Cheatography
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Brief overview of some, common and not so common behavioural design patterns.
OBSERVERone-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 QuestionsWhat is it? many objects need to be notified of changes in the state of one object | Where have I seen it before? RSS feeds or any pub/sub system you might have used/coded |
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 ExampleA 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. |
| | MEMENTOprovides the ability to restore an object to its previous state |
a memento is like a magic cookie that encapsulates a checkpoint capability QuestionsWhat problem does it solve? you want to save the state of an object so that you can restore it later. | Where have I seen it before? Git or any version control system for that matter. A memento makes rollbacks possible. |
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. ExampleA 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. |
| | INTERPRETERRepresent the grammar of a language with a hierarchical object-oriented design. |
The language is usually a domain specific language. QuestionsWhat problem does it solve? A language must be parsed and interpreted. | Where have I seen it before? Parsers |
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. ExampleA roman numeral (context) is converted into decimal notation by the parser (Abstract Expression).
Derivation: LIV => 50 + IV => 50 + (-1 + 5) => 50 + 4 => 54 |
AdvantagesObserver: 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 |
DisadvantagesObserver: 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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