ALAsset Photo Library Image Performance Improvements When Its Loading Slow

Question

Hi i\'ve got a problem with display an image on my scrollView.

At first i create new UIImageView with asset url:

-(void) findLargeImage:(NSNumber*)          


        

Answer 1

You are manipulating the view from another thread. You must use the main thread for manipulating the view.

Add image to scrollView using:

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

or using:

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

Please refer:

1. NSObject Class Reference
2. GCD

Answer 2

ALAssetsLibrary block will execute in separate thread. So I suggest to do the UI related stuffs in main thread.

To do this either use dispatch_sync(dispatch_get_main_queue() or performSelectorOnMainThread

Some Important Notes:

1. Use AlAsset aspectRatioThumbnail instead of fullResolutionImage for high performance

   Example:

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

Example:

-(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];

}

Also change the order of viewWillAppear()

- (void) viewWillAppear:(BOOL)animated {

    self.scrollView.delegate = self;
    [self.view addSubview:self.scrollView];
    [self findLargeImage:self.actualPhotoIndex];

}