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When looking beyond the RAD (drag-drop and configure) way of building user interfaces that many tools encourage you are likely to come across three design patterns called Model-View-Controller, Model-View-Presenter and Model-View-ViewModel. My question has three parts to it:
In MVP, the Presenter contains the UI business logic for the View. All invocations from the View delegate directly to the Presenter. The Presenter is also decoupled directly from the View and talks to it through an interface. This is to allow mocking of the View in a unit test. One common attribute of MVP is that there has to be a lot of two-way dispatching. For example, when someone clicks the "Save" button, the event handler delegates to the Presenter's "OnSave" method. Once the save is completed, the Presenter will then call back the View through its interface so that the View can display that the save has completed.
MVP tends to be a very natural pattern for achieving separated presentation in WebForms. The reason is that the View is always created first by the ASP.NET runtime. You can find out more about both variants.
Passive View: The View is as dumb as possible and contains almost zero logic. A Presenter is a middle man that talks to the View and the Model. The View and Model are completely shielded from one another. The Model may raise events, but the Presenter subscribes to them for updating the View. In Passive View there is no direct data binding, instead, the View exposes setter properties that the Presenter uses to set the data. All state is managed in the Presenter and not the View.
Supervising Controller: The Presenter handles user gestures. The View binds to the Model directly through data binding. In this case, it's the Presenter's job to pass off the Model to the View so that it can bind to it. The Presenter will also contain logic for gestures like pressing a button, navigation, etc.
In the MVC, the Controller is responsible for determining which View to display in response to any action including when the application loads. This differs from MVP where actions route through the View to the Presenter. In MVC, every action in the View correlates with a call to a Controller along with an action. In the web, each action involves a call to a URL on the other side of which there is a Controller who responds. Once that Controller has completed its processing, it will return the correct View. The sequence continues in that manner throughout the life of the application:
Action in the View
-> Call to Controller
-> Controller Logic
-> Controller returns the View.
One other big difference about MVC is that the View does not directly bind to the Model. The view simply renders and is completely stateless. In implementations of MVC, the View usually will not have any logic in the code behind. This is contrary to MVP where it is absolutely necessary because, if the View does not delegate to the Presenter, it will never get called.
One other pattern to look at is the Presentation Model pattern. In this pattern, there is no Presenter. Instead, the View binds directly to a Presentation Model. The Presentation Model is a Model crafted specifically for the View. This means this Model can expose properties that one would never put on a domain model as it would be a violation of separation-of-concerns. In this case, the Presentation Model binds to the domain model and may subscribe to events coming from that Model. The View then subscribes to events coming from the Presentation Model and updates itself accordingly. The Presentation Model can expose commands which the view uses for invoking actions. The advantage of this approach is that you can essentially remove the code-behind altogether as the PM completely encapsulates all of the behavior for the view. This pattern is a very strong candidate for use in WPF applications and is also called Model-View-ViewModel.
There is a MSDN article about the Presentation Model and a section in the Composite Application Guidance for WPF (former Prism) about Separated Presentation Patterns
I blogged about this a while back, quoting on Todd Snyder's excellent post on the difference between the two:
Here are the key differences between the patterns:
MVP Pattern
- View is more loosely coupled to the model. The presenter is responsible for binding the model to the view.
- Easier to unit test because interaction with the view is through an interface
- Usually view to presenter map one to one. Complex views may have multi presenters.
MVC Pattern
- Controller are based on behaviors and can be shared across views
- Can be responsible for determining which view to display
It is the best explanation on the web I could find.
MVP is not necessarily a scenario where the View is in charge (see Taligent's MVP for example).
I find it unfortunate that people are still preaching this as a pattern (View in charge) as opposed to an anti-pattern as it contradicts "It's just a view" (Pragmatic Programmer). "It's just a view" states that the final view shown to the user is a secondary concern of the application. Microsoft's MVP pattern renders re-use of Views much more difficult and conveniently excuses Microsoft's designer from encouraging bad practice.
To be perfectly frank, I think the underlying concerns of MVC hold true for any MVP implementation and the differences are almost entirely semantic. As long as you are following separation of concerns between the view (that displays the data), the controller (that initialises and controls user interaction) and the model (the underlying data and/or services)) then you are achieving the benefits of MVC. If you are achieving the benefits then who really cares whether your pattern is MVC, MVP or Supervising Controller? The only real pattern remains as MVC, the rest are just differing flavours of it.
