let register = Toolkit => Toolkit.Desktop =

// Layered window manager
class Desktop extends Toolkit.Component {

    //////////////////////////////// Constants ////////////////////////////////

    // Comparator for ordering child windows
    static CHILD_ORDER(a, b) {
        return b.element.style.zIndex - a.element.style.zIndex;
    }



    ///////////////////////// Initialization Methods //////////////////////////

    constructor(app, options = {}) {
        super(app, Object.assign({
            class: "tk desktop"
        }, options, { style: Object.assign({
            position: "relative",
            zIndex  : "0"
        }, options.style || {})} ));

        // Configure event listeners
        this.addEventListener("resize", e=>this.onResize());
    }



    ///////////////////////////// Event Handlers //////////////////////////////

    // Element resized
    onResize() {

        // The element is hidden: its size is indeterminate
        if (!this.isVisible())
            return;

        // Don't allow children to be out-of-frame
        if (this.children != null) {
            for (let child of this.children) {
                child.left = child.left;
                child.top  = child.top;
            }
        }

    }



    ///////////////////////////// Public Methods //////////////////////////////

    // Add a child component
    add(comp) {
        super.add(comp);
        this.bringToFront(comp);
    }

    // Retrieve the foremost visible window
    getActiveWindow() {
        if (this.children != null) {
            for (let child of this.children) {
                if (child.isVisible())
                    return child;
            }
        }
        return null;
    }



    ///////////////////////////// Package Methods /////////////////////////////

    // Reorder children so that a particular child is in front
    bringToFront(child) {
        this.children.splice(this.children.indexOf(child), 1);
        this.children.push(child);
        let z = 1 - this.children.length;
        for (let child of this.children)
            child.element.style.zIndex = z++;
    }

}

export { register };