Question

我有一个填充数组的方法，但是里面的计算非常大而且很慢，我想使用多线程来加速它。

unsigned char array[500*500*4];
for(int i=0; i<500; i++) {
    for(int j=0; j<500; j++) {

        CGFloat element = [self veryLongCalculationWithI:i andJ:j];

        array[k] = drand48()*255; 
        array[k+1] = drand48()*255; 
        array[k+2] = drand48()*255;
        array[k+3] = element*255; 
        k+=4;
    }
}

我该怎么做？我一直在调查GDC与调度异步，但我不确定这是否是最佳方式。有什么建议吗？

Answer 1

帮助并行化计算的一个解决方案是使用并发NSOperationQueue。

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
// Be sure to call free() on array when appropriate
unsigned char *array = calloc(500 * 500 * 4, sizeof(unsigned char));
int k = 0;
for (int i = 0; i < 500; i++) {
    for (int j = 0; j < 500; j++) {
        [queue addOperationWithBlock:^{
            CGFloat element = [self veryLongCalculationWithI:i andJ:j];
            array[k + 3] = element * 255;
        }];

        [queue addOperationWithBlock:^{
            array[k] = drand48() * 255; 
            array[k + 1] = drand48() * 255; 
            array[k + 2] = drand48() * 255;
        }];

        k += 4;
    }
}

// optional, if needed:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    [queue waitUntilAllOperationsAreFinished];

    dispatch_async(dispatch_get_main_queue(). ^{
        // Update UI as needed
    });
});

以上是打破它的一种可能方法。尝试一些变化并做一些时间测试。另请注意，结果取决于设备及其拥有的核心数。

更新 - 上述代码最终创建了500,000个操作。该开销可能比原始代码更糟糕。尝试这样的事情：

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
// Be sure to call free() on array when appropriate
unsigned char *array = calloc(500 * 500 * 4, sizeof(unsigned char));
int k = 0;
for (int i = 0; i < 500; i++) {
    [queue addOperationWithBlock:^{
        for (int j = 0; j < 500; j++) {
            CGFloat element = [self veryLongCalculationWithI:i andJ:j];
            array[k + 3] = element * 255;

            array[k] = drand48() * 255; 
            array[k + 1] = drand48() * 255; 
            array[k + 2] = drand48() * 255;
            k += 4;
        }
    }];
}

// optional, if needed:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    [queue waitUntilAllOperationsAreFinished];

    dispatch_async(dispatch_get_main_queue(). ^{
        // Update UI as needed
    });
});

Answer 2

我们在swift中有GCD的一般语法在后台进行计算并更新前景中的UI

BASIC SYNTAX

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0)) {
    // do background task
    dispatch_async(dispatch_get_main_queue()) {
        // update some UI
    }
}

将所有计算代码保留在后台任务中。这不会阻止您的用户界面。

IOS中多线程填充数组

2 个答案: