PhotoKit相簿的效能優化

有關PhotoKit的基礎知識，參考我的另一篇博文： 優雅的建立一個相簿管理類。
這篇文章記錄一下在實際專案中，自定義相簿的優化工作。

之前的文章提到過，獲取照片高清原圖使用下面的方法：

    PHImageManager *manger = [PHImageManager defaultManager];

    PHImageRequestOptions * options = [[PHImageRequestOptions alloc] init];
    options.deliveryMode = PHImageRequestOptionsDeliveryModeFastFormat;
    options.synchronous = YES;
    options.resizeMode = PHImageRequestOptionsResizeModeFast;
    options.networkAccessAllowed = NO;

[manger requestImageForAsset:asset targetSize:PHImageManagerMaximumSize contentMode:PHImageContentModeDefault options:options resultHandler:^(UIImage * _Nullable result, NSDictionary * _Nullable info) {
           //獲取照片 result
}];

在實際的使用過程中，如果涉及多張照片迴圈獲取原圖，就會出現記憶體大幅度上升的問題。通過Allcation就行記憶體檢測的時候，發現正是該方法會佔用很大的記憶體。

經過查詢資料和自己的驗證後，發現可以通過下面的方法獲取圖片。這樣記憶體會穩定，就不會引起這樣記憶體的問題。

    [manger requestImageDataForAsset:asset options:options resultHandler:^(NSData * _Nullable imageData, NSString * _Nullable dataUTI, UIImageOrientation orientation, NSDictionary * _Nullable info) {
        UIImage *image = [UIImage imageWithData:imageData];
        //獲取照片 image
    }];

該方法是獲取先圖片的二進位制流，然後轉化為UImage。

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另外，對於滑動相簿的時候，記憶體也會出現一定的增大，雖然不會造成明顯的卡頓現象。
但官方也給出了PhotoKit的快取機制介面，更好的優化記憶體，提升滑動的流暢性。

具體方法如下：

在當前擁有UICollectionView的檢視控制器中：

@property (nonatomic, assign) CGRect previousPreheatRect;

@property (nonatomic, strong) PHCachingImageManager *imageManager;

- (PHCachingImageManager *)imageManager
{
    if (_imageManager == nil) {
        _imageManager = [PHCachingImageManager new];
    }

    return _imageManager;
}


- (void)viewDidAppear:(BOOL)animated{
    [super viewDidAppear:animated];
    [self updateCachedAssets];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    [self updateCachedAssets];
}

該快取方案的原理就是快取frame範圍內所對應的顯示到cell上的照片的PHAsset物件。這樣在滑動的時候直接讀取快取裡面的資料，避免了反覆根據index去給cell賦值的操作。

- (void)updateCachedAssets{
    BOOL isViewVisible = [self isViewLoaded] && self.view.window != nil;
    if (!isViewVisible) { return; }

    // The preheat window is twice the height of the visible rect
    CGRect preheatRect = self.collectionView.bounds;
    preheatRect = CGRectInset(preheatRect, 0.0, -0.5 * CGRectGetHeight(preheatRect));

    // If scrolled by a "reasonable" amount...
    CGFloat delta = ABS(CGRectGetMidY(preheatRect) - CGRectGetMidY(self.previousPreheatRect));

    if (delta > CGRectGetHeight(self.collectionView.bounds) / 3.0) {
        // Compute the assets to start caching and to stop caching
        NSMutableArray *addedIndexPaths = [NSMutableArray array];
        NSMutableArray *removedIndexPaths = [NSMutableArray array];

        [self computeDifferenceBetweenRect:self.previousPreheatRect andRect:preheatRect addedHandler:^(CGRect addedRect) {
            NSArray *indexPaths = [self apple_indexPathsForElementsInRect:addedRect];
            [addedIndexPaths addObjectsFromArray:indexPaths];
        } removedHandler:^(CGRect removedRect) {
            NSArray *indexPaths = [self apple_indexPathsForElementsInRect:removedRect];
            [removedIndexPaths addObjectsFromArray:indexPaths];
        }];

        NSArray *assetsToStartCaching = [self assetsAtIndexPaths:addedIndexPaths];
        NSArray *assetsToStopCaching = [self assetsAtIndexPaths:removedIndexPaths];

        CGSize itemSize = CGSizeMake((ScreenWidth-15)/3, (ScreenWidth-15)/3);
        CGSize targetSize = CGSizeScale(itemSize, self.traitCollection.displayScale);

        [self.imageManager startCachingImagesForAssets:assetsToStartCaching
                                            targetSize:targetSize
                                           contentMode:PHImageContentModeAspectFill
                                               options:nil];
        [self.imageManager stopCachingImagesForAssets:assetsToStopCaching
                                           targetSize:targetSize
                                          contentMode:PHImageContentModeAspectFill
                                              options:nil];

        self.previousPreheatRect = preheatRect;
    }
}

- (void)computeDifferenceBetweenRect:(CGRect)oldRect andRect:(CGRect)newRect addedHandler:(void (^)(CGRect addedRect))addedHandler removedHandler:(void (^)(CGRect removedRect))removedHandler
{
    if (CGRectIntersectsRect(newRect, oldRect)) {
        CGFloat oldMaxY = CGRectGetMaxY(oldRect);
        CGFloat oldMinY = CGRectGetMinY(oldRect);
        CGFloat newMaxY = CGRectGetMaxY(newRect);
        CGFloat newMinY = CGRectGetMinY(newRect);

        if (newMaxY > oldMaxY) {
            CGRect rectToAdd = CGRectMake(newRect.origin.x, oldMaxY, newRect.size.width, (newMaxY - oldMaxY));
            addedHandler(rectToAdd);
        }
        if (oldMinY > newMinY) {
            CGRect rectToAdd = CGRectMake(newRect.origin.x, newMinY, newRect.size.width, (oldMinY - newMinY));
            addedHandler(rectToAdd);
        }
        if (newMaxY < oldMaxY) {
            CGRect rectToRemove = CGRectMake(newRect.origin.x, newMaxY, newRect.size.width, (oldMaxY - newMaxY));
            removedHandler(rectToRemove);
        }
        if (oldMinY < newMinY) {
            CGRect rectToRemove = CGRectMake(newRect.origin.x, oldMinY, newRect.size.width, (newMinY - oldMinY));
            removedHandler(rectToRemove);
        }
    } else {
        addedHandler(newRect);
        removedHandler(oldRect);
    }
}

- (NSArray *)apple_indexPathsForElementsInRect:(CGRect)rect
{
    NSArray *allLayoutAttributes = [self.collectionView.collectionViewLayout layoutAttributesForElementsInRect:rect];
    if (allLayoutAttributes.count == 0) { return nil; }

    NSMutableArray *indexPaths = [NSMutableArray arrayWithCapacity:allLayoutAttributes.count];
    for (UICollectionViewLayoutAttributes *layoutAttributes in allLayoutAttributes) {
        NSIndexPath *indexPath = layoutAttributes.indexPath;
        [indexPaths addObject:indexPath];
    }
    return indexPaths;
}

- (NSArray *)assetsAtIndexPaths:(NSArray *)indexPaths
{
    if (indexPaths.count == 0) { return nil; }
    NSMutableArray *assets = [NSMutableArray arrayWithCapacity:indexPaths.count];
    for (NSIndexPath *indexPath in indexPaths) {
        if (indexPath.item < self.fetchResult.count && indexPath.item != 0) {
            PHAsset *asset = self.fetchResult[self.fetchResult.count-indexPath.item];
            [assets addObject:asset];
        }
    }
    return assets;
}

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END.