GCD多執行緒下,實現執行緒同步的方式有如下幾種:

1.序列佇列 2.並行佇列 3.分組 4.訊號量

例項: 去網上獲取一張圖片並展示在檢視上. 實現這個需求,可以拆分成兩個任務,一個是去網上獲取圖片,一個是展示在檢視上. 這兩個任務是有關聯的,所以需要同步處理.

下面看這幾種方式如何實現.

一、

1.序列佇列

1.1[GCD相關:]

(1)GCD下的dispatch_queue佇列都是FIFO佇列,都會按照提交到佇列的順序執行.

只是根據佇列的性質,分為<1>序列佇列:使用者佇列、主執行緒佇列 <2>並行佇列. 

(2)同步(dispatch_sync)、非同步方式(dispatch_async). 配合序列佇列和並行佇列使用.

1.2同步佇列直接提交兩個任務就可以.

    // 串形佇列
    dispatch_queue_t serilQueue = dispatch_queue_create("com.quains.myQueue", 0);
    
    //開始時間
    NSDate *startTime = [NSDate date];
    
    
    __block UIImage *image = nil;
    
    //1.先去網上下載圖片
    dispatch_async(serilQueue, ^{
        NSString *urlAsString = @"
 
http://avatar.csdn.net/B/2/2/1_u010013695.jpg";
        NSURL *url = [NSURL URLWithString:urlAsString];
        
        NSError *downloadError = nil;
        
        NSData *imageData = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:url] returningResponse:nil error:&downloadError];
        
        
 
if (downloadError == nil && imageData != nil) {
            image = [[UIImage imageWithData:imageData] retain];
        }
        else if(downloadError != nil){
            NSLog(@"error happened = %@", downloadError);
        }
        else{
            NSLog(@"No data download");
        }
    });
    
    //2.在主執行緒展示到介面裡
    dispatch_async(serilQueue, ^{
        
        NSLog(@"%@",[NSThread currentThread]);
        
        // 在主執行緒展示
        dispatch_async(dispatch_get_main_queue(), ^{
        if (image != nil) {
            
            UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
            
            [imageView setImage:image];
            
            [imageView setContentMode:UIViewContentModeScaleAspectFit];
            [self.view addSubview:imageView];
            [imageView release];
            
            NSDate *endTime = [NSDate date];
            NSLog(@"序列非同步 completed in %f time", [endTime timeIntervalSinceDate:startTime]);
        }
        else{
            NSLog(@"image isn't downloaded, nothing to display");
        }
        });
        
    });
    
    //3.清理
    dispatch_release(serilQueue);
    [image release];

注意:

(1) __block變數分配在棧,retain下,防止被回收.

(2)dispatch要手動create和release.

(3)提交到主執行緒佇列的時候,慎用同步dispatch_sync方法,有可能造成死鎖. 因為主執行緒佇列是序列佇列,要等佇列裡的任務一個一個執行.所以提交一個任務到佇列,如果用同步方法就會阻塞住主執行緒,而主執行緒又要等主執行緒佇列裡的任務都執行完才能執行那個剛提交的,所以主執行緒佇列裡還有其他的任務的話,但他已經被阻塞住了,沒法先完成佇列裡的其他任務,即,最後一個任務也沒機會執行到,於是造成死鎖.

(4)提交到序列佇列可以用同步方式,也可以用非同步方式.

2.並行佇列

採用並行佇列的時候,可以採用同步的方式把任務提交到佇列裡去,即可以實現同步的方式

//新建一個佇列
    dispatch_queue_t concurrentQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    //記時
    NSDate *startTime = [NSDate date];
    
    //加入佇列
    dispatch_async(concurrentQueue, ^{
        __block UIImage *image = nil;
        
        //1.先去網上下載圖片
        dispatch_sync(concurrentQueue, ^{
            NSString *urlAsString = @"http://avatar.csdn.net/B/2/2/1_u010013695.jpg";
            NSURL *url = [NSURL URLWithString:urlAsString];
            
            NSError *downloadError = nil;
            
            NSData *imageData = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:url] returningResponse:nil error:&downloadError];
            
            if (downloadError == nil && imageData != nil) {
                image = [UIImage imageWithData:imageData];
            }
            else if(downloadError != nil){
                NSLog(@"error happened = %@", downloadError);
            }
            else{
                NSLog(@"No data download");
            }
        });
        
        //2.在主執行緒展示到介面裡
        dispatch_sync(dispatch_get_main_queue(), ^{
            if (image != nil) {
                UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
                [imageView setImage:image];
                
                [imageView setContentMode:UIViewContentModeScaleAspectFit];
                [self.view addSubview:imageView];
                [imageView release];
                
                NSDate *endTime = [NSDate date];
                NSLog(@"並行同步 completed in %f time", [endTime timeIntervalSinceDate:startTime]);
            }
            else{
                NSLog(@"image isn't downloaded, nothing to display");
            }
        });
    });

兩個同步的任務用一個非同步的包起來,提交到並行佇列裡去,即可實現同步的方式.

