PerformSelector 相關(guān)

- (id)performSelector:(SEL)aSelector;
- (id)performSelector:(SEL)aSelector withObject:(id)object;
- (id)performSelector:(SEL)aSelector withObject:(id)object1 withObject:(id)object2;

這三個(gè)方法是同步在當(dāng)前線程執(zhí)行的。會(huì)阻塞當(dāng)前線程?？稍谥骶€程或者子線程執(zhí)行。
比如

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self performSelector:@selector(doSomething:) withObject:@"111"];
    NSLog(@"over");
}

- (void)doSomething:(NSString *)aStr {
    NSLog(@"doSomething:__%@__, currentThread = %@", aStr, [NSThread currentThread]);
    sleep(2);
    NSLog(@"doSomething sleep over");
}

打印結(jié)果是

doSomething:__111__, currentThread = <NSThread: 0x7fe18c904ac0>{number = 1, name = main}
(2s later...)
doSomething sleep over
over

如果在子線程里做會(huì)立即打印 over。子線程嘛，當(dāng)然不會(huì)阻塞主線程。

PerformSelector:afterDelay 相關(guān)

- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay;
- (void)performSelector:(SEL)aSelector withObject:(nullable id)anArgument afterDelay:(NSTimeInterval)delay inModes:(NSArray<NSString *> *)modes;

這兩個(gè)方法類似，只是第二個(gè)方法里指定了線程中 Runloop 的 Modes，這個(gè)先不說(shuō)。
這個(gè)方法是異步的，而且只能在主線程中執(zhí)行。什么意思呢？

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self performSelector:@selector(doSomething:) withObject:@"ahahaha" afterDelay:0];
    NSLog(@"before sleep");
    sleep(4);
    NSLog(@"after sleep");
}

- (void)doSomething:(NSString *)aStr {
    NSLog(@"doSomething:__%@__, currentThread = %@", aStr, [NSThread currentThread]);
}

打印結(jié)果是

before sleep
(4s later...)
after sleep
doSomething:__ahahaha__, currentThread = <NSThread: 0x7fdf73706c70>{number = 1, name = main}

能看出來(lái)，這個(gè)方法不會(huì)阻塞當(dāng)前線程，而是把 Selector 加到主隊(duì)列里，當(dāng) delay 之后執(zhí)行 Selector。如果主線程在執(zhí)行業(yè)務(wù)，那只能等到執(zhí)行完所有業(yè)務(wù)之后才會(huì)去執(zhí)行 Selector，就算 delay 等于 0。
那 delay 從什么時(shí)候開始計(jì)算呢？從 performSelector 的時(shí)候。就算這時(shí)主線程在阻塞也會(huì)計(jì)算時(shí)間，當(dāng)阻塞結(jié)束之后，如果到了 delay 那就執(zhí)行 Selector，如果沒到就繼續(xù) delay。

PerformSelectorOnMainThread 相關(guān)

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait modes:(nullable NSArray<NSString *> *)array;
- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;
// 相關(guān)的指定線程方法就不提了。一個(gè)意思。

這倆方法意思很明確，可以在主或者子線程中執(zhí)行，但 Selector 肯定是在主線程中執(zhí)行。這里只提一下 waitUntilDone。

- (void)viewDidLoad {
    [super viewDidLoad];
    
    [self performSelectorOnMainThread:@selector(doSomething:) withObject:@"1111" waitUntilDone:YES];
    NSLog(@"before sleep");
    sleep(2);
    NSLog(@"after sleep");
}

- (void)doSomething:(NSString *)aStr {
    NSLog(@"doSomething:__%@__, currentThread = %@", aStr, [NSThread currentThread]);
    sleep(2);
    NSLog(@"doSomething sleep over");
}

打印結(jié)果是

doSomething:__1111__, currentThread = <NSThread: 0x7fc90b5038b0>{number = 1, name = main}
(2s later...)
doSomething sleep over
before sleep
(2s later);
after sleep

很明顯，waitUntilDone == YES 會(huì)阻塞當(dāng)前線程，當(dāng)主線程執(zhí)行完 Selector 之后才會(huì)繼續(xù)執(zhí)行當(dāng)前線程。如果是 NO 呢？

before sleep
(2s later...)
after sleep
doSomething:__1111__, currentThread = <NSThread: 0x7fc90b5038b0>{number = 1, name = main}
(2s later);
doSomething sleep over

主線程不會(huì)阻塞當(dāng)前線程，大家互不干擾。但上述代碼中的當(dāng)前線程是主線程，那么和 performSelector:afterDelay 類似，主線程順序執(zhí)行完業(yè)務(wù)之后才會(huì)繼續(xù)執(zhí)行 Selector 的業(yè)務(wù)。如果當(dāng)前線程是子線程呢？

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_queue_t t1 = dispatch_queue_create("sub queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(t1, ^{
        [self performSelectorOnMainThread:@selector(doSomething:) withObject:@"1111" waitUntilDone:NO];
        NSLog(@"before sleep");
        sleep(2);
        NSLog(@"after sleep");
    });
}

- (void)doSomething:(NSString *)aStr {
    NSLog(@"doSomething:__%@__, currentThread = %@", aStr, [NSThread currentThread]);
    sleep(2);
    NSLog(@"doSomething sleep over");
}

打印結(jié)果是

before sleep
doSomething:__1111__, currentThread = <NSThread: 0x7ff7c2603a00>{number = 1, name = main}
(after 2s...)
after sleep
doSomething sleep over

當(dāng)前線程和主線程互不干擾，大家是并行的。那如果 waitUntilDone == YES 呢？
當(dāng)然是等主線程執(zhí)行完之后再接著執(zhí)行子線程里的業(yè)務(wù)了。

doSomething:__1111__, currentThread = <NSThread: 0x7fd3a1f03510>{number = 1, name = main}
(2s later...)
doSomething sleep over
before sleep
(2s later...)
after sleep

大致就是這樣。