“Delegation pattern”的意思、由来-开放百科全书

词条 Delegation pattern

释义

Definition
Example
Language support
See also
References
External links

In software engineering, the delegation pattern is an object-oriented design pattern that allows object composition to achieve the same code reuse as inheritance.

In delegation, an object handles a request by delegating to a second object (the delegate). The delegate is a helper object, but with the original context. With language-level support for delegation, this is done implicitly by having self in the delegate refer to the original (sending) object, not the delegate (receiving object). In the delegate pattern, this is instead accomplished by explicitly passing the original object to the delegate, as an argument to a method.^[1] Note that "delegation" is often used loosely to refer to the distinct concept of forwarding, where the sending object simply uses the corresponding member on the receiving object, evaluated in the context of the receiving object, not the original object.

Note that this article uses "sending object/receiving object" for the two objects, rather than "receiving object/delegate", emphasizing which objects send and receive the delegation call, not the original call.

Definition

In the Introduction to Gamma et al. 1994, Grady Booch defined delegation as:

{{quote
|Delegation is a way to make composition as powerful for reuse as inheritance [Lie86, JZ91]. In delegation, two objects are involved in handling a request: a receiving object delegates operations to its delegate. This is analogous to subclasses deferring requests to parent classes. But with inheritance, an inherited operation can always refer to the receiving object through the this member variable in C++ and self in Smalltalk. To achieve the same effect with delegation, the receiver passes itself to the delegate to let the delegated operation refer to the receiver.^[2]}}

Example

In the example below (using the Kotlin programming language), the class Window delegates the area() call to its internal Rectangle object (its delegate).

class Rectangle(val width: Int, val height: Int) {

}

class Window(val bounds: Rectangle) {

    // Delegation    fun area() = bounds.area()

}

Language support

Some languages have special support for delegation built in.^[3]

References

1. ^Gamma et al. 1994
2. ^{{cite book|last1=Gamma|first1=Erich|last2=Helm|first2=Richard|last3=Johnson|first3=Ralph|last4=Vlissides|first4=John|title=Design patterns : elements of reusable object-oriented software|date=1995|publisher=Addison-Wesley|location=Reading, Mass.|isbn=0-201-63361-2|page=20|edition=14. print.}}
3. ^{{Cite web|url=https://kotlinlang.org/docs/reference/delegation.html|title=Delegation - Kotlin Programming Language|website=Kotlin|access-date=2019-03-23}}

External links

What Is Delegation, WikiWikiWeb
Delegation on Rosetta Code

3 : Articles with example C++ code|Articles with example Java code|Software design patterns

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Definition

Example

Language support

See also

References

External links