I've mulled over Design Patterns for some time now and I am just starting to see how I might actually begin incorporating some of these more deliberately in my development work. However, I am still confused about their treatment of MVC in the beginning of the book and how it relates to the rest of the book.
Most of the frameworks I have worked with - Spring, Yii, ASP.NET, and even Objective-C Cocoa (UIKit) - cater to the MVC paradigm. I get MVC because to me it is a useful way of classifying objects and how they should message or interact with each other. Plus, these frameworks kind of force it upon you even if you are not setting out to think in the MVC way.
I also feel that I understand the premise of Design Patterns: they really don't like subclassing, they love abstract interfaces, and they strive for loose coupling. I can't say I fully understand all of the patterns yet or how they are useful, but I am getting a feel for it.
My question is this: what is the interplay between MVC and design patterns? What were they getting at in the first chapter of the book with the MVC application example? Are certain design patterns just not relevant in the MVC paradigm? I wonder, for example, how the Command pattern is supposed to fit into MVC. It seems incredibly useful, but do we create a CommandModel and CommandController to send to other controllers? Do we just create a Command object as prescribed in the book? Basically, I am wondering if the ideas of MVC and Design Patterns are wholly disjoint and I just don't understand, or if there are some patterns that do not fit into the mold.
My personnal opinion is that MVC is a simplified version of the Observer Pattern which is a simplified version of the Mediator Pattern.
MVC: One Model, One view, the Controler manages the communication between them.
Observer Pattern: One Model, Multiples views ( observers/subscribers ), and the publisher manages the communication
Mediator Pattern: Several different Models, Several views, and the mediator manages the communications between them.
The MVC in the GoF book is for the desktop, it uses the observer pattern to update views. The command example in the GoF book is for an editor.
There are other flavors of MVC where the use of other design patterns may not be obvious:
What is the difference between MVC and MVVM?
Presentation abstraction control
The GoF book says:
...
Taken at face value, this example reflects a design that decouples views from models. But the design is applicable to a more general problem: decoupling objects so that changes to one can affect any number of others without requiring the changed object to know details of the others. This more general design is described by the Observer (page 293) design pattern.
Another feature of MVC is that views can be nested. For example, a control panel of buttons might be implemented as a complex view containing nested button views. The user interface for an object inspector can consist of nested views that may be reused in a debugger. MVC supports nested views with the CompositeView class, a subclass of View. CompositeView objects act just like View objects; a composite view can be used wherever a view can be used, but it also contains and manages nested views.
Again, we could think of this as a design that lets us treat a composite view just like we treat one of its components. But the design is applicable to a more general problem, which occurs whenever we want to group objects and treat the group like an individual object. This more general design is described by the Composite (163) design pattern. It lets you create a class hierarchy in which some subclasses define primitive objects (e.g., Button) and other classes define composite objects (CompositeView) that assemble the primitives into more complex objects.
MVC also lets you change the way a view responds to user input without changing its visual presentation. You might want to change the way it responds to the keyboard, for example, or have it use a pop-up menu instead of command keys. MVC encapsulates the response mechanism in a Controller object. There is a class hierarchy of controllers, making it easy to create a new controller as a variation on an existing one.
A view uses an instance of a Controller subclass to implement a particular response strategy; to implement a different strategy, simply replace the instance with a different kind of controller. It's even possible to change a view's controller at run-time to let the view change the way it responds to user input. For example, a view can be disabled so that it doesn't accept input simply by giving it a controller that ignores input events.
The View-Controller relationship is an example of the Strategy (315) design pattern. A Strategy is an object that represents an algorithm. It's useful when you want to replace the algorithm either statically or dynamically, when you have a lot of variants of the algorithm, or when the algorithm has complex data structures that you want to encapsulate.
MVC uses other design patterns, such as Factory Method (107) to specify the default controller class for a view and Decorator (175) to add scrolling to a view. But the main relationships in MVC are given by the Observer, Composite, and Strategy design patterns.
...
MVC is a pattern. But it only covers a small aspect of a web application. Another common pattern that gets used with MVC is the Repository. These are more architectural patterns.... their scope has a bigger impact on the overall project.
the GOF patterns will introduce themselves in little ways all over the place. They can help build MVC infrastructure depending on design choices. eg, Strategy gets used a lot so you can plug in different ways of doing things like "authentication" etc.
