Question

我正在使用Photos Framework填充UICollectionView。它应该是一个简单的模式，但我在取消对隐藏单元格的请求时遇到了问题。

这是我的代码，取自几个例子：

#pragma mark - UICollectionView DataSource
- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    return 1;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    return [_photos count];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    GalleryViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"Cell" forIndexPath:indexPath];
    NSLog(@">>>>>>SHOWING CELL %d WITH MEMADDRESS %p", indexPath.row, cell);
    PHAsset *asset = [_photos objectAtIndex:indexPath.row];
    PHImageRequestOptions *option = [PHImageRequestOptions new];
    option.synchronous = NO;
    option.version = PHImageRequestOptionsVersionCurrent;
    option.networkAccessAllowed = YES;
    option.deliveryMode = PHImageRequestOptionsDeliveryModeOpportunistic;
    option.resizeMode = PHImageRequestOptionsResizeModeFast;

    PHImageRequestID requestId = cell.tag;
    if(requestId != 0)
    {
        [_imgManager cancelImageRequest:requestId];
        NSLog(@">>>>>>1ST CANCEL CELL %d WITH MEMADDRESS %p AND REQUESTID %d", indexPath.row, cell, requestId);
    }
    [cell.photoIV setImage:nil];
    cell.tag = [_imgManager requestImageForAsset:asset
                                      targetSize:CGSizeMake(CAMERA_GALLERY_SIZE, CAMERA_GALLERY_SIZE)
                //                          contentMode:PHImageContentModeAspectFit
                                     contentMode:PHImageContentModeDefault
                                         options:option
                                   resultHandler:^(UIImage *result, NSDictionary *info) {
                                       //if([self isRowVisible:indexPath])
                                       //{
                                         [cell.photoIV setImage:result];
                                         NSLog(@">>>>>>Assigning photo to cell %d with memaddress %p", indexPath.row, cell);
                                       //}
                                       //else
                                       //{
                                       //    NSLog(@">>>>>>We should assign photo to cell %d with memaddress %p but this cell is not visible", indexPath.row, cell);
                                       //}
                                   }];
    NSLog(@">>>>>>LOADING CELL %d WITH MEMADDRESS %p AND REQUESTID %d", indexPath.row, cell, requestId);
    if([_selectedPhotos containsObject:asset])
        [cell.selectedIV setHidden:NO];
    else
        [cell.selectedIV setHidden:YES];
    [cell.isMineIV setHidden:YES];

    return cell;
}

- (void)collectionView:(UICollectionView *)collectionView didEndDisplayingCell:(UICollectionViewCell *)cell forItemAtIndexPath:(NSIndexPath *)indexPath
{
    NSLog(@">>>>>>HIDDING CELL %d WITH MEMADDRESS %p", indexPath.row, cell);
    [((GalleryViewCell *)cell).photoIV setImage:nil];
    PHImageRequestID requestId = cell.tag;
    if(requestId != 0)
    {
        [_imgManager cancelImageRequest:requestId];
        NSLog(@">>>>>>2ND CANCEL CELL %d WITH MEMADDRESS %p AND REQUESTID %d", indexPath.row, cell, requestId);
    }
}

-(BOOL) isRowVisible:(NSIndexPath *) indexPath
{
    NSArray *indexes = [_photosCV indexPathsForVisibleItems];
    for (NSIndexPath *index in indexes)
    {
        if (index.row == indexPath.row)
        {
            return YES;
        }
    }

    return NO;
}

好吧，如果我在集合视图中向下快速向下滚动，每个单元格会刷新几次，在1或2秒内显示2个甚至3个图像。当单元格不再可见或单元格再次可见但是查看结果并查看日志时，我正试图在同一单元格中取消先前的请求，似乎没有请求被取消。

