How to capture an int value in a closure?

Question

I have an app with an arbitrary number of elements that all call a function that needs to take an argument, defined on creation. This is a simplified example but here I'd be hoping to make 3 buttons that print 0,1,2 but just makes 3 buttons that print 3.

    var application_window = new Gtk.ApplicationWindow (this);
    var grid = new Gtk.Grid ();

    for (int i = 0; i < 3; i++) {
        var button = new Gtk.Button() {expand=true};
        button.clicked.connect (() => {
                print(i.to_string());
        });

        grid.add(button);
    }

    application_window.add(grid);
    application_window.show_all ();

How can I change my app to print 123 instead?

Answer 1

Here is my base code:

public class MyApplication : Gtk.Application {
    public MyApplication () {
        Object(application_id: "testing.my.application",
            flags : ApplicationFlags.FLAGS_NONE);
    }

    protected override void activate () {
        var application_window = new Gtk.ApplicationWindow (this);
        var grid = new Gtk.Grid ();

        for (int i = 0; i < 3; i++) {
            var button = new Gtk.Button() {expand=true};
            button.clicked.connect (() => {
                print(i.to_string());
            });

            grid.add(button);
        }

        application_window.add(grid);
        application_window.show_all ();
    }

    public static int main (string[] args) {
        MyApplication app = new MyApplication ();
        return app.run (args);
    }
}

If you execute it like that, you get 333 as stdout.

The problem is in the capturing code:

for (int i = 0; i < 3; i++) {
    var button = new Gtk.Button() {expand=true};
    button.clicked.connect (() => {
        print(i.to_string());
    });

The closure is capturing the variable i by location. That means when you change the i variable after creating the closures the change will be visibile in the closure as well.

Other programming languages (e. g. C++) have explicit capture lists to avoid this problem.

A quick and dirty solution would be using a local variable inside the scope of the loop:

for (int i = 0; i < 3; i++) {
    var captured_i = i;
    var button = new Gtk.Button() {expand=true};
    button.clicked.connect (() => {
        print(captured_i.to_string());
    });

This prints: 012 as intended.

A better solution would be using a function that returns the closure as a delegate. I just tried that, but for some reason it does not work:

public class MyApplication : Gtk.Application {
    public MyApplication () {
        Object(application_id: "testing.my.application",
            flags : ApplicationFlags.FLAGS_NONE);
    }

    delegate void ButtonClick();

    private ButtonClick make_print_event (int i) {
        return () => print (i.to_string());
    }    

    protected override void activate () {
        var application_window = new Gtk.ApplicationWindow (this);
        var grid = new Gtk.Grid ();

        for (int i = 0; i < 3; i++) {
            var button = new Gtk.Button() { expand=true };
            var print_event = make_print_event (i);
            button.clicked.connect (print_event);
            grid.add(button);
        }

        application_window.add (grid);
        application_window.show_all ();
    }

    public static int main (string[] args) {
        MyApplication app = new MyApplication ();
        return app.run (args);
    }
}

The compiler (valac-0.52) warns:

three_buttons.vala:20.37-20.47: warning: copying delegates is not supported
three_buttons.vala:20.37-20.47: warning: Connecting delegates to signals is experimental
            button.clicked.connect (print_event);
                                    ^^^^^^^^^^^