gnome-shell extension

// Sample extension code, makes clicking on the panel show a message
const St = imports.gi.St;
const Mainloop = imports.mainloop;
const Tweener = imports.ui.tweener;

const Main = imports.ui.main;

function _showHello() {
    let text = new St.Label({ style_class: 'helloworld-label',
                              text: "Hello, world!" });
    text.opacity = 0;
    let monitor = global.get_primary_monitor();
    global.stage.add_actor(text);
    text.set_position
            (Math.floor (monitor.width / 2 - text.width / 2),
            Math.floor(monitor.height / 2 - text.height / 2));

    Tweener.addTween (text, {opacity:255,
                             time: 1.5,
                             transition:"linear"});
    global.get_windows ().forEach (function (w) {
        Tweener.addTween (w, 
            {time: 1.5, transition: "linear",
            scale_x: 0.3,
            scale_y: 0.3,
            opacity: 50});
    });
    Mainloop.timeout_add(3000, function () { 
        Tweener.addTween (text,
            {
                scale_x: 2,
                scale_y: 2,
                opacity: 0,
                time: 0.2,
                transition: "linear",
                onComplete: text.destroy
            }
        );
        global.get_windows ().forEach (function (w) {
            Tweener.addTween (w, {time: 0.2, transition: "linear",
                scale_x: 1,
                scale_y: 1,
                opacity: 255});
        });
    });
}

// Put your extension initialization code here
function main() {
    Main.panel.actor.reactive = true;
    Main.panel.actor.connect('button-release-event', _showHello);
}