在我向下滚动并向上滚动到列表顶部后，这些是我的日志。

2016-10-24 22:10:46.284 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 17 WITH MEMADDRESS 0x17351660
2016-10-24 22:10:46.286 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 17 WITH MEMADDRESS 0x17351660 AND REQUESTID 351
2016-10-24 22:10:46.288 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 15 WITH MEMADDRESS 0x1736a890
2016-10-24 22:10:46.289 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 15 WITH MEMADDRESS 0x1736a890 AND REQUESTID 349
2016-10-24 22:10:46.291 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 0 WITH MEMADDRESS 0x1736a890
2016-10-24 22:10:46.291 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 0 WITH MEMADDRESS 0x1736a890 AND REQUESTID 349
2016-10-24 22:10:46.293 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 0 with memaddress 0x1736a890
2016-10-24 22:10:46.293 Fotoloop[9834:4281410] >>>>>>LOADING CELL 0 WITH MEMADDRESS 0x1736a890 AND REQUESTID 349
2016-10-24 22:10:46.298 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 1 WITH MEMADDRESS 0x17351660
2016-10-24 22:10:46.298 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 1 WITH MEMADDRESS 0x17351660 AND REQUESTID 351
2016-10-24 22:10:46.301 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 1 with memaddress 0x17351660
2016-10-24 22:10:46.303 Fotoloop[9834:4281410] >>>>>>LOADING CELL 1 WITH MEMADDRESS 0x17351660 AND REQUESTID 351
2016-10-24 22:10:46.310 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 2 WITH MEMADDRESS 0x1744d350
2016-10-24 22:10:46.311 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 2 WITH MEMADDRESS 0x1744d350 AND REQUESTID 350
2016-10-24 22:10:46.312 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 2 with memaddress 0x1744d350
2016-10-24 22:10:46.312 Fotoloop[9834:4281410] >>>>>>LOADING CELL 2 WITH MEMADDRESS 0x1744d350 AND REQUESTID 350
2016-10-24 22:10:46.316 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 3 WITH MEMADDRESS 0x171c0fd0
2016-10-24 22:10:46.317 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 3 WITH MEMADDRESS 0x171c0fd0 AND REQUESTID 348
2016-10-24 22:10:46.318 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 3 with memaddress 0x171c0fd0
2016-10-24 22:10:46.318 Fotoloop[9834:4281410] >>>>>>LOADING CELL 3 WITH MEMADDRESS 0x171c0fd0 AND REQUESTID 348
2016-10-24 22:10:46.325 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 4 WITH MEMADDRESS 0x15f15b60
2016-10-24 22:10:46.339 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 4 WITH MEMADDRESS 0x15f15b60 AND REQUESTID 347
2016-10-24 22:10:46.363 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 4 with memaddress 0x15f15b60
2016-10-24 22:10:46.365 Fotoloop[9834:4281410] >>>>>>LOADING CELL 4 WITH MEMADDRESS 0x15f15b60 AND REQUESTID 347
2016-10-24 22:10:46.371 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 5 WITH MEMADDRESS 0x1710b1b0
2016-10-24 22:10:46.373 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 5 WITH MEMADDRESS 0x1710b1b0 AND REQUESTID 346
2016-10-24 22:10:46.376 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 5 with memaddress 0x1710b1b0
2016-10-24 22:10:46.377 Fotoloop[9834:4281410] >>>>>>LOADING CELL 5 WITH MEMADDRESS 0x1710b1b0 AND REQUESTID 346
2016-10-24 22:10:46.399 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 23 with memaddress 0x171d3600
2016-10-24 22:10:46.401 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 16 with memaddress 0x1744d350
2016-10-24 22:10:46.408 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 12 WITH MEMADDRESS 0x15feda90
2016-10-24 22:10:46.408 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 12 WITH MEMADDRESS 0x15feda90 AND REQUESTID 352
2016-10-24 22:10:46.409 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 13 WITH MEMADDRESS 0x171d7d00
2016-10-24 22:10:46.409 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 13 WITH MEMADDRESS 0x171d7d00 AND REQUESTID 353
2016-10-24 22:10:46.410 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 14 WITH MEMADDRESS 0x171ef8d0
2016-10-24 22:10:46.410 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 14 WITH MEMADDRESS 0x171ef8d0 AND REQUESTID 354
2016-10-24 22:10:46.430 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 19 with memaddress 0x15f15b60
2016-10-24 22:10:46.490 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 15 with memaddress 0x1736a890
2016-10-24 22:10:46.504 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 0 with memaddress 0x1736a890
2016-10-24 22:10:46.551 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 17 with memaddress 0x17351660
2016-10-24 22:10:46.614 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 13 with memaddress 0x171d7d00
2016-10-24 22:10:46.677 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 9 with memaddress 0x1735b730
2016-10-24 22:10:46.743 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 12 with memaddress 0x15feda90
2016-10-24 22:10:46.808 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 10 with memaddress 0x171e3660
2016-10-24 22:10:46.865 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 14 with memaddress 0x171ef8d0
2016-10-24 22:10:46.927 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 11 with memaddress 0x15f27020
2016-10-24 22:10:46.996 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 6 with memaddress 0x171d3600
2016-10-24 22:10:47.053 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 7 with memaddress 0x17133010
2016-10-24 22:10:47.088 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 3 with memaddress 0x171c0fd0
2016-10-24 22:10:47.114 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 4 with memaddress 0x15f15b60
2016-10-24 22:10:47.118 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 2 with memaddress 0x1744d350
2016-10-24 22:10:47.122 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 1 with memaddress 0x17351660
2016-10-24 22:10:47.125 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 8 with memaddress 0x15f06e30
2016-10-24 22:10:47.213 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 5 with memaddress 0x1710b1b0

