access NSMutableDictionary property in block - objective-c-blocks

Question

I want to save the UIImages into a NSMutableDictionary which is a property in the method of a block,
__block UIImage *headImage = [[UIImage alloc] init];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
headImage = [self getImageFromURL:status.user.profile_image_url];
dispatch_async(dispatch_get_main_queue(), ^{
[headImageView setImage:headImage];
[[self contentView] addSubview:headImageView];
});
});
When debugging the getImageFromURL I found every time the iconDict is empty, and the method returns before setObject:forKey.
-(UIImage *) getImageFromURL:(NSString *)imageUrl {
UIImage * image = [[UIImage alloc] init];
if(iconDict)
{
image = [_iconDict objectForKey:imageUrl];
if (image) {
return image;
}
} else {
_iconDict = [NSMutableDictionary dictionaryWithCapacity:3];
}
image = [self loadImage:imageUrl];
[_iconDict setObject:image forKey:imageUrl];
return image;
}
What's worry about this?

Answer 1

I would say you need to change the semantics of your code. There is no reason to store a UIImage instance in the calling function, so just make it so that getImageFromURL: simply returns boolean success status. If that is YES then get the images from _iconDict:
dispatch_async(dispatch_get_global_queue(0, 0), ^{
if ([self getImageFromURL:status.user.profile_image_url]) {
dispatch_async(dispatch_get_main_queue(), ^{
[headImageView setImage:_iconDict[status.user.profile_image_url]];
[[self contentView] addSubview:headImageView];
});
}
});
...
- (BOOL)getImageFromURL:(NSString *)imageUrl {
UIImage *image = [_iconDict objectForKey:imageUrl];
if (!image) {
image = [self loadImage:imageUrl];
if (image) {
if (!_iconDict)
_iconDict = [NSMutableDictionary dictionaryWithCapacity:3];
_iconDict[imageUrl] = image;
}
}
return image != nil;
}