mon vs on

A common method when developing an application is to be event driven. This allows your code to be very flexible. Not being event driven can lead to code that is very intimate to the current state of your application. Change or add something later on and you must go through your application and change the different parts where as being event driven allows an event to be fired and forgotten about. Anything listening to that event can then take action on that event but not be dependant on what fired the event.

A simple example is an email application. Say a user sends an email, when the email has been sent and the server returns success, the callback may fire an event to tell anything in the application that the user sent an email. The Sent folder list may listen to this event in order to reload the list. This email may have been sent from different forms, a create email form may be different than a quick reply form but that doesn't matter, either form may fire the same event and the Sent folder list doesn't care where it came from. This is flexibility and in my eyes, creates much safer code.

Adding listeners in Ext JS

Ext JS allows many different ways to add listeners but today I want to speak about mon and on and the differences. First, let's start with on.

on

The simplest of the two is the on method which can simply add an event listener:

component.on('foo', someFunc, component);

You can also pass an object for a convenient way to add multiple listeners:

component.on({
    scope : component,
    foo   : someFunc,
    bar   : someOtherFunc
});

This will add listeners for the foo and bar events and scope them to the component variable. In Ext JS 4 and newer, listeners defined this way will automatically get removed when the component is destroyed thanks to the clearListeners method being executed in the destroy method of Ext.Component.

mon

mon works just like on when defining a listener only the first argument must be a class to add the listeners too. That's a bit confusing of a statement right? Let's look at an example:

component.mon(subClass, 'foo', someFunc, component);

What this is doing, is adding a listener for the foo event that will be fired on the subClass variable. Once fired, it will execute the someFunc function scoped to the component variable but the listener is added to the component but the event will be fired on the subClass.

The benefit here is the component can listen to an event on something else but when the component is destroyed, this listener is removed. The component may be destroyed but the subClass variable may still live on; the component is managing the foo event listener.

Like the on method, you can also pass an object:

component.mon(subClass, {
    scope : component,
    foo   : someFunc,
    bar   : someOtherFunc
});

Difference

Let's think of an Ext.Component instance that wants to listen to a store load. You can use on like this:

store.on('load', this.onStoreLoad, this);

However, when the Ext.Component is destroyed, the listener will not be removed because the event listener is on the store not the Ext.Component. This causes a memory leak and the next store load after the component is destroyed will likely throw an error, both bad things. This is where mon would be a better use:

this.mon(store, 'load', this.onStoreLoad, this);

This adds a load listener on the store but is being managed by the Ext.Component. When the Ext.Component is destroyed, the listener is removed. The store is then free to live on and fire it's load event without errors being thrown and no memory leaks are present.

Mitchell Simoens

Mitchell has held many positions within Sencha currently maintaining the support portal and Sencha Fiddle. Anything expressed on this website are Mitchell's alone and do not represent his employer.

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