (null);
}

Compile with valac --pkg gio-2.0.

Properties and signals are looked up by name through GObject when the program runs. Because of that, the compiler can no longer help you: if you misspell a property name, the program compiles and GLib reports a critical warning when that line is executed.

4.16.1. Dynamic Signals

A signal on a dynamic object is connected to with connect (), like any other signal. To invoke it, use emit (). The emit () call makes it clear that a signal is meant rather than a method with the same name:

dynamic Gst.Element appsink = Gst.ElementFactory.make ("appsink", "my_src");
appsink.max_buffers = 0;
appsink.eos.connect (on_eos);
Gst.Sample? res = appsink.pull_sample.emit ();

This example sets a property, connects a handler to a signal and invokes the pull_sample signal, which returns a value.

The .emit () syntax is not limited to dynamic objects. It also works on signals the compiler knows about, if you prefer it to calling the signal like a method:

t1.sig_1.emit (5);   // same as: t1.sig_1 (5);

Note

Lambda expressions can't be used as handlers for dynamic signals; the compiler reports "Lambdas are not allowed for dynamic signals". Pass the name of a method instead, as on_activate and on_eos are above.