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CMSC 433 – Programming Language
Technologies and Paradigms
Mediator PatternSpring 2007
Apr. 12, 2007
Example
and a text entry field.several buttons, two list boxesConsider a program which has
disabled.Copy and Clear buttons areWhen the program starts, the
editing, and theit is copied into the text field fornames in the left-hand list box,When you select one of the
(^) Copy
(^) button is
enabled.
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Example (cont…)
When you click on
(^) Copy,
Clear right hand list box, and the that text is added to the
(^) button is enabled.
If you click on the
(^) Clear
buttons are again disabled.deselected and the twocleared, the list box isbox and the text field are button, the right hand list
Relationship Diagram
pretty complex.the visual controls areThe interactions between
relationship diagram.quite a tangledmore others, leading toto know about two orEach visual object needs
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Mediator
is aware of the other classes in the system.The Mediator pattern simplifies this system by being the only class that
The controls that the Mediator communicates with is a Colleague.
informed of this event.event, and the Mediator decides which other classes should beEach Colleague informs the Mediator when it has received a user
Why Mediator?
– it is the only class that knows of the other classes, andThe advantage of the Mediator is clear
classes are added.of the other classes changes or if other interface controlthus the only one that would need to be changed if one
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The Code!
– You start by creating an instance of the Mediator andthe Mediator.Each class needs to be aware of the existence of
its constructor.then pass the instance of the Mediator to each class in
KTextField
The text field registers itself with the mediator.
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The Mediator Code!
– It supports the Copy, Clear and Select methods, and hasThe Mediator itself is very simple.
register methods for each of the controls.
More Mediator Code!
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Yet More Mediator Code!
System Initialization
is the initialization of all the controls to the desired state.One further operation that is best delegated to the Mediator
the program evolves, we simply create anknown, default state, and since these states may change asWhen we launch the program, each control must be in a
(^) init
(^) method in
the Mediator, which sets them all to the desired state.
public void init() {Clear button and we simply call that method.In this case, that state is the same as is achieved by the
Clear();
}
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Concluding Remarks
- objects in a program.The Mediator makes loose coupling possible between among several objects.It also localizes the behavior that otherwise would be distributed
changing the Mediator.You can change the behavior of the program by simply
part of the program.Colleagues to a system without having to change any otherThe Mediator approach makes it possible to add new
in the rest of a user interface.needing to know too much about the objects and methodsThe Mediator solves the problem of each Command object
More Concluding Remarks
– Sometimes you can improve this situation by revisinghard to change and maintain.The Mediator can become complex, making it
the responsibilities you have given the Mediator.
- Each object should carry out it’s own tasks and the objects.Mediator should only manage the interaction between
- Colleague has available.for each Colleague to call and knows what methods eachEach Mediator is a custom-written class that has methods This makes it difficult to reuse Mediator code in different projects.
interactions any other way.this code is far easier than managing the complex objectOn the other hand, most Mediators are quite simple and writing
