Android onClick in XML vs. OnClickListener

Question

I realize that a similarly-worded question has been asked before, but this is different. I am pretty new at developing android apps and I have three questions regarding the difference(s) between the android:onclick="" XML attribute and the setOnClickListener method.

1. What are the differences between the two? Is the difference between the two implementations found at compile time or run time or both?

2. What use cases are favorable to which implementation?

3. What difference(s) does the use of fragments in Android make in implementation choice?

Answer 1

Difference Between OnClickListener vs OnClick:

• OnClickListener is the interface you need to implement and can be set to a view in java code.
• OnClickListener is what waits for someone to actually click, onclick determines what happens when someone clicks.
• Lately android added a xml attribute to views called android:onclick, that can be used to handle clicks directly in the view's activity without need to implement any interface.
• You could easily swap one listener implementation with another if you need to.
• An OnClickListener enable you to separate the action/behavior of the click event from the View that triggers the event. While for simple cases this is not such a big deal, for complex event handling, this could mean better readability and maintainability of the code
• Since OnClickListener is an interface, the class that implements it has flexibilities in determining the instance variables and methods that it needs in order to handle the event. Again, this is not a big deal in simple cases, but for complex cases, we don't want to necessary mix up the variables/methods that related to event handling with the code of the View that triggers the event.
• The onClick with function binding in XML Layout is a binding between onClick and the function that it will call. The function have to have one argument (the View) in order for onClick to function.

Both function the same way, just that one gets set through java code and the other through xml code.

setOnClickListener Code Implementation:

Button btn = (Button) findViewById(R.id.mybutton);

btn.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
    myFancyMethod(v);
    }
});

// some more code

public void myFancyMethod(View v) {
    // does something very interesting
}

XML Implementation:

<?xml version="1.0" encoding="utf-8"?>
<!-- layout elements -->
<Button android:id="@+id/mybutton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Click me!"
    android:onClick="myFancyMethod" />
<!-- even more layout elements -->

Performance:

Both are the same in performance. Xml is pre-parsed into binary code while compiling. so there is no over-head in Xml.

Limitation:

android:onClick is for API level 4 onwards, so if you're targeting < 1.6, then you can't use it.

Answer 2

I'm shocked nobody talked about this but be careful, although android:onClick XML seems to be a convenient way to handle click, the setOnClickListener implementation do something additional than adding the onClickListener. Indeed, it put the view property clickable to true.

While it's might not be a problem on most Android implementations, according to the phone constructor, button is always default to clickable = true but other constructors on some phone model might have a default clickable = false on non Button views.

So setting the XML is not enough, you have to think all the time to add android:clickable="true" on non button, and if you have a device where the default is clickable = true and you forget even once to put this XML attribute, you won't notice the problem at runtime but will get the feedback on the market when it will be in the hands of your customers !

In addition, we can never be sure about how proguard will obfuscate and rename XML attributes and class method, so not 100% safe that they will never have a bug one day.

So if you never want to have trouble and never think about it, it's better to use setOnClickListener or libraries like ButterKnife with annotation @OnClick(R.id.button)

Answer 3

Simply:

If you have android:onClick = "someMethod" in xml, it looks for the public void someMethod in your Activity class. OnClickListener is called right from your Activity and it is linked to some particular View. For example someButton.setOnClickListener and in the code below is said what has to be done when someButton is pressed.

Hope it helps :)

Answer 4

As said before: they both are a way to add logic in response to an event, in this case a 'click' event.

I would go for a separation between logic and presentation, just like we do in the HTML/JavaScript world: Leave the XML for presentation and add event listeners by means of code.

Answer 5

There are a couple of reasons why you might want to programmatically set an OnClickListener. The first is if you ever want to change the behaviour of your button while your app is running. You can point your button at another method entirely, or just disable the button by setting an OnClickListener that doesn't do anything.

When you define a listener using the onClick attribute, the view looks for a method with that name only in its host activity. Programmatically setting an OnClickListener allows you to control a button's behaviour from somewhere other than its host activity. This will become very relevant when we use Fragments, which are basically mini activities, allowing you to build reusable collections of views with their own lifecycle, which can then be assembled into activities. Fragments always need to use OnClickListeners to control their buttons, since they're not Activities, and won't be searched for listeners defined in onClick.