1、明确方法所在位置
分别在NSObject.h 、NSRunloop.h 、NSThread.h 三个类中，是不是很神奇。（其中在NSRunloop.h 、NSThread.h文件中都是NSObject的分类）
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2、在NSObject.h中的方法
1、 - (id)performSelector:(SEL)aSelector;
2、 - (id)performSelector:(SEL)aSelector withObject:(id)object;
3、 - (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;
三个方法均为同步执行，与线程无关，在主线程和子线程中均可调用。等同于直接调用该方法。
与直接调用方法区别：直接调用编译时会自动校验。performSelector在运行时去找方法，在编译时不做校验。
3、在NSRunloop.h中的方法
1、- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray *)modes;
2、- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
3.1、这两个方法为异步执行，delay值为0，仍为异步执行。
1、在主线程执行，方法调用成功。
2、在子线程执行，需要开启子线程Runloop，方法才可以调用成功。
注意：调用该方法之前或在该方法所在的VC释放时主动调用取消函数，以确保不会内存泄露。
3.2、可控制延迟调用方法的取消操作：
1、+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget selector:(SEL)aSelector object:(nullable id)anArgument;
2、+ (void)cancelPreviousPerformRequestsWithTarget:(id)aTarget;
4、在NSThread.h中的方法
4.1、在主线程调用方法
1、- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray *)array;
2、- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
在主线程和子线程中均可执行，都会在主线程调用Selector方法。
4.1.1、在子线程中设置wait值
wait=YES：当前线程被阻塞，主线程执行完Selector，接着执行。
-(void)viewDidLoad{[superviewDidLoad];dispatch_queue_t queue=dispatch_queue_create("cwwng-queue",DISPATCH_QUEUE_CONCURRENT);dispatch_async(queue,^{NSLog(@"1");[selfperformSelectorOnMainThread:@selector(testAction)withObject:nil waitUntilDone:YES];NSLog(@"2");});}-(void)testAction{sleep(3);NSLog(@"testAction");}2020-11-2120:26:12.252984+0800CwwngDemo[28225:462843]12020-11-2120:26:15.289730+0800CwwngDemo[28225:462744]testAction2020-11-2120:26:15.289991+0800CwwngDemo[28225:462843]2
wait=NO：当前线程不被阻塞。
-(void)viewDidLoad{[superviewDidLoad];dispatch_queue_t queue=dispatch_queue_create("cwwng-queue",DISPATCH_QUEUE_CONCURRENT);dispatch_async(queue,^{NSLog(@"1");[selfperformSelectorOnMainThread:@selector(testAction)withObject:nil waitUntilDone:NO];NSLog(@"2");});}-(void)testAction{sleep(3);NSLog(@"testAction");}2020-11-2120:28:50.383884+0800CwwngDemo[28347:465953]12020-11-2120:28:50.384127+0800CwwngDemo[28347:465953]22020-11-2120:28:53.410808+0800CwwngDemo[28347:465859]testAction
4.1.2、在主线程中设置wait值
wait=YES：等待Selector执行完，再接着执行。
-(void)viewDidLoad{[superviewDidLoad];NSLog(@"1");[selfperformSelectorOnMainThread:@selector(testAction)withObject:nil waitUntilDone:YES];NSLog(@"2");}-(void)testAction{sleep(3);NSLog(@"testAction");}2020-11-2120:33:53.966768+0800CwwngDemo[28608:472716]12020-11-2120:33:56.967413+0800CwwngDemo[28608:472716]testAction2020-11-2120:33:56.967738+0800CwwngDemo[28608:472716]2
wait=NO：不等待Selector执行完，接着执行。
-(void)viewDidLoad{[superviewDidLoad];NSLog(@"1");[selfperformSelectorOnMainThread:@selector(testAction)withObject:nil waitUntilDone:NO];NSLog(@"2");}-(void)testAction{sleep(3);NSLog(@"testAction");}2020-11-2120:38:55.042160+0800CwwngDemo[28838:477847]12020-11-2120:38:55.042508+0800CwwngDemo[28838:477847]22020-11-2120:38:58.087345+0800CwwngDemo[28838:477847]testAction
4.2、在指定线程调用方法
(void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
(void)performSelector:(SEL)aSelector onThread:(NSThread *)thr withObject:(nullable id)arg waitUntilDone:(BOOL)wait API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
4.3、开启子线程在后台运行
(void)performSelectorInBackground:(SEL)aSelector withObject:(nullable id)arg API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));
作者：Cwwng
链接：https://www.jianshu.com/p/8db250c1a566
来源：简书
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