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docs: 内容
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杭城小刘
@@ -200,7 +200,7 @@
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**__CFRunLoopModeIsEmpty**
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此函数的作用就是判断这个 Mode 下面有没有 source0、source1、timer，只要存在就说明当前 Mode 不是空的，同时看看这个 Mode 是不是属于当前的 RunLoop
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```objective-c
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@@ -768,3 +768,341 @@
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}
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```
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- performSelector 是在 source0 上实现的
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- RunLoopTimer 外部接口设置的精度，精度大于0，则使用 dispatch_source_set_timer，精度小于0，则使用 mk_timer_arm
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- timer_source 使用 dispatch_timer_STRICT 创建，则系统会尽最大努力遵守设置的 leeway 值
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- NSTimer 不准的原因：底层 RunLoop Timer 底层使用的 timer 的精度不高（mk_timer）；与 RunLoop 底层的调用机制有关系
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- 那么为什么存在 RunLoopTimer？意义是什么？应用场景
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```c++
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// Data structure to hold TSD data, cleanup functions for each
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typedef struct __CFTSDTable {
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uint32_t destructorCount;
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uintptr_t data[CF_TSD_MAX_SLOTS];
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tsdDestructor destructors[CF_TSD_MAX_SLOTS];
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} __CFTSDTable;
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```
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```c++
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// 主线程 RunLoop
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CFRunLoopRef CFRunLoopGetMain(void) {
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CHECK_FOR_FORK();
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// 局部静态变量
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static CFRunLoopRef __main = NULL; // no retain needed
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// 创建主线程对应的 RunLoop
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if (!__main) __main = _CFRunLoopGet0(pthread_main_thread_np()); // no CAS needed
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return __main;
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}
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// 子线程 RunLoop
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// 先从 __CFTSDTable 获取 RunLoop，如果有则 return，没有则调用 _CFRunLoopGet0，_CFRunLoopGet0 内部调用 __CFRunLoopCreate 创建 CFRunLoop，然后写入 __CFTSDTable
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CFRunLoopRef CFRunLoopGetCurrent(void) {
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CHECK_FOR_FORK();
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// __CFTSDTable
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CFRunLoopRef rl = (CFRunLoopRef)_CFGetTSD(__CFTSDKeyRunLoop);
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if (rl) return rl;
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// 没有则创建
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return _CFRunLoopGet0(pthread_self());
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}
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CF_EXPORT CFRunLoopRef _CFRunLoopGet0(_CFThreadRef t) {
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if (pthread_equal(t, kNilPthreadT)) {
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t = pthread_main_thread_np();
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}
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__CFLock(&loopsLock);
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if (!__CFRunLoops) {
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CFMutableDictionaryRef dict = CFDictionaryCreateMutable(kCFAllocatorSystemDefault, 0, NULL, &kCFTypeDictionaryValueCallBacks);
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CFRunLoopRef mainLoop = __CFRunLoopCreate(pthread_main_thread_np());
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CFDictionarySetValue(dict, pthreadPointer(pthread_main_thread_np()), mainLoop);
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if (!OSAtomicCompareAndSwapPtrBarrier(NULL, dict, (void * volatile *)&__CFRunLoops)) {
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CFRelease(dict);
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}
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CFRelease(mainLoop);
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}
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CFRunLoopRef newLoop = NULL;
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CFRunLoopRef loop = (CFRunLoopRef)CFDictionaryGetValue(__CFRunLoops, pthreadPointer(t));
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if (!loop) {
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newLoop = __CFRunLoopCreate(t);
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CFDictionarySetValue(__CFRunLoops, pthreadPointer(t), newLoop);
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loop = newLoop;
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}
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__CFUnlock(&loopsLock);
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// don't release run loops inside the loopsLock, because CFRunLoopDeallocate may end up taking it
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if (newLoop) { CFRelease(newLoop); }
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if (pthread_equal(t, pthread_self())) {
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_CFSetTSD(__CFTSDKeyRunLoop, (void *)loop, NULL);
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if (0 == _CFGetTSD(__CFTSDKeyRunLoopCntr)) {
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#if _POSIX_THREADS
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_CFSetTSD(__CFTSDKeyRunLoopCntr, (void *)(PTHREAD_DESTRUCTOR_ITERATIONS-1), (void (*)(void *))__CFFinalizeRunLoop);
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#else
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_CFSetTSD(__CFTSDKeyRunLoopCntr, 0, &__CFFinalizeRunLoop);
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#endif
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}
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}
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return loop;
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}
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```
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- 为什么只有当 RunLoop 中存在 Timer Sourcrs、Input Sources 时，才能保证 RunLoop 不退出？