3.使用分組方式

3.1 group本身是將幾個有關聯的任務組合起來,然後提供給開發者一個知道這個group結束的點.

雖然這個只有一個任務,但是可以利用group的結束點,去阻塞執行緒,從而來實現同步方式.

dispatch_group_t group = dispatch_group_create();
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    NSDate *startTime = [NSDate date];
    
    __block UIImage *image = nil;
    
    dispatch_group_async(group, queue, ^{
        
        //1.先去網上下載圖片
            NSString *urlAsString = @"http://avatar.csdn.net/B/2/2/1_u010013695.jpg";
            NSURL *url = [NSURL URLWithString:urlAsString];
            
            NSError *downloadError = nil;
            
            NSData *imageData = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:url] returningResponse:nil error:&downloadError];
            
            if (downloadError == nil && imageData != nil) {
                image = [[UIImage imageWithData:imageData] retain];
            }
            else if(downloadError != nil){
                NSLog(@"error happened = %@", downloadError);
            }
            else{
                NSLog(@"No data download");
            }
        
        });
    
    // 2.等下載好了再在重新整理主執行緒
    dispatch_group_notify(group, queue, ^{
        
        //在主執行緒展示到介面裡
        dispatch_async(dispatch_get_main_queue(), ^{
            if (image != nil) {
                UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
                [imageView setImage:image];
                [image release];
                
                [imageView setContentMode:UIViewContentModeScaleAspectFit];
                [self.view addSubview:imageView];
                [imageView release];
                
                NSDate *endTime = [NSDate date];
                NSLog(@"分組同步 completed in %f time", [endTime timeIntervalSinceDate:startTime]);
            }
            else{
                NSLog(@"image isn't downloaded, nothing to display");
            }
        });
        
    });
    
    // 釋放掉
    dispatch_release(group);

dispatch_group 也要手動建立和釋放.

dispatch_notify()提供了一個知道group什麼時候結束的點. 當然也可以使用dispatch_wait()去阻塞.

4.訊號量

訊號量 和 瑣 的作用差不多,可以用來實現同步的方式. 

但是訊號量通常用在 允許幾個執行緒同時訪問一個資源,通過訊號量來控制訪問的執行緒個數.

// 訊號量初始化為1
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    NSDate *startTime = [NSDate date];
    
    __block UIImage *image = nil;
    
    
    //1.先去網上下載圖片
    dispatch_async(queue, ^{
        
        // wait操作-1
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        
        // 開始下載
        NSString *urlAsString = @"http://avatar.csdn.net/B/2/2/1_u010013695.jpg";
        NSURL *url = [NSURL URLWithString:urlAsString];
        
        NSError *downloadError = nil;
        
        NSData *imageData = [NSURLConnection sendSynchronousRequest:[NSURLRequest requestWithURL:url] returningResponse:nil error:&downloadError];
        
        if (downloadError == nil && imageData != nil) {

            image = [[UIImage imageWithData:imageData] retain];
            //NSLog(@"heap %@", image);
            //NSLog(@"%d",[image retainCount]);
        }
        else if(downloadError != nil){
            NSLog(@"error happened = %@", downloadError);
        }
        else{
            NSLog(@"No data download");
        }

        // signal操作+1
        dispatch_semaphore_signal(semaphore);
    });
    
  
    // 2.等下載好了再在重新整理主執行緒
    dispatch_async(dispatch_get_main_queue(), ^{
        
        // wait操作-1
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        
        if (image != nil) {
            
            UIImageView *imageView = [[UIImageView alloc] initWithFrame:self.view.bounds];
            
            [imageView setImage:image];
            NSLog(@"%d",[image retainCount]);
            [image release];
            
            [imageView setContentMode:UIViewContentModeScaleAspectFit];
            [self.view addSubview:imageView];
            [imageView release];
            
            NSDate *endTime = [NSDate date];
            NSLog(@"訊號量同步 completed in %f time", [endTime timeIntervalSinceDate:startTime]);
        }
        else{
            NSLog(@"image isn't downloaded, nothing to display");
        }
        
        // signal操作+1
        dispatch_semaphore_signal(semaphore);
    });

dispatch_wait會阻塞執行緒並且檢測訊號量的值,直到訊號量值大於0才會開始往下執行,同時對訊號量執行-1操作.

dispatch_signal則是+1操作.

二、

以上幾種方式，都是通過阻塞執行緒的方式去實現同步。