装饰

//组件接口定义了可以被decorator修改的操作。interface DataSource is method writeData(data) method readData():data //具体组件为//操作提供默认实现。在一个程序中，这些//类可能有几种变体。类FileDataSource实现DataSource是构造函数FileDataSource(文件名){…} method writeData(data) is //将数据写入文件。method readData():data is //从文件中读取数据。//基本装饰器类遵循与//其他组件相同的接口。该类的主要目的是为所有具体装饰器定义包装接口。//包装代码的默认实现可能包括//用于存储包装组件的字段和//初始化它的方法。DataSource构造函数DataSourceDecorator(source: DataSource) is wrap = source //基本装饰器简单地将所有工作委托给//包装组件。 Extra behaviors can be added in // concrete decorators. method writeData(data) is wrappee.writeData(data) // Concrete decorators may call the parent implementation of // the operation instead of calling the wrapped object // directly. This approach simplifies extension of decorator // classes. method readData():data is return wrappee.readData() // Concrete decorators must call methods on the wrapped object, // but may add something of their own to the result. Decorators // can execute the added behavior either before or after the // call to a wrapped object. class EncryptionDecorator extends DataSourceDecorator is method writeData(data) is // 1. Encrypt passed data. // 2. Pass encrypted data to the wrappee's writeData // method. method readData():data is // 1. Get data from the wrappee's readData method. // 2. Try to decrypt it if it's encrypted. // 3. Return the result. // You can wrap objects in several layers of decorators. class CompressionDecorator extends DataSourceDecorator is method writeData(data) is // 1. Compress passed data. // 2. Pass compressed data to the wrappee's writeData // method. method readData():data is // 1. Get data from the wrappee's readData method. // 2. Try to decompress it if it's compressed. // 3. Return the result. // Option 1. A simple example of a decorator assembly. class Application is method dumbUsageExample() is source = new FileDataSource("somefile.dat") source.writeData(salaryRecords) // The target file has been written with plain data. source = new CompressionDecorator(source) source.writeData(salaryRecords) // The target file has been written with compressed // data. source = new EncryptionDecorator(source) // The source variable now contains this: // Encryption > Compression > FileDataSource source.writeData(salaryRecords) // The file has been written with compressed and // encrypted data. // Option 2. Client code that uses an external data source. // SalaryManager objects neither know nor care about data // storage specifics. They work with a pre-configured data // source received from the app configurator. class SalaryManager is field source: DataSource constructor SalaryManager(source: DataSource) { ... } method load() is return source.readData() method save() is source.writeData(salaryRecords) // ...Other useful methods... // The app can assemble different stacks of decorators at // runtime, depending on the configuration or environment. class ApplicationConfigurator is method configurationExample() is source = new FileDataSource("salary.dat") if (enabledEncryption) source = new EncryptionDecorator(source) if (enabledCompression) source = new CompressionDecorator(source) logger = new SalaryManager(source) salary = logger.load() // ...