Question

您好我在我的scrollView上显示图像时出现问题。

首先，我使用资产网址创建新的UIImageView：

-(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是接口中的UIImageView定义：

__block UIImageView *itemToAdd;

并且scrollView定义为属性：

@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];
}

但是图像没有出现，我应该在将图像添加到scrollView之后刷新self.view，还是应该做其他事情？

Answer 1

ALAssetsLibrary 块将在单独的帖子中执行。所以我建议在主线程中执行与UI相关的内容。

要执行此操作，请使用 dispatch_sync（dispatch_get_main_queue（）或 performSelectorOnMainThread

一些重要说明：

使用AlAsset aspectRatioThumbnail而不是fullResolutionImage来获得高性能

示例：

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

示例：

-(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]; }

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

Answer 2

您正在操纵另一个线程的视图。您必须使用主线程来操纵视图。

使用以下方法将图像添加到scrollView：

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：

ALAsset照片库图像性能改进加载缓慢

2 个答案: