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News Become a contributor. Categories Web development Programming Data Security. Subscription Go to Subscription. Subtotal 0. Title added to cart. Subscription About Subscription Pricing Login. 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.
What makes the Singleton is the global access to the instance generally through 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.
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.
The update functionality here will be overwritten later with custom behaviour. See below for inline comments on what these components do in the context of our example. Whilst very similar, there are differences between these patterns worth noting. The Observer pattern requires that the observer or object wishing to receive topic notifications must subscribe this interest to the object firing the event the subject.
This event system allows code to define application specific events which can pass custom arguments containing values needed by the subscriber. The idea here is to avoid dependencies between the subscriber and publisher. This differs from the Observer pattern as it allows any subscriber implementing an appropriate event handler to register for and receive topic notifications broadcast by the publisher.
How are you doing today? Rather than single objects calling on the methods of other objects directly, they instead subscribe to a specific task or activity of another object and are notified when it occurs.
They also help us identify what layers containing direct relationships which could instead be replaced with sets of subjects and observers. This effectively could be used to break down an application into smaller, more loosely coupled blocks to improve code management and potentials for re-use.
Further motivation behind using the Observer pattern is where we need to maintain consistency between related objects without making classes tightly coupled.
Disadvantages Consequently, some of the issues with these patterns actually stem from their main benefits. For example, publishers may make an assumption that one or more subscribers are listening to them.
Say that we're using such an assumption to log or output errors regarding some application process.
If the subscriber performing the logging crashes or for some reason fails to function , the publisher won't have a way of seeing this due to the decoupled nature of the system.