Using Events

The Components and Classes of Ext JS fire a broad range of events at various points in their lifecycle. Events allow your code to react to changes around your application. They are a key concept within Ext JS.

What Are Events?

Events fire whenever something interesting happens to one of your Classes. For example, when Ext.Component renders to the screen, Ext JS fires an event after the render completes. We can listen for that event by configuring a simple listeners object:

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Ext.create('Ext.Panel', {
    html: 'My Panel',
    renderTo: Ext.getBody(),
    listeners: {
        afterrender: function() {
            Ext.Msg.alert('Success!', 'We have been rendered');
        }
    }
});

In this example, when you click the Preview button, the Panel renders to the screen, followed by the defined alert message. All events fired by a class are listed in the class’s API page - for example, Ext.panel.Panel currently has 45 events.

Listening to Events

While Ext.Component#event-afterrender is useful in some cases, you may use other events more frequently. For instance, Ext.button.Button fires click events when clicked:

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Ext.create('Ext.Button', {
    text: 'Click Me',
    renderTo: Ext.getBody(),
    listeners: {
        click: function() {
            Ext.Msg.alert('Success!', 'I was clicked!');
        }
    }
});

A component may contain as many event listeners as needed. In the following example, we confound users by calling this.hide() inside our mouseover listener to hide a Button. We then display the button again a second later. When this.hide() is called, the Button is hidden and the hide event fires. The hide event triggers our hide listener, which waits one second and displays the Button again:

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Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'My Button',
    listeners: {
        mouseover: function() {
            this.hide();
        },
        hide: function() {
            // Waits 1 second (1000ms), then shows the button again
            Ext.defer(function() {
                this.show();
            }, 1000, this);
        }
    }
 });

Event listeners are called every time an event is fired, so you can continue hiding and showing the button for as long as you desire.

Adding Listeners Later

In previous examples, we passed listeners to the component when the class was instantiated. However, If we already have an instance, we can add listeners using the on function:

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var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'My Button'
});
button.on('click', function() {
    Ext.Msg.alert('Success!', 'Event listener attached by .on');
});

You can also specify multiple listeners by using the .on method, similar to using a listener configuration. The following revisits the previous example that set the button’s visibility with a mouseover event:

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var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'My Button'
});
button.on({
    mouseover: function() {
        this.hide();
    },
    hide: function() {
        Ext.defer(function() {
            this.show();
        }, 1000, this);
    }
});

Removing Listeners

Just as we can add listeners at any time, we can also remove them. This time we use the un function. To remove a listener, we need a reference to its function. In the previous examples, we passed a function into the listener’s object or the on call. This time, we create the function earlier and link it into a variable called doSomething, which contains our custom function. Since we initially pass the new doSomething function into our listeners object, the code begins as before. With the eventual addition of an Ext#method-defer function, clicking the button in the first 3 seconds yields an alert message. However, after 3 seconds the listener is removed so nothing happens:

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var doSomething = function() {
    Ext.Msg.alert('Success!', 'listener called');
};
var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'My Button',
    listeners: {
        click: doSomething,
    }
});
Ext.defer(function() {
    button.un('click', doSomething);
}, 3000);

Scope Listener Option

Scope sets the value of this inside your handler function. By default, this is set to the instance of the class firing the event. This is often, but not always, the functionality that you want. This functionality allows us to call this.hide() to hide the button in the second example earlier in this guide. In the following example, we create a Button and a Panel. We then listen to the Button’s click event with the handler running in Panel’s scope. In order to do this, we need to pass in an object instead of a handler function. This object contains the function AND the scope:

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var panel = Ext.create('Ext.Panel', {
    html: 'Panel HTML'
});
var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'Click Me'
});
button.on({
    click: function() {
        Ext.Msg.alert('Success!', this.getXType());
    },
    scope: panel
});

You can also use a more verbose declaration if your listeners do not have the same options, or, if you’re not into the whole brevity thing.

