RunLoop

RunLoop 概念

    能够让线程随时处理事件但并不退出
  1.  The purpose of a run loop is to keep your thread busy when there is work to do and put your thread to sleep when there is none.
  2.  The RunLoop class is generally not considered to be thread-safe and its methods should only be called within the context of the current thread. You should never try to call the methods of an RunLoop object running in a different thread, as doing so might cause unexpected results.

线程和 RunLoop 之间是一一对应的，其关系是保存在一个全局的 Dictionary 里。线程刚创建时并没有 RunLoop，如果你不主动获取，那它一直都不会有。RunLoop 的创建是发生在第一次获取时，RunLoop 的销毁是发生在线程结束时。你只能在一个线程的内部获取其 RunLoop（主线程除外）

CFRunLoopRef

struct _block_item {
    struct _block_item *_next;
    CFTypeRef _mode;    // CFString or CFSet
    void (^_block)(void);
};

struct __CFRunLoop {
    CFRuntimeBase _base;
    pthread_mutex_t _lock;          /* locked for accessing mode list */
    __CFPort _wakeUpPort;           // used for CFRunLoopWakeUp 
    Boolean _unused;
    volatile _per_run_data *_perRunData;              // reset for runs of the run loop
    pthread_t _pthread;
    uint32_t _winthread;
    CFMutableSetRef _commonModes;
    CFMutableSetRef _commonModeItems;
    CFRunLoopModeRef _currentMode;
    CFMutableSetRef _modes;
    struct _block_item *_blocks_head;
    struct _block_item *_blocks_tail;
    CFTypeRef _counterpart;
};

struct __CFRunLoopMode {
    CFRuntimeBase _base;
    pthread_mutex_t _lock;  /* must have the run loop locked before locking this */
    CFStringRef _name;
    Boolean _stopped;
    char _padding[3];
    CFMutableSetRef _sources0;
    CFMutableSetRef _sources1;
    CFMutableArrayRef _observers;
    CFMutableArrayRef _timers;
    CFMutableDictionaryRef _portToV1SourceMap;
    __CFPortSet _portSet;
    CFIndex _observerMask;
#if USE_DISPATCH_SOURCE_FOR_TIMERS
    dispatch_source_t _timerSource;
    dispatch_queue_t _queue;
    Boolean _timerFired; // set to true by the source when a timer has fired
    Boolean _dispatchTimerArmed;
#endif
#if USE_MK_TIMER_TOO
    mach_port_t _timerPort;
    Boolean _mkTimerArmed;
#endif
#if DEPLOYMENT_TARGET_WINDOWS
    DWORD _msgQMask;
    void (*_msgPump)(void);
#endif
    uint64_t _timerSoftDeadline; /* TSR */
    uint64_t _timerHardDeadline; /* TSR */
};

struct __CFRunLoopSource {
    CFRuntimeBase _base;
    uint32_t _bits;
    pthread_mutex_t _lock;
    CFIndex _order;         /* immutable */
    CFMutableBagRef _runLoops;
    union {
    CFRunLoopSourceContext version0;    /* immutable, except invalidation */
        CFRunLoopSourceContext1 version1;   /* immutable, except invalidation */
    } _context;
};

struct __CFRunLoopObserver {
    CFRuntimeBase _base;
    pthread_mutex_t _lock;
    CFRunLoopRef _runLoop;
    CFIndex _rlCount;
    CFOptionFlags _activities;      /* immutable */
    CFIndex _order;         /* immutable */
    CFRunLoopObserverCallBack _callout; /* immutable */
    CFRunLoopObserverContext _context;  /* immutable, except invalidation */
};

struct __CFRunLoopTimer {
    CFRuntimeBase _base;
    uint16_t _bits;
    pthread_mutex_t _lock;
    CFRunLoopRef _runLoop;
    CFMutableSetRef _rlModes;
    CFAbsoluteTime _nextFireDate;
    CFTimeInterval _interval;       /* immutable */
    CFTimeInterval _tolerance;          /* mutable */
    uint64_t _fireTSR;          /* TSR units */
    CFIndex _order;         /* immutable */
    CFRunLoopTimerCallBack _callout;    /* immutable */
    CFRunLoopTimerContext _context; /* immutable, except invalidation */
};

CFRunLoop

一个RunLoop包含若干个Model,每个Model又包含若干个 Source/Timer/Observer
Three types of objects can be monitored by a run loop:

1. sources ([CFRunLoopSource]
2. timers ([CFRunLoopTimer]
3. observers [CFRunLoopObserver]
   (https://developer.apple.com/documentation/corefoundation/cfrunlooptimer?language=objc)

CFRunLoopObserverRef 是观察者，每个Observer都包含一个回调(函数指针),当runLoop的状态发生变化时，观察者就能通过回调接收到这个变化
typedef CF_OPTIONS(CFOptionFlags, CFRunLoopActivity) {
kCFRunLoopEntry 即将进入Loop
kCFRunLoopBeforeTimers 即将处理Timer
kCFRunLoopBeforeSources 即将处理
kCFRunLoopBeforeWaiting 即将进入休眠
kCFRunLoopAfterWaiting 刚从休眠中被唤醒
kCFRunLoopExit 即将推出Loop
kCFRunLoopAllActivities = 0x0FFFFFFFU
};
上面的 Source/Timer/Observer 被统称为 mode item，一个 item 可以被同时加入多个 mode。但一个 item 被重复加入同一个 mode 时是不会有效果的。如果一个 mode 中一个 item 都没有，则 RunLoop 会直接退出，不进入循环。

参考链接

深入理解RunLoop
深入研究 Runloop 与线程保活
RunLoop问题集