如您所见，单元格17（0x17351660）被隐藏，我取消了它的请求。

2016-10-24 22:10:46.284 Fotoloop[9834:4281410] >>>>>>HIDDING CELL 17 WITH MEMADDRESS 0x17351660
2016-10-24 22:10:46.286 Fotoloop[9834:4281410] >>>>>>2ND CANCEL CELL 17 WITH MEMADDRESS 0x17351660 AND REQUESTID 351

在此之后，显示了单元格1（0x17351660），并为此单元格初始化了一个新请求。

2016-10-24 22:10:46.298 Fotoloop[9834:4281410] >>>>>>SHOWING CELL 1 WITH MEMADDRESS 0x17351660
2016-10-24 22:10:46.298 Fotoloop[9834:4281410] >>>>>>1ST CANCEL CELL 1 WITH MEMADDRESS 0x17351660 AND REQUESTID 351
2016-10-24 22:10:46.301 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 1 with memaddress 0x17351660
2016-10-24 22:10:46.303 Fotoloop[9834:4281410] >>>>>>LOADING CELL 1 WITH MEMADDRESS 0x17351660 AND REQUESTID 351

但过了一段时间我就有了这个：

2016-10-24 22:10:46.551 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 17 with memaddress 0x17351660
2016-10-24 22:10:47.122 Fotoloop[9834:4281410] >>>>>>Assigning photo to cell 1 with memaddress 0x17351660

我可以在一秒内看到该单元格中显示的2张图像。如果我取消注释isRowVisible块我没有看到这种丑陋的效果，但我一直认为用我的代码取消请求可能会导致大量照片的性能不佳。我在这里缺少什么？

Answer 1

我正在将照片实现到UICollectionView中，并发现了与您相同的结果。起初，我认为cancelImageRequest不起作用，因为当快速滚动时，性能受到影响，我会看到2-3个图像在一个单元格中闪烁，直到最终落在正确的图像上。我还注意到，即使我在collectionView didEndDisplaying上取消，PHImageCancelledKey也不会存在于resultHandler信息字典中。

然而，我确实解决了我现在要解释的问题。由于PHImageManager requestImage是异步的，因此很可能在后台队列上运行，这可能会在执行任何与UI相关的操作时导致奇怪的结果。在resultHandler中，确保处理主队列上的所有内容，它应该解决您看到的问题。至少它对我有用。

