Android Setonclicklistener Method - How Does It Work

Android setOnClickListener method - How does it work?

It works like this. View.OnClickListenere is defined -

public interface OnClickListener {
    void onClick(View v);
}

As far as we know you cannot instantiate an object OnClickListener, as it doesn't have a method implemented. So there are two ways you can go by - you can implement this interface which will override onClick method like this:

public class MyListener implements View.OnClickListener {
    @Override
    public void onClick (View v) {
         // your code here;
    }
}

But it's tedious to do it each time as you want to set a click listener. So in order to avoid this you can provide the implementation for the method on spot, just like in an example you gave.

setOnClickListener takes View.OnClickListener as its parameter.

How does Android's setOnCLickListener() works?

Each View contains ListenerInfo static class which holds callbacks, OnClickListener too actually.

How it works?

System always holds all views on screen.
When user tap on screen we have a recursive foreach cycle :

switch(event) {
    ...
    case ON_CLICK:
    process(ViewRoot);
}

void process(View view) {
    
    for(View view : view.getChilds()) {
        if(view instanceOf ViewGroup && ((ViewGroup)view).getChildCount() > 0) {
             process(view);
        }
        if(view.getListenerInfo().mOnClickListener != null)
             view.getListenerInfo().mOnClickListener.onClick(view)
    }
}

When you call setOnClickListener you actually say "hey Android! here it callback. And when user make click, please use it."

View.class also have getListenerInfo method which returns ListenerInfo object.

System use this method to dispatch events.

So no Observer pattern here. It just simple check of existing callback.

Android - setOnClickListener vs OnClickListener vs View.OnClickListener

The logic is simple. setOnClickListener belongs to step 2.

1. You create the button
2. You create an instance of OnClickListener* like it's done in that example and override the onClick-method.
3. You assign that OnClickListener to that button using btn.setOnClickListener(myOnClickListener); in your fragments/activities onCreate-method.
4. When the user clicks the button, the onClick function of the assigned OnClickListener is called.

*If you import android.view.View; you use View.OnClickListener. If you import android.view.View.*; or import android.view.View.OnClickListener; you use OnClickListener as far as I get it.

Another way is to let you activity/fragment inherit from OnClickListener. This way you assign your fragment/activity as the listener for your button and implement onClick as a member-function.

Android- Logic behind setOnClickListener

View.OnClickListener is an interface which you need to implement when you want to handle click events. In your code, you are doing that by doing new View.OnClickListener(). Here you are actually creating an anonymous class that implements View.OnClickListener. And any class that implements View.OnClickListener must also implement all the methods declared in it (e.g. the onClick method). Also, in public void onClick(View v) {..},
the View v denotes the view or the button that was clicked so that you can perform whatever you want with it. For example, get it's id, change it's color etc.

setOnClickListener method can't get EditText value

Remove this

final String question = question_EditText.getText().toString();

it only get value 1 time when initial activity.

Move String question = question_EditText.getText().toString(); into onClick method.

Try this to get current string in edittext when you click button.

        

        add_question.setOnClickListener(new View.OnClickListener()

        {

            @Override

            public void onClick(View v)

            {

                String question = question_EditText.getText().toString();

                Log.d("finds","question:"+question);

                if (question.length()==0)

                {

                    new AlertDialog.Builder(AddQuestionActivity.this)

                        .setTitle(R.string.add_error)

                        .setMessage(R.string.add_blank)

                        .setPositiveButton(R.string.ok, null)

                        .setNeutralButton(R.string.cancel, null)

                        .show();

                }

            }

        });

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