Question

A Handler allows you to send and process Message and Runnable objects associated with a thread’s MessageQueue. Each Handler instance is associated with a single thread and that thread’s message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it — from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.

http://developer.android.com/reference/android/os/Handler.html

Does this actually mean that the Runnable object will block the Messages posted after it?
The messages and Runnable objects actually in one queue?

The manual is little ambiguous to me.

Thanks!

Answer 1

It’s not so hard to check the code path of this. So in Activity.java you have the following code to pass Runnable to Ui thread:

public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

Then for Handler.post:

public final boolean post(Runnable r)
{
   return  sendMessageDelayed(getPostMessage(r), 0);
}

Basically sendMessageDelayed just simply sends a message with delay 0 ms. So the most interesting part is how to getPostMessage from Runnable:

private final Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    return m;
}

So what does it mean? This means that the handler of the Ui thread if it gets message to run Runnable will try to execute it. And if this runnable operation is very long it will block the Ui thread that can cause ANR error.