Registering Appropriate Listeners

Registering listeners is a fundamental of many programming languages and event driven applications. Being an event driven framework ZK is no different. To accomplish our goal of a clear button we need to do two things:

1. Capture clicks on our "target"
2. Fire a custom onClear event when the target is clicked

Capturing Clicks on our Target

Firstly, let’s concentrate on capturing clicks on our target div. To do so we need to register appropriate listeners in the bind_ method and remove the listeners in the unbind_ method to avoid any memory leak. Here is the code to do so.

bind_ : function(evt) {
		this.$supers('bind_', arguments);
		this.domListen_(this.$n().lastChild, "onClick", '_doClear');
	},

unbind_ : function(evt) {
		this.domUnlisten_(this.$n().lastChild, "onClick", '_doClear');
		this.$supers('unbind_', arguments);
	},

The key to this is the domListen method which takes the target as the first parameter, in this case we pass it the lastChild which is our target, the name of the event you want to listen for as the second parameter and finally the name of the callback.

Please note that you also are required to call $supers so that parent classes can register and remove their events successfully. Now we need to move on to firing our custom "onClear" event. Let’s take a look at how to do this.

Firing a Custom Event (onClear)

The key lies in the registering the callback of the event. In our case we registered a callback named "_doClear". In this _doClear method we need to clear/restore the text depending on the widget state and fire the onClear method. The code is as follows:

_doClear: function(evt) {

		this._cleared = !(this._cleared);
		
		if(this._cleared) {
			this.$n().firstChild.innerHTML = this._value;
		} else {
			this.$n().firstChild.innerHTML = "";
		}
		
		this.fire("onClear", {cleared: this._cleared});
	}

We have a data member named _cleared which contains the state of the application. Depending on the state the method either shows or clears the displayed value. The method onClear is then fired and the cleared state is sent along with the instruction to fire the event.

The client side widget will now communicate with the component at the server side. This is handled by ZK. The following section explores how this communication works.