You don't have to use the patterns as they are, they don't even have to be the exact same code structure. Its more the design principle / goal of the pattern that you employ in the design.
MVC is an architectural pattern. It perfectly fits with other design patterns like Command Pattern. But you do not apply patterns just because they exist and they are written in an authoritative book. You apply patterns when you have a programming/design problem and there is a way to solve that problem that was discovered by someone else and was written down. The way to solve a problem is a pattern. For example, you have an application that saves data to the database. Data to be saved is quite complex: some records must be inserted, some records updated and some deleted. The sequence of steps is important because the records to be inserted into one table depend on records to be inserted into another table. So, a database transaction must be used. One of possible ways to implement the transaction is to use Command Pattern. The way to do it is very well explained in Larman's "Applying UML and Patterns" book (chapter "Designing a Persistence Framework wth Patterns", section "Designing a Transaction with the Command Pattern" - scroll down to the page 556). PersistentObject is an abstract Model class there. All other Model classes extend it. In that example MVC is implemented in the UI, Application and Domain layers but Command is implemented in the Persistence layer. These patterns help to solve different problems and they are mutually supportive in that example.
Related
While using a custom MVC framework I found that the view can actually access data in the model. That was a bit of a surprise because I always thought the V must go through the C. It was something like
//this is completely made up but not far off
serverside foreach(var v in Model.GetSomeList()) {
<div>#v.name</div>
}
Do many MVC frameworks in any programming language allow the view to access anything in the model? When do i choose what should go through the controller and what is ok to access from the view?
Usually that "Model" is really like viewmodel, not the business layer Model object, although it could be the POCO version of the business object. Basically, the view is bound to some poco without any business logic.
Information flow in classical MVC.
Data in MVC should not go from model through controller to view. That is violation of the original concept.
If you read the original definition of MVC design pattern you will notice that Views are meant to request the data from Model. And views know when to do it, because they are observing Model for changes.
Modern interpretations.
In the original concept you were meant to have small MVC triad for each element in the application. In modern interpretation (as per Martin Fowler), the model is not anymore any single object or class. Model is a layer, which contains several groups of objects. Each with a different set of responsibilities.
Also, with the rise of Web there was another problem. You cannot use classical MVC for websites. Theoretically now you could achieve it by keeping an open socket and pushing a notification to the browser every time you changed something in the model layer.
But in practice even a site with 100 concurrent users will start having problems. And you would not use MVC for making just a blog. Using such an approach for even minor social network would be impossible.
And that was not the only divergence from the original concept.
MVC-inspired patterns
Currently, along with classical MVC (which is not even all so classical anymore). There are three major MVC inspired patterns:
Model2 MVC
This is basically same classical MVC patter, but there is not observer relationship between model later and view(s). This pattern is meant to me more web-oriented. Each time you receive a user request, you know that something is gonna change in model layer. Therefore each user request cause view instance to request information from the model layer.
MVP
This pattern, instead replaces controller with a presenter. The presenter request data from model layer and passes it to current view. You can find patterns definition here. It is actually a lot more complex, and I, honestly, do not fully understand it.
In this case the View is passive and will not request any data from model layer.
MVVM
This pattern is closer to MVP hen to classical MVC. In this case the controller-like structure (which actually would be more then a monolith class) request data from model layer and then alters it in such a way as it is expected by the (passive) view.
This pattern is mostly aimed at situation where developer does not have full controller over views or/and model layer. For example, when you are developing some application where model layer is SAP. Or when you have to work with an existing frontend infrastructure.
FYI: what is called "MVVM" in ASP.NET MVC is actually a good Model2 implementation .. what they call "viewmodels" are actually view instances and "views" are just templates that are used by views.
This is common. If you look at the Wikipedia page for MVC, this is what it says for the view:
A view requests from the model the information that it needs to generate an output representation.
MVC is an architectural style, so some people change it as they see fit. From the design intentions of the architecture this particular question is certainly not frowned upon.
I try to understand the main differences between MVC / MVP and MVVM patterns. I found these 3 diagrams but I'm not sure to understand them. Coul you help me and explain me what is the legend of the dashed line and continuous line.
MVC from Wikipedia definition
MVP from Microsoft MSDN website
MVVM from Microsoft MSDN website
Solid lines are direct calls.
