iOS 多线程 GCD-同步、异步、串行、并发

  • GCD中的两个核心概念
    任务：执行操作
    队列：用来存放任务

  • GCD的使用就2个步骤
    1.定制任务：确定想要做的事
    2.将任务添加到队列中：
    GCD 会自动将队伍中的任务取出，放到对应的线程中执行
    任务的取出遵循队列的FIFO原则：先进先出，后进后出。
    队列按照任务程序员指定的方式调度任务，执行任务的方法：同步/异步
    同步：一个任务没有结束就不会执行下一个任务。
    异步：不用等待任务执行完毕，就会执行下一个任务。

  • GCD中有两个用来执行任务的常用函数
    用同步方法执行任务，其中 queue 是队列，block 是任务
    dispatch_sync(dispatch_queue_t queue, dispatch_block_t blcok);

    用异步方法执行任务，其中 queue 是队列，block 是任务
    dispatch_async(dispatch_queue_t queue, dispatch_block_t blcok);

  • 同步和异步的区别
    同步：只能在当前线程中执行任务，不具备开启新线程的能力
    异步：可以在新线程中执行任务，具备开启新线程的能力。

对于串行队列执行同步任务 和并行队列执行异步任务这里就不再过多介绍了。
废话不多说 直接上代码

  • 串行队列+多个异步任务
//串行队列 异步
- (void)serialQueueTest
{
 dispatch_queue_t serialQueue = dispatch_queue_create("serialQueue", DISPATCH_QUEUE_SERIAL);
    
    NSLog(@" %@", [NSThread currentThread]);
    dispatch_async(serialQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片0 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_async(serialQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片1 %@ %d",[NSThread currentThread],i);
        }];

    });
    
    dispatch_async(serialQueue, ^{

        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片2 %@ %d",[NSThread currentThread],i);
        }];
        
    });
}

- (void)forNumIncrementCondition:(NSUInteger )num  actionBlock:(void(^)(int i))actionBlcok
{
    for (int a = 0; a < num; a ++)
    {
        if (actionBlcok) {
            actionBlcok(a);
        }
    }
}

打印结果
<NSThread: 0x600000061e80>{number = 1, name = main}
下载图片0 <NSThread: 0x60400007f480>{number = 4, name = (null)} 0
下载图片0 <NSThread: 0x60400007f480>{number = 4, name = (null)} 1
下载图片0 <NSThread: 0x60400007f480>{number = 4, name = (null)} 2
下载图片0 <NSThread: 0x60400007f480>{number = 4, name = (null)} 3
下载图片0 <NSThread: 0x60400007f480>{number = 4, name = (null)} 4

  下载图片1 <NSThread: 0x60400007f480>{number = 4, name = (null)} 0
  下载图片1 <NSThread: 0x60400007f480>{number = 4, name = (null)} 1
  下载图片1 <NSThread: 0x60400007f480>{number = 4, name = (null)} 2
  下载图片1 <NSThread: 0x60400007f480>{number = 4, name = (null)} 3
  下载图片1 <NSThread: 0x60400007f480>{number = 4, name = (null)} 4

  下载图片2 <NSThread: 0x60400007f480>{number = 4, name = (null)} 0
  下载图片2 <NSThread: 0x60400007f480>{number = 4, name = (null)} 1
  下载图片2 <NSThread: 0x60400007f480>{number = 4, name = (null)} 2
  下载图片2 <NSThread: 0x60400007f480>{number = 4, name = (null)} 3
  下载图片2 <NSThread: 0x60400007f480>{number = 4, name = (null)} 4

结论：串行队列 异步任务，会在一个新的子线程当中顺序执行（注意串行队列同步任务会直接在当前线程执行）

  • 并发队列+同步任务
//并发队列 同步
- (void)concurrentQueueSyncTest
{
    
    dispatch_queue_t concurrentQueue = dispatch_queue_create("concurrentQueue", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"%@",[NSThread currentThread]);
    dispatch_sync(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片0 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_sync(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片1 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_sync(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片2 %@ %d",[NSThread currentThread],i);
        }];
    });

}

- (void)forNumIncrementCondition:(NSUInteger )num  actionBlock:(void(^)(int i))actionBlcok
{
    for (int a = 0; a < num; a ++)
    {
        if (actionBlcok) {
            actionBlcok(a);
        }
    }
}

