PHPhotoLibraryChangeObserver 方法执行太频繁，为什么？答案

【问题标题】：PHPhotoLibraryChangeObserver method executes too often, why?PHPhotoLibraryChangeObserver 方法执行太频繁，为什么？
【发布时间】：2016-08-10 11:18:45
【问题描述】：

当有一个图库应用程序时，我可以在其中显示相机胶卷中的所有图像和视频。如果我截取屏幕截图，该应用程序将无法正常运行，因为相机胶卷发生了变化。它崩溃的原因是photoLibraryDidChange函数。
如果我退出程序拍照并重新打开它，一切正常，但是当我截取屏幕截图时，我的程序会多次进入此功能，而不仅仅是一次。我该如何解决？

【问题讨论】：

标签： ios photosframework

【解决方案1】：

您必须为 PhotoLibrary 添加更改观察者，并实现相关功能以接收更改。例如：

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];

    [[PHPhotoLibrary sharedPhotoLibrary] registerChangeObserver:self];
}

- (void)viewWillDisappear:(BOOL)animated
{
    [super viewWillDisappear:animated];

    [[PHPhotoLibrary sharedPhotoLibrary] unregisterChangeObserver:self];
}

然后，覆盖协议PHPhotoLibraryChangeObserver的didChange方法。

#pragma mark - <PHPhotoLibraryChangeObserver>

- (void)photoLibraryDidChange:(PHChange *)changeInstance
{
    // Check if there are changes to the assets we are showing.
    PHFetchResultChangeDetails *collectionChanges = [changeInstance changeDetailsForFetchResult:_fetchResult];
    if (collectionChanges == nil) {
        return;
    }

    // Get the new fetch result.
    _fetchResult = [collectionChanges fetchResultAfterChanges];

    /*
     Change notifications may be made on a background queue. Re-dispatch to the
     main queue before acting on the change as we'll be updating the UI.
     */
    dispatch_async(dispatch_get_main_queue(), ^{

        if (!_isCollectionViewLoaded) {
            return ;
        }

        UICollectionView *collectionView = _collectionView;

        if (![collectionChanges hasIncrementalChanges] || [collectionChanges hasMoves]) {
            // Reload the collection view if the incremental diffs are not available
            [collectionView reloadData];

        } else {
            /*
             Tell the collection view to animate insertions and deletions if we
             have incremental diffs.
             */

            NSArray<NSIndexPath *> * removedPaths = [[collectionChanges removedIndexes] aapl_indexPathsFromIndexesWithSection:0];
            NSArray<NSIndexPath *> * insertedPaths = [[collectionChanges insertedIndexes] aapl_indexPathsFromIndexesWithSection:0];
            NSArray<NSIndexPath *> * changedPaths = [[collectionChanges changedIndexes] aapl_indexPathsFromIndexesWithSection:0];

            BOOL shouldReload = NO;

            if ((changedPaths != nil) + (removedPaths != nil) + (insertedPaths!= nil) > 1) {
                shouldReload = YES;
            }

            if (shouldReload) {
                [collectionView reloadData];

            } else {

                @try {
                    [collectionView performBatchUpdates:^{
                        if ([removedPaths count] > 0) {
                            [collectionView deleteItemsAtIndexPaths:removedPaths];
                        }

                        if ([insertedPaths count] > 0) {
                            [collectionView insertItemsAtIndexPaths:insertedPaths];
                        }

                        if ([changedPaths count] > 0) {
                            [collectionView reloadItemsAtIndexPaths:changedPaths];
                        }
                    } completion:^(BOOL finished) {
                        if (_fetchResult.count == 0) {
                            MTLog(@"There is no photo in this album yet!!!");
                            [self.navigationController popViewControllerAnimated:YES];
                        }
                    }];

                }
                @catch (NSException *exception) {
                    [collectionView reloadData];
                }
            }
        }
    });
}

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