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Pixel Perfect Precision Handbook v. Designing for the Web By Mark Boulton A Practical Guide to Designing for the Web aims to teach you the techniques for designing your website using the principles of graphic design. Pay Me… Or Else! It covers tactics and tips that will help you recover your hard earned money and how to avoid similar situations in the future.
This usabilty guide sheds some light on some common interface elements and mistakes people often make with them. This is by no means a complete guide or solid set of rules, but it is definitely a good start.
For more information on dojo. For those using Sencha's ExtJS, an example demonstrating how to correctly use the Module pattern with the framework can be found below.
Oh, and thanks to David Engfer for the joke. The disadvantages of the Module pattern are that as we access both public and private members differently, when we wish to change visibility, we actually have to make changes to each place the member was used. We also can't access private members in methods that are added to the object at a later point.
That said, in many cases the Module pattern is still quite useful and when used correctly, certainly has the potential to improve the structure of our application. Other disadvantages include the inability to create automated unit tests for private members and additional complexity when bugs require hot fixes. It's simply not possible to patch privates.
Instead, one must override all public methods which interact with the buggy privates. Developers can't easily extend privates either, so it's worth remembering privates are not as flexible as they may initially appear.
For further reading on the Module pattern, see Ben Cherry's excellent in-depth article on it. The Revealing Module pattern came about as Heilmann was frustrated with the fact that he had to repeat the name of the main object when we wanted to call one public method from another or access public variables.
The result of his efforts was an updated pattern where we would simply define all of our functions and variables in the private scope and return an anonymous object with pointers to the private functionality we wished to reveal as public. The pattern can also be used to reveal private functions and properties with a more specific naming scheme if we would prefer:.
This pattern allows the syntax of our scripts to be more consistent. It also makes it more clear at the end of the module which of our functions and variables may be accessed publicly which eases readability. A disadvantage of this pattern is that if a private function refers to a public function, that public function can't be overridden if a patch is necessary. This is because the private function will continue to refer to the private implementation and the pattern doesn't apply to public members, only to functions.
Public object members which refer to private variables are also subject to the no-patch rule notes above. As a result of this, modules created with the Revealing Module pattern may be more fragile than those created with the original Module pattern, so care should be taken during usage.
The Singleton pattern 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.
Here, getInstance becomes a little like a Factory method and we don't need to update each point in our code accessing it. FooSingleton above would be a subclass of BasicSingleton and implement the same interface. It is important to note the difference between a static instance of a class object and a Singleton: 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.
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 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.
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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.
Consequently, some of the issues with these patterns actually stem from their main benefits. For example, publishers may make an assumption that one or more subscribers are listening to them.
Say that we're using such an assumption to log or output errors regarding some application process. If the subscriber performing the logging crashes or for some reason fails to function , the publisher won't have a way of seeing this due to the decoupled nature of the system.
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. Below we can see some examples of this:. Links to just a few of these can be found below. This demonstrates the core concepts of subscribe, publish as well as the concept of unsubscribing.
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.
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.
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.
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?
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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:What do I get with a Video? Its views and routers act a little similar to a controller, but neither are actually controllers on their own.
When a model is changed it notifies its observers Views that something has been updated - this is perhaps the most important relationship in MVC. 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.
It just fires the event and moves on. When a photo entry gets updated, we re-render the view to reflect the changes to the meta-data.