Two Phase Composite Operation

If you have an operation that has a validation phase and a execution phase, you can write it as:

  Leaf>>operation
    self validate ifTrue: [self execute]

When you use CompositePattern, the simplest implementation is to distribute the operation to the children:

  Branch>>operation
    children do: [:each ¦ each operation]

The problem is when execute modifies state in one child, and validate fails in a later child. Therefore, you should make validate and execute the composite operations, and push operation up to a common superclass:

  Branch>>validate
    ^children conform: [:each ¦ each validate]

  Branch>>execute
    children do: [:each ¦ each execute]

This pattern was first expressed by RobBrown?

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CategoryPattern