Android: why must use getBaseContext() instead of

问题:

this often to reference to current context. But, at some case, why we must use getBaseContext() instead of this. (It means when use this will notice error).

Here is my example:

Spinner spinner = (Spinner) findViewById(R.id.spinner);
spinner.setAdapter(adapter);            
spinner.setOnItemSelectedListener(new OnItemSelectedListener() {
    @Override
    public void onItemSelected(AdapterView<?>arg0, View arg1, int arg2, long arg3){
       Toast.makeText(getBaseContext(),"SELECTED", Toast.LENGTH_SHORT).show(); //this line
    }

At above code, when I change getBaseContext() to this will receive error.

Who can explain for me, please.

回答1:

1. getApplicationContext () returns the application context of the entire application life cycle,when application will destroy then it will destroy also.

2. this the context returns the current context of the activity, belong to the activity, the activity is destroyed then it will destroy also.but in your case it will refers to the Spinner instance because we are using this within onItemSelected(AdapterView<?>arg0, View arg1, int arg2, long arg3) method which is from Spinner class and Spinner inherit this method from AdapterView.OnItemSelectedListener interface

3. getBaseContext() is the method of ContextWrapper. And ContextWrapper is, "Proxying implementation of Context that simply delegates all of its calls to another Context. Can be subclassed to modify behavior without changing the original Context." (as per javadocs)..

and in your case :Spinner class is not subclass of Context or ContextWrapper class*

Toast.makeText(getBaseContext(),"SELECTED", Toast.LENGTH_SHORT).show();

means getBaseContext() is method of ContextWrapper and ContextWrapper is Proxying implementation of Context so indirectly we are passing an Context Class Object.

or we can also pass 'Activity.this' because Activity class is subclass of ContextWrapper class .

if you go with android documention then this method require an Context class object:
public static Toast makeText (Context context, int resId, int duration)

so we are not able to pass an activity or class context means this to Toast.makeText which don't have a subclass of either Context or ContextWrapper class.

回答2:

In your example this refers to newly created OnItemSelectedListener not to any context object. If this code is in activity you can write YourActivity.this instead of getBaseContext().

OnItemSelectedListener listener = new OnItemSelectedListener() {
    @Override
    public void onItemSelected(AdapterView<?>arg0, View arg1, int arg2, long arg3){
       // this.equals(listener) == true;

       // getBaseContext() here means YourActivity.this.getBaseContext()
       // getBaseContext() called from outer context object (activity, application, service)
    }
}

回答3:

If this code is in the Activity MyActivity, you could also replace getBaseContext() by MyActivity.this.

This is because this refers to the OnItemSelectedListener instance, not to the Activity. getBaseContext() refers to the Activity context.

回答4:

OnItemSelected method this refers to the new OnItemSelectedListener instance that you used. getBaseContext is you outer class.

回答5:

getBaseContext() refers to Activity.this

like we want to showing Toast on click of button, we never user this we use Activty.this. So that our Toast display till we are on the same activity. But if we use getApplicationContext() than our Toast will display even we switch the activity.

回答6:

If you use this refers OnItemSelectedListener .And its Interface not Class .so it's gives you error ,,, Always Use if you are in Activity YourActivityName.this

回答7:

The getBaseContext() Sometimes confuses new comers to android, instead one could also use the ActivityName which is the current activity in which your are working. so ActivityName.this will replace getBaseContext()