self [self Class]

self是实例的指针，[self class]是类的指针，静态方法得用类的指针来调用

- (NSString*) pathForImageURL:(NSString*)imageURL {

if ([imageURL hasPrefix:@"http://"] || [imageURL hasPrefix:@"https://"] || [imageURLhasPrefix:@"ftp://"])

return [[self class] tmpFilePathForResourceAtURL:imageURL];

return imageURL;

}

+ (NSString*) tmpFilePathForResourceAtURL:(NSString*)url {

NSString * fileName = [self fileNameForResourceAtURL:url];

NSString * path = [self filePathForTemporaryStorage];

return [path stringByAppendingPathComponent:fileName];

}