iOS多线程系列（三）----Dispatch Queues

dispatch_queue_t aQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_queue_t queue;  
queue = dispatch_queue_create("com.example.MyQueue", NULL); 

shows a custom finalizer function and a function that creates a queue and installs that finalizer.   
The queue uses the finalizer function to release the data stored in the queue’s context pointer.   
(The myInitializeDataContextFunction and myCleanUpDataContextFunction functions referenced from the code are custom functions that   
you would provide to initialize and clean up the contents of the data structure itself.)   
The context pointer passed to the finalizer function contains the data object associated with the 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);  
}
  
dispatch_queue_t createMyQueue()  
{  
    MyDataContext* data = (MyDataContext*)             malloc(sizeof(MyDataContext));  
    myInitializeDataContextFunction(data);  
    // Create the queue and set the context data.  
    dispatch_queue_t serialQueue = dispatch_queue_create("com.example.CriticalTaskQueue", NULL);  

    if (serialQueue)  
    {  
        dispatch_set_context(serialQueue, data);  
        dispatch_set_finalizer_f(serialQueue, &myFinalizerFunction);  
    }  
return serialQueue;  
} 

void average_async(int *data, size_t len,  
 
dispatch_queue_t queue, void (^block)(int))  
{  
    // Retain the queue provided by the user to make  
    // sure it does not disappear before the completion  
    // block can be called.  
    dispatch_retain(queue);  

    // 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);});  
    　　// Release the user-provided queue when done  
   　　 dispatch_release(queue);  
    });  
}     

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
dispatch_apply(count, queue, ^(size_t i) {  
    printf("%un",i);  
}); 

double delayInSeconds = 0.5;
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);

dispatch_after(time, globalParallelQueue, ^{
        [_friendList addObject:@"234"];
        NSLog(@"testParallel, 111 count = %lu",(unsigned long)_friendList.count);
});

int i;  
int count = 10;  
for (i = 0; i < count; i++) {  
   printf("%d  ",i);  
}  

// 获得全局并发queue  
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
size_t count = 10;  
dispatch_apply(count, queue, ^(size_t i) {  
    printf("%zd ", i);  
});  

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// 根据url获取UIImage
- (UIImage *)imageWithURLString:(NSString *)urlString {
    NSURL *url = [NSURL URLWithString:urlString];
    NSData *data = [NSData dataWithContentsOfURL:url];
    return [UIImage imageWithData:data];
}

- (void)downloadImages {
    // 异步下载图片
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 下载第一张图片
        NSString *url1 = @"http://car0.autoimg.cn/upload/spec/9579/u_20120110174805627264.jpg";
        UIImage *image1 = [self imageWithURLString:url1];
        
        // 下载第二张图片
        NSString *url2 = @"http://hiphotos.baidu.com/lvpics/pic/item/3a86813d1fa41768bba16746.jpg";
        UIImage *image2 = [self imageWithURLString:url2];
        
        // 回到主线程显示图片
        dispatch_async(dispatch_get_main_queue(), ^{
            self.imageView1.image = image1;
            
            self.imageView2.image = image2;
        });
    });
}

// 根据url获取UIImage
- (UIImage *)imageWithURLString:(NSString *)urlString {
NSURL *url = [NSURL URLWithString:urlString];
NSData *data = [NSData dataWithContentsOfURL:url];
// 这里并没有自动释放UIImage对象
return [[UIImage alloc] initWithData:data];
}

- (void)downloadImages {
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

// 异步下载图片
dispatch_async(queue, ^{
// 创建一个组
dispatch_group_t group = dispatch_group_create();

__block UIImage *image1 = nil;
__block UIImage *image2 = nil;

// 关联一个任务到group
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 下载第一张图片
NSString *url1 = @"http://car0.autoimg.cn/upload/spec/9579/u_20120110174805627264.jpg";
image1 = [self imageWithURLString:url1];
});

// 关联一个任务到group
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 下载第一张图片
NSString *url2 = @"http://hiphotos.baidu.com/lvpics/pic/item/3a86813d1fa41768bba16746.jpg";
image2 = [self imageWithURLString:url2];
});

// 等待组中的任务执行完毕,回到主线程执行block回调
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            self.imageView1.image = image1;
            self.imageView2.image = image2;
            
            // 千万不要在异步线程中自动释放UIImage，因为当异步线程结束，异步线程的自动释放池也会被销毁，那么UIImage也会被销毁
            
            // 在这里释放图片资源
            [image1 release];
            [image2 release];
        });

// 释放group
dispatch_release(group);
});
}