When to use handler.post() & when to new Thread()

Question

I'm wondering when should I use handler.post(runnable); and when should I use new Thread(runnable).start();

It is mentioned in developers documentation for Handler:

Causes the Runnable r to be added to the message queue. The runnable will be run on the thread to which this handler is attached.

Does this mean if I write in the onCreate() of Activity class:

Handler handler = new Handler();
handler.post(runnable);

then runnable will be called in a separate thread or in the Activity's thread?

Answer 1

use handler.post() when you want to post the code (usually from background thread) to the main thread. Yea, POST,just like you, post a letter to someone. With the help of handler the code will be executed ASAP i.e. almost immediately.

Answer 2

To answer you specific question:

Does this mean if in the onCreate of Activity class I write:

Handler handler = new Handler() hanlder.post(runnable); then, runnable will be called in a separate thread or on the Activity's thread?

No it won't be. The Runnable will be called on the Main Thread itself. Handler is simply used for posting a message to the thread to which it is attached (where its is created). It does not create a thread on its own. In your example, you created a Handler in the main Thread (that where Activity.OnCreate() is called) and hence any message posted on such a Handler will be run on the Main Thread only.

Answer 3

Example is jacked:

mHandler = new Handler();
new Thread(new Runnable(){
  @Override
  public void run () {
    mHandler.post(new Runnable() {
     @Override
     public void run () {
       mUiView.setX(x);
     }
    });
  }
}).start();

Alternatively you can skip the handler and use the post method on the view directly:

new Thread(new Runnable(){
  @Override
  public void run () {
    mUiView.post(new Runnable() {
     @Override
     public void run () {
       mUiView.setX(x);
     }
    });
  }
}).start();

This is a good post that outlines the difference: What exactly does the post method do?

Answer 4

You should use Handler.post() whenever you want to do operations in the UI thread.

So let's say in the callback (which is running in separate thread) you want to change a TextView's text, you should use Handler.post().

In Android, as in many other UI frameworks, UI elements (widgets) can be only modified from main thread.

---

Edit: the example of long-running tasks

mHandler = new Handler();

new Thread(new Runnable() {
  @Override
  public void run () {
    // Perform long-running task here
    // (like audio buffering).
    // you may want to update some progress
    // bar every second, so use handler:
    mHandler.post(new Runnable() {
     @Override
     public void run () {
       // make operation on UI - on example
       // on progress bar.
     }
    });
  }
}).start();

Of course, if the task you want to perform is really long and there is a risk that user might switch to some another app in the meantime, you should consider using a Service.