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.
What are the differences between the two? Is the difference between the two implementations found at compile time or run time or both?
What use cases are favorable to which implementation?
What difference(s) does the use of fragments in Android make in implementation choice?
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.
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)
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 :)
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.
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.
If you have several buttons using only one method, I suggest doing it in java. But if you have a button with one specific method, onClick in XML would be better.
It's more convenient to always use android:onClick attribute unless you have a good reason not to, for example, if you instantiate the Button at runtime or you need to declare the click behavior in a Fragment subclass.
I think main difference between them is:
OnClick: When you click on the button with your finger.
OnClickListner: It is may be a wider choice that be implemented in various codes.
For example when you type url "ymail.com", yahoo finds your username and your password from your browser and enable click state button to open your mail. This action should be implemented only in onClickListener.
This is my idea!
Its possible to have a listener to all buttons without setting the onClickListener in each button across all activities? and without making a extends button with the listener already set.
No. Each view has to be told what to listen to. You can specify it in xml if you prefer with the onClick attribute, but you'll still need to specify it on each object.
If all the buttons are doing the exact same thing, you can include that in the List kind of a layout and have one button with just one onCLick() event
Is it a bad programming practice to set onClick() method in the layout XML file instead of setting OnClickListener() in the java code in an android activity?
No,not at all,
Whenever you set some listener by setOnClickListener() in java
class,then you are doing it yourself
but when you specify onClick in xml file,you are commanding to
android to implement the setOnClickListener for you
.However in both cases the setOnClickListener is getting implemented.
You may decide according to your requirements which approach you want to follow.
I want to create a custom view with some text and two buttons all on one line. I need to be able to add multiple (any number) of these views to an existing layout dynamically (needs to be able to scroll). I want to pass a custom object to the view and set the text and buttons. I need access to the button event handlers from the activity. I've looked a little into custom views but I'm still at a loss for how to do what I want. I'm used to .NET custom controls, and I'm looking for the same effect. Any help or example code would be greatly appreciated.
What you want is custom compound view. You should write you own class (usually extending one of the layouts) and whole behavior, inflate the layout the way you want etc.
More of it: http://developer.android.com/guide/topics/ui/custom-components.html
This one helped me a lot too: http://javatechig.com/android/creating-custom-and-compound-views-in-android-tutorial
If you use list activity or list fragment, you will automatically have the many features you have asked for. You only need to create a adapter class for your listview. You can define your layout for your row view(buttons, text etc..) Try to look at the examples on the web for cusom adapter and lists.
I am building a Android app and I am a bit struggling with custom Views.
I would like to have a reusable View that consist of a few standard layout elements. Let's say a relativelayout with some buttons in it.
How should I proceed. Should I create a custom view class that extends RelativeLayout and programmaticly add those buttons? I would think that's a bit overkill?
What's the way to do it properly in Android?
Here are some rough steps regarding one way to create a custom aggregate view:
extend RelativeLayout
Provide a constructor in your new class that accepts Context and AttributeSet, making sure to call the superclass first. Do no add anything at this point. Wait until the next step.
override the onFinishInflate method, where you can add your contents through Java code or inflating an XML resource
Add any event handlers, etc
Optionally create a resources file if your widget will require attributes to be set.