Develop Reference

Adding functionality to onClick in Android

I want to run a function each time a button is pressed in my app. I know I can make a new button which extends Android's button and override onClick() but I have already made my entire app and want this new functionality as an afterthought. Can I somehow add this functionality without changing the class of all buttons in my app?
Lets say I want to add something like
runthisfunction();
before every onClick() code executes. There are many buttons in various activities and so they have their own onClick functions.
Is there a way that this function is run before each and every onClick() without the need to make a new class and changing all the existing buttons.

Add to your xml resource for whatever button/view you want to have an onClick listener:
android:onClick="fncClick"
For example:
<Button
android:id="#+id/btnClicker"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:onClick="fncClick" />
Then in the activity that utilizes that xml resource, include:
public void fncClick(View view) {
//TODO: Code here
}
This ONLY attaches an onClick listener for this specific button - not all buttons

Which is better in implementing click listener?

Which is a good practice in implementing click listener and why? Or is there a better way other than the two? Thanks.
First :
sampleButton.setOnClickListener(new View.OnClickListener(){
public void onClick(View v){
// do something
}
});
Second : implement OnClickListener then override onClick method?

The third option is to set the listener directly in your XML layout:
android:onClick="myClickHandler"
and then implement it in your Activity:
public void myClickHandler(View v){
// do something
}

You're technically doing the 2nd thing with the 1st one. The 1st case uses whats called an anonymous class which implements OnClickListener, but since is anonymous, doesn't have a class name and isn't editable from external classes. Explicitably implementing OnClickListener is useful if you expect to use the same onClick functionality in multiple different locations, or if the click code is long

The first approach is used when you want to perform the action only for a particular case, if many click events require the same action then use the second one.

Clickable TextView only works once

I made a textview clickable then it triggers an intent, it works but only once. After clicking the textview the first time it's no longer clickable and I have no idea why. Your help will be appreciated.
<TextView android:text="Click Me" android:layout_height="wrap_content"
android:layout_width="match_parent" android:id="#+id/textView1"
android:textSize="50dp" android:focusable="false" android:longClickable="true"></TextView>
TextView txt = (TextView) findViewById(R.id.textView1);
txt.setOnLongClickListener(new OnLongClickListener() {
public boolean onLongClick(View v) {
// TODO Auto-generated method stub
startActivity(new Intent(Example.this, Alert.class));
return false;
}
});

What does the alert class do? If it is an activity it could be that it is being laid over the top of your current activity so although you can see your activity, it's not at the top of the stack/in the foreground so you're not actually pressing the TextView, you're pressing a transparent activity that is over the top of it.
The easiest way to check that is to press the TextView, then press your device's back key and see if the TextView responds to the click.
Or are you sure you're not setting the same layout in Alert.class? That would make it look like it's the same activity but if the Alert class doesn't set the click listener, nothing is going to happen.
The fact that you're starting an activity with an intent and that's making an instance of another class (which I assume is also an activity) stops the click working to me is seriously suggesting that Alert is getting the click somehow instead of Example. When you say things work fine if you remove the intent backs that up as well. Maybe you could post the full source of both classes?

Do you have an onClickListener that disables the textview?
By returning false from onLongClick Android would also invoke the onClick listener if you have one.
Also you could try to remove android:focusable="false"

how to change the view of a button onClick in android

In my app I am trying to calculate an operation using timer. For controlling those operations I am using four buttons as Start, Stop, Pause and resume.
But I want to show only 2 buttons. At the beginning I have only two buttons Start and Pause.
When the start button is clicked timer gets started and immediately in Start button's place I want to show the Stop button.
I want to do the same for the other stop and pause buttons. How to do this please help me......

Using ToggleButton is a good solution for you. Do something like:
ToggleButton first = new ToggleButton(getContext());
ToggleButton second = new ToggleButton(getContext());
first.setTextOff("start");
first.setTextOn("stop");
second.setTextOff("pause");
second.setTextOn("resume");
and use setOnCheckedChangeListener() to implement your actions.

In your onClick(View v), v is the button that gets clicked. You can cast it like:
Button b = (Button) v;
so you can change its text with setText(), and set another listener. You can declare the alternate listeners once as members of the activity, and set them without re-declaring them each time.

Your application needs to maintain states, such as "Idle/Stopped", "In Progress", "Paused", etc. If you want to hide buttons, you can use View.setVisibility, and dynamically show and hide the buttons when your state changes (when other buttons are pressed). You would need to set your layout appropriately so that the buttons display nicely as they are shown/hidden dynamically
Or, you can change the text of the buttons, and their associated click listeners dynamically. This method is not very ideal becuase you may run in to cases where you want different amount of buttons for all your different states, and also, you're associating variable behavior with a single control. Also, you must manage your click listeners, adding and removing them dynamically.

here is a simple implementation
public class Demo extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
final Button button=(Button)findViewById(R.id.button);
button.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
button.setText("stop");
}
});
}
}
In the main.xml have a Button widget like this,
<Button android:id="#+id/button"
android:text="start"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
/>

How to simulate a button click using code?

How can I trigger a button click event using code in Android? I want to trigger the button click programmatically when some other event occurs.
Same Problem I am Facing
public void onDateSelectedButtonClick(View v){
/*Something Alarm Management
http://www.java2s.com/Code/Android/Core-Class/Alarmdemo.htm
copied code from this site*/
}
Button code:
<Button
android:onClick="onDateSelectedButtonClick"
android:text="Set notification for this date" />
But I want to call that function OnLoadLayout without OnClickEvent

there is a better way.
View.performClick();
http://developer.android.com/reference/android/view/View.html#performClick()
this should answer all your problems. every View inherits this function, including Button, Spinner, etc.
Just to clarify, View does not have a static performClick() method. You must call performClick() on an instance of View.
For example, you can't just call
View.performClick();
Instead, do something like:
View myView = findViewById(R.id.myview);
myView.performClick();

Just to clarify what moonlightcheese stated:
To trigger a button click event through code in Android
provide the following:
buttonName.performClick();

you can do it this way
private Button btn;
btn = (Button)findViewById(R.id.button2);
btn.performClick();

Just write this simple line of code :-
button.performClick();
where button is the reference variable of Button class and defined as follows:-
private Button buttonToday ;
buttonToday = (Button) findViewById(R.id.buttonToday);
That's it.

Android's callOnClick() (added in API 15) can sometimes be a better choice in my experience than performClick(). If a user has selection sounds enabled, then performClick() could cause the user to hear two continuous selection sounds that are somewhat layered on top of each other which can be jarring. (One selection sound for the user's first button click, and then another for the other button's OnClickListener that you're calling via code.)

Starting with API15, you can use also callOnClick() that directly call attached view OnClickListener. Unlike performClick(), this only calls the listener, and does not do any associated clicking actions like reporting an accessibility event.

If you do not use the sender argument, why not refactor the button handler implementation to separate function, and call it from wherever you want (from the button handler and from the other place).
Anyway, it is a better and cleaner design - a code that needs to be called on button handler AND from some other places deserves to be refactored to own function. Plus it will help you separate UI handling from application logic code. You will also have a nice name to the function, not just onDateSelectedButtonClick().