Objective-C GCD(Dispatch Queue)

GCD

Reference:《Objective-C高级编程》

Dispatch Queue

一、特点：Dispatch Queue按照追加的顺序处理任务。

二、种类：Serial Dispatch Queue 和 Concurrent Dispatch Queue

Serial Dispatch Queue	Concurrent Dispatch Queue	对比项
等待	不等待	是否等待执行中任务完成
1	>=1	队列执行需要线程数
多个Serial Dispatch Queue将并行执行	1个Concurrent Dispatch Queue可并行执行多任务	并行执行多任务
为每个Serial Dispatch Queue分配一个线程	取决于当前系统状态，XNU内核决定所需线程数，应当执行的处理数减少时，XNU内核会结束不再需要的线程	分配线程数

多线程开发注意：
1、数据竞争：多线程更新同一数据资源导致数据不一致。
2、死锁：停止等待的线程会导致多线程相互持续等待。
3、使用太多线程会消耗大量内存。

举例：Serial Dispatch Queue

typedef void(^blk)(void);
-(blk)getTask:(int)x
{
    return ^{
        NSLog(@"非ui线程%d  -->%@",x,[NSThread currentThread]);
        [NSThread sleepForTimeInterval:1];
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"ui线程%d",x);
        });
    };
}
//test
dispatch_queue_t mySerialQueue=dispatch_queue_create("SerialQueue",DISPATCH_QUEUE_SERIAL);
blk blk1=[self getTask:1];
blk blk2=[self getTask:2];
blk blk3=[self getTask:3];
blk blk4=[self getTask:4];
dispatch_async(mySerialQueue, blk1);
dispatch_async(mySerialQueue, blk2);
dispatch_async(mySerialQueue, blk3);
dispatch_async(mySerialQueue, blk4);
/*输出：等待、按顺序执行、同一线程
2019-03-10 10:19:38.655232+0800 Block[97332:7774955] 非ui线程1  -->{number = 3, name = (null)}
2019-03-10 10:19:39.659269+0800 Block[97332:7774918] ui线程1
2019-03-10 10:19:39.659381+0800 Block[97332:7774955] 非ui线程2  -->{number = 3, name = (null)}
2019-03-10 10:19:40.664956+0800 Block[97332:7774918] ui线程2
2019-03-10 10:19:40.664963+0800 Block[97332:7774955] 非ui线程3  -->{number = 3, name = (null)}
2019-03-10 10:19:41.670519+0800 Block[97332:7774918] ui线程3
2019-03-10 10:19:41.670520+0800 Block[97332:7774955] 非ui线程4  -->{number = 3, name = (null)}
2019-03-10 10:19:42.676126+0800 Block[97332:7774918] ui线程4
*/

举例：Concurrent Dispatch Queue

dispatch_queue_t myConcurrentqueue=dispatch_queue_create("ConcurrentQueue", DISPATCH_QUEUE_CONCURRENT);
blk blk1=[self getTask:1];
blk blk2=[self getTask:2];
blk blk3=[self getTask:3];
blk blk4=[self getTask:4];
dispatch_async(myConcurrentqueue, blk1);
dispatch_async(myConcurrentqueue, blk2);
dispatch_async(myConcurrentqueue, blk3);
dispatch_async(myConcurrentqueue, blk4);
/*输出：不等待、完成顺序被打乱、多线程
2019-03-10 11:16:59.346941+0800 Block[97725:7855969] 非ui线程4  -->{number = 5, name = (null)}
2019-03-10 11:16:59.346941+0800 Block[97725:7855971] 非ui线程1  -->{number = 3, name = (null)}
2019-03-10 11:16:59.346941+0800 Block[97725:7855970] 非ui线程3  -->{number = 4, name = (null)}
2019-03-10 11:16:59.347041+0800 Block[97725:7855968] 非ui线程2  -->{number = 6, name = (null)}
2019-03-10 11:17:00.352596+0800 Block[97725:7855927] ui线程4
2019-03-10 11:17:00.352838+0800 Block[97725:7855927] ui线程3
2019-03-10 11:17:00.353006+0800 Block[97725:7855927] ui线程2
2019-03-10 11:17:00.353147+0800 Block[97725:7855927] ui线程1
*/

Main Dispatch Queue 和Global Dispatch Queue

名称	种类	说明
Main Dispatch Queue	Serial Dispatch Queue	主线程执行
Global Dispatch Queue（High Priority）	Concurrent Dispatch Queue	执行优先级：高
Global Dispatch Queue（Default Priority）	Concurrent Dispatch Queue	执行优先级：默认
Global Dispatch Queue（Low Priority）	Concurrent Dispatch Queue	执行优先级：低
Global Dispatch Queue（Background Priority）	Concurrent Dispatch Queue	执行优先级：后台

//获取高优先级Global Dispath Queue
dispatch_queue_t globalDispatchQueueHigh= dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);