Send an android.os.Message whose what is 0 and then remove it, which unexpectedly removes all posted Runnables

Scenario

I write a Runnable and post it to the main thread, however the Runnable is not executed.

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// callback from the background thread
private void callback() {
    mHandler.post(new Runnable() {
        // update UI in the main thread
    });
}

Bug

After posting the Runnable, someone executes the following code:

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// CUSTOM_MESSAGE_WHAT is 0
mHandler.removeMessages(CUSTOM_MESSAGE_WHAT)

and it not only removes the custom message but also removes all posted Runnables in the MessageQueue. The reason is that android.os.Handler wraps every posted Runnable in an android.os.Message whose what is 0 which conflicts with the what value of the custom message.

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/** Handler.java **/
public final boolean post(Runnable r) {
    return sendMessageDelayed(getPostMessage(r), 0);
}

private static Message getPostMessage(Runnable r) {
    Message m = Message.obtain();
    m.callback = r;
    // if not specified otherwise, m.what is 0 which is the default value
    return m;
}

Solution

Never use 0 as the what value of custom messages

Reference

The source code of android.os.Handler, android.os.Message and android.os.MessageQueue