iOS併發,串行,異步,同步

如何充分的利用計算機的有效資源，在併發以前使用多線程解決，然而不少東西須要程序員來考慮，如何建立合理數目的線程，如何控制線程之間有效運行互不影響。併發很好的解決了這樣的問題。在學習的過程當中，咱們常常看到併發，串行，異步，同步這樣的自研。它們之間有以下4種組合：

---

1.串行+同步

- (void)serialSync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_SERIAL);
    
    NSLog(@"1 - %@", [NSThread currentThread]);

    dispatch_sync(serialQueue, ^{
        NSLog(@"2 - %@", [NSThread currentThread]);
    });   
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"3 - %@", [NSThread currentThread]);
    });
    
    NSLog(@"4 - %@", [NSThread currentThread]);
}

結果：

2017-08-21 08:37:44.619 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.619 Chapter2[1772:68997] 2 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 3 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}

能夠看到該組合，沒有建立新的線程，在當前線程執行任務。首先隊列是先進先出的原則，當第一個同步任務插入隊列時，阻塞線程，須要執行隊列中的任務。由於隊列中只有它本身，因此執行。同理，第二個第三個...也是這樣的狀況。

---

2.串行+異步

- (void)serialAsync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_SERIAL);
    
    NSLog(@"1 - %@", [NSThread currentThread]);
    
    dispatch_async(serialQueue, ^{
        NSLog(@"2 - %@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"3 - %@", [NSThread currentThread]);
    });
    
    NSLog(@"4 - %@", [NSThread currentThread]);
}

結果：

2017-08-21 08:37:44.620 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.620 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.621 Chapter2[1772:69315] 2 - <NSThread: 0x608000262480>{number = 3, name = (null)}
2017-08-21 08:37:44.621 Chapter2[1772:69315] 3 - <NSThread: 0x608000262480>{number = 3, name = (null)}

當前組合，只建立了一條新的線程。由於是異步任務，因此在插入隊列時，不會阻塞，待插入結束後，而後依次執行隊列中的任務。

---

3.並行+同步

- (void)concurrentSync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_CONCURRENT);
    
    NSLog(@"1 - %@", [NSThread currentThread]);
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"2 - %@", [NSThread currentThread]);
    });
    
    dispatch_sync(serialQueue, ^{
        NSLog(@"3 - %@", [NSThread currentThread]);
    });
    
    NSLog(@"4 - %@", [NSThread currentThread]);
}

結果

2017-08-21 08:37:44.621 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.621 Chapter2[1772:68997] 2 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.622 Chapter2[1772:68997] 3 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.622 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}

在當前隊列執行任務，沒有建立新的線程。

---

4.並行+異步

- (void)concurrentAsync {
    dispatch_queue_t serialQueue = dispatch_queue_create("com.tian.lawrence", DISPATCH_QUEUE_CONCURRENT);
    
    NSLog(@"1 - %@", [NSThread currentThread]);

    dispatch_async(serialQueue, ^{
        NSLog(@"2 - %@", [NSThread currentThread]);
    });
    
    dispatch_async(serialQueue, ^{
        NSLog(@"3 - %@", [NSThread currentThread]);
    });
    
    NSLog(@"4 - %@", [NSThread currentThread]);
}

結果

2017-08-21 08:37:44.624 Chapter2[1772:68997] 1 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.624 Chapter2[1772:68997] 4 - <NSThread: 0x608000074840>{number = 1, name = main}
2017-08-21 08:37:44.624 Chapter2[1772:69315] 2 - <NSThread: 0x608000262480>{number = 3, name = (null)}
2017-08-21 08:37:44.624 Chapter2[1772:69312] 3 - <NSThread: 0x600000260000>{number = 4, name = (null)}

當前組合，按照子任務的數目建立相應數目的線程，達到了併發的目的。由於異步，在插入到隊列的時候，不會阻塞。彈出隊列的執行時，會建立不一樣的線程去執行子任務，子任務執行的結果的輸出是無序的，子任務之間相互不影響。

附圖一張，便於理解：