在iOS中NSRunLoop是一個對象，run是它的一個對象方法，runloop嵌套一般指的是在NSRunLoop的run方法中再調用一次run方法，runloop的run方法內部都是開啟了一個while循環，因此在run方法中再調用一次run方法則相當于在一個while循環中開啟另一個while循環，內層while循環會導致外層while循環的阻塞。

    BOOL r1_run = YES;
    BOOL r2_run = YES;

    while (r1_run) {
        NSLog(@"---1-");
        while (r2_run) {
            
        }
        NSLog(@"---2-");
    }

另外，在runloop 中，每開啟一個run方法都是在處理runloop 指定mode下的modeItem的事件，如果外層run方法與內層run方法運行的是在同一mode下，那在外層run方法中沒來得及處理的modeItem的事件在進入內層run方法后可以繼續被處理。

1、練習一

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSTimer *timer1 = [NSTimer timerWithTimeInterval:5 target:self selector:@selector(timerMethod) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:timer1 forMode:NSDefaultRunLoopMode];

}

- (void)timerMethod {
    NSLog(@"---1");
    [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
    NSLog(@"---2");
}

2、練習二：求下列代碼打印順序

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
 
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"1");
    });
    [[NSRunLoop currentRunLoop] performBlock:^{
        NSLog(@"2");
    }];
    NSTimer *timer = [[NSTimer alloc] initWithFireDate:[NSDate date] interval:10 repeats:YES block:^(NSTimer * _Nonnull timer) {
        NSLog(@"3");
        CFRunLoopStop(CFRunLoopGetCurrent());
    }];
    [[NSRunLoop currentRunLoop] addTimer:timer forMode:@"mode1"];
    [[NSRunLoop currentRunLoop] runMode:@"mode1" beforeDate:[NSDate distantFuture]];
    NSLog(@"4");
}

3、練習三：求下列代碼打印內容

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
 
    NSThread *thread = [[NSThread alloc] initWithTarget:self selector:@selector(threadStartMethod) object:nil];
    [thread start];
}

- (void)threadStartMethod {
    NSTimer *timer1 = [NSTimer timerWithTimeInterval:5 target:self selector:@selector(timerMethod) userInfo:nil repeats:YES];
    NSTimer *timer2 = [NSTimer timerWithTimeInterval:3 target:self selector:@selector(timerMethod2) userInfo:nil repeats:YES];
    
    [[NSRunLoop currentRunLoop] addTimer:timer1 forMode:@"mode1"];
    [[NSRunLoop currentRunLoop] addTimer:timer2 forMode:@"mode1"];
    [[NSRunLoop currentRunLoop] runMode:@"mode1" beforeDate:[NSDate distantFuture]];
    
}

- (void)timerMethod {
    NSLog(@"---1");
    NSTimer *timer4 = [NSTimer timerWithTimeInterval:10 target:self selector:@selector(timerMethod4) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:timer4 forMode:@"mode1"];
    [[NSRunLoop currentRunLoop] runMode:@"mode1" beforeDate:[NSDate distantFuture]];
    NSLog(@"---2");
}

- (void)timerMethod2 {
    NSLog(@"---3");
    NSTimer *timer3 = [NSTimer timerWithTimeInterval:10 target:self selector:@selector(timerMethod3) userInfo:nil repeats:YES];
    [[NSRunLoop currentRunLoop] addTimer:timer3 forMode:@"mode1"];
    [[NSRunLoop currentRunLoop] runMode:@"mode1" beforeDate:[NSDate distantFuture]];
    NSLog(@"---4");
}

- (void)timerMethod3 {
    NSLog(@"---5");
    //    CFRunLoopStop(CFRunLoopGetCurrent());
}

- (void)timerMethod4 {
    NSLog(@"---6");
}