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A real-world analogy could be a typical airport traffic control system. My Collection. Imagine that we have a UI factory where we are asked to create a type of UI component. Composite A structure of simple and composite objects which makes the total object more than just the sum of its parts. It's considered a decoration as the original Macbook objects constructor methods which are not overridden e.
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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. 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. Taking a look at the above code, it would be trivial to invoke our carManager methods by directly accessing the object.
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 pattern provides a convenient higher-level interface to a larger body of code, hiding its true underlying complexity.
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. This isn't very practical and certainly isn't manageable when we factor in a growing number of different abilities. The Decorator pattern isn't heavily tied to how objects are created but instead focuses on the problem of extending their functionality.
Rather than just relying on prototypal inheritance, we work with a single base object and progressively add decorator objects which provide the additional capabilities. The idea is that rather than sub-classing, we add decorate properties or methods to a base object so it's a little more streamlined.
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. What we're doing here is overriding the addCase and getPrice methods that need to be decorated and we're achieving this by first calling these methods on the original macbook and then simply appending a string or numeric value e.
As there's been quite a lot of information presented in this section so far, let's try to bring it all together in a single example that will hopefully highlight what we have learned. As decorators are able to modify objects dynamically, they're a perfect pattern for changing existing systems. Occasionally, it's just simpler to create decorators around an object versus the trouble of maintaining individual sub-classes for each object type.
This makes maintaining applications that may require a large number of sub-classed objects significantly more straight-forward. A functional version of this example can be found on JSBin.
As with other patterns we've covered, there are also examples of the Decorator pattern that can be implemented with jQuery. In the following example, we define three objects: The aim of the task is to decorate the defaults object with additional functionality found in options settings.
We must:. Developers enjoy using this pattern as it can be used transparently and is also fairly flexible - as we've seen, objects can be wrapped or "decorated" with new behavior and then continue to be used without needing to worry about the base object being modified. In a broader context, this pattern also avoids us needing to rely on large numbers of subclasses to get the same benefits. There are however drawbacks that we should be aware of when implementing the pattern.
If poorly managed, it can significantly complicate our application architecture as it introduces many small, but similar objects into our namespace. The concern here is that in addition to becoming hard to manage, other developers unfamiliar with the pattern may have a hard time grasping why it's being used. Sufficient commenting or pattern research should assist with the latter, however as long as we keep a handle on how widespread we use the decorator in our applications we should be fine on both counts.
The Flyweight pattern is a classical structural solution for optimizing code that is repetitive, slow and inefficiently shares data.
It aims to minimize the use of memory in an application by sharing as much data as possible with related objects e. The pattern was first conceived by Paul Calder and Mark Linton in and was named after the boxing weight class that includes fighters weighing less than lb.
The name Flyweight itself is derived from this weight classification as it refers to the small weight memory footprint the pattern aims to help us achieve. In practice, Flyweight data sharing can involve taking several similar objects or data constructs used by a number of objects and placing this data into a single external object.
We can pass through this object to those depending on this data, rather than storing identical data across each one. There are two ways in which the Flyweight pattern can be applied. The first is at the data-layer, where we deal with the concept of sharing data between large quantities of similar objects stored in memory. The second is at the DOM-layer where the Flyweight can be used as a central event-manager to avoid attaching event handlers to every child element in a parent container we wish to have some similar behavior.
As the data-layer is where the flyweight pattern is most used traditionally, we'll take a look at this first. For this application, there are a few more concepts around the classical Flyweight pattern that we need to be aware of.
In the Flyweight pattern there's a concept of two states - intrinsic and extrinsic. Intrinsic information may be required by internal methods in our objects which they absolutely cannot function without. Extrinsic information can however be removed and stored externally. Objects with the same intrinsic data can be replaced with a single shared object, created by a factory method. This allows us to reduce the overall quantity of implicit data being stored quite significantly.
The benefit of this is that we're able to keep an eye on objects that have already been instantiated so that new copies are only ever created should the intrinsic state differ from the object we already have.
Flyweight corresponds to an interface through which flyweights are able to receive and act on extrinsic states Concrete Flyweight actually implements the Flyweight interface and stores intrinsic state. Concrete Flyweights need to be sharable and capable of manipulating state that is extrinsic Flyweight Factory manages flyweight objects and creates them too. It makes sure that our flyweights are shared and manages them as a group of objects which can be queried if we require individual instances.
If an object has been already created in the group it returns it, otherwise it adds a new object to the pool and returns it. Flyweight CoffeeFlavor: Concrete Flyweight CoffeeOrderContext: Helper CoffeeFlavorFactory: Flyweight Factory testFlyweight: Utilization of our Flyweights.
Next, let's continue our look at Flyweights by implementing a system to manage all of the books in a library. The important meta-data for each book could probably be broken down as follows:. We'll also require the following properties to keep track of which member has checked out a particular book, the date they've checked it out on as well as the expected date of return. Each book would thus be represented as follows, prior to any optimization using the Flyweight pattern:. This probably works fine initially for small collections of books, however as the library expands to include a larger inventory with multiple versions and copies of each book available, we may find the management system running slower and slower over time.
