IOS多线程GCD

一、GCD几种常用方法:

1、界面UI操作GCD

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
    // 耗时的操作  
    dispatch_async(dispatch_get_main_queue(), ^{  
        // 更新界面  
    });  
}); 

2、多线程同时执行任务不同步骤

        //创建多线程队列
        dispatch_queue_t myQueue=dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        dispatch_group_t group=dispatch_group_create();

 　　　　dispatch_group_async(group, myQueue, ^{
        //多线程执行内容
　　　　 });

　　　　dispatch_group_async(group, myQueue, ^{
        //多线程执行内容
　　　　 });

//等待信号 等待上述所有group线程结束
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
        
        dispatch_release(group);
        dispatch_release(myQueue);

3、简单分离多线程操作先后顺序

dispatch_queue_t queue = dispatch_queue_create("wait", DISPATCH_QUEUE_CONCURRENT);  
dispatch_async(queue, ^{   
    NSLog(@"dispatch_async1");  
});  
dispatch_async(queue, ^{   
    NSLog(@"dispatch_async2");  
});  
//等待前面线程执行结束
dispatch_barrier_async(queue, ^{  
    NSLog(@"dispatch_barrier_async");    
});  
//等待dispach_barrier_async执行结束
dispatch_async(queue, ^{   
    NSLog(@"dispatch_async3");  
});  

4、重复执行代码

dispatch_apply(5, globalQ, ^(size_t index) {
    // 执行5次
});

二、判断内核数目

　　通过内核数判断是否使用GCD，避免Iphone4等单核手机，多线程执行反而变慢的情况...

//判断内核数
+ (unsigned int) countCores
{
    host_basic_info_data_t hostInfo;
    mach_msg_type_number_t infoCount;
    
    infoCount = HOST_BASIC_INFO_COUNT;
    host_info(mach_host_self(), HOST_BASIC_INFO,
              (host_info_t)&hostInfo, &infoCount);
    
    return (unsigned int)(hostInfo.max_cpus);
}