Delegate method not being called

问题

I am making an OpenSource (github) helper class for downloading images asynchronously (I had major trouble with).

However, I have delegate methods set up to alert the delegate that a image has finished downloading. The problem is that the delegate method is not getting called. I am setting the delegate and everything, but I don't have a clue why the problem is occurring.

Please take a look at my code! I have only posted the relevant code.

MKAsyncImageDownloader.h

@protocol MKAsyncImageDownloaderDelegate <NSObject>
@required
- (void)imageShouldFinishDownloading;
@end
@interface MKAsyncImageDownloader : NSObject {
    id <MKAsyncImageDownloaderDelegate> delegate;
}
- (id) initWithDelegate:(id <MKAsyncImageDownloaderDelegate>) delegat;
@property (retain, nonatomic) id <MKAsyncImageDownloaderDelegate> delegate;
@end

MKAsyncImageDownloader.m

- (id) initWithDelegate:(id<MKAsyncImageDownloaderDelegate>) delegat {
    self = [super init];
    if (self) {
        delegate = delegat;
    }
    return self;
}
- (void)imageAtURLHasDownloaded:(NSDictionary *)dict {
    [downloadedImageArray addObject:[dict objectForKey:@"image"]];
    [[self delegate] imageShouldFinishDownloading];
}

MKOperation.m Subclass of NSOperation. I alloc/init MKAsynImageDownloader to perform the selector only. Code:

- (void)start {
    UIImage *image = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:self.targetURL]];
    if (image) {
        NSDictionary *dict = [[NSDictionary alloc] initWithObjects:[NSArray arrayWithObjects:image, self.targetURL, nil] forKeys:[NSArray arrayWithObjects:@"image", @"url", nil]];
        MKAsyncImageDownloader *downloader = [[MKAsyncImageDownloader alloc] init];
        [downloader performSelectorOnMainThread:@selector(imageAtURLHasDownloaded:) withObject:dict waitUntilDone:YES];
        [dict release];
        [downloader release];
     }
    [image release];
}

RootViewController.h

MKAsyncImageDownloader *loader;

RootViewController.m Just to show how I am setting the delegate.

 loader = [[MKAsyncImageDownloader alloc] initWithDelegate:self];

回答1:

In your start method you are never calling your proper init method, you are calling:

MKAsyncImageDownloader *downloader = [[MKAsyncImageDownloader alloc] init];

you should be calling

MKAsyncImageDownloader *downloader = [[MKAsyncImageDownloader alloc] initWithDelegate:myDelegate];

Then you say you set a loader object somewhere else? These are two separate objects, the one you actually seem to use is what is referenced above.

That is:

loader = [[MKAsyncImageDownloader alloc] initWithDelegate:self];

does not make

MKAsyncImageDownloader *downloader = [[MKAsyncImageDownloader alloc] init];

work. Your MKOperation has no reference to your loader class that has the delegate set.

You state:

MKOperation.m Subclass of NSOperation. I alloc/init MKAsynImageDownloader to perform the selector only.

I think you misunderstand what you are doing here. You are create a brand new instance of MKAsynImageDownloader and performing the selector on that instance, not the loader instance that lives in your RootController. You probably want to have MKOperation take a MKAsyncImageDownloader object during it init.

EDIT:

This is the "downloader" I'm referring to. In your MKOperation's start

if (image) {
    NSDictionary *dict = [[NSDictionary alloc] initWithObjects:[NSArray arrayWithObjects:image, self.targetURL, nil] forKeys:[NSArray arrayWithObjects:@"image", @"url", nil]];
    -->> MKAsyncImageDownloader *downloader = [[MKAsyncImageDownloader alloc] init];
    [downloader performSelectorOnMainThread:@selector(imageAtURLHasDownloaded:) withObject:dict waitUntilDone:YES];
    [dict release];
    [downloader release];
 }

That is the separate instance from the one that lives in your RootViewController, this one does not have a delegate set. THIS is the one you are doing work on, therefore, this one is the one attempting to notify the delegate... but again, it doesn't have a delegate set.