Factory Pattern. When to use factory methods?

什么时候在对象内使用工厂方法而不是Factory类是一个好主意？

---

I like thinking about design pattens in terms of my classes being 'people,' and the patterns are the ways that the people talk to each other.

So, to me the factory pattern is like a hiring agency. You've got someone that will need a variable number of workers. This person may know some info they need in the people they hire, but that's it.

So, when they need a new employee, they call the hiring agency and tell them what they need. Now, to actually hire someone, you need to know a lot of stuff - benefits, eligibility verification, etc. But the person hiring doesn't need to know any of this - the hiring agency handles all of that.

In the same way, using a Factory allows the consumer to create new objects without having to know the details of how they're created, or what their dependencies are - they only have to give the information they actually want.

public interface IThingFactory
{
    Thing GetThing(string theString);
}

public class ThingFactory : IThingFactory
{
    public Thing GetThing(string theString)
    {
        return new Thing(theString, firstDependency, secondDependency);
    }
}

So, now the consumer of the ThingFactory can get a Thing, without having to know about the dependencies of the Thing, except for the string data that comes from the consumer.

---

Factory methods should be considered as an alternative to constructors - mostly when constructors aren't expressive enough, ie.

class Foo{
  public Foo(bool withBar);
}

is not as expressive as:

class Foo{
  public static Foo withBar();
  public static Foo withoutBar();
}

Factory classes are useful when you need a complicated process for constructing the object, when the construction need a dependency that you do not want for the actual class, when you need to construct different objects etc.

---

One situation where I personally find separate Factory classes to make sense is when the final object you are trying to create relies on several other objects. Eg, in PHP: Suppose you have a House object, which in turn has a Kitchen and a LivingRoom object, and the LivingRoom object has a TV object inside as well.

The simplest method to achieve this is having each object create their children on their construct method, but if the properties are relatively nested, when your House fails creating you will probably spend some time trying to isolate exactly what is failing.

The alternative is to do the following (dependency injection, if you like the fancy term):

$TVObj = new TV($param1, $param2, $param3);
$LivingroomObj = new LivingRoom($TVObj, $param1, $param2);
$KitchenroomObj = new Kitchen($param1, $param2);
$HouseObj = new House($LivingroomObj, $KitchenroomObj);

Here if the process of creating a House fails there is only one place to look, but having to use this chunk every time one wants a new House is far from convenient. Enter the Factories:

class HouseFactory {
    public function create() {
        $TVObj = new TV($param1, $param2, $param3);
        $LivingroomObj = new LivingRoom($TVObj, $param1, $param2);
        $KitchenroomObj = new Kitchen($param1, $param2);
        $HouseObj = new House($LivingroomObj, $KitchenroomObj);

        return $HouseObj;
    }
}

$houseFactory = new HouseFactory();
$HouseObj = $houseFactory->create();

Thanks to the factory here the process of creating a House is abstracted (in that you don't need to create and set up every single dependency when you just want to create a House ) and at the same time centralized which makes it easier to maintain. There are other reasons why using separate Factories can be beneficial (eg testability) but I find this specific use case to illustrate best how Factory classes can be useful.

链接地址: http://www.djcxy.com/p/77842.html

上一篇: 如何在控制器中使用过滤器？

下一篇: 工厂模式。 何时使用工厂方法？