异步调度的递归块 [英] Asynchronously dispatched recursive blocks

问题描述

假设我运行以下代码:

__block int step = 0;

__block dispatch_block_t myBlock;

myBlock = ^{
     if(step == STEPS_COUNT)
     {
         return;
     }

     step++;
     dispatch_time_t delay = dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC / 2);
     dispatch_after(delay, dispatch_get_current_queue(), myBlock);
};

dispatch_time_t delay = dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC / 2);
dispatch_after(delay, dispatch_get_current_queue(), myBlock);

该块从外部被调用一次.到达内部调用后，程序将崩溃，没有任何详细信息.如果我在所有地方都使用直接调用而不是GCD调度，那么一切都会很好.

The block is invoked once from outside. When the inner invocation is reached, the program crashes without any details. If I use direct invocations everywhere instead of GCD dispatches, everything works fine.

我也尝试过使用该块的副本调用dispatch_after.我不知道这是否是朝着正确方向迈出的一步，但这还不足以使其正常工作.

I've also tried calling dispatch_after with a copy of the block. I don't know if this was a step in the right direction or not, but it wasn't enough to make it work.

想法?

推荐答案

尝试解决此问题时，我发现了一段代码，该代码片段解决了许多与递归块相关的问题.我无法再次找到源，但是仍然有代码:

When trying to solve this problem, I found a snippet of code that solves much of the recursive block related issues. I have not been able to find the source again, but still have the code:

// in some imported file
dispatch_block_t RecursiveBlock(void (^block)(dispatch_block_t recurse)) {
    return ^{ block(RecursiveBlock(block)); };
}

// in your method
dispatch_block_t completeTaskWhenSempahoreOpen = RecursiveBlock(^(dispatch_block_t recurse) {
    if ([self isSemaphoreOpen]) {
        [self completeTask];
    } else {
        double delayInSeconds = 0.3;
        dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
        dispatch_after(popTime, dispatch_get_main_queue(), recurse);
    }
});

completeTaskWhenSempahoreOpen();

RecursiveBlock允许使用非参数块.可以为单个或多个参数块重写它.使用此结构简化了内存管理，例如，没有保留周期的机会.

RecursiveBlock allows for non-argument blocks. It can be rewritten for single or multiple argument blocks. The memory management is simplified using this construct, there is no chance of a retain cycle for example.

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