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RunLoop 本质就是一个有条件的 do...while 循环。__CFRunLoopModeIsEmpty 里面去判断 source0、source1、timers 不存在则 while 循环条件不满足，RunLoop 退出
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```c++
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void CFRunLoopRun(void) { /* DOES CALLOUT */
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int32_t result;
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do {
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result = CFRunLoopRunSpecific(CFRunLoopGetCurrent(), kCFRunLoopDefaultMode, 1.0e10, false);
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CHECK_FOR_FORK();
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} while (kCFRunLoopRunStopped != result && kCFRunLoopRunFinished != result);
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}
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// expects rl and rlm locked
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static Boolean __CFRunLoopModeIsEmpty(CFRunLoopRef rl, CFRunLoopModeRef rlm, CFRunLoopModeRef previousMode) {
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// ...
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if (NULL != rlm->_sources0 && 0 < CFSetGetCount(rlm->_sources0)) return false;
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if (NULL != rlm->_sources1 && 0 < CFSetGetCount(rlm->_sources1)) return false;
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if (NULL != rlm->_timers && 0 < CFArrayGetCount(rlm->_timers)) return false;
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// ...
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return true;
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}
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SInt32 CFRunLoopRunSpecific(CFRunLoopRef rl, CFStringRef modeName, CFTimeInterval seconds, Boolean returnAfterSourceHandled) { /* DOES CALLOUT */
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// ...
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CFRunLoopModeRef currentMode = __CFRunLoopFindMode(rl, modeName, false);
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if (NULL == currentMode || __CFRunLoopModeIsEmpty(rl, currentMode, rl->_currentMode)) {
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Boolean did = false;
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if (currentMode) __CFRunLoopModeUnlock(currentMode);
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__CFRunLoopUnlock(rl);
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return did ? kCFRunLoopRunHandledSource : kCFRunLoopRunFinished;
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}
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// ...
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result = __CFRunLoopRun(rl, currentMode, seconds, returnAfterSourceHandled, previousMode);
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// ...
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return result;
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}
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```
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## 线程保活技术：
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```objective-c
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- (void)viewDidLoad {
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[super viewDidLoad];
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[self testKeepAliveThread];
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}
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- (void)testKeepAliveThread
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{
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static dispatch_once_t onceToken;
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dispatch_once(&onceToken, ^{
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self.thread = (ThreadKeepLive *)[[NSThread alloc] initWithTarget:self selector:@selector(threadTask) object:nil];
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[self.thread setName:@"LongLiveThread"];
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[self.thread setQualityOfService:[NSThread mainThread].qualityOfService];
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[self.thread start];
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});
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}
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- (void)threadTask
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{
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[NSTimer scheduledTimerWithTimeInterval:1 repeats:YES block:^(NSTimer * _Nonnull timer) {
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NSLog(@"come");
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}];
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[[NSRunLoop currentRunLoop] addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
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while (true) {
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[[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
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}
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}
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```
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- come 会打印几次？1次，因为 RunLoop 跑起来之后，后面的代码不会执行，内部不断的 do...while
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- NSRunLoop 常驻线程可以运行在 NSRunLoopCommonModes 下吗？
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不可以。因为在跑的时候如果 modeName 等于 kCFRunLoopCommonModes 则直接 kCFRunLoopRunFinished，则 RunLoop 的 while 循环条件失败
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```objective-c
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SInt32 CFRunLoopRunSpecific(CFRunLoopRef rl, CFStringRef modeName, CFTimeInterval seconds, Boolean returnAfterSourceHandled) { /* DOES CALLOUT */
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CHECK_FOR_FORK();
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if (modeName == NULL || modeName == kCFRunLoopCommonModes || CFEqual(modeName, kCFRunLoopCommonModes)) {
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static dispatch_once_t onceToken;
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dispatch_once(&onceToken, ^{
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CFLog(kCFLogLevelError, CFSTR("invalid mode '%@' provided to CFRunLoopRunSpecific - break on _CFRunLoopError_RunCalledWithInvalidMode to debug. This message will only appear once per execution."), modeName);
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_CFRunLoopError_RunCalledWithInvalidMode();
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});
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return kCFRunLoopRunFinished;
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}
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// ...
