iOS多线程---GCD中线程的通信

在子线程的任务完成后，有时候需要从子线程回到主线程，刷新UI。 从子线程中回到主线程，以前已经写过一种方法：

   [self.imageView performSelectorOnMainThread:@selector(setImage:) withObject:image waitUntilDone:NO];

现在GCD又提供了一种方法：

  dispatch_async(dispatch_get_main_queue(), ^{
          self.imageView.image=image;
  });

示例代码：

//
//  ViewController.m
//  GCDTest
//
//  Created by 登 on 2017/6/16.
//  Copyright ? 2017年 登. All rights reserved.
//

#import "ViewController.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    NSLog(@"主线程----%@",[NSThread mainThread]);

}

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event
{
    //1 获取一个全局队列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    // 2 把任务添加到队列中执行
    dispatch_async(queue, ^{

        //打印当前的线程
        NSLog(@"%@",[NSThread currentThread]);

        //3.从网络下载图片
        NSURL *urlStr = [NSURL URLWithString:@"http://h.hiphotos.baidu.com/baike/w%3D268/sign=30b3fb747b310a55c424d9f28f444387/1e30e924b899a9018b8d3ab11f950a7b0308f5f9.jpg"];
        NSData *data = [NSData dataWithContentsOfURL:urlStr];
        UIImage *image = [UIImage imageWithData:data];

        //提示
        NSLog(@"图片加载完毕");

        //4.回到主线程，展示图片
       // [self.imageView performSelectorOnMainThread:@selector(setImageView:) withObject:image waitUntilDone:NO];
        dispatch_async(dispatch_get_main_queue(), ^{
            _imageView.image = image;

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

}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
}

@end

打印结果：

2017-06-16 17:55:45.848 GCDTest[15011:2269875] 主线程----<NSThread: 0x60800007f600>{number = 1, name = main}

2017-06-16 17:56:43.391 GCDTest[15011:2269966] <NSThread: 0x60000026b980>{number = 3, name = (null)}

2017-06-16 17:56:43.463 GCDTest[15011:2269966] 图片加载完毕

2017-06-16 17:56:43.463 GCDTest[15011:2269875] <NSThread: 0x60800007f600>{number = 1, name = main}

本文参考：http://www.cnblogs.com/wendingding/p/3807265.html