Ios 如何使用PhotoKit减少内存使用？

我正在使用

PhotoKit

从系统相册中获取照片，并将其放入

UICollectionView

中

对于

UICollectionViewCell

，我将其设置为：

cell.imageView.contentMode = UIViewContentModeScaleAspectFill;

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    MYCell *cell = (MYCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"reuseCell" forIndexPath:indexPath];

    CGFloat scale = [UIScreen mainScreen].scale;
    CGSize targetSize = CGSizeMake(_layout.itemSize.width*scale, _layout.itemSize.height*scale);

    PHAsset *asset = _fetchedPhotos[indexPath.item];
    [_manager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options resultHandler:^(UIImage *result, NSDictionary *info) {
         cell.imageView.image = result;
    }];

    return cell;
}

初始化我的

UICollectionView

时，我仅从

collection

中获取照片

PHAsset

：

摄像机卷

：

PHFetchResult *fetchResult = [PHAssetCollection fetchAssetCollectionsWithType:PHAssetCollectionTypeSmartAlbum subtype:PHAssetCollectionSubtypeAlbumRegular options:nil];
[fetchResult enumerateObjectsUsingBlock:^(PHAssetCollection *collection, NSUInteger idx, BOOL *stop) {
    NSLog(@"ALBUM NAME: %@", collection.localizedTitle);
    if ([collection.localizedTitle isEqualToString:@"Camera Roll"]) {
        _fetchedPhotos = [PHAsset fetchAssetsInAssetCollection:collection options:nil];
        _pickedStatuses = @[].mutableCopy;            

        NSMutableArray *assets = @[].mutableCopy;

        _manager = [[PHCachingImageManager alloc] init];
        _options = [[PHImageRequestOptions alloc] init];
        _options.resizeMode = PHImageRequestOptionsResizeModeExact;
        _options.deliveryMode = PHImageRequestOptionsDeliveryModeOpportunistic;

        CGFloat scale = [UIScreen mainScreen].scale;
        CGSize targetSize = CGSizeMake(layout.itemSize.width*scale, layout.itemSize.height*scale);

        //I'm not sure if this api should be called here
        [_manager startCachingImagesForAssets:assets targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options];
    }
}];

然后我从上面的

PHFetchResult

请求

UIImage

，如下所示：

cell.imageView.contentMode = UIViewContentModeScaleAspectFill;

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    MYCell *cell = (MYCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"reuseCell" forIndexPath:indexPath];

    CGFloat scale = [UIScreen mainScreen].scale;
    CGSize targetSize = CGSizeMake(_layout.itemSize.width*scale, _layout.itemSize.height*scale);

    PHAsset *asset = _fetchedPhotos[indexPath.item];
    [_manager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options resultHandler:^(UIImage *result, NSDictionary *info) {
         cell.imageView.image = result;
    }];

    return cell;
}

但当我运行它并足够快地滚动

UICollectionView

时，我发现内存使用变得如此急剧：

cell.imageView.contentMode = UIViewContentModeScaleAspectFill;

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    MYCell *cell = (MYCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"reuseCell" forIndexPath:indexPath];

    CGFloat scale = [UIScreen mainScreen].scale;
    CGSize targetSize = CGSizeMake(_layout.itemSize.width*scale, _layout.itemSize.height*scale);

    PHAsset *asset = _fetchedPhotos[indexPath.item];
    [_manager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options resultHandler:^(UIImage *result, NSDictionary *info) {
         cell.imageView.image = result;
    }];

    return cell;
}

---

如果内存不足，它会崩溃，我如何减少它呢？

我现在所做的是

减少获取的

PHAsset

对象的目标大小

尝试采用缓存。我的代码如下：

CGFloat scale = 1.5; //or an even smaller one
if (!_manager) {
    _manager = [[PHCachingImageManager alloc] init];
}
if (!_options) {
    _options = [[PHImageRequestOptions alloc] init];
}
_options.resizeMode = PHImageRequestOptionsResizeModeExact;
_options.deliveryMode = PHImageRequestOptionsDeliveryModeOpportunistic;

NSRange range = NSMakeRange(0, _fetchedPhotos.count);
NSIndexSet *set = [NSIndexSet indexSetWithIndexesInRange:range];
NSArray *assets = [_fetchedPhotos objectsAtIndexes:set];

CGSize targetSize = CGSizeMake(_layout.itemSize.width*scale, _layout.itemSize.height*scale);

[_manager startCachingImagesForAssets:assets targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options];

现在，即使我以足够快的速度滚动

UICollectionView

，顶部内存也将小于

50MB

，多亏了

缓存

（我猜它是基于我的代码工作的），它的波动性不会太大，内存使用如下：

cell.imageView.contentMode = UIViewContentModeScaleAspectFill;

-(UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    MYCell *cell = (MYCell *)[collectionView dequeueReusableCellWithReuseIdentifier:@"reuseCell" forIndexPath:indexPath];

    CGFloat scale = [UIScreen mainScreen].scale;
    CGSize targetSize = CGSizeMake(_layout.itemSize.width*scale, _layout.itemSize.height*scale);

    PHAsset *asset = _fetchedPhotos[indexPath.item];
    [_manager requestImageForAsset:asset targetSize:targetSize contentMode:PHImageContentModeAspectFill options:_options resultHandler:^(UIImage *result, NSDictionary *info) {
         cell.imageView.image = result;
    }];

    return cell;
}

更新

---

根据另一篇帖子，建议不要指定

PHImageRequestOptions

对象。相反，你可以让iOS来决定什么是最适合你的，以最好的质量和最短的时间展示照片。

我建议你看看我提到的答案。这不仅可以帮助您进行缓存，苹果的照片框架示例也可以帮助您解决任何问题。

我期望这种模式，因为在PhotoKit创建缩略图之前，它必须将整个图像加载到内存中。另外，您是在设备上测试还是在模拟器中测试？两者之间的内存行为会有很大的不同，所以总是在设备上测试，以获得应用程序性能的真实图片。我总是在我的iPhone 6 Plus设备上测试它。