Why do we set content view in onCreate() and not in onStart() or onResume()?

Question

I've created a small program to set the content view on onResume() method instead of onCreate() and its working fine.

onResume()
{
        setContentView(R.layout.activity_main);
        editText1 = (EditText) findViewById(R.id.ed1);
        editText2 = (EditText) findViewById(R.id.ed2);
        Button button = (Button) findViewById(R.id.button1);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(MainActivity.this, "Button Clicked", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

Answer 1

You don't setContentView in onStart or onResume because it's inefficient. Setting up the layout is a pretty heavyweight task. You have to parse the XML, create the Views, measurements, drawing, etc. It's the sort of task that you want to run at most once.

During the lifetime of your Activity class onCreate gets called exactly once. onResume and onStart will get called multiple times without your Activity getting destroyed.

Answer 2

As onCreate() of an Activity is called only once, this is the point where most initialization should go: calling setContentView(int) to inflate the activity's UI, using findViewById to programmatically interact with widgets in the UI, calling managedQuery(android.net.Uri, String[], String, String[], String) to retrieve cursors for data being displayed, etc.

It is inefficient to set the content in onResume() or onStart() (which are called multiple times) as the setContentView() is a heavy operation.

Answer 3

onResume you may call it serval times. onCreate is just one time. Try log sth in they and skip to other activity than back.