non-ARC代码转 ARC 排除 “Existing instance variable 'delegate' for property with assign attribute must be ...

原来非ARC代码是

@interface MHWebImageDownloader : NSObject
{
    id<MHWebImageDownloaderDelegate> delegate;
}

.
.
.

@property (nonatomic, weak) id<MHWebImageDownloaderDelegate> delegate;

变为 ARC 该代码报错，Existing instance variable 'delegate' for property with assign attribute must be _unsafe _unretained

根据Stack Overflow上这篇帖子

http://stackoverflow.com/questions/7021852/arc-error-when-declaring-delegate-ivar

应改为

@interface MHWebImageDownloader : NSObject
{
    _weak id<MHWebImageDownloaderDelegate> delegate;
}

.
.
.

@property (nonatomic, weak) id<MHWebImageDownloaderDelegate> delegate;