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Different way to handle events in Android

· Mobile Zone

Typically, events respond to user interactions. Android supports multiple ways to handle events on views. When a user clicks on an Android View, some method is getting called by the Android framework and then past the control to the application listeners.

For example, when a user clicks on a view like as a button, the onTouchEvent() method is called on that button object. In order to make our application respond to the event, we must extend the class and override the method. But extending every View object in order to handle such an event would not be practical. Each View class in Android provides a collection of nested interfaces called listeners with callbacks that you can much more easily define in order to handle the event.

1. Defining a listener programatically on the OnCreate method

button.setOnClickListener(new OnClickListener(){
    @Override
    public void onClick(View v) {
        //do stuff
    }
});

This method will create an anonymous class for each button you create. This is recommended only if you have fewer listeners in your class. But if we have a complex screen layout with many views, then writing a listener programatically for each view will make the code messy. It's costly and less readable.

2. Setting the android:OnClick property in XML

<Button android:id="@+id/btnView"
    ...............
    ...............
    android:OnClick="btnViewOnClick"/>

Many people use this method of handling click events by writing an OnClick attribute in XML. But usually it is not preferable, because it is better to keep listeners inside the code. Internally, Android is using the Java reflection concept behind the scenes to handle this. It is less readable, and confuses some developers.

3. Implementing the OnClickListener interface on the Activity class and passing a reference to the Button


public class MainActivity extends Activity implements OnClickListener{
    @Override
    public void onClick(View v) {
        //do stuff
    }
 
    protected void onCreate(Bundle savedInstanceState) {
        ...
        button.setOnClickListener(this);
    } 
 				}

Here, we are implementing the OnClickListener interface on the activity class and passing a self reference to the button. This way, the OnClick listener will hold the reference to the activity object, and is a heavy operation to keep the whole activity’s object in it.

This way we can handle the click event for all views. However, we need to differentiate views using their IDs. We can use the view.getId() method to see which button was clicked. Again, this is preferable only when we have fewer views to handle. This way, all the click event handling codes are done in one place.

This way is hard to navigate because you can’t determine the type of the listener you are using with the current button (I know Eclipse will highlight the methods this is pointing at, but with lots of code I think it will be hard to find).

4. Create a field with the OnClickListener type

private OnClickListener onClickHandler = new OnClickListener(){
 
    @Override
    public void onClick(View v) {
        //stuff
    }
};
 
protected void onCreate(Bundle savedInstanceState) {
    ...
    button.setOnClickListener(onClickHandler);
}

The best practice is the create a local variable with the OnClickListener type. This way it is easy to navigate and more readable. But it doesn't stop you from implementing the other three options provided above. Everyone has different way of looking at the problem.

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