  <NSThread: 0x604000078100>{number = 1, name = main}
  下载图片0 <NSThread: 0x604000078100>{number = 1, name = main} 0
  下载图片0 <NSThread: 0x604000078100>{number = 1, name = main} 1
  下载图片0 <NSThread: 0x604000078100>{number = 1, name = main} 2
  下载图片0 <NSThread: 0x604000078100>{number = 1, name = main} 3
  下载图片0 <NSThread: 0x604000078100>{number = 1, name = main} 4
  下载图片1 <NSThread: 0x604000078100>{number = 1, name = main} 0
  下载图片1 <NSThread: 0x604000078100>{number = 1, name = main} 1
  下载图片1 <NSThread: 0x604000078100>{number = 1, name = main} 2
  下载图片1 <NSThread: 0x604000078100>{number = 1, name = main} 3
  下载图片1 <NSThread: 0x604000078100>{number = 1, name = main} 4
  下载图片2 <NSThread: 0x604000078100>{number = 1, name = main} 0
  下载图片2 <NSThread: 0x604000078100>{number = 1, name = main} 1
  下载图片2 <NSThread: 0x604000078100>{number = 1, name = main} 2
  下载图片2 <NSThread: 0x604000078100>{number = 1, name = main} 3
  下载图片2 <NSThread: 0x604000078100>{number = 1, name = main} 4

结论：并发队列 里放置同步任务会在当前线程顺序执行

  • 并发队列+2异步+2同步+2异步
//并发队列 2异步 2同步 2 异步
- (void)concurrentQueueAsyncAndSyncTest
{
    
    dispatch_queue_t concurrentQueue = dispatch_queue_create("concurrentQueue", DISPATCH_QUEUE_CONCURRENT);
    
    
     NSLog(@"%@",[NSThread currentThread]);
    dispatch_async(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片0 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_async(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片1 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_sync(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片2 %@ %d",[NSThread currentThread],i);
        }];
    });
    
    dispatch_sync(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片3 %@ %d",[NSThread currentThread],i);
        }];
    });
    
    dispatch_async(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片4 %@ %d",[NSThread currentThread],i);
        }];
        
    });
    
    dispatch_async(concurrentQueue, ^{
        
        [self forNumIncrementCondition:5 actionBlock:^(int i) {
            NSLog(@"下载图片5 %@ %d",[NSThread currentThread],i);
        }];
        
    });
}

- (void)forNumIncrementCondition:(NSUInteger )num  actionBlock:(void(^)(int i))actionBlcok
{
    for (int a = 0; a < num; a ++)
    {
        if (actionBlcok) {
            actionBlcok(a);
        }
    }
}

打印结果
{number = 1, name = main}
下载图片2 {number = 1, name = main} 0
下载图片0 {number = 5, name = (null)} 0
下载图片1 {number = 6, name = (null)} 0
下载图片2 {number = 1, name = main} 1
下载图片0 {number = 5, name = (null)} 1
下载图片1 {number = 6, name = (null)} 1
下载图片2 {number = 1, name = main} 2
下载图片0 {number = 5, name = (null)} 2
下载图片1 {number = 6, name = (null)} 2
下载图片2 {number = 1, name = main} 3
下载图片0 {number = 5, name = (null)} 3
下载图片1 {number = 6, name = (null)} 3
下载图片2 {number = 1, name = main} 4
下载图片0 {number = 5, name = (null)} 4
下载图片1 {number = 6, name = (null)} 4

  下载图片3  {number = 1, name = main} 0
  下载图片3  {number = 1, name = main} 1
  下载图片3  {number = 1, name = main} 2
  下载图片3  {number = 1, name = main} 3
  下载图片3  {number = 1, name = main} 4

  下载图片4  {number = 6, name = (null)} 0
  下载图片5  {number = 5, name = (null)} 0
  下载图片4  {number = 6, name = (null)} 1
  下载图片4  {number = 6, name = (null)} 2
  下载图片5  {number = 5, name = (null)} 1
  下载图片4  {number = 6, name = (null)} 3
  下载图片5  {number = 5, name = (null)} 2
  下载图片4  {number = 6, name = (null)} 4
  下载图片5  {number = 5, name = (null)} 3
  下载图片5  {number = 5, name = (null)} 4

结论：并发队列中同步任务会直接在当前线程执行（相当于耗时操作堵塞了当前线程），执行完成后才会执行下一行代码，而异步任务则会直接开辟另外的线程来执行任务，不会因为任务而堵塞当前线程。

注意：同一个串行队列不能在任务中再添加同步任务,因为同步会在当前线程直接执行，会堵塞当前线程用来执行刚刚添加的同步任务，而根据先入先出原则，后添加的任务应该要放在后边执行，所以会造成线程锁死。

//同一个串行队列不能在任务中再添加同步任务
- (void)globalQueueThreadtest
{
    dispatch_queue_t q = dispatch_queue_create("PPD", DISPATCH_QUEUE_SERIAL);
//    q = dispatch_get_main_queue();
//    q = dispatch_get_global_queue(0, 0);
    dispatch_sync(q, ^{
        NSLog(@"start  %@",[NSThread currentThread]);
        dispatch_sync(q, ^{
            NSLog(@"continue %@",[NSThread currentThread]);
        });
        NSLog(@"end %@",[NSThread currentThread]);
    });
    
    NSLog(@"end %@ end",[NSThread currentThread]);
    
}
GCD 运行
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