看看下面这个程序:
public class BusinessObject {
private FloppyWriter writer = new FloppyWriter();
.
public void Save() {
writer.SaveToFloppy();
}
}
BusinessObject 依赖于实际的 FloppyWriter,为了让 BusinessObject 获得重用性,不让 BusinessObject 直接依赖于实际的 FloppyWriter,而是依赖于抽象的接口:
private FloppyWriter writer = new FloppyWriter();
.
public void Save() {
writer.SaveToFloppy();
}
}
public interface IDeviceWriter {
public void SaveToDevice();
}
public class BusinessObject {
private IDeviceWriter writer;
public DeviceWriter
{
Set
{
this.writer = value;
}
}
public void Save() {
.
writer.SaveToDevice();
}
}
public class FloppyWriter : IDeviceWriter {
public void SaveToDevice() {
.
// 实现储存至Floppy的程序代码
}
}
public class UsbDiskWriter : IDeviceWriter {
public void SaveToDevice() {
.
// 实现储存至UsbDisk的程序代码
}
}
如果今天BusinessObject想要与UseDiskWriter对象发生依赖关系,可以这么建立: businessObject.SetDeviceWriter(new UsbDiskWriter());
由于BusinessObject依赖于抽象接口,在需要建立依赖关系时,可以通过抽象接口注入依赖的实际对象。
依赖注入在Martin Fowler的文章中谈到了三种实现方式:Interface injection、Setter injection 与 Constructor injection。并分别称其为Type 1 IoC、Type 2 IoC 与 Type 3 IoC。
上面的BusinessObject所实现的是Type 2 IoC,通过属性注入依赖关系,而Type 3 IoC,则在是构造函数上注入依赖关系,例如:
public class BusinessObject {
private IDeviceWriter writer;
public BusinessObject(IDeviceWriter writer) {
this.writer = writer;
}
public void Save() {
.
writer.SaveToDevice();
}
}