在UITableView中，取消已离开屏幕的单元格的GCD操作的最佳方法是什么？ [英] In a UITableView, best method to cancel GCD operations for cells that have gone off screen?

问题描述

我有一个 UITableView ，可以使用GCD将URL中的图像异步加载到单元格中。问题是如果用户轻弹超过150行，150个操作排队并执行。我想要的是将已经过去和离开屏幕的那些出列/取消。

I have a UITableView that loads images from a URL into cells asynchronously using GCD. Problem is if a user flicks past 150 rows, 150 operations queue up and execute. What I want is to dequeue/cancel the ones that blew past and went off screen.

我该怎么做？

我的代码（非常标准）：

My code at this point (pretty standard):

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath

    // after getting the cell...

    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        if (runQ) {
            NSString *galleryTinyImageUrl = [[self.smapi getImageUrls:imageId imageKey:imageKey] objectForKey:@"TinyURL"];
            NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:galleryTinyImageUrl]];
            dispatch_async(dispatch_get_main_queue(), ^{
                if (imageData != nil) {
                    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
                    cell.imageView.image = [UIImage imageWithData:imageData];
                }
            });
        }
    });

runQ是 BOOL ivar我设置为否在 viewWillDisappear 上，我认为这可以在此<$ c $时快速刷新队列c> UITableView 弹出导航控制器。

runQ is a BOOL ivar I set to NO on viewWillDisappear, which (I think) has the effect of flushing out the queue rapidly when this UITableView pops off the navigation controller.

所以，回到我原来的问题：如何取消单元格的图像获取操作屏幕已关闭？谢谢。

So, back to my original question: how do I cancel the image fetch operations for cells that have gone off screen? Thanks.

推荐答案

首先，滚动时不要排队操作。相反，只为 viewDidLoad 中的可见行加载图像，当用户停止滚动时：

First, don't queue operations while scrolling. Instead, load images for just the visible rows in viewDidLoad and when the user stops scrolling:

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView {
    for (NSIndexPath *indexPath in [self.tableView indexPathsForVisibleRows]) {
        [self loadImageForCellAtPath:indexPath];
    }
}

如果您仍希望能够取消加载不可见的单元格，您可以使用 NSBlockOperation 而不是GCD：

If you still want to be able to cancel loading for invisible cells, you could use NSBlockOperation instead of GCD:

self.operationQueue = [[[NSOperationQueue alloc] init] autorelease];
[self.operationQueue setMaxConcurrentOperationCount:NSOperationQueueDefaultMaxConcurrentOperationCount];

// ...

-(void)loadImageForCellAtPath:(NSIndexPath *)indexPath {
    __block NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        if (![operation isCancelled]) {
            NSString *galleryTinyImageUrl = [[self.smapi getImageUrls:imageId imageKey:imageKey] objectForKey:@"TinyURL"];
            NSData *imageData = [[NSData alloc] initWithContentsOfURL:[NSURL URLWithString:galleryTinyImageUrl]];
            dispatch_async(dispatch_get_main_queue(), ^{
                if (imageData != nil) {
                    UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
                    cell.imageView.image = [UIImage imageWithData:imageData];
                }
            });
        }
    }];

    NSValue *nonRetainedOperation = [NSValue valueWithNonretainedObjectValue:operation];
    [self.operations addObject:nonRetainedOperation forKey:indexPath];
    [self.operationQueue addOperation:operation];
}

此处操作是一个 NSMutableDictionary 。如果要取消操作，可以通过单元格的 indexPath 检索它，取消它，并将其从字典中删除：

Here operations is an NSMutableDictionary. When you want to cancel an operation, you retrieve it by the cell's indexPath, cancel it, and remove it from the dictionary:

NSValue *operationHolder = [self.operations objectForKey:indexPath];
NSOperation *operation = [operationHolder nonretainedObjectValue];
[operation cancel];

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