Dashed lines are only event callbacks.
Main differences between MVC and MVP (Passive view) patterns:
In MVC view knows about model (calls getData() etc. to display data)
In MVP (Passive view) the view does not know about model. Presenter passes data from model to view.
More details in:
MVC vs MVP vs MVVM
In depth description by Martin Fowler: GUI Architectures
I think the dotted lines are indirect references
I'm not as familiar with MVC or MVP, but in MVVM a View references a ViewModel, and the ViewModel references the Model, which is represented by solid lines.
The Models can broadcast messages or raise event notifications which are picked up by the ViewModel, and ViewModels can publish events that are picked up by the View, however these objects should never directly reference the other object, so they're indirect references. For example, a programmer is aware that the purpose for raising an event notification on a Model is so that the ViewModel can hook into the event and process something, however the Model itself never references the ViewModel.
It should be noted if you're comparing the patterns, that they are very different patterns that just happen to share the same naming convention for some objects. For example, a Model in MVC is not the same as a Model in MVVM. Instead, MVC's M+C is equal to MVVM's VM, and MVC's M contains a mix of both MVVM's M and VM pieces
The dotted lines are notifications (e.g. observer pattern) and the solid lines are direct knowledge (i.e. compile time dependencies). Data change notifications are flowing on the dotted lines. A solid line with an arrow says that one component has knowledge of the other and can directly push data. A dotted line is looser coupling as the sender is firing out an event but doesn't know the nature of the receiver of that event which is hidden behind an event listener interface (if you are doing event driven versions of those patterns).
The point of the patterns is to create order by avoiding spaghetti code where everything directly interacts with everything else. So the diagrams are really only hints about what should be decoupled from what. Like any such diagrams they are hard to grok without a detailed explanation and they are only really indicative of what you should aim for; certain frameworks have more or less support for doing things in a "pure way". How components get loaded and wired together is not in the scope of those diagrams; only what happens when the user inputs data or the model is updated through a different view component. So the actual classes may have compile time dependencies and code to initialise the objects which seem to violate the diagrams; yet so long as it is just "initialisation" code which is connecting things together it may not be material.
Here is a presentation which tries to explain MVP, MVC (or possibly MVVMP) and MVVM (aka MVB) in terms of some less formal diagrams which show what compiles to what and who notifies whom with observer pattern event listeners. It is relevant to your question as it sets the context about what the patterns aim to achieve which helps in interpreting the diagrams in your question:
Design Patterns in ZK: Java MVVM as Model-View-Binder, Simon Massey
Here is an article which doesn't have the diagrams in it but which does the same simple screen three times using three different GUI event driven desktop patterns (which can be loosely described as MVP, MVVM and MVC/MVVMP). One key point of confusion about the M__ patterns is that they are overloaded monikers and hardly very descriptive or indicative of the actual pattern. The article is relevant to your question as it follows Martin Fowlers formal description of the patterns which are clearer and less confusing than their "M__" names:
Implementing event-driven GUI patterns using the ZK Java AJAX framework, Simon Massey
Whilst that article does not specifically answer your question it does give a comparison of three implementations of the patterns you are asking about and compares them; so it is likely to shed some light on what the choices the patterns are making which the diagrams are meant to describe. Of course if you pick a different framework to implement the three patterns the example code would look different; but hopefully the same trade-offs would be seen as with the examples shown in that article.
MVC is used in java architectures such as Spring, Struts etc..
MVC stands for Model view and container.
it is very good to use this strategy in Web application
Model–view–controller (MVC) is a software architecture,[1] currently considered an architectural pattern, used in software engineering. The pattern isolates domain logic (the application logic for the user) from the user interface (input and presentation), permitting independent development, testing and maintenance of each (separation of concerns).
Use of the MVC pattern results in separating the different aspects of the application (input logic, business logic, and UI logic), while providing a loose coupling between these elements.[2]
Data binding establishes a direct coupling between the view and the model, thereby bypassing the controller. Fundamentally this breaks with the Model-View Controller architectural pattern, am I right in thinking this? Does this make data binding a "bad thing"?
Edit: As example, angular claims to be a MVC framework, yet one of its main features is data binding.
In my opinion Data Binding can be a valid implementation of the MVC Pattern since the data binding mechanism itself acts as the controller in that case.
