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Design patterns which are used for architectural purposes. Design Pattern Examples Each of the design patterns represents a specific type of solution to a specific type of problem. There is no universal set of patterns that is always the best fit. We need to learn when a particular pattern will prove useful and whether it will provide actual value. Once we are familiar with the patterns and scenarios they are best suited for, we can easily determine whether or not a specific pattern is a good fit for a given problem. Remember, applying the wrong pattern to a given problem could lead to undesirable effects such as unnecessary code complexity, unnecessary overhead on performance, or even the spawning of a new anti-pattern.
This pattern has been referred to as a self-executing anonymous function, but cowboy Ben Alman introduced the term IIFE as a more semantically accurate term for the pattern.
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The engine integrates the tiled map format making level design easier. Elements of Reusable Object-Oriented Software , is:. 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. We can now expand on what we've learned to implement the Observer pattern with the following components:.
Next, let's model the Subject and the ability to add, remove or notify observers on the observer list.
We then define a skeleton for creating new Observers. The update functionality here will be overwritten later with custom behaviour. We then define ConcreteSubject and ConcreteObserver handlers for both adding new observers to the page and implementing the updating interface. See below for inline comments on what these components do in the context of our example. In this example, we looked at how to implement and utilize the Observer pattern, covering the concepts of a Subject, Observer, ConcreteSubject and ConcreteObserver.
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.
The general idea here is the promotion of loose coupling. 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. For example, when an object needs to be able to notify other objects without making assumptions regarding those objects. Dynamic relationships can exist between observers and subjects when using either pattern. This provides a great deal of flexibility which may not be as easy to implement when disparate parts of our application are tightly coupled.
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. Another draw-back of the pattern is that subscribers are quite ignorant to the existence of each other and are blind to the cost of switching publishers.
Due to the dynamic relationship between subscribers and publishers, the update dependency can be difficult to track.
Next, let's imagine we have a web application responsible for displaying real-time stock information. The application might have a grid for displaying the stock stats and a counter for displaying the last point of update. When the data model changes, the application will need to update the grid and counter.
When our subscribers receive notification that the model itself has changed, they can update themselves accordingly. In our implementation, our subscriber will listen to the topic "newDataAvailable" to find out if new stock information is available.
If a new notification is published to this topic, it will trigger gridUpdate to add a new row to our grid containing this information. It will also update a last updated counter to log the last time data was added.
Notice how submitting a rating only has the effect of publishing the fact that new user and rating data is available. It's left up to the subscribers to those topics to then delegate what happens with that data.
In our case we're pushing that new data into existing arrays and then rendering them using the Underscore library's. Quite often in Ajax-heavy applications, once we've received a response to a request we want to achieve more than just one unique action. One could simply add all of their post-request logic into a success callback, but there are drawbacks to this approach.
What this means is that although keeping our post-request logic hardcoded in a callback might be fine if we're just trying to grab a result set once, it's not as appropriate when we want to make further Ajax-calls to the same data source and different end-behavior without rewriting parts of the code multiple times.
Using Observers, we can also easily separate application-wide notifications regarding different events down to whatever level of granularity we're comfortable with - something which can be less elegantly done using other patterns.
Notice how in our sample below, one topic notification is made when a user indicates they want to make a search query and another is made when the request returns and actual data is available for consumption.
It's left up to the subscribers to then decide how to use knowledge of these events or the data returned. The benefits of this are that, if we wanted, we could have 10 different subscribers utilizing the data returned in different ways but as far as the Ajax-layer is concerned, it doesn't care.
Its sole duty is to request and return data then pass it on to whoever wants to use it. This separation of concerns can make the overall design of our code a little cleaner. The Observer pattern is useful for decoupling a number of different scenarios in application design and if you haven't been using it, I recommend picking up one of the pre-written implementations mentioned today and just giving it a try out.
It's one of the easier design patterns to get started with but also one of the most powerful. In the section on the Observer pattern, we were introduced to a way of channeling multiple event sources through a single object. It's common for developers to think of Mediators when faced with this problem, so let's explore how they differ. The dictionary refers to a mediator as a neutral party that assists in negotiations and conflict resolution. In our world, a mediator is a behavioral design pattern that allows us to expose a unified interface through which the different parts of a system may communicate.
