Android: how add opacity on ImageButton click?

In case that when user click on button and this cause to change the opacity of that button you can do:

In your xml file on button declaration add this line:

android:onClick = "clickMethod"

and in the java file you need to implement the clickMethod,

public void clickMethod(View view)
{
  // change opacity
}

so, if you want to do the same process(change button opacity) for each button, so in the xml file for each button add the line

android:onClick="clickMethod"

If you want to give users better expirince when clicking ImageButton, I recommend you to use selection drawable as the background of your ImageButtons. It gives better user experience, then setting opacity while performing click and it’s really easy to achieve.

First you need to create in your drawable folder file with name f.e. image_button_selection.xml. In which you should define:

<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_focused="true" android:state_pressed="false" android:drawable="@drawable/your_focused_background_drawable" />
    <item android:state_pressed="true" android:drawable="@drawable/your_pressed_background_drawable " />
    <item android:drawable="@android:color/transparent" />
</selector>

You should also put into drawable two png with alpha channel that will be displayed on focused button and pressed button. In this example they should be named respectively your_focused_background_drawable and your_pressed_background_drawable. When you do this you should in every use of your ImageButton in xml use following statement:

android:background="@drawable/image_button_selection"