For example in the mentioned angular it seems like the $watch function is a shortcut to implement features that are typical Controller responsibilities and features in an MVC-style way.
So in my opinion data binding is an evolution step that implements best practices learned by implementing classic MVC controllers.
UPDATE
But in original pattern sense I would characterize data binding more like MVP or Passive View.
But the differences aren't that sharp in my opinion since it always also depends on your UI technology.
Not necessarily, since you don't have to bind your Model objects to the view.
What I usually do is create simple DTOs (or Presentation Objects) that contain only the data I want to display, and that's what the View layer displays.
In that case, the Controller retains its function as a translator between actions performed on the DTOs and actions on the underlying Model entities.
Actually, when your data is abstracted properly, the act of pushing the content of your models to your UI is a repetitive task that normally lead to some kind of "helpers".
Let's say to push a list of items to a combobox. This is not necessarily part of the controller as you may want to share such functionality. Also pushing the value of the control (to keep it simple, let's say the text of a textbox) is repetitive and bi-directional.
Also here you repeat your self (think of DRY) and do the same thing over and
over again.
That's exactly the point where databinding comes into play. This can take over the tasks that anyway are identical for simple controls (checkbox, textbox, combobox). For grid control and the like it may be specific.
Have a look at mvc & databinding: what's the best approach?. Here I discuss what could be the optimum when using databinding in combination with MVC.
Data Binding does not directly couple the view and model, so it is not a Bad Thing®. It is an integral feature of the MVC architecture, which the GoF Design Patterns book describes succinctly in chapter 1.
MVC decouples views and models by establishing a subscribe/notify protocol between them. A view must ensure that its appearance reflects the state of the model. Whenever the model's data changes, the model notifies views that depend on it. In response, each view gets an opportunity to update itself. This approach lets you attach multiple views to a model to provide different presentations. You can also create new views for a model without rewriting it.
It's a common misconception that MVC is a layered (3-tier) architecture. It is not. The model updates the view(s) directly. But this does not mean the two are coupled! A publish/subscribe design keeps the model and view decoupled.
This more general design is described by the Observer design pattern.
I am contemplating about using an MVC pattern in my new project and I can clearly see the main advantage of being able to put the data layer (the model) a little closer to the presentation layer (the view), which will allow a little increase in application speed. But apart from performance stand point are there any other advantages of MVC over the view-logic-data layered type pattern?
EDIT:
For those who's interested I just uploaded a sample PHP code that I created to test the use of MVC. I purposly omitted all the security checks to make the code a little easier to read. Please don't critisize it too much, because I know it could be a lot more refined and advanced, but nevertheless - it works!!! I will welcome questions and suggestions: Here is the link: http://www.sourcecodester.com/sites/default/files/download/techexpert/test_mvc.zip
The separation of concerns that's quoted as being an advantage of MVC is actually also an advance of a 3-layer/3-tier system. There too, the business logic is independent and can be used from different presentation tiers.
A main difference is that in classic MVC the model can have a reference back to the view. This means when data is updated the model can push this data back to possibly multiple views. The prime example is a desktop application where data is visualized in multiple ways. This can be as simple as a table and graph. A change in the table (which is a change in one view) is first pushed via the controller to the model, which then pushes it back to the graph (the other view). The graph then updates itself.
Since desktop development is on the decline, a lot of programmers have only come in touch with MVC in some web variant, e.g. via JSF in Java EE.
In those cases the model almost never has a reference to the view. This is because the web is mainly request/response based and after a request has been served, the server cannot send additional information. I.e. an update pushed from the model to the client would be meaningless. With reverse ajax/comet this is changing, but many web based MVC frameworks still don't fully utilize this.
Thus, in the case of web based MVC, the typical "triangle" between M, V and C is less there and that MVC variant is actually closer to an n-tier model than 'true' MVC is.
Also note that some web MVC frameworks have an intermediate plumbing part between M, V and C called a backing bean (Java/JSF) or code behind (ASP.NET). In JSF the controller is provided by the framework, and the view often doesn't bind directly to the model but uses this backing bean as an intermediary. The backing bean is very slim and basically just pre-fetches data from the model one way and translates model specific messages (e.g. exceptions) into view specific messages (e.g. some human readable text).
