What does zero mean in Handler.sendEmptyMessage(0)

Question

I am learning Android and I am stuck at this statement. From google:

Sends a Message containing only the what value.

Returns

Returns true if the message was successfully placed in to the message queue. Returns false on failure, usually because the looper processing the message queue is exiting.

Someone please explains for me what the Message containing zero will do. Thanks

Answer 1

It means the what value. what is basically an integer that allows the receiver to identify the messages it receives.

Your handleMessage function looks like this

public void handleMessage (Message msg)

you are passed a Message object and you can check the public field what to figure out what the message is about. (msg.what)

Eg.

you send two types of messages, with what value 1 for success and 0 for failure

so your handleMessage function would look something like this

public void handleMessage (Message msg) {
    switch (msg.what) {
        case 1:
            //success handling
            break;

        case 0:
            //failure handling
            break;
    }
}

Now you can have sendEmptyMessage(0) for success and sendEmptyMessage(1) for failure.

Remember, that you dont have to send an empty message, you can send a Message object with more data attached to it too

for example to send a message with some text on success you can do

Message.obtain(mHandler, 0, "Success text")

and similarly for failure

Now as per what the zero means, It is just sending an empty message and the 0 could be replaced by any value. The idea is in this case you have only one type of messages and the Handler does understands that. So it does not need to check what kind of message it has received it just needs to receive a message. So sendEmptyMessage(AnyInteger) would work fine. 0 is just by convention