What's message queue in Android?

Question

Can someone explain what's the message queue in Android? Is it the list of processes running? I can't find a good source explaining it.

I am asking because I was reading about the method post of the class View.

POST

added in API level 1 boolean post (Runnable action)

Causes the Runnable to be added to the message queue. The runnable will be run on the user interface thread.

Thank you in advance.

Metencephalon

Answer 1

In simple terms a MessageQueue is a list of tasks (Messages, runnables) that will be executed in a certain thread. The Android system has a very known main thread (the UI one). The method you just saw simply adds a runnable to the list of processes that will be executed in the UI thread. Alongside with Looper and Handler, MessageQueues are part of the bulding blocks of threading in Android and they are used virtually everywhere in the system.

When would you use this method?

Whenever you want to update some UI element (View element) from another thread. Maybe you're doing some heavy lifting in another thread and want to update the UI element, you can't update the UI elements in others threads but the UI thread so you post changes to be executed in the UI thread.

You can learn more about MessageQueues here and here.

Embay

In summary: In the context of post method, Is it the list of processes running in the main thread? – Metencephalon

it's the list of process waiting to be run, they'll still be dispatched to the Looper to be executed – Embay

Don't get why everyone starts talking about UI thread whenever Handler is the topic. They are not specifically UI-coupled components. Handlers will send the task to be executed in a thread that was used to instantiate the new Handler(Looper) if no looper was provided it will take the associated thread's looper if any (and will throw an exception if no looper is not associate with the thread) – Tipstaff

@LeviMoreira Hi. Is there a limit in MessageQueue, so if there will be many messages and the handler can't process them all, MessageQueue will be blocked? – Winger

Answer 2

to Understand MessageQueue, you need understand executing model of android app;

Just like Javascript, Cocoa in iOS, to avoid cocurrency access racing, many App related framework adapts a single thread model.

that means there is a main thread, you put your work that need be done into the queue(MessageQueue) dedicated to this thread, there is a worker(Looper) that will reteive your work from the queue, run them one by one;

this model avoid cocurrency collision in the app;

when you need do a longtime job , you should put the work into the main thread queue, when it's to do your work in the message , you create a new thread to do this longtime job, after job is done , you put a new message into the main thread message queue from your new thread;

this picture will help you understand the running model

Faber