Handling Click events in Button | Android

There are 2 ways to handle the click event in button

• Onclick in xml layout
• Using an OnClickListener

Onclick in XML layout

When the user clicks a button, the Button object receives an on-click event.

To make click event work add android:onClick attribute to the Button element in your XML layout. The value for this attribute must be the name of the method you want to call in response to a click event. The Activity hosting the layout must then implement the corresponding method.

NOTE:
If you use this event handler in your code, make sure that you are having that button in your MainActivity. It won’t work if you use this event handler in fragment because onClick attribute only works in Activity or MainActivity.

Example:

<Button xmlns:android="http:// schemas.android.com/apk/res/android"
    android:id="@+id/button_send"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/button_send"
    android:onClick="sendMessage"
/>

In MainActivity class

/** Called when the user touches the button */
public void sendMessage(View view)
{
    // Do something in response to button click
}

Make sure that your sendMessage method should have the following :

Be public

Return void

Define a View as its only parameter (this will be the View that was clicked)

Using an OnClickListener

You can also declare the click event handler programmatically rather than in an XML layout. This event handler code is mostly preferred because it can be used in both Activities and Fragments.

There are two ways to do this event handler programmatically :

• Implementing View.OnClickListener in your Activity or fragment.
• Creating new anonymous View.OnClickListener.

Implementing View.OnClickListener in your Activity or fragment

To implement View.OnClickListener in your Activity or Fragment, you have to override onClick method on your class.

Firstly, link the button in xml layout to java by calling findViewById() method.
R.id.button_send refers the button in XML.

mButton.setOnClickListener(this); means that you want to assign listener for your Button “on this instance” this instance represents OnClickListener and for this reason your class have to implement that interface.

<RelativeLayout
    xmlns:android="http:// schemas.android.com/apk/res/android"
    xmlns:app="http:// schemas.android.com/apk/res-auto"
    xmlns:tools="http:// schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.sample.MainActivity" >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/button_send" />
</RelativeLayout>

MainActivity code:

public class MainActivity extends AppCompatActivity
    implements View.OnClickListener {
    private Button mButton;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mButton = findViewById(R.id.button_send);
        mButton.setOnClickListener(this);
    }

    @Override
    public void onClick(View view)
    {
        switch (view.getId()) {
        case R.id.button_send:
            // Do something
        }
    }
}

If you have more than one button click event, you can use switch case to identify which button is clicked.

Creating Anonymous View.OnClickListener

Link the button from the XML by calling findViewById() method and set the onClick listener by using setOnClickListener() method.

MainActivity code:

public class MainActivity extends AppCompatActivity {

    private Button mButton;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mButton = findViewById(R.id.button_send);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view)
            {
                // Do something
            }
        });
    }
}

setOnClickListener takes an OnClickListener object as the parameter. Basically it’s creating an anonymous subclass OnClickListener in the parameter.

It’s like the same in java when you can create a new thread with an anonymous subclass.