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Module Pattern Variations Import mixins This variation of the pattern demonstrates how globals e. This effectively allows us to import them and locally alias them as we wish. This takes as its first argument a dot-separated string such as myObj.
Ben Cherry previously suggested an implementation where a function wrapper is used around module definitions in the event of there being a number of commonalities between modules.
Secondly, it supports private data - so, in the Module pattern, public parts of our code are able to touch the private parts, however the outside world is unable to touch the class's private parts no laughing! Oh, and thanks to David Engfer for the joke. Disadvantages The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used.
We also can't access private members in methods that are added to the object at a later point.
That said, in many cases the Module pattern is still quite useful and when used correctly, certainly has the potential to improve the structure of our application. Other disadvantages include the inability to create automated unit tests for private members and additional complexity when bugs require hot fixes.
It's simply not possible to patch privates. Instead, one must override all public methods which interact with the buggy privates.
Developers can't easily extend privates either, so it's worth remembering privates are not as flexible as they may initially appear. For further reading on the Module pattern, see Ben Cherry's excellent in-depth article on it. The Revealing Module pattern came about as Heilmann was frustrated with the fact that he had to repeat the name of the main object when we wanted to call one public method from another or access public variables.
The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public.
It also makes it more clear at the end of the module which of our functions and variables may be accessed publicly which eases readability. Disadvantages A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary. This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions.
Public object members which refer to private variables are also subject to the no-patch rule notes above.
As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage. The Singleton Pattern The Singleton pattern is thus known because it restricts instantiation of a class to a single object.
Classically, the Singleton pattern can be implemented by creating a class with a method that creates a new instance of the class if one doesn't exist. In the event of an instance already existing, it simply returns a reference to that object. Singletons differ from static classes or objects as we can delay their initialization, generally because they require some information that may not be available during initialization time.
They don't provide a way for code that is unaware of a previous reference to them to easily retrieve them. This is because it is neither the object or "class" that's returned by a Singleton, it's a structure. Think of how closured variables aren't actually closures - the function scope that provides the closure is the closure.
In the GoF book, the applicability of the Singleton pattern is described as follows: There must be exactly one instance of a class, and it must be accessible to clients from a well-known access point. When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code.
The second of these points refers to a case where we might need code such as: mySingleton. FooSingleton above would be a subclass of BasicSingleton and implement the same interface.
Why is deferring execution considered important for a Singleton? It is important to note the difference between a static instance of a class object and a Singleton: whilst a Singleton can be implemented as a static instance, it can also be constructed lazily, without the need for resources nor memory until this is actually needed.
If we have a static object that can be initialized directly, we need to ensure the code is always executed in the same order e. Both Singletons and static objects are useful but they shouldn't be overused - the same way in which we shouldn't overuse other patterns. In practice, the Singleton pattern is useful when exactly one object is needed to coordinate others across a system. They're often an indication that modules in a system are either tightly coupled or that logic is overly spread across multiple parts of a codebase.
Singletons can be more difficult to test due to issues ranging from hidden dependencies, the difficulty in creating multiple instances, difficulty in stubbing dependencies and so on. Miller Medeiros has previously recommended this excellent article on the Singleton and its various issues for further reading as well as the comments to this article, discussing how Singletons can increase tight coupling.
I'm happy to second these recommendations as both pieces raise many important points about this pattern that are also worth noting. The Observer Pattern The Observer is a design pattern where an object known as a subject maintains a list of objects depending on it observers , automatically notifying them of any changes to state.
February 26, 2012
When a subject needs to notify observers about something interesting happening, it broadcasts a notification to the observers which can include specific data related to the topic of the notification.
When we no longer wish for a particular observer to be notified of changes by the subject they are registered with, the subject can remove them from the list of observers. It's often useful to refer back to published definitions of design patterns that are language agnostic to get a broader sense of their usage and advantages over time.
The definition of the Observer pattern provided in the GoF book, Design Patterns: Elements of Reusable Object-Oriented Software, is: "One or more observers are interested in the state of a subject and register their interest with the subject by attaching themselves.
When something changes in our subject that the observer may be interested in, a notify message is sent which calls the update method in each observer.
When the observer is no longer interested in the subject's state, they can simply detach themselves. If you want to keep your code efficient, more manageable, and up-to-date with the latest best practices, this book is for you. Explore many popular design patterns, including Modules, Observers, Facades, and Mediators. This book also walks you through modern module formats, how to namespace code effectively, and other essential topics.
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