多线程

线程
多线程GCD
https://www.jianshu.com/p/667ec9e612ad

#----打印当前线程
    NSLog(@"currentThread:%@",[NSThread currentThread]);

#----判断是否主线程
 if (strcmp(dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL), dispatch_queue_get_label(dispatch_get_main_queue())) == 0) {
            // do something in main thread
            NSLog(@"--main thread");
        } else {
            // do something in other thread
            NSLog(@"--other thread");
            
        }



dispatch_async(dispatch_get_global_queue(0, 0), ^{
       //费时操作
        NSInteger size = [[SDImageCache sharedImageCache] getSize];
        
        dispatch_async(dispatch_get_main_queue(), ^{
            
            NSString *sizeStr = [self fileSizeWithInterge:size];
            
        });
        
    });

线程组

日志分析：
1：系统会为每个异步任务开辟一条子线程；
2：每个任务在开辟的子线程中执行；
3：在dispatch_group_notify监听到回调之后，此时还在某条子线程中；需要编写回到主线程的语句
4：不会造成主线程堵塞；
强狂三：解决场景：执行完多个异步任务之后才执行某一任务 （举例：在异步任务中发起多个获取网络图片的异步请求，等到图片都获取成功之后再进行UI界面的刷新）
解决办法：

dispatch_queue_t queueT = dispatch_queue_create("group.queue", DISPATCH_QUEUE_CONCURRENT);//一个并发队列
dispatch_group_t grpupT = dispatch_group_create();//一个线程组

dispatch_group_async(grpupT, queueT, ^{
    NSLog(@"group——当前线程一");
    //模仿网络请求代码
    dispatch_group_enter(grpupT);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for (int i = 0; i < 10; i++) {
            [NSThread sleepForTimeInterval:1];
            NSLog(@"网络图片请求中 ···%d", i);
        }
        dispatch_group_leave(grpupT);
    });
    
});

dispatch_group_async(grpupT, queueT, ^{
    NSLog(@"group——当前线程二");
    //模仿网络请求代码
    dispatch_group_enter(grpupT);
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for (int i = 0; i < 10; i++) {
            [NSThread sleepForTimeInterval:1];
            NSLog(@"网络图片2请求中 ···2_%d", i);
        }
        dispatch_group_leave(grpupT);
    });
    
});

dispatch_group_async(grpupT, queueT, ^{
    NSLog(@"group——当前线程三");
    //模仿网络请求代码
    dispatch_group_enter(grpupT);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        for (int i = 0; i < 10; i++) {
            [NSThread sleepForTimeInterval:1];
            NSLog(@"网络图片3请求中 ···3_%d", i);
        }
         dispatch_group_leave(grpupT);
    });
   
});



dispatch_group_notify(grpupT, queueT, ^{
    
    NSLog(@"此时还是在子线程中");
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"回到主线程");
    });
    
});