初始化数组的代码实际上并没有创建UIImageExtra子类的实例。

Your code to initialize the array is not actually creating instances of your UIImageExtra subclass.

UIImageExtra *illustration = (UIImageExtra *)[UIImage imageNamed:illustrationString];

返回UIImage。投射它并不能达到您的目的。

returns a UIImage. Casting it doesn't do what you were intending.

UIImageExtra *scaledIllustration = [illustration adjustForResolution];

仍只是一个UIImage。

is still just a UIImage.

一种直接但又详细的方法是在UIImage周围使UIImageExtra成为包装器。包装器将有一个用于从UIImage初始化的类方法：

One straightforward-but-verbose way to approach this would be to make UIImageExtra a wrapper around UIImage. The wrapper would have a class method for initializing from a UIImage:

+ (UIImageExtra)imageExtraWithUIImage:(UIImage *)image;

然后你要调用的每个UIImage方法都必须转发到包装的UIImage实例 - 也小心重新包装例如的结果 -adjustForResolution 以免你最终得到一个解包的UIImage实例。

And then every UIImage method you want to call would have to forward to the wrapped UIImage instance-- also being careful to re-wrap the result of e.g. -adjustForResolution lest you again end up with an unwrapped UIImage instance.

一个更客观的C复杂方法是在UIImage上的 Category 中添加所需的功能，然后使用方法调配将NSCoding方法替换为您的类别实现。这个棘手的部分（除了所需的Objective-C运行时体操）是存储额外数据的地方，因为您无法在类别中添加实例变量。 [标准答案是有一个由UIImage实例的某个合适表示键入的后备字典（如包含其指针值的NSValue），但正如您可以想象的那样，簿记可以快速复杂化。]

A more Objective-C sophisticated approach would be to add the functionality you want in a Category on UIImage, and then use method swizzling to replace the NSCoding methods with your category implementations. The tricky part of this (apart from the required Objective-C runtime gymnastics) is where to store your "extra" data, since you can't add instance variables in a category. [The standard answer is to have a look-aside dictionary keyed by some suitable representation of the UIImage instance (like an NSValue containing its pointer value), but as you can imagine the bookkeeping can get complicated fast.]

稍微退一步，我对一位新Cocoa程序员的建议是：想想一个更简单的方法。如果你想要做的事情很复杂，那就别管其他了。例如，编写一个简单的 ImageValue 类，它具有 -image 方法和 -extraInfo 方法（并实现NSCoding等），并在数组中存储该实例。

Stepping back for a moment, my advice to a new Cocoa programmer would be: "Think of a simpler way. If what you are trying to do is this complicated, try something else." For example, write a simple ImageValue class that has an -image method and an -extraInfo method (and implements NSCoding, etc.), and store instances of that in your array.