当从 tableView:cellForRowAtIndexPath 中的块调用时，cellForRowAtIndexPath 始终为零：

Question

当从 tableView:cellForRowAtIndexPath 调用时，cellForRowAtIndexPath 总是返回 nil：

问题是我实际上是在 tableView:cellForRowAtIndexPath: 内的 block 内调用 cellForRowAtIndexPath:，我猜这可能是问题所在。

我正在使用临时加载（和缓存）的远程图像填充单元格。当图像在完成处理程序块中完全加载时（在 tableView:cellForRowAtIndexPath:) 中，我需要再次获取单元格，因为它可能已经滚动出视图......所以我调用 cellForRowAtIndexPath 再次获取单元格 - cellForRowAtIndexPath 只会返回一个单元格，如果它是可见的。就我而言，我从来没有让它返回任何东西，但为零。即使我滚动得很慢（或很快，或者我尝试过的任何速度......）我得到的都是零。

我会尽快尝试在这里获取一些代码，但在那之前，任何关于为什么会发生这种情况的建议 - 我猜块的事情会是一个提示。

这里是代码：https://dl.dropboxusercontent.com/u/147970342/stackoverflow/appsappsappsappsapps.zip

-tableView:cellForRowAtIndexPath: 实现：

-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber* numberAppleid = self.appids[indexPath.row];
    UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:@"Mycell" forIndexPath:indexPath];

    cell.textLabel.text = [NSString stringWithFormat:@"Appleid: %@", numberAppleid];

    NSString* appleidasstring = [NSString stringWithFormat:@"%@", numberAppleid];

    cell.imageView.hidden = YES; // Hide any previous until image loads
    [self getAppIconForAppleid:appleidasstring handlerImage:^(UIImage *image) {
        if (image == nil) {
            return;
        }

        DLog(@"cellForRowAtIndexPath tableView: %@, indexPath: %@", tableView, indexPath);
        UITableViewCell* localcell = [tableView cellForRowAtIndexPath:indexPath];
        if (localcell != nil) {
            localcell.imageView.image = image;
            localcell.imageView.hidden = NO;
        }
        else {
            DLog(@"Nah indexpath is not visible for: %@ because localcell is nil.", appleidasstring);
        }
    }];


    return cell;
}

固定版本：

#define KEYAPPLEID  @"keyappleid"
-(UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    NSNumber* numberAppleid = self.appids[indexPath.row];
    UITableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:@"Mycell" forIndexPath:indexPath];

    cell.textLabel.text = [NSString stringWithFormat:@"Appleid: %@", numberAppleid];

    NSString* appleidasstring = [NSString stringWithFormat:@"%@", numberAppleid];

    [cell setAssociativeObject:appleidasstring forKey:KEYAPPLEID];

    cell.imageView.hidden = YES; // Hide any previous until image loads
    [self getAppIconForAppleid:appleidasstring handlerImage:^(UIImage *image) {
        if (image == nil) {
            return;
        }

        NSString* currentappleidofcell = [cell associativeObjectForKey:KEYAPPLEID];
        if ([currentappleidofcell isEqualToString:appleidasstring]) {
            cell.imageView.image = image;
            cell.imageView.hidden = NO;
        }
        else {
            DLog(@"Returning appleid: %@ is not matching current appleid: %@", appleidasstring, currentappleidofcell);
        }
    }];


    return cell;
}

并且需要这个分类：https://stackoverflow.com/a/10319083/129202

Answer 1

如果块在tableView:cellForRowAtIndexPath: 回调中，您可以直接在块中引用单元格。该块将自动保留该单元格，直到该块消失。但是，请注意保留周期。如果块归单元所有，则必须使用对单元的弱引用。

所以你的实现应该是这样的：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSNumber* numberAppleid = self.appids[indexPath.row];

    // Create your own UITableViewCell subclass that has an "appleidasstring" property
    MyTableViewCell* cell = [tableView dequeueReusableCellWithIdentifier:@"Mycell" forIndexPath:indexPath];

    cell.textLabel.text = [NSString stringWithFormat:@"Appleid: %@", numberAppleid];

    NSString* appleidasstring = [NSString stringWithFormat:@"%@", numberAppleid];

    cell.imageView.hidden = YES; // Hide any previous until image loads

    // Set the appleidasstring on the cell to be checked later
    cell. getAppIconForAppleid:appleidasstring = appleidasstring;

    // Modify the handler callback to also callback with the appleidasstring
    [self
        getAppIconForAppleid:appleidasstring
        handlerImage:^(UIImage *image, NSString *imageAppleIdaString)
        {
            if (image == nil) {
                return;
            }

            // Ensure the cell hasn't been repurposed for a
            // different imageAppleIdaString
            if ([cell.appleidasstring isEqualToString:imageAppleIdaString]) {
                cell.imageView.image = image;
                cell.imageView.hidden = NO;
            }
        }
    ];

    return cell;
}