Handler understands the record

Catalog

problem

1、 A thread has several handler

2、 A thread has several looper？ How to ensure ？

3、handler The cause of the memory leak , Why don't other inner classes have this problem

4、 In the child thread new handler（） What preparations should be made before

5、 Maintained in a subthread looper, When there is no message in the message queue , What's the usage? ？

6、 Since there can be more than one handler Go to MessageQueue Add data to （ Messages can be sent in different threads ） How does his internal system ensure thread safety

7、 We use Message How to create

8、Looper.loop（） Why doesn't an endless cycle occur anr

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handler Mechanism

problem

1、 A thread has several handler

handler adopt new establish handler object , The number of objects depends on new The number of

2、 A thread has several looper？ How to ensure ？

A thread has a looper

Whenever executed looper.prepare（） Will judge ThreadLocal.get（）

Looper Class will define final ThreadLocal attribute ThreadLocal only

ThreadLocal Class maintains an inner ThreadLocalmap class

ThreadLocalmap Of key For threads

value by Created looper

according to ThreadLocalmap Of key You can know that there is only one thread looper

3、handler The cause of the memory leak , Why don't other inner classes have this problem

handler The implementation Anonymous inner classes hold this References to ,handler.enqueueMessage(msg) Time will make msg.target hold handler object .

This problem occurs when the generation period of the inner class is no longer greater than that of the outer class

（ Static inner class     Weak reference An external class with a dough period ）

4、 In the child thread new handler（） What preparations should be made before

In the child thread You need to perform looper.prepare（） and  looper.loop()

In the main thread stay ActivityThread Of main It has already been carried out   looper.mainprepare（） and  looper.loop()

5、 Maintained in a subthread looper, When there is no message in the message queue , What's the usage? ？

call looper.quit（）

perform nativewake Wake up the thread   

end looper.loop The dead cycle in

6、 Since there can be more than one handler Go to MessageQueue Add data to （ Messages can be sent in different threads ） How does his internal system ensure thread safety

Every looper in Message queue   MessageQueue Is the only one.

When executed  MessageQueue.enqeueMessage（） Will use synchronized  Synchronization lock , perform next when Same meeting  synchronized, This ensures thread safety

7、 We use Message How to create

1、 Sure new message

2、 Reuse message.obtain（）

avoid Memory jitter cause oom

Message in msg.target.dispatchMessage Will perform recycleUnchecked  take message Data clearing for

message.next Maintenance queue

8、Looper.loop（） Why doesn't an endless cycle occur anr

anr yes Packaged handler news

Itself is a need adopt Looper.loop（） To deal with

anr Is with the Looper.loop（） Functions unrelated to the loop