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BOOK: Professional PHP Design Patterns
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Old February 7th, 2010, 02:19 PM
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Default Differences between Delegate AND Strategy Pattern

Hi,
First, sorry for mistakes, I'm from Italy.

I don't understand very well the differences between these 2 pattern, they seems to be very similar.
In both of them you move the responsability ( you "Delegate" ... ) of an action to another specific class, in both of them you call a method of a property of the main object.

I look on other Design Patterns book (i.e.: GoF ) and I don't find anything about Delegate Pattern ... could you explain me what are the differences?

By the way, the Delegate pattern seems to be the "simple" version of Strategy Pattern ...!

thanks
 
Old February 8th, 2010, 05:18 PM
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Hi,

Thanks for the question!

The question is a good one. It took me a little bit to understand the difference between these two patterns (as well as similar building patterns like factory).

This is the important thing to notice: The delegate pattern is about objects that are created to help delegate the decision making responsibility. This means a set of qualifications need to be analyzed to determine the end result. When the Strategy pattern is introduced, it is more of a context based object. So instead of more explicit analysis, its more implicit.

You might think of it like this...
In a delegate pattern, you may choose between two classes, one called LowerDelegate for objects with a lower value, and one called HigherDelegate for those with higher values. In the strategy pattern, the object is instantiated with a context, so it knows if its ahigher or lower object. Then, the strategy initiation portion of this may be the new object being created by calling ContextDelegate (where Context is an internal value of either Lower or Higher)... see how there is no decision making? (of course error handling should be in place in case something like NULL comes through - probably isn't a NULLDelegate class!)
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Old April 28th, 2010, 05:40 PM
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So is the Delegate pattern concerned with object instantiation? Is it a variant or synonym for the Factory pattern then? I've tried to figure out .NET delegates (which I thought were more related to the Observer pattern though it's not an exact match), or are they completely unrelated to the Delegate pattern?

I've been in situations where I needed to manage different end products so I created a class for each one and then created a factory to encapsulate the decision making code. Then when you need to instantiate an object, you don't do it in plain sight, you call on the factory, pass in your parameters (if any) that tell the factory what kind of object you need, and let the factory worry about instantiation.

My understanding of the Strategy patten was that it allows you to encapsulate a particular behavior that a class has, then compose the behavior into the main class. This gives you the option to build variations of the behavior to 1) define different behaviors for different subclasses and 2)
swap / change behaviors at run time if desired.
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