我知道有很多关于这两种模式之间差异的帖子，但有一些东西我找不到。
From what I have been reading, I see that the factory method pattern allows you to define how to create a single concrete product but hiding the implementation from the client as they will see a generic product. My first question is about the abstract factory. Is its role to allow you to create families of concrete objects in (that can depend on what specific factory you use) rather than just a single concrete object? Does the abstract factory only return one very large object or many objects depending on what methods you call?
我最后两个问题是关于一句我在很多地方都见过的引语，我不能完全理解:
两者之间的一个区别是
使用抽象工厂模式，a
类委托的责任
对象实例化到另一个对象
通过合成，而工厂
方法模式使用继承和
类依赖于子类来处理
所需的对象实例化。
我的理解是，工厂方法模式有一个Creator接口，它将使ConcreteCreator负责知道要实例化哪个ConcreteProduct。这就是使用继承来处理对象实例化的意思吗?
现在，关于引用，抽象工厂模式是如何通过组合将对象实例化的责任委托给另一个对象的?这是什么意思?在我看来，抽象工厂模式似乎也使用继承来完成构造过程，但我仍然在学习这些模式。
任何帮助，特别是最后一个问题，将非常感激。
Understand the differences in the motivations:
Suppose you’re building a tool where you’ve objects and a concrete implementation of the interrelations of the objects. Since you foresee variations in the objects, you’ve created an indirection by assigning the responsibility of creating variants of the objects to another object (we call it abstract factory). This abstraction finds strong benefit since you foresee future extensions needing variants of those objects.
Another rather intriguing motivation in this line of thoughts is a case where every-or-none of the objects from the whole group will have a corresponding variant. Based on some conditions, either of the variants will be used and in each case all objects must be of same variant. This might be a bit counter intuitive to understand as we often tend think that - as long as the variants of an object follow a common uniform contract (interface in broader sense), the concrete implementation code should never break. The intriguing fact here is that, not always this is true especially when expected behavior cannot be modeled by a programming contract.
A simple (borrowing the idea from GoF) is any GUI applications say a virtual monitor that emulates look-an-feel of MS or Mac or Fedora OS’s. Here, for example, when all widget objects such as window, button, etc. have MS variant except a scroll-bar that is derived from MAC variant, the purpose of the tool fails badly.
These above cases form the fundamental need of Abstract Factory Pattern.
On the other hand, imagine you’re writing a framework so that many people can built various tools (such as the one in above examples) using your framework. By the very idea of a framework, you don’t need to, albeit you could not use concrete objects in your logic. You rather put some high level contracts between various objects and how they interact. While you (as a framework developer) remain at a very abstract level, each builders of the tool is forced to follow your framework-constructs. However, they (the tool builders) have the freedom to decide what object to be built and how all the objects they create will interact. Unlike the previous case (of Abstract Factory Pattern), you (as framework creator) don’t need to work with concrete objects in this case; and rather can stay at the contract level of the objects. Furthermore, unlike the second part of the previous motivations, you or the tool-builders never have the situations of mixing objects from variants. Here, while framework code remains at contract level, every tool-builder is restricted (by the nature of the case itself) to using their own objects. Object creations in this case is delegated to each implementer and framework providers just provide uniform methods for creating and returning objects. Such methods are inevitable for framework developer to proceed with their code and has a special name called Factory method (Factory Method Pattern for the underlying pattern).
Few Notes:
If you’re familiar with ‘template method’, then you’d see that factory methods are often invoked from template methods in case of programs pertaining to any form of framework. By contrast, template methods of application-programs are often simple implementation of specific algorithm and void of factory-methods.
Furthermore, for the completeness of the thoughts, using the framework (mentioned above), when a tool-builder is building a tool, inside each factory method, instead of creating a concrete object, he/she may further delegate the responsibility to an abstract-factory object, provided the tool-builder foresees variations of the concrete objects for future extensions.
Sample Code:
//Part of framework-code
BoardGame {
Board createBoard() //factory method. Default implementation can be provided as well
Piece createPiece() //factory method
startGame(){ //template method
Board borad = createBoard()
Piece piece = createPiece()
initState(board, piece)
}
}
//Part of Tool-builder code
Ludo inherits BoardGame {
Board createBoard(){ //overriding of factory method
//Option A: return new LudoBoard() //Lodu knows object creation
//Option B: return LudoFactory.createBoard() //Lodu asks AbstractFacory
}
….
}
//Part of Tool-builder code
Chess inherits BoardGame {
Board createBoard(){ //overriding of factory method
//return a Chess board
}
….
}