我在iPhone 5s上测试滚动800张图片。

以下是我当前测试的collectionView代码示例。

我还在下面提供了我加载照片获取结果并缓存它们的位置。我使用的self.imageManager是一个PHCachingImageManager。

只是想补充一点，你会注意到我对wasCancelled和isSameRequest进行了布尔检查。这些都是无关紧要的，因为取消的密钥永远不会存在，现在一切都在主队列上处理，不再出现闪烁的图像。我离开那些你想要亲眼看看的人。

// MARK: UICollectionViewDelegate

public func numberOfSections(in collectionView: UICollectionView) -> Int
{
    return 1
}

public func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int
{
    if let assetFetchResult = self.assetFetchResult
    {
        return assetFetchResult.count
    }

    return 0
}

public func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
{
    let cell: PhotoLibraryCell = self.itemsCollectionView.dequeueReusableCell(withReuseIdentifier: PhotoLibraryCell.ReuseIdentifier, for: indexPath) as! PhotoLibraryCell

    cell.backgroundColor = .white
    cell.drawBorder(color: .lightGray, borderWidth: 1)
    cell.imageView.image = nil

    if let assetFetchResult = self.assetFetchResult
    {
        let asset: PHAsset = assetFetchResult.object(at: indexPath.row)
        let itemSize: CGSize = Settings.ItemSize
        let targetSize: CGSize = CGSize(width: itemSize.width * 2, height: itemSize.height * 2)

        if (asset.mediaType == .image)
        {
            cell.imageRequestID = self.imageManager.requestImage(for: asset, targetSize: targetSize, contentMode: .aspectFill, options: nil, resultHandler: { (image: UIImage?, info: [AnyHashable: Any]?) in

                DispatchQueue.main.async {

                    var isDegraded: Bool = false
                    var wasCancelled: Bool = false
                    var isSameRequest: Bool = false

                    if let degraded = (info?[PHImageResultIsDegradedKey] as? NSNumber)?.boolValue
                    {
                        isDegraded = degraded
                    }

                    if let cancelled = (info?[PHImageCancelledKey] as? NSNumber)?.boolValue
                    {
                        wasCancelled = cancelled
                    }

                    if let id = info?[PHImageResultRequestIDKey] as? NSNumber, let cellRequestID = cell.imageRequestID
                    {
                        let requestID: Int = id.intValue

                        isSameRequest = Int(cellRequestID) == requestID
                    }

                    if (!isDegraded && !wasCancelled && isSameRequest)
                    {
                        cell.imageView.image = image
                    }
                }
            })                
        }
        else if (asset.mediaType == .video)
        {

        }
    }

    return cell
}

public func collectionView(_ collectionView: UICollectionView, didEndDisplaying cell: UICollectionViewCell, forItemAt indexPath: IndexPath)
{
    if let photoLibraryCell = cell as? PhotoLibraryCell
    {
        if let imageRequestID = photoLibraryCell.imageRequestID
        {
            self.imageManager.cancelImageRequest(imageRequestID)
        }
    }
}

public func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize
{
    return Settings.ItemSize
}

// MARK: Assets

public func reloadAssets()
{
    let photoResult: PHFetchResult<PHAsset> = PHAsset.fetchAssets(with: .image, options: nil)

    self.assetFetchResult = photoResult

    self.itemsCollectionView.reloadData()

    //enumerate assets and start caching them
    let backgroundQueue: DispatchQueue = DispatchQueue(label: "enumerate-phassets-background-queue", qos: .background, target: nil)
    backgroundQueue.async {

        var assets: [PHAsset] = Array()
        let itemSize: CGSize = Settings.ItemSize
        let targetSize: CGSize = CGSize(width: itemSize.width * 2, height: itemSize.height * 2)

        photoResult.enumerateObjects({ (asset: PHAsset, index: Int, stopEnumeration: UnsafeMutablePointer<ObjCBool>) -> Void in
            assets.append(asset)

            let reachedEnd: Bool = index == photoResult.count - 1
            if (reachedEnd)
            {
                self.imageManager.startCachingImages(for: assets, targetSize: targetSize, contentMode: .aspectFill, options: nil)
            }
        })
    }
}

iOS PHImageManager cancelImageRequest无法正常工作？

1 个答案: