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Confused by asynchronous mechanism

berlusca Published in 2017-12-06 11:35:49Z

I am confused by the fact that android is multi-threaded. If the code of a particular activity method is executing, can it be interrupted in the middle by onPause(), onStop() or onDestroy() or by another method like the onPostExecute() of an asynctask?

Edit 1

This question is not about activities life-cycle. What i am really asking is if the onPause() method can interrupt the onClick() method (just an example) and i would like to read more about how exactly android manages activities, calls their async methods etc...

Edit 2

Ok, it seems that i have found the relevant information i needed in the developer docs (don't know if it is explained better somewhere else): Threading Performance:


The main thread has a very simple design: Its only job is to take and execute blocks of work from a thread-safe work queue until its app is terminated. The framework generates some of these blocks of work from a variety of places. These places include callbacks associated with lifecycle information, user events such as input, or events coming from other apps and processes. In addition, app can explicitly enqueue blocks on their own, without using the framework.

It states that callbacks relative to activity lifecycle, user events such as input and other code are all managed using a "thread-safe work queue". This is central to understanding Android asynchronous programming. It explains a lots of things, such as why onClick() will be never be interrupted by activity onPause(), or why onClick() will be never be interrupted by a runnable posted using a Handler object (allocated in the main thread). So, for example, the onPostExecute() of an AsyncTask cannot interrupt the onClick() method (or onStart(), onResume(), onPause() etc...). As a novice android programmer, it was a bit confusing at first.

nihal_softy Reply to 2017-12-06 11:48:16Z

According to your question, I think you need to understand basic about Android Application's life cycle.

Basically, the life cycle deals with the state of application in different different situation. Like what will be state when application goes, in foreground or background and all that So These cycles state that you had mentioned like onPause(),onDestroy(), onStart(), onCreate().

Now talk for AsynchTask(), If you want to execute long running process like downloading image or listening musing or any other then we will use BackGround services. So don't get confusion between life cycle method and background services. .

Third, App will close only some rare condition like if memory is not sufficient or some other fatal issue occur in App then only it will terminate.

So, findings are that Thread is meant for long running process and Activity Life Cycles method are meant for various activity state

Please follow this tutorial and I hope you will get clarification

Hari N Jha
Hari N Jha Reply to 2017-12-06 13:20:51Z

This is a good question to ask I think. However, Here is my understanding:

When an Application starts, a Dalvik Virtual Machine (DVM) is assigned to it and works separately. So no two Applications interrupt each other as each of the application is managed by separate DVM.

Coming to Activity, it is a component of Android and runs on main thread. Android OS manages priority level to it's components. It gives high priority level to Activity, Services and may be other component(s). The OS maintains the non interruption behaviour of main thread.

Further when you run an AsyncTask, it is a long running task in background but dependent on Application context. So the OS does not guarantee to manage it.

Now regarding activity life cycle. As @nihal_softy said, activity has it's own life cycle which start from onCreate() and encounters onStart(), onResume(), onPause(), onStop() and onDestroy(). You can call these methods from anywhere of your Activity or from an AsyncTask and it will called but does not mean that the Activity will go in background or will destroy. In a simpler word, if you call onPause() from an AsyncTask, it does not mean that the Activity will be sent to background (if in foreground). Because when an Activity will go in background then it calls it's life cycle methods such as onPause() and onStop() as callback. But converse is not true.

I hope my understanding will help you to get the answer.

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