CocoaAsyncSocket 源码学习摘要：
1. [NSObject cancelPreviousPerformRequestsWithTarget:self]
//AsyncSocket 的dealloc说明。
- (void)dealloc
{
[self close];
[theReadQueue release];
[theWriteQueue release];
[theRunLoopModes release];
[partialReadBuffer release];
[NSObject cancelPreviousPerformRequestsWithTarget:theDelegate selector:@selector(onSocketDidDisconnect:) object:self];
[NSObject cancelPreviousPerformRequestsWithTarget:self];
[super dealloc];
}
一般我们都知道调用了[instance performSelector:@selector() withObject:nil afterDelay:]会导致instance引用计数+1。如果我们调用了[instance performSelector:@selector() withObject:nil afterDelay:]。在某些情况下我们就需要调用[NSObject cancelPreviousPerformRequestsWithTarget:instance]或者功能类似API，来取消延迟。否则有些时候会造成内存问题（实例都释放了还在调用实例方法）导致crash，或者导致某个类延迟释放。
问题：
如果有一个类，在类中有一个方法调用了[self performSelector:@selector() withObject:nil afterDelay:]。那么在这个类中对应的dealloc方法中是否有必要调用[NSObject cancelPreviousPerformRequestsWithTarget:self]呢？
因为延迟调用会把self引用计数+1，调用了之后，只能等延迟方法执行完或者被[NSObject cancelPreviousPerformRequestsWithTarget:self]类似方法取消掉，否则不可能执行到dealloc方法中。这样看来dealloc方法调用[NSObject cancelPreviousPerformRequestsWithTarget:self]没啥用。
但是最佳实践中我还是会建议在dealloc方法最后加上[NSObject cancelPreviousPerformRequestsWithTarget:self]
理由如下：
如果我们在定义一个类，里面有方法调用了[self performSelector:@selector() withObject:nil afterDelay:]。那么在这个类的dealloc方法最后要调用[NSObject cancelPreviousPerformRequestsWithTarget:self]。因为dealloc里面一旦调用了一些方法，这些方法间接或者直接调用了[self performSelector:@selector() withObject:nil afterDelay:]，那么就等待着crash吧。此时在dealloc方法最后加上[NSObject cancelPreviousPerformRequestsWithTarget:self]。问题就解决了。总之dealloc里面的[NSObject cancelPreviousPerformRequestsWithTarget:self]方法不一定有用，但是加上有益无害。
还是不太明白？请看下面代码：
@interface Person : NSObject
@end
@implementation Person
- (void)personTest{
NSLog(@"personTest");
}
- (void)invoke{
[self performSelector:@selector(personTest) withObject:nil afterDelay:0];
}
- (void)dealloc{
[self invoke];
// [NSObject cancelPreviousPerformRequestsWithTarget:self];
}
@end
Person *person = [Person new];person实例释放的时候必然会奔溃。
2.[NSObject performSelector]系列函数和
[NSRunLoop performSelector]系列函数区别
下面是官方API，都是为了实现延迟调用，但是你们知道他们的区别吗？
/**************** Delayed perform ******************/
@interface NSObject (NSDelayedPerforming)
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray<NSRunLoopMode> *)modes;
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(nullable id)anArgument;
+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;
@end
@interface NSRunLoop (NSOrderedPerform)
- (void)performSelector:(SEL)aSelector target:(id)target argument:(nullable id)arg order:(NSUInteger)order modes:(NSArray<NSRunLoopMode> *)modes;
- (void)cancelPerformSelector:(SEL)aSelector target:(id)target argument:(nullable id)arg;
- (void)cancelPerformSelectorsWithTarget:(id)target;
@end
下面是CocoaAsyncSocket早期基于runLoop实现的部分代码
代码一：
- (BOOL)moveToRunLoop:(NSRunLoop *)runLoop{
NSAssert((theRunLoop == NULL) || (theRunLoop == CFRunLoopGetCurrent()),
@"moveToRunLoop must be called from within the current RunLoop!");
[runLoop performSelector:@selector(maybeDequeueRead) target:self argument:nil order:0 modes:theRunLoopModes];
[runLoop performSelector:@selector(maybeDequeueWrite) target:self argument:nil order:0 modes:theRunLoopModes];
[runLoop performSelector:@selector(maybeScheduleDisconnect) target:self argument:nil order:0 modes:theRunLoopModes];
}
代码二：
- (BOOL)setRunLoopModes:(NSArray *)runLoopModes{
NSAssert((theRunLoop == CFRunLoopGetCurrent()), @"setRunLoopModes must be called from within the current RunLoop!");
[self performSelector:@selector(maybeDequeueRead) withObject:nil afterDelay:0 inModes:theRunLoopModes];
[self performSelector:@selector(maybeDequeueWrite) withObject:nil afterDelay:0 inModes:theRunLoopModes];
[self performSelector:@selector(maybeScheduleDisconnect) withObject:nil afterDelay:0 inModes:theRunLoopModes];
}
为了方便把下面方法定义为“方法1”
[runLoop performSelector:@selector(maybeDequeueRead) target:self argument:nil order:0 modes:theRunLoopModes];
下面方法定义为“方法2”
[self performSelector:@selector(maybeDequeueRead) withObject:nil afterDelay:0 inModes:theRunLoopModes];
"方法1"，"方法2"都干同样一件事，就是延迟执行maybeDequeueRead调用。但是它们是有区别的。区别如下：
NSRunLoop一般是和线程相关的。一个线程最多有一个NSRunLoop也可能没有。在这里可以将runLoop看做一个线程。
假设在“线程A”中调用moveToRunLoop，当线程A执行到moveToRunLoop方法中的[runLoop performSelector:@selector(maybeDequeueRead) target:self argument:nil order:0 modes:theRunLoopModes]这段代码的时候。“线程A”唤起另一个线程（runLoop所在的线程）假设为“线程B”去调用maybeDequeueRead方法。实现效果和下面方法一致*- (void)performSelector:(SEL)aSelector onThread:(NSThread )thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait （只是一个基于RunLoop实现，一个基于Thread调用）。这样就实现了多线程切换调用。
但是如果用下面方法[self performSelector:@selector(maybeDequeueRead) withObject:nil afterDelay:0 inModes:theRunLoopModes]则所有调用都在”线程A“中。
先整理到这吧，后面还会继续分析CocoaAsyncSocket学些心得。
