The Basis of Android Threading: Loopers and Handlers

Transcript

1. The Basis of Android Threading: Loopers and Handlers Matheus C. Candido Android @ Concrete
2. Why not Java threads?
3. Java Threads • Execute their work and quit • Threads have their own stack (which is large and memory intensive) • Creating a new thread takes (precious) time and memory
4. Looper • Creates a message loop for a Thread • Interaction with the Looper happens through a Handler • A Looper basically loops indefinitely dequeuing messages until it is requested to quit
5. class LooperThread extends Thread { public Handler handler; public void run() { Looper.prepare(); handler = new Handler() { public void handleMessage(Message msg) { // process incoming messages here } }; Looper.loop(); } }
6. public static void loop() { final Looper me = myLooper(); final MessageQueue queue = me.mQueue; // ... for (;;) { Message msg = queue.next(); // might block // ... try { msg.target.dispatchMessage(msg); } finally { // ... } // ... } }
7. public static void loop() { final Looper me = myLooper(); final MessageQueue queue = me.mQueue; // ... for (;;) { Message msg = queue.next(); // might block // ... try { msg.target.dispatchMessage(msg); } finally { // ... } // ... } }
8. public static void loop() { final Looper me = myLooper(); final MessageQueue queue = me.mQueue; // ... for (;;) { Message msg = queue.next(); // might block // ... try { msg.target.dispatchMessage(msg); } finally { // ... } // ... } }
9. Handler • Responsible for sending messages to a Looper • Default constructor uses the Looper of the thread it was called on • A different Looper can be passed in as a constructor argument
10. Handler • A message can be queued using many of the obtainMessage and post method overloads val message = handler.obtainMessage(LOAD_SUCCESS, result) message.sendToTarget() or mainThreadHandler.post { // some ui thread related code }
11. Work on the main thread? • Choreographer does not post at the end of the queue but rather at a specific (frame) time • If there’s something blocking the queue at that time, a frame will be dropped!
12. Work on the main thread? final long nextFrameTime = Math.max( mLastFrameTimeNanos / TimeUtils.NANOS_PER_MS + sFrameDelay, now); // … Message msg = mHandler.obtainMessage(MSG_DO_FRAME); // … mHandler.sendMessageAtTime(msg, nextFrameTime);
13. Message & MessageQueue • MessageQueue is not used directly, but through a Handler • Message is a public class but should not be instantiated: there’s a pool of recycled message objects (Message.obtain, Handler.obtainMessage)
14. public final class Message implements Parcelable { public int what; public int arg1; public int arg2; public Object obj; public Messenger replyTo; public int sendingUid = -1; }
15. ActivityThread • Created at the beginning of the app process • Orchestrates many intricacies of the app, like launching and coordinating activities life cycles • Basically all operations use the main handler
16. HandlerThread • It’s wrapper around a Java Thread • A HandlerThread assigns itself a looper and prepares it when run() is called • It can also provide its handler (one is not created automatically)
17. Resources Droidcon NYC 2015 - How the Main Thread works
18. Resources • Android OS classes • Um passeio com Threads no Android — Parte 1