Different ways to handle Click Events in Android

Android: OnClickListener Ways

Overview:

            Android provides Delegate Event Model to handle the click events in different ways.  An Android View in an App will fire an event in response to user or code interaction. The application will not respond to the event unless it is listening for it. To detect the event a class that implements a listener is instantiated and assigned to the View.

          

                There are pros and cons for each method and experienced developers will advocate a particular method over others. But not one of the methods can be regarded as the must do way. Developers need to be familiar with the different ways to code event handlers, they they will come across the different methods in tutorials, samples and online snippets.

             In Event Delegation Model when a View fires an event an Application will not respond to it unless it is listening for it. To detect the event a class that implements a listener is instantiated and assigned to the View. 

       Take for example the onClick event, the most widely used event in Android Apps. Nearly every View that can be added to an App screen will fire the event when the user stabs it with their finger (on touch screens) or presses the trackpad/trackball when the View has focus. This event is listened to by a class implementing the OnClickListener interface. 

The class instance is then assigned to the required View using the View's setOnClickListener method. 

There are different ways of handling the Click event in android.

1. Member Class
2. Interface Type
3. Anonymous Inner Class (Inline method)
4. Implementation in Activity
5. Attribute in View Layout for OnClick Events

Below are the implementation ways:

           1. Member Class

         A class called HandleClick implementing OnClickListener is declared as a member of the Activity (MainActivity). This is useful when several listeners require similar processing than can be handled by a single class.

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        //attach an instance of HandleClick to the Button
        findViewById(R.id.button1).setOnClickListener(new HandleClick());
    }    
    private class HandleClick implements OnClickListener{
        public void onClick(View arg0) {
     Button btn = (Button)arg0; //cast view to a button
     // get a reference to the TextView
     TextView tv = (TextView) findViewById(R.id.textview1);
     // update the TextView text
     tv.setText("You have pressed " + btn.getText());
 }
    }
}

    2. Interface Type 

            

              In Java an Interface can be used as a type, a variable is declared as an OnClickListener and assigned using new OnClickListener(){...}, behind the scenes Java is creating an object (an Anonymous Class) that implements OnClickListener. This has similar benefits to the first method.

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        //use the handleClick variable to attach the event listener
        findViewById(R.id.button1).setOnClickListener(handleClick);
    }    
    private OnClickListener handleClick = new OnClickListener(){
        public void onClick(View arg0) {
     Button btn = (Button)arg0;
     TextView tv = (TextView) findViewById(R.id.textview1);
     tv.setText("You have pressed " + btn.getText());
 }
    };
}

  3. Anonymous Inner Class (Inline method)

           Declaring the OnClickListener within the call to the setOnClickListener method is common. This method is useful when each listener does not have functionality that could be shared with other listeners. Some novice developers find this type of code difficult to understand. This method is very quick but does leave a bit of a mess. I Would only recommend using this for small blocks of code.Again behind the scenes for new OnClickListener(){...} . Java is creating an object that implements the interface.

public class MainActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        findViewById(R.id.button1).setOnClickListener(new OnClickListener(){
            public void onClick(View arg0) {
      Button btn = (Button)arg0;
      TextView tv = (TextView) findViewById(R.id.textview1);
      tv.setText("You have pressed " + btn.getText());
         }
        });
    }     
}

4. Implementation in Activity

        The Activity itself can implement the OnClickListener. Since the Activity object (MainActivity) already exists this saves a small amount of memory by not requiring another object to host the onClickmethod. It does make public a method that is unlikely to be used elsewhere. Implementing multiple events will make the declaration of main long.

public class MainActivity extends Activity implements OnClickListener{
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        findViewById(R.id.button1).setOnClickListener(this);
    }    
    public void onClick(View arg0) {
 Button btn = (Button)arg0;
 TextView tv = (TextView) findViewById(R.id.textview1);
 tv.setText("You have pressed " + btn.getText());
    }
}

5. Attribute in View Layout for OnClick Events

            In Android 1.6 and later (API level 4 and upwards) the name of a method defined in the Activity can be assigned to the android:onClick attribute in a layout file. This can save writing a lot of boilerplate code.

public class MainActivity extends Activity{
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }    
    public void HandleClick(View arg0) {
 Button btn = (Button)arg0;
        TextView tv = (TextView) findViewById(R.id.textview1);
 tv.setText("You have pressed " + btn.getText());
    }
}

In the layout file the Button would be declared with the android:onClick attribute.

<Button android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Button 1"
        android:onClick="HandleClick"/>

    The first four methods of handling events can be used with other event types (onLongClick, onKey, onTouch, onCreateContextMenu, onFocusChange). The fifth method only applies to the onClick event. The layout file below declares an additional two buttons and using the android:onClick attribute no additional code is required than that defined above, i.e. no additional findViewById and setOnClickListener for each button is required.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
  <TextView android:id="@+id/textview1"
            android:layout_width="fill_parent" 
            android:layout_height="wrap_content" 
            android:text="Click a button."
            android:textSize="20dp"/>
  <LinearLayout android:orientation="horizontal"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content">
    <Button android:id="@+id/button1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Button 1"
            android:onClick="HandleClick"/>             
    <Button android:id="@+id/button2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Button 2"
            android:onClick="HandleClick"/>
    <Button android:id="@+id/button3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Button 3"
            android:onClick="HandleClick"/>
  </LinearLayout>      
</LinearLayout>

Conclusion

Deciding which technique to use to wire up a listener will depend on the functionality required, how much code is reusable across Views and how easy the code would be to understand by future maintainers. Ideally the code should be sufficient and easy to view.

    One method not shown here is similar to the first method. In the first method it would be possible to save the listener class in a different class file as a public class. Then instances of that public class could be used by other Activities, passing the Activity's context in via the constructor. However, Activities should try and stay self contained in case they are killed by Android. Sharing listeners across Activities is against the ideals of the Android platform and could lead to unnecessary complexity passing references between the public classes.