加载缓慢时，ALAsset照片库图像性能得到改善

嗨，我有一个问题，我的scrollView上显示图像。

首先我创build新的UIImageView与资产url：

-(void) findLargeImage:(NSNumber*) arrayIndex { ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset) { ALAssetRepresentation *rep; if([myasset defaultRepresentation] == nil) { return; } else { rep = [myasset defaultRepresentation]; } CGImageRef iref = [rep fullResolutionImage]; itemToAdd = [[UIImageView alloc] initWithFrame:CGRectMake([arrayIndex intValue]*320, 0, 320, 320)]; itemToAdd.image = [UIImage imageWithCGImage:iref]; [self.scrollView addSubview:itemToAdd]; }; ALAssetsLibraryAccessFailureBlock failureblock = ^(NSError *myerror) { NSLog(@"Cant get image - %@",[myerror localizedDescription]); }; NSURL *asseturl = [NSURL URLWithString:[self.photoPath objectAtIndex:[arrayIndex intValue] ]]; ALAssetsLibrary* assetslibrary = [[ALAssetsLibrary alloc] init]; [assetslibrary assetForURL:asseturl resultBlock:resultblock failureBlock:failureblock]; 

}

其中itemToAdd是一个在界面中定义的UIImageView：

 __block UIImageView *itemToAdd; 

和scrollView定义为一个属性：

 @property (nonatomic, strong) __block UIScrollView *scrollView; 

然后在我看来，我会这样做：

 - (void) viewWillAppear:(BOOL)animated { self.scrollView.delegate = self; [self findLargeImage:self.actualPhotoIndex]; [self.view addSubview:self.scrollView]; } 

但图像不会出现，我应该刷新self.view后添加图像scrollView，还是应该做点别的？

Answer 1

ALAssetsLibrary块将在单独的线程中执行。 所以我build议在主线程中做UI相关的东西。

要做到这一点，使用dispatch_sync（dispatch_get_main_queue（）或performSelectorOnMainThread

一些重要说明：

1. 使用AlAsset aspectRatioThumbnail而不是FullResolutionImage来获得高性能

   例：

  CGImageRef iref = [myasset aspectRatioThumbnail]; itemToAdd.image = [UIImage imageWithCGImage:iref]; 

例：

 -(void) findLargeImage:(NSNumber*) arrayIndex { ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset) { CGImageRef iref = [myasset aspectRatioThumbnail]; dispatch_sync(dispatch_get_main_queue(), ^{ itemToAdd = [[UIImageView alloc] initWithFrame:CGRectMake([arrayIndex intValue]*320, 0, 320, 320)]; itemToAdd.image = [UIImage imageWithCGImage:iref]; [self.scrollView addSubview:itemToAdd]; });//end block }; ALAssetsLibraryAccessFailureBlock failureblock = ^(NSError *myerror) { NSLog(@"Cant get image - %@",[myerror localizedDescription]); }; NSURL *asseturl = [NSURL URLWithString:[self.photoPath objectAtIndex:[arrayIndex intValue] ]]; ALAssetsLibrary* assetslibrary = [[ALAssetsLibrary alloc] init]; [assetslibrary assetForURL:asseturl resultBlock:resultblock failureBlock:failureblock]; } 

还要改变viewWillAppear（）的顺序

 - (void) viewWillAppear:(BOOL)animated { self.scrollView.delegate = self; [self.view addSubview:self.scrollView]; [self findLargeImage:self.actualPhotoIndex]; } 

Answer 2

您正在操作来自另一个线程的视图。 您必须使用主线程来操作视图。

添加图像到scrollView使用：

 dispatch_async(dispatch_get_main_queue(), ^{ [self.scrollView addSubview:itemToAdd]; } 

或使用：

 [self.scrollView performSelectorOnMainThread:@selector(addSubview:) withObject:itemToAdd waitUntilDone:NO]; 

请参阅：

1. NSObject类参考
2. GCD