在iOS中NSRunLoop是一个对象，run是它的一个对象方法，runloop嵌套一般指的是在NSRunLoop的run方法中再调用一次run方法，runloop的run方法内部都是开启了一个while循环，因此在run方法中再调用一次run方法则相当于在一个while循环中开启另一个while循环，内层while循环会导致外层while循环的阻塞。

    BOOL r1_run = YES;
    BOOL r2_run = YES;

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

另外，在runloop 中，每开启一个run方法都是在处理runloop 指定mode下的modeItem的事件，如果外层run方法与内层run方法运行的是在同一mode下，那在外层run方法中没来得及处理的modeItem的事件在进入内层run方法后可以继续被处理。

1、练习一

- (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、练习二：求下列代码打印顺序

- (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、练习三：求下列代码打印内容

- (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");
}