Abstract Factory-抽象工厂
#########################

.. note:: 工厂模式用来创建不同但是相关类型的对象（继承同一父类或者接口的一组子类），由给定的参数来决定创建哪种类型的对象。



简单工厂模式
============

示例::

    type Person struct {
      Name string
      Age int
    }

    func (p Person) Greet() {
      fmt.Printf("Hi! My name is %s", p.Name)
    }

简单工厂模式是最常用、最简单的。它就是一个接受一些参数，然后返回 Person 实例的函数::

    func NewPerson(name string, age int) *Person {
      return &Person{
        Name: name,
        Age: age,
      }
    }

抽象工厂模式
============

.. note:: groups object factories that have a common theme. 和简单工厂模式的唯一区别，就是它返回的是接口而不是结构体。

.. image:: https://img.zhaoweiguo.com/knowledge/images/architectures/design-modes/Creationals/abstract-factory1.png

通过返回接口，可以在你不公开内部实现的情况下，让调用者使用你提供的各种功能::

    type Person interface {
      Greet()
    }

    // Here, NewPerson returns an interface, and not the person struct itself
    func NewPerson(name string, age int) Person {
      return person{
        name: name,
        age: age,
      }
    }

通过返回接口，我们还可以实现多个工厂函数，来返回不同的接口实现::


    // We define a Doer interface, that has the method signature
    // of the `http.Client` structs `Do` method
    type Doer interface {
      Do(req *http.Request) (*http.Response, error)
    }

    // This gives us a regular HTTP client from the `net/http` package
    func NewHTTPClient() Doer {
      return &http.Client{}
    }

    type mockHTTPClient struct{}

    func (*mockHTTPClient) Do(req *http.Request) (*http.Response, error) {
      // The `NewRecorder` method of the httptest package gives us
      // a new mock request generator
      res := httptest.NewRecorder()

      // calling the `Result` method gives us
      // the default empty *http.Response object
      return res.Result(), nil
    }

    // This gives us a mock HTTP client, which returns
    // an empty response for any request sent to it
    func NewMockHTTPClient() Doer {
      return &mockHTTPClient{}
    }

要测试的实例::

    func QueryUser(doer Doer) error {
      req, err := http.NewRequest("Get", "http://api.zhaoweiguo.com/v1/secrets", nil)
      if err != nil {
        return req
      }

      _, err := doer.Do(req)
      if err != nil {
        return err
      }

      return nil
    }

其测试用例为::

    func TestQueryUser(t *testing.T) {
      doer := NewMockHTTPClient()
      if err := QueryUser(doer); err != nil {
        t.Errorf("QueryUser failed, err: %v", err)
      }
    }