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var panel = Ext.create('Ext.Panel', {
    html: 'Panel HTML'
});
var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'Click Me'
});
button.on({
    click: {
        scope: panel,
        fn: function() {
            Ext.Msg.alert('Success!', this.getXType());
        }
    }
});

When you run this example, the value of the click handler’s this is a reference to the Panel. To see this illustrated, we alert the xtype of the scoped component. When the button is clicked, we should see the Panel xtype being alerted.

Listening to an Event Once

You may want to listen to one event only once. The event itself might fire any number of times, but we only want to listen to it once. The following codes illustrates this situation:

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var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'Click Me',
    listeners: {
        click: {
            single: true,
            fn: function() {
                Ext.Msg.alert('Success!', 'I will say this only once');
            }
        }
    }
});

Using a Buffer Configuration

For events that fire many times in short succession, we can reduce the number of times our

listener is called by using a buffer configuration. In this case our button’s click listener is only invoked once every 2 seconds, regardless of how many times you click it:

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var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: 'Click Me',
    listeners: {
        click: {
            buffer: 2000,
            fn: function() {
                Ext.Msg.alert('Success!', 'I say this only once every 2 seconds');
            }
        }
    }
});

Firing Custom Events

Firing your own events is done by calling fireEvent with an event name. In the following example we fire an event called myEvent that passes two arguments - the button itself and a random number between 1 and 100:

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var button = Ext.create('Ext.Button', {
    renderTo: Ext.getBody(),
    text: "Just wait 2 seconds",
    listeners: {
        myEvent: function(button, points) {
            Ext.Msg.alert('Success!', 'myEvent fired! You score ' + points + ' points');
        }
    }
});
Ext.defer(function() {
    var number = Math.ceil(Math.random() * 100);
    button.fireEvent('myEvent', button, number);
}, 2000);

Once again we used Ext.defer to delay the function that fires our custom event, this time by 2 seconds. When the event fires, the myEvent listener picks up on it and displays the arguments we passed in.

Listening for DOM Events

Not every ExtJS component raises every event. However, by targeting the container’s element, we can attach many native events to which the component can then listen. In this example, we target Ext.container.Container. Containers do not have a click event. Let’s give it one!

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var container = Ext.create('Ext.Container', {
    renderTo: Ext.getBody(),
    html: 'Click Me!',
    listeners: {
        click: function(){
            Ext.Msg.alert('Success!', 'I have been clicked!')  
        }
    }
});
container.getEl().on('click', function(){ 
    this.fireEvent('click', container); 
}, container);

Without the second block of code, the container’s click listener would not fire. Since we have targeted the container’s element and attached a click listener, we have extended the container’s event capabilities.

Event Normalization

Event normalization is the key to allowing Ext JS 5+ applications to run on touch-screen devices. This normalization occurs behind the scenes and is a simple translation from standard mouse events to their equivalent touch and pointer events.

Pointer events are a w3c standard for dealing with events that target a specific set of coordinates on the screen, regardless of input device (mouse, touch, stylus, etc.)

When your code requests a listener for a mouse event, the framework attaches a similar touch or pointer event as needed. For example, if the application attempts to attach a mousedown listener:

myElement.on('mousedown', someFunction);

The event system translates this to touchstart in the case of a device that supports touch events:

myElement.on('touchstart', someFunction);

Or, pointerdown in the case of a device that supports pointer events:

myElement.on('pointerdown', someFunction);

This translation is in place so that you may achieve tablet and touch-screen support without any additional coding. Translation can be disabled using the translate event option:

myElement.on({
    mousedown: someFunction,
    // only listen to mousedown.  Do not call the handler in response to touchstart or pointerdown.
    translate: false
});

More Events

In addition to DOM events and Component events, Ext JS provides a powerful system to recognize meaningful gestures since as pinch, rotate, drag and more. For more on these events, see the Gestures guides.