startActivty in a different thread

Question

I'm very very new to android. Hopefully this is not a dumb question.

protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    new Thread(new Runnable()
    {
        public void run()
        {
            Intent intent = new Intent(MainActivity.this, SecondActivity.class);
            startActivity(intent);
        }
    }).start();
}

Question: why does this work? is it normal that startActivity can be called from a secondary thread?

I thought that all UI related things had to be done in the UI thread.

Answer 1

startActivity is not immediate. It schedules the start of the activity to run at the next available cycle on the main thread, so you can call it from anywhere. (That doesn't necessarily mean it's a good idea, though.)

You are not supposed to touch the view hierarchy anywhere except the main thread. That's a different issue.

Answer 2

startActivity method can be called from any Activity Context

A new instance of the thread has access to the Context, which in turn has an Activity. That is why your code is working.

Note that if this method is being called from outside of an Activity Context, then the Intent must include the FLAG_ACTIVITY_NEW_TASK launch flag. This is because, without being started from an existing Activity, there is no existing task in which to place the new activity and thus it needs to be placed in its own separate task.

This method throws ActivityNotFoundException if there was no Activity found to run the given Intent.

For more info, read this developer manual