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return result;
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}
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```
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### Mach Port 跨线程通信
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1. Mach IPC 基于 Mach 内核实现进程间通讯。
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2. Mach IPC 被抽象为3种操作：messages、ports、and port sets
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3. Mach port 跨线程通信
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线程 B 有个 port 在等待消息，具有消息接收权限，等待消息到来的时候会阻塞当前线程
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线程 A 有个 port 在发送消息，具有发送消息权限，要把发送的消息包装成消息，通过消息队列传递，message 包括：header（目的地 port、size）、data。
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线程 B 收到消息后，解除 block，线程继续向下运行
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4. Mach port 如何进行跨线程通信？
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线程开启一个 port，然后给 port 申请接收、发送的权限。mach_msg 是通信函数，在等待消息的时候不加 timeout 则会一直阻塞，等到消息到来
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5. 在测试工作中 main.m 文件中打印当前的 RunLoop
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```objective-c
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int main(int argc, char * argv[]) {
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NSString * appDelegateClassName;
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@autoreleasepool {
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appDelegateClassName = NSStringFromClass([AppDelegate class]);
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}
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NSLog(@"%@", NSRunLoop.currentRunLoop);
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return UIApplicationMain(argc, argv, nil, appDelegateClassName);
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}
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//
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2020-08-13 09:49:41.326621+0800 ***[52423:2383402] <CFRunLoop 0x6000015f0000 [0x7fff8062d750]>{wakeup port = 0x1103, stopped = false, ignoreWakeUps = true,
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current mode = (none),
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common modes = <CFBasicHash 0x6000027f1320 [0x7fff8062d750]>{type = mutable set, count = 1,
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entries =>
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2 : <CFString 0x7fff80640a20 [0x7fff8062d750]>{contents = "kCFRunLoopDefaultMode"}
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}
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,
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common mode items = (null),
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modes = <CFBasicHash 0x6000027f1290 [0x7fff8062d750]>{type = mutable set, count = 1,
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entries =>
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2 : <CFRunLoopMode 0x6000012fc0d0 [0x7fff8062d750]>{name = kCFRunLoopDefaultMode, port set = 0x1003, queue = 0x6000007f0100, source = 0x6000007f0280 (not fired), timer port = 0xe03,
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sources0 = (null),
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sources1 = (null),
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observers = (null),
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timers = (null),
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currently 618976181 (257342842892563) / soft deadline in: 1.84464867e+10 sec (@ -1) / hard deadline in: 1.84464867e+10 sec (@ -1)
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},
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}
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}
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```
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可以看到 main.m 中，还没有 return 的时候当前 RunLoop 的内部结构中存在一个 **wakeup port** 的端口。查看 RunLoop 源代码 **wakeup port** 就是 mach port 的一种。
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```c++
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typedef mach_port_t __CFPort;
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struct __CFRunLoop {
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CFRuntimeBase _base;
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_CFRecursiveMutex _lock; /* locked for accessing mode list */
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__CFPort _wakeUpPort; // used for CFRunLoopWakeUp
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Boolean _unused;
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volatile _per_run_data *_perRunData; // reset for runs of the run loop
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_CFThreadRef _pthread;
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uint32_t _winthread;
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CFMutableSetRef _commonModes;
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CFMutableSetRef _commonModeItems;
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CFRunLoopModeRef _currentMode;
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CFMutableSetRef _modes;
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struct _block_item *_blocks_head;
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struct _block_item *_blocks_tail;
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CFAbsoluteTime _runTime;
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CFAbsoluteTime _sleepTime;
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CFTypeRef _counterpart;
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_Atomic(uint8_t) _fromTSD;
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CFLock_t _timerTSRLock;
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};
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```
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```c++
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void CFRunLoopWakeUp(CFRunLoopRef rl) {
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// ...
