Objective C类继承与工厂方法 [英] Objective C class inheritance with factory methods

问题描述

我想从具有工厂方法的框架类继承。如何使工厂方法返回我继承的类类型的对象？我发现这篇有用的文章描述了类似的情况，但在他们的情况下，你可以控制超类。我怎么能写一个 UIImage 的子类， imageNamed：会返回我的子类类型的对象？ / p>

I would like to inherit from a framework class that has a factory method. How can I make the factory method return an object of my inherited class type? I found this useful article which describe a similar situation but in their case you have control over the superclass. How could I write, say, a subclass of UIImage that imageNamed: would return an object of my subclass type?

推荐答案

我想从具有工厂方法的框架类继承。如何让工厂方法返回我继承的类类型的对象？

I would like to inherit from a framework class that has a factory method. How can I make the factory method return an object of my inherited class type?

这就是应该 >必须这样做：

This is all you should have to do:

@interface MONImage : UIImage
@end

@implementation MONImage
@end

然后：

MONImage * image = [MONImage imageNamed:name];

---

---

我怎么能写一个UIImage的子类，imageNamed：会返回我的子类类型的对象吗？

How could I write, say, a subclass of UIImage that imageNamed: would return an object of my subclass type?

+ [UIImage imageNamed：] 的实现用这种方法写了子类。因此，您需要自己实现此方法。

+[UIImage imageNamed:]'s implementation wrote subclassers out of this approach. Consequently, you would need to implement this method yourself.

以下是应声明工厂方法的方式：

Here's how one should declare a factory method:

+ (instancetype)imageNamed:(NSString *)pName;

以及应如何实施它：

+ (instancetype)imageNamed:(NSString *)pName
{
  MONImage * image = [[self alloc] initWithThisDesignatedInitializer:pName];
                       ^^^^ NOTE: self, not a concrete class
  ...set up image...
  return image;
}

但他们没有这样做 - + [UIImage imageNamed：] 编写子类并在编写时返回 UIImage MONImage * img = [MONImage imageNamed： PNAME]; 。有时这是有充分理由的。有些方法应该具有'最终'语义。当您的方法可能返回多个类型时，通常会出现这种情况，如在类集群中。该语言不表达最终方法 - 但至少应该记录这种方法。

but they did not do it that way -- +[UIImage imageNamed:] wrote subclasses out and returns a UIImage when you write MONImage * img = [MONImage imageNamed:pName];. Sometimes that is done for a good reason. Some methods should have 'final' semantics. This often appears when your method may return multiple types, as in a class cluster. The language does not express 'final' methods -- but such a method should at least be documented.

所以来大约这个 UIImage 案例：

@interface MONImage : UIImage

+ (instancetype)imageNamed:(NSString *)pName;

@end

@implementation MONImage

+ (instancetype)imageNamed:(NSString *)pName
{
    UIImage * source = [UIImage imageNamed:pName];
    CGImageRef cgImage = source.CGImage;
    if (cgImage)
        return [[self alloc] initWithCGImage:cgImage];
    // try it another way
    return nil;
}

@end

请注意 UIImage s和 CGImage s是不可变的。这不应导致图像数据的深层复制。

Note that UIImages and CGImages are immutable. This should not result result in a deep copy of the image data.

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