线程间的通信（3种方式）

三种方法都是通过touchesBegin监听屏幕的触摸实现

一、performSelector方式

#import "ViewController.h"
@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@end
@implementation ViewController
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    //放入子线程
    [self performSelectorInBackground:@selector(download3) withObject:nil];
}

//下载放入子线程，显示图片应该放在主线程！！！否则会导致刷新问题
- (void)download3
{
    //图片的网络路径
    NSURL *url = [NSURL URLWithString:@"http://ww2.sinaimg.cn/mw690/63e6fd01jw1f3f3rf75goj20qo0zkagy.jpg"];
    //下载图片数据
    NSData *data = [NSData dataWithContentsOfURL:url];
    
    //生成图片
    UIImage *image = [UIImage imageWithData:data];
    //回到主线程显示图片方法一：
//    [self performSelectorOnMainThread:@selector(showImage:) withObject:image waitUntilDone:YES];
    //回到主线程显示图片方法二：
    //waitUntilDone:表示是否等待主线程做完事情后往下走，YES表示做完后执行下面事，NO表示跟下面事一起执行
    [self.imageView performSelectorOnMainThread:@selector(setImage:) withObject:image waitUntilDone:YES];
    //回到主线程显示图片方法三：
    [self.imageView performSelector:@selector(setImage:) onThread:[NSThread mainThread] withObject:image waitUntilDone:YES];
}
//主线程显示图片
- (void)showImage:(UIImage *)image
{
    self.imageView.image = image;
}

二、GCD方式

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //图片的网络途径
        NSURL *url = [NSURL URLWithString:@"http://ww2.sinaimg.cn/mw1024/75614297jw1f34e5llyz4j20qo0zj0zl.jpg"];
        //加载图片
        NSData *data = [NSData dataWithContentsOfURL:url];
        //生成图片
        UIImage *image = [UIImage imageWithData:data];
        //回到主线程
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView.image = image;
        });
    });
    
}

三、operation方式(此种方式更具有面向对象特性！)

- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    //直接开始子线程执行任务
    [[[NSOperationQueue alloc] init] addOperationWithBlock:^{
        NSURL *url = [NSURL URLWithString:@"http://ww4.sinaimg.cn/mw690/63e6fd01jw1ezxz499hy5j21gt0z94qq.jpg"];
        NSData *data = [NSData dataWithContentsOfURL:url];
        UIImage *image = [UIImage imageWithData:data];
        //回到主线程
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            //显示图片
            self.imageView.image = image;
        }];
    }];
}

以上三种方式都需要在main storyboard中拖一个imageView，然后设置自动布！！