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kern_return_t ret;
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/* We unconditionally try to send the message, since we don't want
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* to lose a wakeup, but the send may fail if there is already a
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* wakeup pending, since the queue length is 1. */
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ret = __CFSendTrivialMachMessage(rl->_wakeUpPort, 0, MACH_SEND_TIMEOUT, 0);
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// ...
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int ret;
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do {
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ret = eventfd_write(rl->_wakeUpPort, 1);
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} while (ret == -1 && errno == EINTR);
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// ...
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SetEvent(rl->_wakeUpPort);
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// ...
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}
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```
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`CFRunLoopWakeUp(CFRunLoopGetCurrent());` 可以唤醒 RunLoop，函数的底层实现如上。核心实现就是 `__CFSendTrivialMachMessage` 函数。在iOS 中，除了 source1 可以自己唤醒 RunLoop 之外，其他的事件都需要用户手动唤醒 RunLoop 才可以。RunLoop 提供了专门的方法来实现这个功能。其核心部分就是调用 mach_msg 来向指定的 **_wakeUpPort** 端口发送消息，从而唤醒线程继续工作。
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为什么 Source1 可以唤醒 RunLoop？因为 Source1 本质上就是针对 Mach Port 的封装
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6. 做个实验检测 _wakeUpPort 端口
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```objective-c
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- (void)viewDidLoad {
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[super viewDidLoad];
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[self listenWakeUpPort];
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}
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- (void)listenWakeUpPort
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{
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NSArray<NSString *> *array = [NSRunLoop.currentRunLoop.description componentsSeparatedByString:@"wakeup port = "];
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NSString *wakeupPort = [array.lastObject substringToIndex:[array.lastObject rangeOfString:@","].location];
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dispatch_queue_t queue = dispatch_queue_create("com.test.wake_up_port_queue", DISPATCH_QUEUE_CONCURRENT);
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dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_MACH_RECV, [self numberWithHexString:wakeupPort], 0, queue);
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dispatch_source_set_event_handler(source, ^{
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mach_port_t port = (mach_port_t)dispatch_source_get_handle(source);
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NSLog(@"%u--wakeUp", port);
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});
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dispatch_activate(source);
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}
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- (NSInteger)numberWithHexString:(NSString *)hexString
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{
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const char *hexChar = [hexString cStringUsingEncoding:NSUTF8StringEncoding];
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int hexNumber;
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sscanf(hexChar, "%x", &hexNumber);
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return (NSInteger)hexNumber;
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}
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- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
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{
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CFRunLoopWakeUp(CFRunLoopGetCurrent());
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}
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```
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可以看到每次在点击屏幕时调用 `CFRunLoopWakeUp` 尝试唤醒 RunLoop，然后监听 RunLoop 的 _wakeUpPort，都可以在回调中获取到消息。
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7. RunLoop lifecycle
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休眠时 RunLoop 被 mach_msg 阻塞，等到消息到来，可以是手动给 RunLoop 的 wakeUpPort 发消息 mach_msg，或者是 timer、Source1。然后继续走到 RunLoop run，不断循环
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Timer 不准确的原因：1. RunLoop 底层对 Timer 实现的精度不高，所以需要指定 Timer 的精度；2. RunLoop 在 DoSource0、DoSource1 的时候耗时较多，则导致 Timer 下次执行会不准确
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Reference in New Issue
Block a user