If it appears a system has too many direct relationships between components, it may be time to have a central point of control that components communicate through instead. The Mediator promotes loose coupling by ensuring that instead of components referring to each other explicitly, their interaction is handled through this central point.
This can help us decouple systems and improve the potential for component reusability. A real-world analogy could be a typical airport traffic control system. A tower Mediator handles what planes can take off and land because all communications notifications being listened out for or broadcast are done from the planes to the control tower, rather than from plane-to-plane. A centralized controller is key to the success of this system and that's really the role a Mediator plays in software design.
Another analogy would be DOM event bubbling and event delegation. If all subscriptions in a system are made against the document rather than individual nodes, the document effectively serves as a Mediator. Instead of binding to the events of the individual nodes, a higher level object is given the responsibility of notifying subscribers about interaction events.
When it comes to the Mediator and Event Aggregator patterns, there are some times where it may look like the patterns are interchangeable due to implementation similarities.
However, the semantics and intent of these patterns are very different. And even if the implementations both use some of the same core constructs, I believe there is a distinct difference between them. I also believe they should not be interchanged or confused in communication because of the differences.
A Mediator is an object that coordinates interactions logic and behavior between multiple objects. It makes decisions on when to call which objects, based on the actions or inaction of other objects and input.
It is an object that handles the workflow between many other objects, aggregating the responsibility of that workflow knowledge into a single object. The result is workflow that is easier to understand and maintain. The similarities boil down to two primary items: These differences are superficial at best, though. When we dig into the intent of the pattern and see that the implementations can be dramatically different, the nature of the patterns become more apparent.
The difference, then, is why these two patterns are both using events. The event aggregator, as a pattern, is designed to deal with events. Both the event aggregator and mediator, by design, use a third-party object to facilitate things. The event aggregator itself is a third-party to the event publisher and the event subscriber. It acts as a central hub for events to pass through. The mediator is also a third party to other objects, though. So where is the difference? The answer largely comes down to where the application logic and workflow is coded.
In the case of an event aggregator, the third party object is there only to facilitate the pass-through of events from an unknown number of sources to an unknown number of handlers. All workflow and business logic that needs to be kicked off is put directly into the object that triggers the events and the objects that handle the events.
In the case of the mediator, though, the business logic and workflow is aggregated into the mediator itself. The mediator decides when an object should have its methods called and attributes updated based on factors that the mediator knows about. It encapsulates the workflow and process, coordinating multiple objects to produce the desired system behaviour. The individual objects involved in this workflow each know how to perform their own task. It just fires the event and moves on.
A mediator pays attention to a known set of input or activities so that it can facilitate and coordinate additional behavior with a known set of actors objects. Understanding the similarities and differences between an event aggregator and mediator is important for semantic reasons. The basic semantics and intent of the patterns does inform the question of when, but actual experience in using the patterns will help you understand the more subtle points and nuanced decisions that have to be made.
In general, an event aggregator is used when you either have too many objects to listen to directly, or you have objects that are entirely unrelated. When two objects have a direct relationship already — say, a parent view and child view — there may be benefit in using an event aggregator. Have the child view trigger an event and the parent view can handle the event. A Collection often uses model events to modify the state of itself or other models.
This could quickly deteriorate performance of the application and user experience. Indirect relationships are also a great time to use event aggregators. In modern applications, it is very common to have multiple view objects that need to communicate, but have no direct relationship. For example, a menu system might have a view that handles the menu item clicks.
Having the content and menu coupled together would make the code very difficult to maintain, in the long run. A mediator is best applied when two or more objects have an indirect working relationship, and business logic or workflow needs to dictate the interactions and coordination of these objects. There are multiple views that facilitate the entire workflow of the wizard. Rather than tightly coupling the view together by having them reference each other directly, we can decouple them and more explicitly model the workflow between them by introducing a mediator.
The mediator extracts the workflow from the implementation details and creates a more natural abstraction at a higher level, showing us at a much faster glance what that workflow is. We no longer have to dig into the details of each view in the workflow, to see what the workflow actually is. The crux of the difference between an event aggregator and a mediator, and why these pattern names should not be interchanged with each other, is illustrated best by showing how they can be used together.
