display image from URL retrieved from ALAsset in iPhone

Question

I am using a ALAsset Framework for accessing the files in the device's photo gallery.

So far I am able to access the thumbnail and display it.
I want to display the actual image in an image view but I am unable to figure out how to do this.

I tried using the URLs field in the ALAsset object but was unsuccessful.

Anybody knows how this can be done?

Here's some code where I was able to access the thumbnail and place it in a table cell:

- (UITableViewCell *)tableView:(UITableView *)tableView 
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {


  static NSString *CellIdentifier = @"Cell";

  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
  if (cell == nil) {
    cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier] autorelease];
  }
  //here 'asset' represents the ALAsset object


  asset = [assets objectAtIndex:indexPath.row];
       //i am accessing the thumbnail here
  [cell.imageView setImage:[UIImage imageWithCGImage:[asset thumbnail]]];
  [cell.textLabel setText:[NSString stringWithFormat:@"Photo %d", indexPath.row+1]];

  return cell;
}

Answer 1

The API has changed the rules slightly and you dont get direct file system access to the iPhoto library any more. Instead you get asset library URL's like this.

assets-library://asset/asset.JPG?id=1000000003&ext=JPG

You use the ALAssetLibrary object to access the ALAsset object via the URL.

so from the docs for ALAssetLibrary throw this in a header (or your source)

typedef void (^ALAssetsLibraryAssetForURLResultBlock)(ALAsset *asset);
typedef void (^ALAssetsLibraryAccessFailureBlock)(NSError *error);

which isnt strictly needed but keeps things pretty.
and then in your source.

-(void)findLargeImage
{
    NSString *mediaurl = [self.node valueForKey:kVMMediaURL];

    //
    ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
    {
        ALAssetRepresentation *rep = [myasset defaultRepresentation];
        CGImageRef iref = [rep fullResolutionImage];
        if (iref) {
            largeimage = [UIImage imageWithCGImage:iref];
            [largeimage retain];
        }
    };

    //
    ALAssetsLibraryAccessFailureBlock failureblock  = ^(NSError *myerror)
    {
        NSLog(@"booya, cant get image - %@",[myerror localizedDescription]);
    };

    if(mediaurl && [mediaurl length] && ![[mediaurl pathExtension] isEqualToString:AUDIO_EXTENSION])
    {
        [largeimage release];
        NSURL *asseturl = [NSURL URLWithString:mediaurl];
        ALAssetsLibrary* assetslibrary = [[[ALAssetsLibrary alloc] init] autorelease];
        [assetslibrary assetForURL:asseturl 
                       resultBlock:resultblock
                      failureBlock:failureblock];
    }
}

A couple of things to note are that this uses blocks which were new to me before I started my iOS4 porting but you might like to look at

https://www.mikeash.com/pyblog/friday-qa-2008-12-26.html

and

https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Blocks/Articles/00_Introduction.html

They bend your head a little but if you think of them as notification selectors or callbacks it kind of helps.

Also

• when findLargeImage returns the resultblock wont have run yet as its a callback. So largeImage wont be valid yet.
• largeImage needs to be an instance variable not scoped to the method.

I use this construct to do this when using the method but you may find something more suitable to your use.

[node.view findLargeImage];
UIImage *thumb = node.view.largeImage;
if (thumb) { blah blah }

Thats what I learned while trying to get this working anyway.

iOS 5 update

When the result block fires seems to be a bit slower with iOS5 & maybe single core devices so I couldnt rely on the image to be available directly after calling findLargeImage. So I changed it to call out to a delegate.

@protocol HiresImageDelegate <NSObject>
@optional
-(void)hiresImageAvailable:(UIImage *)aimage;
@end

and comme cá

//
    ALAssetsLibraryAssetForURLResultBlock resultblock = ^(ALAsset *myasset)
    {
        ALAssetRepresentation *rep = [myasset defaultRepresentation];
        CGImageRef iref = [rep fullResolutionImage];
        if (iref) {
            UIImage *largeimage = [UIImage imageWithCGImage:iref];
            [delegate hiresImageAvailable:large];
        }
    };