在开发中我们有可能会遇到这样的需求，在一个UIImageView来回切换俩张图片来达到动态效果。我也是实验了好久，后来经过指导实现了，效果如下：

图片来回切换.gif

iOS中定时器有三种：(1)NSTimer (2)CADisplayLink (3)dispatch_source,而我使用到的定时器是dispatch_source，下面就来看看代码吧。（可以实现暂停和播放）

//初始化UIImageView
UIImage *img = [UIImage imageNamed:@"image_huoyan_1"];
backImage = [[UIImageView alloc] initWithFrame:CGRectMake(50, 60, self.view.frame.size.width/5, self.view.frame.size.width/5 * img.size.height/img.size.width)]; 
[backImage setImage:img];
[self.view addSubview:backImage];

//加载按钮
self.button = [UIButton buttonWithType:UIButtonTypeCustom];
self.button.frame = CGRectMake(100, 300, 100, 100);
self.button.backgroundColor = [UIColor blueColor];
[self.button addTarget:self action:@selector(isAnimotion) forControlEvents:UIControlEventTouchUpInside]; 
[self.button setTitle:@"开始" forState:UIControlStateNormal];
[self.view addSubview:self.button];

//按钮的点击事件
-(void)isAnimotion {
    if([self.button.titleLabel.text isEqualToString:@"停止"]) {
        [self stopNS];
        [self.button setTitle:@"开始" forState:UIControlStateNormal];
    } else {
        [self startNS];
        [self.button setTitle:@"停止" forState:UIControlStateNormal];
    }
}

接下来就是主要代码啦，其中需要注意的是：需要将dispatch_source_t timer设置为成员变量，不然会立即释放。

@property (nonatomic, strong) dispatch_source_t timer;

//加载动画
-(void)startNS {
    __block NSString *str =@"image_huoyan_2";
    backImage.image = [UIImage imageNamed:str];
    //定时器开始执行的延时时间
    NSTimeInterval delayTime = 0.0f;
    //定时器间隔时间
    NSTimeInterval timeInterval = 0.1f;
    //创建子线程队列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    //使用之前创建的队列来创建计时器
    _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    //设置延时执行时间，delayTime为要延时的秒数
    dispatch_time_t startDelayTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayTime * NSEC_PER_SEC));
    //设置计时器
    dispatch_source_set_timer(_timer, startDelayTime, timeInterval * NSEC_PER_SEC, 0.1 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(_timer, ^{
        //执行事件
        if ( [str isEqualToString:@"image_huoyan_1"]) {
            str = @"image_huoyan_2";
        } else {
            str = @"image_huoyan_1";
        }
        dispatch_async(dispatch_get_main_queue(), ^{
            [backImage setImage:[UIImage imageNamed:str]];
        }) ;
    });
    // 启动计时器
    dispatch_resume(_timer);
}

//停止动画
-(void)stopNS {
    dispatch_source_cancel(_timer);
}

上面提到的"image_huoyan_1"和"image_huoyan_2"就是要用到的俩张图片。写的比较简单，如果有错误请多多指教。