Using thousands of book objects may overwhelm the available memory, but we can optimize our system using the Flyweight pattern to improve this. We can now separate our data into intrinsic and extrinsic states as follows: Effectively this means that only one Book object is required for each combination of book properties. The following single instance of our book meta-data combinations will be shared among all of the copies of a book with a particular title.
As we can see, the extrinsic states have been removed. Everything to do with library check-outs will be moved to a manager and as the object data is now segmented, a factory can be used for instantiation. Let's now define a very basic factory. What we're going to have it do is perform a check to see if a book with a particular title has been previously created inside the system; if it has, we'll return it - if not, a new book will be created and stored so that it can be accessed later.
This makes sure that we only create a single copy of each unique intrinsic piece of data:. Next, we need to store the states that were removed from the Book objects somewhere - luckily a manager which we'll be defining as a Singleton can be used to encapsulate them. Combinations of a Book object and the library member that's checked them out will be called Book records. Our manager will be storing both and will also include checkout related logic we stripped out during our flyweight optimization of the Book class.
The result of these changes is that all of the data that's been extracted from the Book class is now being stored in an attribute of the BookManager singleton BookDatabase - something considerably more efficient than the large number of objects we were previously using. Methods related to book checkouts are also now based here as they deal with data that's extrinsic rather than intrinsic.
This process does add a little complexity to our final solution, however it's a small concern when compared to the performance issues that have been tackled. Data wise, if we have 30 copies of the same book, we are now only storing it once. Also, every function takes up memory.
With the flyweight pattern these functions exist in one place on the manager and not on every object, thus saving on memory use. For the above-mentioned flyweight unoptimized version we store just link to the function object as we used Book constructor's prototype but if it was implemented in other way, functions would be created for every book instance.
The DOM Document Object Model supports two approaches that allow objects to detect events - either top down event capture or bottom up event bubbling. In event capture, the event is first captured by the outer-most element and propagated to the inner-most element. In event bubbling, the event is captured and given to the inner-most element and then propagated to the outer-elements. One of the best metaphors for describing Flyweights in this context was written by Gary Chisholm and it goes a little like this:.
Try to think of the flyweight in terms of a pond. A fish opens its mouth the event , bubbles rise to the surface the bubbling a fly sitting on the top flies away when the bubble reaches the surface the action.
In this example we can easily transpose the fish opening its mouth to a button being clicked, the bubbles as the bubbling effect and the fly flying away to some function being run.
Bubbling was introduced to handle situations where a single event e. Where this happens, event bubbling executes event handlers defined for specific elements at the lowest level possible. From there on, the event bubbles up to containing elements before going to those even higher up.
For our first practical example, imagine we have a number of similar elements in a document with similar behavior executed when a user-action e. Normally what we do when constructing our own accordion component, menu or other list-based widget is bind a click event to each link element in the parent container e. Instead of binding the click to multiple elements, we can easily attach a Flyweight to the top of our container which can listen for events coming from below.
These can then be handled using logic that is as simple or complex as required. As the types of components mentioned often have the same repeating markup for each section e. We'll use this information to construct a very basic accordion using the Flyweight below. A stateManager namespace is used here to encapsulate our flyweight logic whilst jQuery is used to bind the initial click to a container div. In order to ensure that no other logic on the page is attaching similar handles to the container, an unbind event is first applied.
Now to establish exactly what child element in the container is clicked, we make use of a target check which provides a reference to the element that was clicked, regardless of its parent.
We then use this information to handle the click event without actually needing to bind the event to specific children when our page loads. The benefit here is that we're converting many independent actions into a shared ones potentially saving on memory.
In our second example, we'll reference some further performance gains that can be achieved using Flyweights with jQuery. James Padolsey previously wrote an article called 76 bytes for faster jQuery where he reminded us that each time jQuery fires off a callback, regardless of type filter, each, event handler , we're able to access the function's context the DOM element related to it via the this keyword.
James had wanted to use jQuery's jQuery. Now with respect to redundant wrapping, where possible with jQuery's utility methods, it's better to use jQuery. This avoids the need to call a further level of abstraction or construct a new jQuery object each time our function is called as as jQuery. Because however not all of jQuery's methods have corresponding single-node functions, Padolsey devised the idea of a jQuery.
It's important to understand what the original MVC pattern was aiming to solve as it's mutated quite heavily since the days of its origin. Back in the 70's, graphical user-interfaces were few and far between and a concept known as Separated Presentation began to be used as a means to make a clear division between domain objects which modeled concepts in the real world e.
The Smalltalk implementation of MVC took this concept further and had an objective of separating out the application logic from the user interface. The idea was that decoupling these parts of the application would also allow the reuse of models for other interfaces in the application.
There are some interesting points worth noting about Smalltalk's MVC architecture:. As mentioned in the bullet point above, anytime the Model changes, the Views react.
