在子線程的任務完成後，有時候需要從子線程回到主線程，刷新UI。 從子線程中回到主線程，以前已經寫過一種方法：

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

現在GCD又提供了一種方法：

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

示例代碼：

//
//
 
  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(
 
@"主線程----%@",[NSThread mainThread]);

}

-(void)touchesBegan:(NSSet*)touches withEvent:(UIEvent *)event
{
    //1 獲取一個全局隊列
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    // 2 把任務添加到隊列中執行
    dispatch_async(queue, ^{
        
        //打印當前的線程
        NSLog(@"
 
%@",[NSThread currentThread]);
        
        //3.從網絡下載圖片
        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(@"圖片加載完畢");
        
        //4.回到主線程，展示圖片
       // [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

打印結果：

2017-06-16 17:55:45.848 GCDTest[15011:2269875] 主線程----<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] 圖片加載完畢

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

本文參考：http://www.cnblogs.com/wendingding/p/3807265.html

iOS多線程---GCD中線程的通信