notes/docs/lectures/mdp/04_threads.md

Threads (and Services)

Activities are regularly transitioned to the background

• i.e. when they are stopped or destroyed
• As the user continues with the task

Thread of Execution

Android applications use a single thread model

• A single thread of execution called main
  • Started when a process is created
• Handles and dispatches user interface events
  • Drawing the interface, responding to onClick()
• Handles activity life cycle events
  • onCreate(), onDestroy()
  • For all components in an application

Looper and Handler

HandlerThread - extension of thread with support for looper

Looper

• Each HandlerThread can have one looper
• A Java thread dies when the run method returns
• Maintains a message queue
• looper.loop() - loops through the message queue and processes waiting messages

Message

• A task to be completed
  • Might contain data or a reference to a Runnable object

Handler

• Attached to a looper
• Enqueues messages in the looper message queue
• Handles messages from the message queue
• Thread-safe
• One looper can have many handlers associated with it

If we run code that takes >5 s, the application will appear to hang.

• Put longer-running / non-instantaneous code in a separate thread of execution
  • Network access, file access
• 2 rules
  1. Do not block the UI thread
  2. Do not access the UI thread from outside the UI thread
     • Android view objects are not thread-safe
     • Post messages via the Handler to the UI thread's looper

We can programmatically interact with UI components

• e.g. textField.setText(string)
• But cannot call this method from outside the UI thread (rule 2)

Instead, split code into two parts

1. Long(ish)-running code that does not involve the UI
2. Instantaneous code that does involve the UI
   • Which needs to be posted to the UI thread responsible for the view

We need to remember the activity life-cycle; otherwise, we risk having orphaned threads.

AsyncTask and others

Accessing the UI thread from another thread

Activity.runOnUiThread(Runnable);
View.post(Runnable);
View.postDelayed(Runnable, long);
MutableLiveData.postValue;