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CHAPTER 8 DATA PERSISTENCE
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created. The Memento instance never contains references that both Memento and Simple share. When Memento is assigned the state of a reference object, a clone is made.
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Realizing a Memento Pattern Implementation
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Implementing the Memento pattern using the classical pattern definition isn t useful, and will prove complicated. The main problem has to do with the method CreateMemento. The idea behind this method is to instantiate the type that will transfer the state. The trouble with this approach is that the class that transfers the state might not always know what the state object is. Think of it as follows. Let s say you have an immutable object. The immutable object can be serialized using binary serialization. The immutable object can t be serialized using XML serialization because of the differences in serialization implementation. To be able to serialize using both binary and XML, the immutable object has to be converted into a mutable object. Through the Memento pattern, the possibility exists whereby the immutable object transfers its state to an XML serializable state class. But let s get back to the original problem of serializing both to binary and XML. To do that, two different state classes are needed, as was shown in previous examples. The question is which state class CreateMemento instantiates. The answer is that the IOriginator implementation has no idea. A better approach to state transfer isn t to use static types, but to use .NET Generics, and replace the CreateMemento method with a GetState method. Using GetState, it s simpler to substitute implementations that can be used in different serialization contexts. This moves the responsibility of instantiation from the IOriginator implementation to the caller of IOriginator, and it allows the IOriginator to focus on the transfer of state and not care what happens to that state. The modified IOriginator<> interface is defined as follows: interface IOriginator< type> { void GetState( type state); void SetState( type state); } Continuing this serialization line of thinking, the Memento pattern is going to be used to serialize the object using binary or XML serialization. The focus this time will be to make the state transfer for serialization as flexible as possible. The first modification is to convert Memento into an interface, which is defined as follows: interface IMemento { string Buffer { get; set; } int Value { get; set; } } The interface IMemento has two properties that represent the state of SampleClass. The modified SampleClass that uses IMemento and IOriginator<> is defined as follows: class SampleClass : IOriginator< IMemento> { private int _value; private string _buffer;
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public SampleClass() { } public SampleClass(int value, string buffer) { _value = value; _buffer = buffer; } public int Value { get { return _value; } } public string Buffer { get { return _buffer; } } public void GetState( IMemento state) { state.Buffer = _buffer; state.Value = _value; } public void SetState( IMemento state) { _buffer = state.Buffer; _value = state.Value; } } SampleClass implements the IOriginator<> interface, and the interface that represents the state is IMemento. Then in the implementation of the method GetState, the state from SampleClass is transferred to the IMemento instance. And in the implementation of SetState, the state is transferred from IMemento to SampleClass. SampleClass has no idea what the final destination is of the state. From the viewpoint of SampleClass, there is no such thing as serialization, only transfer of state. Serialization is possible because the IMemento implementation could implement it. That extra step is the responsibility of IMemento. What is interesting about this transfer of state is that only the state that pertains to the unique definition of the object is transferred. For example, a database connection wouldn t be transferred, as a database connection is specific to the instance of the object. The Memento pattern could be combined with the Object Pool pattern to transfer the state of objects to already existing objects that are immutable. Or the Memento pattern could transfer the state between multiple versions of object instances (for example, version 1 and version 2 of a type). And finally, the Memento pattern could be used for integration purposes of two dissimilar applications. Getting back to the serialization example, following is an implementation of IMemento that contains the state of SampleClass. (The declaration of the class will remarkably resemble the XML serialization example, other than the implementation of IMemento. This is a coincidence, because back in the XML serialization example, the class contained only data members.)
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