Beside
code reuse,
separating of concerns,
less coupling between the layers,
already mentioned by #bakoyaro and #arjan
i think that MVC is better than 3-tier when combined with the "convention over configuration" pattern. (i.e. "ruby on rails" or Microsofts "MVC for asp.net").
In my opinion this combination leads to to better and easier code maintanance.
In the first place it makes learning the mvc-framework a bit more difficuilt since you have to learn the conventions (a la controllers go into the controllers folder and must be named xxxxxcontroller)
But after you learned the conventions it is easier to maintain your own and foreign code.
Forget increasing application speed by moving to MVC. I have found the biggest benefit to be ease of code reuse. Once you move to MVC, there are no dependencies on the presentation of your data or the storage of the actual data.
For example you could write a servlet that served up .jsp pages as your presentation layer one day, and the next day write a web service as another presentation layer to your existing Model and Controller. Like wise if you want or need to switch your DBMS. Since accessing the Model is completely separate from everything else, you would just need to re-write just your data access objects to return the data in a way your Controller can handle it.
By separating concerns into 3 distinct pieces, you also facilitate true unit testing. Your Presentation layer can be tested free of the Model or Controller, and vice-a-versa.
On a side note, I've often felt that the MVC abbreviation was inaccurate. Whenever I see it I think of it as View->Controller->Model. The presentation layer will never have DAO code in it, and the model will never have presentation logic in it. The Controller is forced to act as a go-between.
Where 3-tier separates presentation from business and data access, MVC is a presentation layer pattern which further separates Model (data) from View (screen) and Controller (input).
There is no choosing MVC over 3-tier/3-layered. Use them both.
Question number three in my quest to properly understand MVC before I implement it:
I have two cases in mind:
The primary application
window needs to launch the
preferences window. (One View
invoking another View.)
The primary Model for an application
needs to access a property
in the preferences Model. (One Model
accessing another Model.)
These questions are related in that they both involve communication across Model-View-Controller triplets, a topic that I haven't found much discussion of in my Googling.
The obvious way to fix this is to wrap everything in a top-level "application" object that handles transactions between Models and allows Controllers to invoke one another's methods. I have seen this implemented, but I'm not convinced its a good idea. I can also see possibilities involving Controllers observing more than one Model and responding to more than one View, but this seems like its going to become very cluttered and difficult to follow.
Suggestions on how best to implement this sort of cross-talk? I feel like its a very obvious question, but I've been unable to find a well-documented solution.
On a broader note, if anyone has a link that shows typical approaches to these sorts of MVC issues, I would love to see it. I haven't had much luck finding solid, non-trivial references. Examples in Python would be lovely, but I'll gladly read anything.
Edit 1:
I see some pretty interesting things being said below and in general no-one seems to have a problem with the approach I've described. It is already almost a lazy form of the FrontController design that Vincent is describing. I certainly don't foresee any problems in implementing that pattern, however, it doesn't seem that anyone has really addressed the question in regards to communication amongst Models. All the answers seem to be addressing communication among objects in a single Model. I'm more interested in maintaining separate Models for separate components of the application, so that I'm not stuffing fifty state properties into a single Model class. Should I be maintaining them as sub-Models instead?
With respect to (1), views don't invoke other views. They invoke controller actions that may result in other views being rendered. In your case, the primary application window contains a user interface element (button, link) that invokes a controller action to display the preferences window.
With respect to (3), model components certainly could be related to one another. This isn't unexpected nor to be avoided, necessarily. For instance your Customer model may have an associated set of Orders. It would be perfectly natural to access the customer's orders via a method in the Customer class.
You might want to take a look at the MVC page on wikipedia for an overview.
You may want to consider looking up the Front Controller design pattern.
The Front Controller pattern defines a single component that is responsible for processing application requests. A front controller centralizes functions such as view selection, security, and templating, and applies them consistently across all pages or views. Consequently, when the behavior of these functions need to change, only a small part of the application needs to be changed: the controller and its helper classes.
This way all requests from the view goes to the FrontController who then decides which specific action (controller) to invoke. Sometimes, it could forward straight to another view as in your first case.
There is no problem with multiple objects in the model talking to each other. In fact that will be very common. The way I see it, all the objects in the Model act like one component to represent the data and operations on the data.
This article might help. And this one.
Model does not mean a single model object. The model is the subset of the entirety of your domain model which is directly related to the controller actions and the views in question.