queue types

• Serial
• Concurrent
• Main dispatch queue

Queue-Related Technologies

• Dispatch groups
• Dispatch semaphores
• Dispatch sources

1. 全局队列(Concurrent)

可选的优先级：

• DISPATCH_QUEUE_PRIORITY_HIGH
• DISPATCH_QUEUE_PRIORITY_DEFAULT
• DISPATCH_QUEUE_PRIORITY_LOW
• DISPATCH_QUEUE_PRIORITY_BACKGROUND

//参数1：队列优先级
//参数2：保留字段，传 0
dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

2. 创建队列

• DISPATCH_QUEUE_SERIAL
• DISPATCH_QUEUE_CONCURRENT

//参数1：队列的名字 com.example.queue
//队列属性：DISPATCH_QUEUE_SERIAL（orNULL）或者  DISPATCH_QUEUE_CONCURRENT
dispatch_queue_t dispatch_queue_create(const char *label, dispatch_queue_attr_t attr);

3. 运行时获取队列
   dispatch_get_current_queue
4. 内存管理

Dispatch queues and other dispatch objects are reference-counted data types.
也是一种引用技术的类型

MRC下管理内存使用：

• dispatch_retain
• dispatch_release

5. 设置和获取队列上下文数据

• dispatch_set_context
• dispatch_get_context
• dispatch_set_finalizer_f 提供队列销毁时的处理

创建队列上下文，并提供清理函数，example：

typedef struct {
    char *name;
    int age;
    char *sex;
} MyDataContext;

- (void)testCreateQueueWithContext {
    
    dispatch_queue_t queue = [self createQueueWithContext];
    //add tasks
    dispatch_async(queue, ^{
        
        printf("task1\n");
        MyDataContext *context = dispatch_get_context(queue);
        printf("name = %s, age = %d, sex = %s\n",context->name, context->age, context->sex);
        //modify the context
        context->age = 11;
        
    });
    
    dispatch_async(queue, ^{
        printf("task2\n");
        MyDataContext *context = dispatch_get_context(queue);
        printf("name = %s, age = %d, sex = %s\n",context->name, context->age, context->sex);
    });
}



- (dispatch_queue_t)createQueueWithContext {
    
    MyDataContext *data = (MyDataContext *)malloc(sizeof(MyDataContext));
    data->name = "context";
    data->age = 10;
    data->sex = "male";
    dispatch_queue_t queue = dispatch_queue_create("com.example.queue.withcontext", DISPATCH_QUEUE_SERIAL);
    dispatch_set_context(queue, data);
    
    dispatch_set_finalizer_f(queue, &myFinalizerFunction);
    return queue;
}



void myFinalizerFunction(void *context)
{
    MyDataContext* theData = (MyDataContext*)context;
    // Clean up the contents of the structure
    myCleanUpDataContextFunction(theData);
    // Now release the structure itself.
    free(theData);
}

void myCleanUpDataContextFunction(void *context)
{
    MyDataContext* theData = (MyDataContext*)context;
    printf("%s",__FUNCTION__);
    printf("name = %s, age = %d, sex = %s",theData->name, theData->age, theData->sex);
}

6. 任务完成时，执行一个block

- (void)testPerformingCompletionBlockWhenTaskIsDone {
    
    dispatch_queue_t queue = dispatch_queue_create("com.example.queue.completion", DISPATCH_QUEUE_SERIAL);
    int a[5] = {1, 2, 3, 4, 5};
    average_async(a, 5, queue, ^(int average) {
        NSLog(@"done, average = %d", average);
    });
}

void average_async(int *data, size_t len,
                   dispatch_queue_t queue, void (^block)(int))
{
    // Do the work on the default concurrent queue and then
    // call the user-provided block with the results.
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        int avg = average(data, len);
        dispatch_async(queue, ^{ block(avg);});  
    });
}
int average(int *data, size_t length) {
    
    int sum = 0;
    for (int i = 0; i< length; i++) {
        sum += data[i];
    }
    return sum/length;
}

7. 使用dispatch_apply来遍历

- (void)testApply {
    
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    int count = 100;
    dispatch_apply(count, queue, ^(size_t i) {
        printf("%u\n",i);
    });
}

8. 使用信号量

//创建 count = 1
dispatch_semaphore_t  semaphore = dispatch_semaphore_create(1);
//wait, count--
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER); 

//do something 
//临界区代码

//signal, count++
dispatch_semaphore_signal(semaphore); //count++

9. 使用group

使用dispatch_group_wait方式，会阻塞当前的线程，当group里的任务全部完成之后，当前线程继续执行

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
 
// Add a task to the group
dispatch_group_async(group, queue, ^{
   // Some asynchronous work
});

// Add another task to the group
dispatch_group_async(group, queue, ^{
   // Some asynchronous work
});
 
// Do some other work while the tasks execute.
 
// When you cannot make any more forward progress,
// wait on the group to block the current thread.
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
//all task done, continue to do other tasks

使用dispatch_group_notify不阻塞当前的线程，group里的任务全部完成之后，会通知到对应的block去执行。

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
 
// Add a task to the group
dispatch_group_async(group, queue, ^{
   // Some asynchronous work
});

// Add another task to the group
dispatch_group_async(group, queue, ^{
   // Some asynchronous work
});
 
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"all tasks are done");
});