A simple example of this is an application backed by stock market data - in order for the application to be useful, any change to the data in our Models should result in the View being refreshed instantly. Martin Fowler has done an excellent job of writing about the origins of MVC over the years and if interested in some further historical information about Smalltalk's MVC, I recommend reading his work. We've reviewed the 70's, but let us now return to the here and now. In modern times, the MVC pattern has been applied to a diverse range of programming languages including of most relevance to us: These frameworks include the likes of Backbone, Ember.
Below we can see an example of a very simplistic model implemented using Backbone. The built-in capabilities of models vary across frameworks, however it is quite common for them to support validation of attributes, where attributes represent the properties of the model, such as a model identifier.
When using models in real-world applications we generally also desire model persistence.
Persistence allows us to edit and update models with the knowledge that its most recent state will be saved in either: In addition, a model may also have multiple views observing it. If say, our photo model contained meta-data such as its location longitude and latitude , friends that were present in the photo a list of identifiers and a list of tags, a developer may decide to provide a single view to display each of these three facets.
Design pattern literature commonly refers to views as "dumb" given that their knowledge of models and controllers in an application is limited. Users are able to interact with views and this includes the ability to read and edit i. As the view is the presentation layer, we generally present the ability to edit and update in a user-friendly fashion. For example, in the former photo gallery application we discussed earlier, model editing could be facilitated through an "edit' view where a user who has selected a specific photo could edit its meta-data.
One may wonder where user-interaction comes into play here. When users click on any elements within the view, it's not the view's responsibility to know what to do next. It relies on a controller to make this decision for it. In our sample implementation, this is achieved by adding an event listener to photoEl which will delegate handling the click behavior back to the controller, passing the model information along with it in case it's needed.
The benefit of this architecture is that each component plays its own separate role in making the application function as needed. It has long been considered and proven a performance bad practice to manually create large blocks of HTML markup in-memory through string concatenation.
Developers doing so have fallen prey to inperformantly iterating through their data, wrapping it in nested divs and using outdated techniques such as document. As this typically means keeping scripted markup inline with our standard markup, it can quickly become both difficult to read and more importantly, maintain such disasters, especially when building non-trivially sized applications.
The role of navigation thus falls to a "router", which assists in managing application state e. As routers are, however, neither a part of MVC nor present in every MVC-like framework, I will not be going into them in greater detail in this section.
Controllers are an intermediary between models and views which are classically responsible for updating the model when the user manipulates the view. In our photo gallery application, a controller would be responsible for handling changes the user made to the edit view for a particular photo, updating a specific photo model when a user has finished editing. Remember that the controllers fulfill one role in MVC: In the Strategy pattern regard, the view delegates to the controller at the view's discretion.
So, that's how the strategy pattern works. The view could delegate handling user events to the controller when the view sees fit.
The reasons for this vary, but in my honest opinion, it is that framework authors initially look at the server-side interpretation of MVC, realize that it doesn't translate 1: The issue with this however is that it is subjective, increases the complexity in both understanding the classical MVC pattern and of course the role of controllers in modern frameworks.
As an example, let's briefly review the architecture of the popular architectural framework Backbone. Backbone contains models and views somewhat similar to what we reviewed earlier , however it doesn't actually have true controllers. Its views and routers act a little similar to a controller, but neither are actually controllers on their own.
We now know that controllers are traditionally responsible for updating the model when the user updates the view. It can thus be useful for us to review the controller from another MVC framework to appreciate the difference in implementations and further demonstrate how nontraditionally frameworks approach the role of the controller. For this, let's take a look at a sample controller from Spine.
In this example, we're going to have a controller called PhotosController which will be in charge of individual photos in the application. It will ensure that when the view updates e.
We won't be delving heavily into Spine. In Spine, controllers are considered the glue for an application, adding and responding to DOM events, rendering templates and ensuring that views and models are kept in sync which makes sense in the context of what we know to be a controller. What we're doing in the above example is setting up listeners in the update and destroy events using render and remove.
When a photo entry gets updated, we re-render the view to reflect the changes to the meta-data. Similarly, if the photo gets deleted from the gallery, we remove it from the view. What this provides us with is a very lightweight, simple way to manage changes between the model and the view.
Later on in this section we're going to revisit the differences between Backbone and traditional MVC, but for now let's focus on controllers. In Backbone, one shares the responsibility of a controller with both the Backbone. View and Backbone. Some time ago Backbone did once come with its own Backbone. Controller , but as the naming for this component didn't make sense for the context in which it was being used, it was later renamed to Router.
Routers handle a little more of the controller responsibility as it's possible to bind the events there for models and have our view respond to DOM events and rendering. As Tim Branyen another Bocoup-based Backbone contributor has also previously pointed out, it's possible to get away with not needing Backbone. Router at all for this, so a way to think about it using the Router paradigm is probably:. To summarize, the takeaway from this section is that controllers manage the logic and coordination between models and views in an application.