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Contribute to dieuph/SE__Object_Oriented_Software_Development development by creating an account on GitHub. An ebook on design patterns and the principles behind them .. The book will be available to you in PDF, ePub and Mobi formats immediately upon purchase. Modern C++ Design: Generic Programming and Design Patterns Applied. Addison-. Wesley, Design Patterns Explained Simply.
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Design patterns were created as a bible for avoiding problems related to software design. be available to you in PDF. book Design Patterns Explained Simply;. Put simply, design patterns help a designer get a design "right" faster. None of the design patterns in this book describes new or unproven designs. We. A design pattern is a time tested solution to a common software problem. Patterns enable a common design vocabulary, improving communication, easing.
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You'll be referring to this for years to come. Design Pattern Videos Create objects on demand yet keep that process abstracted away for easy testing. 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.
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.
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.
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.
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. Both the event aggregator and mediator use events, in the above examples.
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.
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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.
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. All Command objects with the same interface can easily be swapped as needed and this is considered one of the larger benefits of the pattern.
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 pattern provides a convenient higher-level interface to a larger body of code, hiding its true underlying complexity.
Think of it as simplifying the API being presented to other developers, something which almost always improves usability.
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.
Each new object we define has a prototype from which it can inherit further properties. Prototypes can inherit from other object prototypes but, even more importantly, can define properties for any number of object instances. We can leverage this fact to promote function re-use.
They can be viewed as objects with attributes and methods that can be easily shared across a number of other object prototypes. 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.
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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. A common reason why developers use them is their applications may contain features requiring a large quantity of distinct types of object.
This type of simplistic implementation is functional, but it doesn't really demonstrate all of the strengths Decorators have to offer. For this, we're first going to go through my variation of the Coffee example from an excellent book called Head First Design Patterns by Freeman, Sierra and Bates, which is modeled around a Macbook purchase.
In the above example, our Decorators are overriding the MacBook super-class objects. It's considered a decoration as the original Macbook objects constructor methods which are not overridden e. There isn't really a defined interface in the above example and we're shifting away the responsibility of ensuring an object meets an interface when moving from the creator to the receiver. This particular variation of the Decorator pattern is provided for reference purposes. If finding it overly complex, I recommend opting for one of the simpler implementations covered earlier.
Lightweight interfaces can be used without a great performance cost however and we will next look at Abstract Decorators using this same concept.
To demonstrate the structure of this version of the Decorator pattern, we're going to imagine we have a superclass that models a Macbook once again and a store that allows us to "decorate" our Macbook with a number of enhancements for an additional fee. Now if we were to model this using an individual sub-class for each combination of enhancement options, it might look something like this:. This would be an impractical solution as a new subclass would be required for every possible combination of enhancements that are available.
As we would prefer to keep things simple without maintaining a large set of subclasses, let's look at how decorators may be used to solve this problem better. Rather than requiring all of the combinations we saw earlier, we should simply have to create five new decorator classes.
Methods that are called on these enhancement classes would be passed on to our Macbook class. In our next example, decorators transparently wrap around their components and can interestingly be interchanged as they use the same interface. To make it easier for us to add as many more options as needed later on, an Abstract Decorator class is defined with default methods required to implement the Macbook interface, which the rest of the options will sub-class. Abstract Decorators ensure that we can decorate a base class independently with as many decorators as needed in different combinations remember the example earlier?
What's happening in the above sample is that the Macbook Decorator accepts an object a Macbook to use as our base component. It's using the Macbook interface we defined earlier and for each method is just calling the same method on the component. We can now create our option classes for what can be added, just by using the Macbook Decorator.Below we can see an example of a very simplistic model implemented using Backbone. While these are details, they make the information much more readable and enhances the whole experience.
Thanks for the great material! Mixins assist in decreasing functional repetition and increasing function re-use in a system. I am now preparing a report on Design Patterns at work. For example, I have heard the command pattern can be used to radio groups or for writing wizards. Solicited by a view, presenters perform any work to do with user requests and pass data back to them. What makes a difference between an expert and a novice?
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