You need to communicate with the server to retrieve the model from the server, and to put the updates to the model on the server. These libraries typically should provide a higher level of abstraction than xhr (or even $.ajax). Typically they should handle
- The model serialization / de-serialization (ex: date format issues). Most of the community as settled on JSON as the default format here.
- Protocol used to communicate with the server (REST / DDP – See below)
- Provide a higher level of abstraction to deal with asynchronous ajax calls like jQuery’s Deferred / Promise abstraction.
- Change tracking?
- Isolate and abstract the server communication from the rest of the application modules.
In the Requirement Specific Parameters post I talked about live / real-time updates. Meteor implements real-time updates on top of a protocol called Distributed Data Protocol (DDP). This alleviates us from a need to poll the server for updates. Other frameworks like Derby also support this real-time updates, they call it Subscriptions. I am hoping in the future everyone will settle on a standard protocol.
Some of my notes from links given below:
Explicit URL Design
URL Design is the process of designing the structure of your application’s URL. Do not leave URL Design to the framework / library you are using. Make an explicit choice about the URLs used by your application.
In a Single Page Application you want the URL to change as you navigate between views. This will provide the application the ability to deep link into specific parts of the application. Routing is the process of selecting a view based on the URL.
In a Single Page Application, when the URL changes you do not want to go the server to get the page when the URL changes. This is achieved typically by using location hashes in the URLs. Location hashes are supported in all the browsers, but they are not understood by the server.
When when the Search Engine Bot sends a Hash url to the server the server the server ignores the hash and will not be returning the right page. People workaround this by using a hack called hash bangs that is supported by Google’s Search bot. Hash bangs have been criticized by several Smart people and are not recommended.
HTML5 History API – pushState API
The alternative that is acceptable is HTML5 History – pushState based URLs. This allow to server pages in the client without going to the server on a URL change. But the issue with HTML5 history – pushState API is that it is not supported in all the browsers. So the routing engine / history library you select should be a Polyfill for unsupported browsers.
What ever polyfill hack / workaround the client-side library uses needs to be understood by the Web Server and the right pages should be served for SEO scenarios.
Traditionally these routing engine implementations were inspired their counterparts on the server-side. Server-side is a stateless environment so these routing engines on the server were designed with that limitation in mind. The client-side on the other hand is a different story where you can leverage the stateful nature. Ember’s routing library implements a stateful routing. It is something to keep an eye on.
Can you deep link to specific views in your Single Page Application?
If you can deep link are the URLs SEO friendly, are they readable?
Should you use location hashes which are supported in pretty much all browsers or should you use HTML5 Push state based navigation bowser support for which is still poor?
Do you need SEO? SEO requirement has an impact on location hashes. Can you avoid Hash bangs if you choose the right routing / navigation / history library?
What is a Template Engine?
- DOM based, DSL based (ex: Angular, Knockout)
- DOM based, Non DSL – (ex: Future browsers that will natively support DOM templating)
- String based, DSL based (ex: Handlebars, Mustache, dustjs)
- String based, Non DSL – (Plain Old Html ex: Plates)
Why is the categorization relevant?
- Using a DSL based template engine is not so user interface designer friendly, these templates are no longer Html.
- Using a DOM based engine makes it difficult to execute the template on the server-side. These factors have a significant impact on template engine selection.
I have written about some requirement specific parameters here, that are problematic if we use a client-side template engine. For the cases where we need
- SEO / Accessibility / Progressive Enhancements
- Faster initial load times
a workable solution available today is to
- Detect these scenarios
- Render the pages on the server-side rather than on the client-side.
Of course we do not want to duplicate the templates once for the server and once for the client. So for these scenarios we need a template that is isomorphic (can execute both on the server and client).
A string based template engine is the best choice for the scenario, But if you use a DOM based template engine it is still not the end of the road, there are server-side DOM simulations available.
Keep in mind
- These are DOM simulations are not real browser DOMs
- And are bleeding edge
Template engine performance
Template engine performance has a context associated to it. That is where the template is rendered.
On the client
- String based: string => template engine => string => browser => DOM
- DOM based: DOM => template engine => DOM
On the server
- String based: string => template engine => string
- DOM based: string => DOM Simulation => DOM => template engine => string Of these operations string => DOM is an expensive operation. So in theory string based template engines are faster on the server and DOM based template engines are faster on the browser. But its good to actually measure it.
The other point to keep in mind is when considering a string based template engine on the client we also need to take into account the time taken to load the template output into the DOM.
String based: string => template engine => string => browser => DOM
Notes summarized from
Updates / Refresh
Another feature that you may want to consider when you evaluate a template engine is whether the template engine can update the views automatically when the (view )model changes. Can the template engine re-render when the model changes and can that be localized to partial portions of the view or the entire view has to be refreshed. Different libraries use different terminology here, Spark – Meteor uses the term Live Page Update, Ember calls this Auto updating templates.
- Use a DOM-based template engine if SEO / Accessibility readers / Progressive enhancement are not a concern & you do not need server-side rendering. Ex: Knockout
- When both client & server-side rendering are required choose a string based template engine. Ex: Handlebars / dustjs.
- Even if you use a string based template engine it is worthwhile to keep a watch on the following categories. DOM based template engines (Angular, Knockout). Non DSL based template engine that lets you write plain old html markup and uses standard json (Plates).
Logic in templates & Reuse
- If you come from a server-side scripting environment you may be familiar with logic getting embedded in the templates. Logic-less templates are an answer to this. DOM-based template engines are generally not Turing-Complete Programming Languages so it is hard to embed logic in DOM-based templates
- If you come from ASP.NET Web Forms side you may be familiar with the Controls model which offers template reuse at control / widget level. Angular has support for something like this, they call it as directives.
