Meaning of "View v" in Android Studio

Question

I am trying to implement button click handling in my Android app.

In the XML layout file that includes my button, I added the following line to my Button XML element:

android:onClick="handleClick"

I also defined a method with the following signature in the Activity that uses this layout:

public void handleClick() { /* ... */ }

However, when I ran my app with this code, it crashed. I was able to fix this crash by updating my method signature to:

public void handleClick(View v) { /* ... */ }

but I don't understand why I am required to include this View parameter?

score 6 · Accepted Answer · edited Jul 24 '17 at 13:20

This is because you might want to use your handleClick method for 2 or more Buttons in your XML.

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

<Button
    android:id="@+id/button2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:onClick="handleClick"/>

In that case it could be unclear which button triggers the callback. The View v helps you identify that e.g.

public void handleClick(View v) {
    if (v.getId() == R.id.button1) {

    } else if(v.getId() == R.id.button2) {

    }
}

stkent · Answer 2 · 2017-07-24T13:21:13.800

3

The View argument supplied represents the View that received the click event. This may be useful if you reuse the handleClick method for multiple Views (in which case, you could inspect the id of the View passed to the method to determine which View was clicked, as illustrated by Enzokie's answer).

You must include this View parameter when defining your method even if you do not use it in your click logic. This is because reflection is used to locate the method corresponding to the name you supplied in XML, and the method name, parameter count, and parameter types are all required to uniquely define a method in Java. Check out this section of the View source code to see exactly how this reflective lookup works!

edited Jul 24 '17 at 13:21

answered Jul 24 '17 at 00:16

stkent

19,772
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Thanks! Awesome answer. – Florian Walther Jul 24 '17 at 00:19
Holy cow, this is the source code behind "View"? Its huge. – Florian Walther Jul 24 '17 at 00:21
Haha, yes, the `View` class is a behemoth! – stkent Jul 24 '17 at 00:21

score 1 · Answer 3 · answered Jul 24 '17 at 00:25

1

The View v is the object of your xml file which is referred in your onCreate method. To refer any component from xml you have to use v to gets its id of the component.

Condition

You have give id to the component in xml if you want to use onClick in your class file.

answered Jul 24 '17 at 00:25

Shoeb Surve

64
7

Thank you! I understand. – Florian Walther Jul 24 '17 at 00:44

Meaning of "View v" in Android Studio

3 Answers3