相关疑难解决方法(0)

这与我之前提出的问题有关,但不同之处,我认为我会把它扔进一个新问题.我有一些代码在自定义队列上运行异步,然后在完成时在主线程上执行完成块.我想围绕这种方法编写单元测试.我的方法MyObject看起来像这样.

+ (void)doSomethingAsyncThenRunCompletionBlockOnMainQueue:(void (^)())completionBlock {

    dispatch_queue_t customQueue = dispatch_queue_create("com.myObject.myCustomQueue", 0);

    dispatch_async(customQueue, ^(void) {

        dispatch_queue_t currentQueue = dispatch_get_current_queue();
        dispatch_queue_t mainQueue = dispatch_get_main_queue();

        if (currentQueue == mainQueue) {
            NSLog(@"already on main thread");
            completionBlock();
        } else {
            dispatch_async(mainQueue, ^(void) {
                NSLog(@"NOT already on main thread");
                completionBlock();
        }); 
    }
});

}

我投入了主队列测试以获得额外的安全性,但它总是能够击中dispatch_async.我的单元测试如下所示.

- (void)testDoSomething {

    dispatch_semaphore_t sema = dispatch_semaphore_create(0);

    void (^completionBlock)(void) = ^(void){        
        NSLog(@"Completion Block!");
        dispatch_semaphore_signal(sema);
    }; 

    [MyObject doSomethingAsyncThenRunCompletionBlockOnMainQueue:completionBlock];

    // Wait for async code to finish
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); …

我想知道如果我从viewDidLoad方法在主队列中异步调用某些内容会发生什么.一个小实验向我展示了这些结果:

这段代码:

override func viewDidLoad() {
    super.viewDidLoad()

    firstSelector()
    DispatchQueue.main.async {
        self.secondSelector()
    }
    for i in 1...10 {
        print(i)
    }
    thirdSelector()

}

func firstSelector() {
    print("First selector fired")
}

func secondSelector() {
    print("Second selector fired")
}

func thirdSelector() {
    print("Third selector fired")
}

给出这些印刷品:

First selector fired
1
2
3
4
5
6
7
8
9
10
Third selector fired
Second selector fired

所以最后一个被调用的方法是secondSelector.我认为这是因为主队列是串行的,当我异步调用某个方法(在本例中为secondSelector)时,它立即返回并等待直到所有其他方法都完成.在队列没有任务之后,它完成了我异步调用的方法.我的想法是对的吗？