Cheatography
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Brief overview of some, common and not so common behavioural design patterns.
OBSERVER
one-to-many dependency between subject and observers, so that when subject changes, the observers are notified and updated. |
a way of notifying change to a number of classes
Questions
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What is it? many objects need to be notified of changes in the state of one object
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Where have I seen it before? RSS feeds or any pub/sub system you might have used/coded
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Ok, this is cool. What do I need to implement it?
1. A Subject Abstract Class and an Observer Abstract Class
2. Concrete subject and observer class that implement above pattern.
The concrete subject registers its observers
Example
A referee (concrete subject) notifies all the players and commentators (concrete observers) about changes in the state of a Soccer match. Each player must be notifiable. |
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MEMENTO
provides the ability to restore an object to its previous state |
a memento is like a magic cookie that encapsulates a checkpoint capability
Questions
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What problem does it solve? you want to save the state of an object so that you can restore it later.
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Where have I seen it before? Git or any version control system for that matter. A memento makes rollbacks possible.
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Ok, this is cool. What do I need to implement it?
1. An originator class (class that has a state that needs to be remembered)
2. A caretaker class (class that wants to modify the state of the originator)
3. A memento class that holds originator information that can't be modified by any other class. It is merely a container.
Example
A programmer (caretaker) asks for a copy (memento) of the code (originator) he/she is modifying. Later he/she decides he doesn't like the new state of the code so he restores it with the copy it still has. |
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INTERPRETER
Represent the grammar of a language with a hierarchical object-oriented design. |
The language is usually a domain specific language.
Questions
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What problem does it solve? A language must be parsed and interpreted.
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Where have I seen it before? Parsers
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Ok, this is cool. What do I need to implement it?
1. A Context class that contains the input.
2. An AbstractExpression class, a composite object of terminals and non-terminals.
3. The client passes the context to the abstract expression, which calls the interpret() function on its children.
Example
A roman numeral (context) is converted into decimal notation by the parser (Abstract Expression).
Derivation: LIV => 50 + IV => 50 + (-1 + 5) => 50 + 4 => 54 |
Advantages
Observer: minimal coupling; easy addition and removal of observers |
Memento: it is an encapsulated copy so it avoids exposing its info; the storage burden is on the caretaker, not on originator |
Interpreter: easy to change/extend/implement/evaluate a language |
Disadvantages
Observer: Possible memory leak; Objects might need to work hard to deduce what changed in the subject. |
Memento: Copy operation to a memento can be costly for the originator; Caretaker might have large storage costs. |
Interpreter: Complex grammars are hard to maintain and debug. |
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