The menu example for an event aggregator is the perfect place to introduce a mediator as well. Clicking a menu item may trigger a series of changes throughout an application.
Some of these changes will be independent of others, and using an event aggregator for this makes sense. Some of these changes may be internally related to each other, though, and may use a mediator to enact those changes. A mediator, then, could be set up to listen to the event aggregator.
It could run its logic and process to facilitate and coordinate many objects that are related to each other, but unrelated to the original event source.
An event aggregator and a mediator have been combined to create a much more meaningful experience in both the code and the application itself. We now have a clean separation between the menu and the workflow through an event aggregator and we are still keeping the workflow itself clean and maintainable through the use of a mediator.
Adding new publishers and subscribers is relatively easy due to the level of decoupling present. Perhaps the biggest downside of using the pattern is that it can introduce a single point of failure. Placing a Mediator between modules can also cause a performance hit as they are always communicating indirectly. Because of the nature of loose coupling, it's difficult to establish how a system might react by only looking at the broadcasts. That said, it's useful to remind ourselves that decoupled systems have a number of other benefits - if our modules communicated with each other directly, changes to modules e.
This problem is less of a concern with decoupled systems. At the end of the day, tight coupling causes all kinds of headaches and this is just another alternative solution, but one which can work very well if implemented correctly. We will be covering the Facade pattern shortly, but for reference purposes some developers may also wonder whether there are similarities between the Mediator and Facade patterns.
They do both abstract the functionality of existing modules, but there are some subtle differences. The Mediator centralizes communication between modules where it's explicitly referenced by these modules. In a sense this is multidirectional. The Facade however just defines a simpler interface to a module or system but doesn't add any additional functionality.
Other modules in the system aren't directly aware of the concept of a facade and could be considered unidirectional. The GoF refer to the prototype pattern as one which creates objects based on a template of an existing object through cloning. We can think of the prototype pattern as being based on prototypal inheritance where we create objects which act as prototypes for other objects. The prototype object itself is effectively used as a blueprint for each object the constructor creates.
With other design patterns, this isn't always the case. Not only is the pattern an easy way to implement inheritance, but it can also come with a performance boost as well: For those interested, real prototypal inheritance, as defined in the ECMAScript 5 standard, requires the use of Object. To remind ourselves, Object. We saw earlier that Object. For example:. Here the properties can be initialized on the second argument of Object. It is worth noting that prototypal relationships can cause trouble when enumerating properties of objects and as Crockford recommends wrapping the contents of the loop in a hasOwnProperty check.
If we wish to implement the prototype pattern without directly using Object. This alternative does not allow the user to define read-only properties in the same manner as the vehiclePrototype may be altered if not careful. One could reference this method from the vehicle function.
Note, however that vehicle here is emulating a constructor, since the prototype pattern does not include any notion of initialization beyond linking an object to a prototype. The Command pattern aims to encapsulate method invocation, requests or operations into a single object and gives us the ability to both parameterize and pass method calls around that can be executed at our discretion.
In addition, it enables us to decouple objects invoking the action from the objects which implement them, giving us a greater degree of overall flexibility in swapping out concrete classes objects. Concrete classes are best explained in terms of class-based programming languages and are related to the idea of abstract classes. An abstract class defines an interface, but doesn't necessarily provide implementations for all of its member functions.
It acts as a base class from which others are derived. A derived class which implements the missing functionality is called a concrete class. The general idea behind the Command pattern is that it provides us a means to separate the responsibilities of issuing commands from anything executing commands, delegating this responsibility to different objects instead.
Implementation wise, simple command objects bind together both an action and the object wishing to invoke the action. They consistently include an execution operation such as run or execute.
There are however scenarios where this may be disadvantageous. For example, imagine if the core API behind the carManager changed. This would require all objects directly accessing these methods within our application to also be modified. This could be viewed as a layer of coupling which effectively goes against the OOP methodology of loosely coupling objects as much as possible.
