Drawing Waveform with AVAssetReader and with ARC

问题:

I'm trying to apply Unsynchronized's answer (Drawing waveform with AVAssetReader) while using ARC. There were only a few modifications required, mostly release statements. Many thanks for a great answer! I'm using Xcode 4.2 targeting iOS5 device.

But I'm getting stuck on one statement at the end while trying to invoke the whole thing.

Method shown here:

-(void) importMediaItem {

    MPMediaItem* item = [self mediaItem];

    waveFormImage = [[UIImage alloc ] initWithMPMediaItem:item completionBlock:^(UIImage* delayedImagePreparation){

        [self displayWaveFormImage];
    }];

    if (waveFormImage) {
       [self displayWaveFormImage];
    }
}

On the call to initWithMPMediaItem I get the following error:

Automatic Reference Counting Issue.  Receiver type 'UIImage' for instance message 
does not declare a method with selector 'initWithMPMediaItem:completionBlock:'

Since I do have the method initWithMPMediaItem declared in the class header, I really don't understand why I'm getting this error.

- (id) initWithMPMediaItem:(MPMediaItem*)item
       completionBlock:(void (^)(UIImage* delayedImagePreparation))completionBlock;

Been trying to wrap my head around this for several hours now but to no avail. Is my method declaration wrong for this type of method? Thanks!

回答1:

It looks like initWithMPMediaItem should be declared as an initializer for UIImage. So you should declare it inside a UIImage category in your header file:

@interface UIImage (MPMedia)

- (id) initWithMPMediaItem:(MPMediaItem*)item
   completionBlock:(void (^)(UIImage* delayedImagePreparation))completionBlock;

@end