动态地给一个对象添加一些额外的职责。就扩展功能而言,Decorator模式比生成子类方式更为灵活。
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以.NET Framework 2.0 System.IO中类为例
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Code
System.IO.Stream
public abstract class Stream : MarshalByRefObject, IDisposable
{
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public abstract int Read([In, Out] byte[] buffer, int offset, int count);
public abstract void Write(byte[] buffer, int offset, int count);
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}
System.IO.MemoryStream
public class MemoryStream : Stream
{
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public MemoryStream(byte[] buffer)
{
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}
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public override int Read([In, Out] byte[] buffer, int offset, int count)
{
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}
public override void Write(byte[] buffer, int offset, int count)
{
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}
}
System.IO.FileStream
public class FileStream : Stream
{
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public FileStream(String path, FileMode mode)
{
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}
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public override int Read([In, Out] byte[] buffer, int offset, int count)
{
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}
public override void Write(byte[] buffer, int offset, int count)
{
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}
}
System.IO.BufferedStream
public class BufferedStream : Stream
{
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private Stream _s;
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public BufferedStream(Stream stream)
{
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}
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public override int Read(byte[] buffer, int offset, int count)
{
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_s.Read();
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}
public override void Write(byte[] buffer, int offset, int count)
{
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_s.Write();
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}
}
调用代码
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Code
public class Client
{
public static void Main()
{
FileStream stream = new FileStream(@"c:\\C:\WINDOWS\system32\drivers\etc\services", FileMode.Open);
BufferedStream bufferedStream = new BufferedStream(stream, 1024);
}
}
在.NET Framework中,所有的流操作都有一个共同的基类System.IO.Stream,它是一个抽象类,主要包含了Read、Write等行为。针对文件流和内存流的操作,定义了FileStream和MemoryStream类,都继承Stream类,分别实现不同的读写操作。为了提高流的读写性能,需要使用缓存Buffer来存放流数据。此时,Buffer的作用对于流的读写操作而言,就相当于一个装饰的作用。
在以下情况下应当使用装饰模式:
1.需要扩展一个类的功能,或给一个类增加附加责任。
2.需要动态地给一个对象增加功能,这些功能可以再动态地撤销。
3.需要增加由一些基本功能的排列组合而产生的非常大量的功能,从而使继承关系变得不现实。
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