add factory method pattern

Author: senghoo

04_factory_method/README.md

@@ -0,0 +1,5 @@
+# 工厂方法模式
+
+工厂方法模式使用子类的方式延迟生成对象到子类中实现。
+
+Go中不存在继承 所以使用匿名组合来实现

04_factory_method/factorymethod.go

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+package factorymethod
+
+//Operator 是被封装的实际类接口
+type Operator interface {
+	SetA(int)
+	SetB(int)
+	Result() int
+}
+
+//OperatorFactory 是工厂接口
+type OperatorFactory interface {
+	Create() Operator
+}
+
+//OperatorBase 是Operator 接口实现的基类，封装公用方法
+type OperatorBase struct {
+	a, b int
+}
+
+//SetA 设置 A
+func (o *OperatorBase) SetA(a int) {
+	o.a = a
+}
+
+//SetB 设置 B
+func (o *OperatorBase) SetB(b int) {
+	o.b = b
+}
+
+//PlusOperatorFactory 是 PlusOperator 的工厂类
+type PlusOperatorFactory struct{}
+
+func (PlusOperatorFactory) Create() Operator {
+	return &PlusOperator{
+		OperatorBase: &OperatorBase{},
+	}
+}
+
+//PlusOperator Operator 的实际加法实现
+type PlusOperator struct {
+	*OperatorBase
+}
+
+//Result 获取结果
+func (o PlusOperator) Result() int {
+	return o.a + o.b
+}
+
+//MinusOperatorFactory 是 MinusOperator 的工厂类
+type MinusOperatorFactory struct{}
+
+func (MinusOperatorFactory) Create() Operator {
+	return &MinusOperator{
+		OperatorBase: &OperatorBase{},
+	}
+}
+
+//MinusOperator Operator 的实际减法实现
+type MinusOperator struct {
+	*OperatorBase
+}
+
+//Result 获取结果
+func (o MinusOperator) Result() int {
+	return o.a - o.b
+}

04_factory_method/factorymethod_test.go

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+package factorymethod
+
+import "testing"
+
+func compute(factory OperatorFactory, a, b int) int {
+	op := factory.Create()
+	op.SetA(a)
+	op.SetB(b)
+	return op.Result()
+}
+
+func TestOperator(t *testing.T) {
+	var (
+		factory OperatorFactory
+	)
+
+	factory = PlusOperatorFactory{}
+	if compute(factory, 1, 2) != 3 {
+		t.Fatal("error with factory method pattern")
+	}
+
+	factory = MinusOperatorFactory{}
+	if compute(factory, 4, 2) != 2 {
+		t.Fatal("error with factory method pattern")
+	}
+}