GObject-Style Construction

Vala supports an alternative construction scheme that is slightly different to the one described earlier. This scheme is closer to the way GObject construction works. Which one you prefer depends on whether you come from the GObject side or from the Java or C# side. The gobject-style construction scheme introduces some new syntax elements:

construct properties, a special Object(...) call and a construct block. Let's take a look at how this works:

public class Person : Object {

    /* Construction properties */
    public string name { get; construct; }
    public int age { get; construct set; }

    public Person(string name) {
        Object(name: name);
    }

    public Person.with_age(string name, int years) {
        Object(name: name, age: years);
    }

    construct {
        // do anything else
        stdout.printf("Welcome %s\n", this.name);
    }
}

With the gobject-style construction scheme each construction method only contains an Object(...) call for setting so-called construct properties. The Object(...) call takes a variable number of named arguments in the form of property: value. These properties must be declared as construct or set properties. They will be set to the given values and afterwards all construct {} blocks in the hierarchy from GLib.Object down to our class will be called.

The construct block is guaranteed to be called when an instance of this class is created, even if it is created as a subtype. It has no parameters or return value. Within this block you can call other methods and set member variables as needed.

Construct properties are defined just as get and set properties, and therefore can run arbitrary code on assignment. If you need to do initialisation based on a single construct property, it is possible to write a custom construct block for the property, which will be executed immediately on assignment, and before any other construction code.

If a construct property is declared without set it is a so-called construct only property, which means it can only be assigned on construction, but no longer afterwards. In the example above name is such a construct only property.

Here's a summary of the various types of properties together with the nomenclature usually found in the documentation of gobject-based libraries:

public int a { get; private set; }    // Read
public int b { private get; set; }    // Write
public int c { get; set; }            // Read / Write
public int d { get; set construct; }  // Read / Write / Construct
public int e { get; construct; }      // Read / Write-Construct-Only

In some cases you may also want to perform some action - not when instances of a class is created - but when the class itself is created by the GObject runtime. In GObject terminology we are talking about a snippet of code run inside the class_init function for the class in question. In Java this is known as static initializer blocks. In Vala this looks like:

/* This snippet of code is run when the class
 * is registered with the type system */
static construct {
  ...
}