Consider this highly exciting article that comprehensively lists a number of these differing implementations. You may note that they're all basically doing the same thing but slightly differently.
I personally think MVP has only been recently re-introduced as a catchy term to either reduce arguments between semantic bigots who argue whether something is truly MVC or not or to justify Microsofts Rapid Application Development tools. Neither of these reasons in my books justify its existence as a separate design pattern.
The view, in most cases, creates its presenter. The presenter will interact with the model and manipulate the view through an interface. The view will sometimes interact with the presenter, usually through some interface. This comes down to implementation; do you want the view to call methods on the presenter or do you want the view to have events the presenter listens to? It boils down to this: The view knows about the presenter. The view delegates to the presenter.
The controller is created or accessed based on some event/request. The controller then creates the appropriate view and interacts with the model to further configure the view. It boils down to: the controller creates and manages the view; the view is slave to the controller. The view does not know about the controller.
MVC = Model-View-Controller
Also worth remembering is that there are different types of MVPs as well. Fowler has broken the pattern into two - Passive View and Supervising Controller.
When using Passive View, your View typically implement a fine-grained interface with properties mapping more or less directly to the underlaying UI widget. For instance, you might have a ICustomerView with properties like Name and Address.
Your implementation might look something like this:
public class CustomerView : ICustomerView
{
public string Name
{
get { return txtName.Text; }
set { txtName.Text = value; }
}
}
Your Presenter class will talk to the model and "map" it to the view. This approach is called the "Passive View". The benefit is that the view is easy to test, and it is easier to move between UI platforms (Web, Windows/XAML, etc.). The disadvantage is that you can't leverage things like databinding (which is really powerful in frameworks like WPF and Silverlight).
The second flavor of MVP is the Supervising Controller. In that case your View might have a property called Customer, which then again is databound to the UI widgets. You don't have to think about synchronizing and micro-manage the view, and the Supervising Controller can step in and help when needed, for instance with compled interaction logic.
The third "flavor" of MVP (or someone would perhaps call it a separate pattern) is the Presentation Model (or sometimes referred to Model-View-ViewModel). Compared to the MVP you "merge" the M and the P into one class. You have your customer object which your UI widgets is data bound to, but you also have additional UI-spesific fields like "IsButtonEnabled", or "IsReadOnly", etc.
I think the best resource I've found to UI architecture is the series of blog posts done by Jeremy Miller over at The Build Your Own CAB Series Table of Contents. He covered all the flavors of MVP and showed C# code to implement them.
I have also blogged about the Model-View-ViewModel pattern in the context of Silverlight over at YouCard Re-visited: Implementing the ViewModel pattern.
Both of these frameworks aim to seperate concerns - for instance, interaction with a data source (model), application logic (or turning this data into useful information) (Controller/Presenter) and display code (View). In some cases the model can also be used to turn a data source into a higher level abstraction as well. A good example of this is the MVC Storefront project.
There is a discussion here regarding the differences between MVC vs MVP.
The distinction made is that in an MVC application traditionally has the view and the controller interact with the model, but not with each other.
MVP designs have the Presenter access the model and interact with the view.
Having said that, ASP.NET MVC is by these definitions an MVP framework because the Controller accesses the Model to populate the View which is meant to have no logic (just displays the variables provided by the Controller).
To perhaps get an idea of the ASP.NET MVC distinction from MVP, check out this MIX presentation by Scott Hanselman.
Both are patterns trying to separate presentation and business logic, decoupling business logic from UI aspects
Architecturally, MVP is Page Controller based approach where MVC is Front Controller based approach. That means that in MVP standard web form page life cycle is just enhanced by extracting the business logic from code behind. In other words, page is the one servicing http request. In other words, MVP IMHO is web form evolutionary type of enhancement. MVC on other hand changes completely the game because the request gets intercepted by controller class before page is loaded, the business logic is executed there and then at the end result of controller processing the data just dumped to the page ("view") In that sense, MVC looks (at least to me) a lot to Supervising Controller flavor of MVP enhanced with routing engine
Both of them enable TDD and have downsides and upsides.
Decision on how to choose one of them IMHO should be based on how much time one invested in ASP NET web form type of web development. If one would consider himself good in web forms, I would suggest MVP. If one would feel not so comfortable in things such as page life cycle etc MVC could be a way to go here.