Instead, we could solve this problem by abstracting the API away further. Let's now expand on our carManager so that our application of the Command pattern results in the following: As per this structure we should now add a definition for the carManager. When we put up a facade, we present an outward appearance to the world which may conceal a very different reality. This was the inspiration for the name behind the next pattern we're going to review - the Facade pattern.
This allows us to interact with the Facade directly rather than the subsystem behind the scenes. The jQuery core methods should be considered intermediate abstractions.
To build on what we've learned, the Facade pattern both simplifies the interface of a class and it also decouples the class from the code that utilizes it. This gives us the ability to indirectly interact with subsystems in a way that can sometimes be less prone to error than accessing the subsystem directly. A Facade's advantages include ease of use and often a small size-footprint in implementing the pattern. This is an unoptimized code example, but here we're utilizing a Facade to simplify an interface for listening to events cross-browser.
Internally, this is actually being powered by a method called bindReady , which is doing this:. Facades don't just have to be used on their own, however. They can also be integrated with other patterns such as the Module pattern. As we can see below, our instance of the module patterns contains a number of methods which have been privately defined. A Facade is then used to supply a much simpler API to accessing these methods:.
In this example, calling module. Facades generally have few disadvantages, but one concern worth noting is performance. Namely, one must determine whether there is an implicit cost to the abstraction a Facade offers to our implementation and if so, whether this cost is justifiable. Did you know however that getElementById on its own is significantly faster by a high order of magnitude?
Take a look at this jsPerf test to see results on a per-browser level: Now of course, we have to keep in mind that jQuery and Sizzle - its selector engine are doing a lot more behind the scenes to optimize our query and that a jQuery object, not just a DOM node is returned.
The challenge with this particular Facade is that in order to provide an elegant selector function capable of accepting and parsing multiple types of queries, there is an implicit cost of abstraction. The user isn't required to access jQuery. That said, the trade-off in performance has been tested in practice over the years and given the success of jQuery, a simple Facade actually worked out very well for the team. When using the pattern, try to be aware of any performance costs involved and make a call on whether they are worth the level of abstraction offered.
The Factory pattern is another creational pattern concerned with the notion of creating objects. Where it differs from the other patterns in its category is that it doesn't explicitly require us to use a constructor. Instead, a Factory can provide a generic interface for creating objects, where we can specify the type of factory object we wish to be created. Imagine that we have a UI factory where we are asked to create a type of UI component. Rather than creating this component directly using the new operator or via another creational constructor, we ask a Factory object for a new component instead.
We inform the Factory what type of object is required e. This is particularly useful if the object creation process is relatively complex, e. Examples of this pattern can be found in UI libraries such as ExtJS where the methods for creating objects or components may be further subclassed. The following is an example that builds upon our previous snippets using the Constructor pattern logic to define cars. It demonstrates how a Vehicle Factory may be implemented using the Factory pattern:.
Car object of color "yellow", doors: Modify a VehicleFactory instance to use the Truck class. Approach 2: Subclass VehicleFactory to create a factory class that builds Trucks.
The Factory pattern can be especially useful when applied to the following situations: When our object or component setup involves a high level of complexity When we need to easily generate different instances of objects depending on the environment we are in When we're working with many small objects or components that share the same properties When composing objects with instances of other objects that need only satisfy an API contract aka, duck typing to work.
This is useful for decoupling. When applied to the wrong type of problem, this pattern can introduce an unnecessarily great deal of complexity to an application. Unless providing an interface for object creation is a design goal for the library or framework we are writing, I would suggest sticking to explicit constructors to avoid the unnecessary overhead.
Due to the fact that the process of object creation is effectively abstracted behind an interface, this can also introduce problems with unit testing depending on just how complex this process might be.
It is also useful to be aware of the Abstract Factory pattern, which aims to encapsulate a group of individual factories with a common goal.
It separates the details of implementation of a set of objects from their general usage. An Abstract Factory should be used where a system must be independent from the way the objects it creates are generated or it needs to work with multiple types of objects. An example which is both simple and easier to understand is a vehicle factory, which defines ways to get or register vehicles types.
