开子线程下载图片,回到主线程刷新UI步骤

-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    [NSThread detachNewThreadSelector:@selector(download) toTarget:self withObject:nil];
}

#pragma mark ----------------------
#pragma Methods

//开子线程下载图片,回到主线程刷新UI
-(void)download
{
    //1.确定URL
    NSURL *url = [NSURL URLWithString:@"http://img4.duitang.com/uploads/blog/201310/18/20131018213446_smUw4.thumb.700_0.jpeg"];

    //2.根据url下载图片二进制数据到本地
    NSData *imageData = [NSData dataWithContentsOfURL:url];

    //3.转换图片格式
    UIImage *image = [UIImage imageWithData:imageData];

    NSLog(@"download----%@",[NSThread currentThread]);

    //4.回到主线程显示UI
    /*
     第一个参数:回到主线程要调用哪个方法
     第二个参数:前面方法需要传递的参数 此处就是image
     第三个参数:是否等待
     */
    //[self performSelectorOnMainThread:@selector(showImage:) withObject:image waitUntilDone:NO];

 [self performSelector:@selector(showImage:) onThread:[NSThread mainThread] withObject:image waitUntilDone:YES];

//    self.imageView.image = image;
    NSLog(@"---end---");
}

//更新UI操作
-(void)showImage:(UIImage *)image
{
    self.imageView.image = image;
    NSLog(@"UI----%@",[NSThread currentThread]);
}

performSelectorOnMainThread:和performSelector:方法都可以
也可以直接用对象self.imageView调用performSelectorOnMainThread：方法，使用对象self.imageView的image属性的setImage:方法来实现（这样可以省略创建showImage方法）。

[self.imageView performSelectorOnMainThread:@selector(setImage:) withObject:image waitUntilDone:YES];