Here's yet another blog post link giving a little bit more details on this topic
I have used both MVP and MVC and although we as developers tend to focus on the technical differences of both patterns the point for MVP in IMHO is much more related to ease of adoption than anything else.
If I’m working in a team that already as a good background on web forms development style it’s far easier to introduce MVP than MVC. I would say that MVP in this scenario is a quick win.
My experience tells me that moving a team from web forms to MVP and then from MVP to MVC is relatively easy; moving from web forms to MVC is more difficult.
I leave here a link to a series of articles a friend of mine has published about MVP and MVC.
http://www.qsoft.be/post/Building-the-MVP-StoreFront-Gutthrie-style.aspx
I think this image by Erwin Vandervalk (and the accompanying article) is the best explanation of MVC, MVP, and MVVM, their similarities, and their differences. The article does not show up in search engine results for queries on "MVC, MVP, and MVVM" because the title of the article does not contain the words "MVC" and "MVP"; but it is the best explanation, I think.
(The article also matches what Uncle Bob Martin said in his one of his talks: that MVC was originally designed for the small UI components, not for the architecture of the system)
In MVC, the Controller is the one in charge! The Controller is triggered or accessed based on some events/requests then, manages the Views.
Views in MVC are virtually stateless, the Controller is responsible for choosing which View to show.
E.g.: When the user clicks on the “Show MyProfile” button, the Controller is triggered. It communicates with the Model to get the appropriate data. Then, it shows a new View that resembles the profile page. The Controller may take the data from the Model and feed it directly to the View -as proposed in the above diagram- or let the View fetch the data from the Model itself.
In MVP, the View is the one in charge! each View calls its Presenter or has some events that the Presenter listens to.
Views in MVP don’t implement any logic, the Presenter is responsible for implementing all the logic and communicates with the View using some sort of interface.
E.g.: When the user clicks the “Save” button, the event handler in the View delegates to the Presenter’s “OnSave” method. The Presenter will do the required logic and any needed communication with the Model then, calls back the View through its interface so that the View can display that the save has been completed.
The simplest answer is how the view interacts with the model. In MVP the view is updated by the presenter, which acts as as intermediary between the view and the model. The presenter takes the input from the view, which retrieves the data from the model and then performs any business logic required and then updates the view. In MVC the model updates the view directly rather than going back through the controller.
There is this nice video from Uncle Bob where he briefly explains MVC & MVP at the end.
IMO, MVP is an improved version of MVC where you basically separate the concern of what you're gonna show (the data) from how you're gonna show (the view). The presenter includes kinda the business logic of your UI, implicitly imposes what data should be presented and gives you a list of dumb view models. And when the time comes to show the data, you simply plug your view (probably includes the same id's) into your adapter and set the relevant view fields using those view models with a minimum amount of code being introduced (just using setters). Its main benefit is you can test your UI business logic against many/various views like showing items in a horizontal list or vertical list.
In MVC, we talk through interfaces (boundaries) to glue different layers. A controller is a plug-in to our architecture but it has no such a restriction to impose what to show. In that sense, MVP is kind of an MVC with a concept of views being pluggable to the controller over adapters.
I hope this helps better.
You forgot about Action-Domain-Responder (ADR).
As explained in some graphics above, there's a direct relation/link between the Model and the View in MVC. An action is performed on the Controller, which will execute an action on the Model. That action in the Model, will trigger a reaction in the View. The View, is always updated when the Model's state changes.
Some people keep forgetting, that MVC was created in the late 70", and that the Web was only created in late 80"/early 90". MVC wasn't originally created for the Web, but for Desktop applications instead, where the Controller, Model and View would co-exist together.
Because we use web frameworks (eg:. Laravel) that still use the same naming conventions (model-view-controller), we tend to think that it must be MVC, but it's actually something else.
Instead, have a look at Action-Domain-Responder.
In ADR, the Controller gets an Action, which will perform an operation in the Model/Domain. So far, the same.
The difference is, it then collects that operation's response/data, and pass it to a Responder (eg:. view()) for rendering.
When a new action is requested on the same component, the Controller is called again, and the cycle repeats itself.
In ADR, there's no connection between the Model/Domain and the View (Reponser's response).
Note: Wikipedia states that "Each ADR action, however, is represented by separate classes or closures.". This is not necessarily true. Several Actions can be in the same Controller, and the pattern is still the same.
In a few words,