The abstract factory can be named abstractVehicleFactory. The Abstract factory will allow the definition of types of vehicle like "car" or "truck" and concrete factories will implement only classes that fulfill the vehicle contract e. For developers unfamiliar with sub-classing, we will go through a brief beginners primer on them before diving into Mixins and Decorators further.
Sub-classing is a term that refers to inheriting properties for a new object from a base or superclass object. In traditional object-oriented programming, a class B is able to extend another class A.
Here we consider A a superclass and B a subclass of A. As such, all instances of B inherit the methods from A. B is however still able to define its own methods, including those that override methods originally defined by A. Should B need to invoke a method in A that has been overridden, we refer to this as method chaining. Should B need to invoke the constructor A the superclass , we call this constructor chaining.
In order to demonstrate sub-classing, we first need a base object that can have new instances of itself created. Next, we'll want to specify a new class object that's a subclass of the existing Person object. Let us imagine we want to add distinct properties to distinguish a Person from a Superhero whilst inheriting the properties of the Person "superclass".
As superheroes share many common traits with normal people e. The Superhero constructor creates an object which descends from Person. Objects of this type have attributes of the objects that are above it in the chain and if we had set default values in the Person object, Superhero is capable of overriding any inherited values with values specific to it's object.
Imagine that we define a Mixin containing utility functions in a standard object literal as follows:. We can then easily extend the prototype of existing constructor functions to include this behavior using a helper such as the Underscore.
As we can see, this allows us to easily "mix" in common behaviour into object constructors fairly trivially. In the next example, we have two constructors: What we're going to do is augment another way of saying extend the Car so that it can inherit specific methods defined in the Mixin, namely driveForward and driveBackward. This time we won't be using Underscore. Instead, this example will demonstrate how to augment a constructor to include functionality without the need to duplicate this process for every constructor function we may have.
Mixins assist in decreasing functional repetition and increasing function re-use in a system. Where an application is likely to require shared behaviour across object instances, we can easily avoid any duplication by maintaining this shared functionality in a Mixin and thus focusing on implementing only the functionality in our system which is truly distinct.
That said, the downsides to Mixins are a little more debatable. Some developers feel that injecting functionality into an object prototype is a bad idea as it leads to both prototype pollution and a level of uncertainty regarding the origin of our functions.
In large systems this may well be the case. I would argue that strong documentation can assist in minimizing the amount of confusion regarding the source of mixed in functions, but as with every pattern, if care is taken during implementation we should be okay.
Decorators are a structural design pattern that aim to promote code re-use. Similar to Mixins, they can be considered another viable alternative to object sub-classing. Classically, Decorators offered the ability to add behaviour to existing classes in a system dynamically.
The idea was that the decoration itself wasn't essential to the base functionality of the class, otherwise it would be baked into the superclass itself. They can be used to modify existing systems where we wish to add additional features to objects without the need to heavily modify the underlying code using them.
The object constructors could represent distinct player types, each with differing capabilities. If we then factored in capabilities, imagine having to create sub-classes for each combination of capability type e. Design Pattern Examples Each of the design patterns represents a specific type of solution to a specific type of problem. There is no universal set of patterns that is always the best fit. We need to learn when a particular pattern will prove useful and whether it will provide actual value.
Once we are familiar with the patterns and scenarios they are best suited for, we can easily determine whether or not a specific pattern is a good fit for a given problem. Remember, applying the wrong pattern to a given problem could lead to undesirable effects such as unnecessary code complexity, unnecessary overhead on performance, or even the spawning of a new anti-pattern.
They help us mimic the behavior of access modifiers through scoping. Since we cannot access the counter variable from outside of the function expression, we made it private through scoping manipulation. Using the closures, we can create objects with private and public parts. These are called modules and are very useful whenever we want to hide certain parts of an object and only expose an interface to the user of the module. However, not everything is so perfect. When you wish to change the visibility of a member, you need to modify the code wherever you have used this member because of the different nature of accessing public and private parts.
Also, methods added to the object after their creation cannot access the private members of the object. Revealing Module Pattern This pattern is an improvement made to the module pattern as illustrated above. The main difference is that we write the entire object logic in the private scope of the module and then simply expose the parts we want to be public by returning an anonymous object.
We can also change the naming of private members when mapping private members to their corresponding public members.