The marriage of Html and your (view )model data does not end with template engines alone, because it is just one side of the story. A traditional template engine is usually about getting your data on to the browser as Html, what about the other way round. When the user inputs a value in the browser can the template engine update your model back. This called bi-directional data-binding. Some people use the term data-linking.
Many of the DOM based template engines support this bi-directional data-binding out of the box. Some of the string based template engines also support this (Ember for example implements this feature on top of handlebars).
This feature is particularly useful if your application is input intensive. Read more about Input intensive applications here.
No discussion about UI Architectural patterns is complete without reading / linking to Martin Fowler’s GUI Architectures & Michael Feather’s The Humble Dialog Box. I will quote extensively from Fowler’s work here, to drive point. If you know MVC & Presentation Model (or MVVM) really well, You can jump directly to the MVC vs. MVVM section
Separation of Concerns
These UI Architectural patterns are all about Separation of Concerns and having more maintainable code – Divide and Conquer
Model View Controller (MVC)
- In MVC, the domain element is referred to as the model. Model objects are completely ignorant of the UI.
- Make a strong separation between presentation (view & controller) and domain (model) –Separated Presentation.
- Divide GUI widgets into a controller (for reacting to user stimulus) and view (for displaying the state of the model). Controller and view should (mostly) not communicate directly but through the model.
- Have views (and controllers) observe the model to allow multiple widgets to update without needed to communicate directly – Observer Synchronization.
(Presentation Model / MVVM)
The essence of a Presentation Model is of a fully self-contained class that represents all the data and behavior of the UI window, but without any of the controls used to render that UI on the screen. A view then simply projects the state of the presentation model onto the glass.
To do this the Presentation Model will have data fields for all the dynamic information of the view. This won’t just include the contents of controls, but also things like whether or not they are enabled. In general the Presentation Model does not need to hold all of this control state (which would be lot) but any state that may change during the interaction of the user. So if a field is always enabled, there won’t be extra data for its state in the Presentation Model.
Since the Presentation Model contains data that the view needs to display the controls you need to synchronize the Presentation Model with the view. This synchronization usually needs to be tighter than synchronization with the domain – screen synchronization is not sufficient, you’ll need field or key synchronization.
Presentation Model is not a GUI friendly facade to a specific domain object. Instead it is easier to consider Presentation Model as an abstract of the view that is not dependent on a specific GUI framework.
MVC vs. MVVM
- MVC is about separating business logic (Model) from the presentation logic (View and Controller) – Separation of Concerns
- It opens up the possibility of having multiple presentations technologies on the same model.
MVVM / Presentation Model
While MVC addressed separation of Business Concerns from that of the Presentation, It did not focus much on Presentation Logic. The Presentation Logic in modern applications is significantly complex. Presentation Model puts additional emphasis on organizing presentation logic (which was just a high level guidance in original MVC – Controllers handle user input and View handle display & display logic. In much of the modern UI Toolkit implementations User Input is sucked into the toolkit itself).
The move to Presentation Model came when the presentation logic in the views (and controllers) became more complex and testability of that logic became a primary concern. Read the Humble Dialog Box article if you haven’t read it. So Presentation Model is all about moving the presentation logic from the View to the View Models (not as is but in an abstract way that is UI framework independent and becomes unit testable) so that the View becomes really dumb / humble.
But it does not come free, along with moving the Presentation Logic from the view we also moved the Presentation State from the View and the View now needs to synchronize with that. This synchronization can be a pain in environments that do not support data-binding.
- MVVM is about separating business logic (Model) from the presentation logic (View and View Model) – Separation of Concerns
- But it also addresses other concerns,
- Having near zero presentation logic & state in the views by moving them to view models
- Having a view model that is a presentation technology agnostic – UI platform-independent abstraction of a View – Josh Smith’s MSDN Article on MVVM
- Having testable presentation logic in view models
MVVM makes much more sense if
- You are towards near zero presentation logic
- You are towards Unit Testable presentation logic
- You are towards two-way data-binding and have a presentation technology that supports it
Some people hate data-binding and its natural for them to use MVC.
- I am biased towards MVVM and all for two way data binding (I also happen to have worked on Windows Presentation Foundation – WPF). So it is a personal preference.
I read this quote somewhere, I cannot recall, But it best echoes the sentiments about modular applications.
The best way to write a large-scale application is by not writing one, instead choose to implement small rightly coupled modules
- Do not pollute the global namespace
- Avoid namespace collisions
- Load modules fast (even out of order, given the asynchronous nature of the browser)
- Execute / Evaluate modules in the right order
- Asynchronous Module Definition format (AMD)
- CommonJS (CJS) module format
- Node (though this is not a standard you can consider this as a slightly modified version of CJS. I hope CJS adopts the additional features from Node)
- write modules that could execute in a server environment in CJS
- write modules that could benefit from AMD’s browser-friendly features in AMD format
What if you need a module to executed in both client and server? Use requirejs to run AMD in node instead of using Node’s inbuilt module loader and module format? The solutions like onejs / browserify seem like too much magic to me.
- Unless you take special care everything ends up as Globals
UI Architectural Patterns
The other thing about User Interface logic is that it is the most difficult to organize. Architectural patterns can help here. Separated Presentation patterns help you separate presentation logic from the business logic (ex: MVC) and provide a way to add structure to your application
So it is recommended that you use write
- Use a MV* framework that lets you implement one of the popular UI Architectural patterns
otherwise you application would end up as a “big ball of mud”.