概要
在很久以前的时候转载了一小篇文章《Android Message Queue、Message、Looper、Handler》白话介绍了一下Android Message Queue、Looper、Handler这几个概念之间的关系。其中Looper居于这几个概念的核心，一个Looper里面有一个MessageQueue。多个Handler可以共享一个Looper，一个线程里面只有一个Looper。今天再深入分析一下Looper的原理源码
源码位置
frameworks/native/lib/utils/Looper.cpp
Looper.h
class Looper : public ALooper, public RefBase {
protected:
virtual ~Looper();
public:
/**
* Creates a looper.
*
* If allowNonCallbaks is true, the looper will allow file descriptors to be
* registered without associated callbacks. This assumes that the caller of
* pollOnce() is prepared to handle callback-less events itself.
*/
Looper(bool allowNonCallbacks);
/**
* Returns whether this looper instance allows the registration of file descriptors
* using identifiers instead of callbacks.
*/
bool getAllowNonCallbacks() const;
/**
* Waits for events to be available, with optional timeout in milliseconds.
* Invokes callbacks for all file descriptors on which an event occurred.
*
* If the timeout is zero, returns immediately without blocking.
* If the timeout is negative, waits indefinitely until an event appears.
*
* Returns ALOOPER_POLL_WAKE if the poll was awoken using wake() before
* the timeout expired and no callbacks were invoked and no other file
* descriptors were ready.
*
* Returns ALOOPER_POLL_CALLBACK if one or more callbacks were invoked.
*
* Returns ALOOPER_POLL_TIMEOUT if there was no data before the given
* timeout expired.
*
* Returns ALOOPER_POLL_ERROR if an error occurred.
*
* Returns a value >= 0 containing an identifier if its file descriptor has data
* and it has no callback function (requiring the caller here to handle it).
* In this (and only this) case outFd, outEvents and outData will contain the poll
* events and data associated with the fd, otherwise they will be set to NULL.
*
* This method does not return until it has finished invoking the appropriate callbacks
* for all file descriptors that were signalled.
*/
int pollOnce(int timeoutMillis, int* outFd, int* outEvents, void** outData);
inline int pollOnce(int timeoutMillis) {
return pollOnce(timeoutMillis, NULL, NULL, NULL);
}
/**
* Like pollOnce(), but performs all pending callbacks until all
* data has been consumed or a file descriptor is available with no callback.
* This function will never return ALOOPER_POLL_CALLBACK.
*/
int pollAll(int timeoutMillis, int* outFd, int* outEvents, void** outData);
inline int pollAll(int timeoutMillis) {
return pollAll(timeoutMillis, NULL, NULL, NULL);
}
/**
* Wakes the poll asynchronously.
*
* This method can be called on any thread.
* This method returns immediately.
*/
void wake();
/**
* Adds a new file descriptor to be polled by the looper.
* If the same file descriptor was previously added, it is replaced.
*
* "fd" is the file descriptor to be added.
* "ident" is an identifier for this event, which is returned from pollOnce().
* The identifier must be >= 0, or ALOOPER_POLL_CALLBACK if providing a non-NULL callback.
* "events" are the poll events to wake up on. Typically this is ALOOPER_EVENT_INPUT.
