If you've programmed with graphical user interface (GUI) libraries in the past 10 years or so, you have likely come across the model-view-controller (MVC) design. MVC was first introduced by, a Smalltalk developer at the Xerox Palo Alto Research Center in 1979, and helps to decouple data access and business logic from the manner in which it is displayed to the user. More precisely, MVC can be broken down into three elements:. Model - The model represents data and the rules that govern access to and updates of this data. In enterprise software, a model often serves as a software approximation of a real-world process.
Aug 30, 2018 - This Java login application follows MVC architecture and consists of Java servlets. Start your programming with New Project -> dynamic web.
View - The view renders the contents of a model. It specifies exactly how the model data should be presented. If the model data changes, the view must update its presentation as needed. This can be achieved by using a push model, in which the view registers itself with the model for change notifications, or a pull model, in which the view is responsible for calling the model when it needs to retrieve the most current data.
Controller - The controller translates the user's interactions with the view into actions that the model will perform. In a stand-alone GUI client, user interactions could be button clicks or menu selections, whereas in an enterprise web application, they appear as GET and POST HTTP requests. Depending on the context, a controller may also select a new view - for example, a web page of results - to present back to the user. This section will take a closer look at one way to implement Figure 1 in the context of an application in the (Java SE 6).
Once the model, view, and controller objects are instantiated, the following occurs:. The view registers as a listener on the model. Any changes to the underlying data of the model immediately result in a broadcast change notification, which the view receives. This is an example of the push model described earlier. Note that the model is not aware of the view or the controller - it simply broadcasts change notifications to all interested listeners. The controller is bound to the view.
This typically means that any user actions that are performed on the view will invoke a registered listener method in the controller class. The controller is given a reference to the underlying model. Once a user interacts with the view, the following actions occur:. The view recognizes that a GUI action - for example, pushing a button or dragging a scroll bar - has occurred, using a listener method that is registered to be called when such an action occurs. The view calls the appropriate method on the controller.
The controller accesses the model, possibly updating it in a way appropriate to the user's action. If the model has been altered, it notifies interested listeners, such as the view, of the change.
In some architectures, the controller may also be responsible for updating the view. This is common in Java technology-based enterprise applications. Figure 2 shows this interaction in more detail. A Java SE Application Using MVC As this article mentioned earlier, the model does not carry a reference to the view but instead uses an event-notification model to notify interested parties of a change. One of the consequences of this powerful design is that the many views can have the same underlying model.
When a change in the data model occurs, each view is notified by a property change event and can update itself accordingly. For example, Figure 3 shows two views that use the same data model.
An MVC Design Placing the Controller Between the Model and the View The primary difference between this design and the more traditional version of MVC is that the notifications of state changes in model objects are communicated to the view through the controller. Hence, the controller mediates the flow of data between model and view objects in both directions.
View objects, as always, use the controller to translate user actions into property updates on the model. In addition, changes in model state are communicated to view objects through an application's controller objects. Thus, when all three components are instantiated, the view and the model will both register with the controller.
Once a user interacts with the view, the events are nearly identical:. The view recognizes that a GUI action - for example, pushing a button or dragging a scroll bar - has occurred, using a listener method that is registered to be called when such an action occurs. The view calls the appropriate method on the controller. The controller accesses the model, possibly updating it in a way appropriate to the user's action. If the model has been altered, it notifies interested listeners of the change. However, in this case, the change is sent to the controller. Why adopt this design?
Using this modified MVC helps to more completely decouple the model from the view. In this case, the controller can dictate the model properties that it expects to find in one or more models registered with the controller. In addition, it can also provide the methods that effect the model's property changes for one or more views that are registered with it.
Once the application is up and running, you immediately run into a problem. Consider the following chain of events:. One of the Swing components in the view receives a change, presumably from the user action.
The appropriate controller method is called. The model is updated.
It notifies the controller of its property change. The view receives a change event from the controller and attempts to reset the value of the appropriate Swing components. The appropriate controller method is called, and the model is updated again. At this point, any of three different scenarios can occur, depending on what Swing component you use and how robust your model is. The Swing component that prompted the initial change refuses to update itself the second time, noting that its property state cannot be updated again while it is in the process of notifying listeners of the initial state change.
This primarily occurs when you use Swing text components. The model notes that the value of the second update matches that of the first, its current value, and refuses to send a change notification. This is always a safe programming practice, and it automatically occurs if you use the class provided in the package. However, it does not keep the model from receiving a redundant update. No safeguards are in place on either the model or the Swing component, and the program enters an infinite loop. This issue occurs because the Swing components are autonomous.
For example, what happens if the user presses the up arrow of the JSpinner component in PropertiesViewPanel, incrementing the spinner's value by one? After the value is updated, a GUI event listener method that is listening for value changes is called, opacitySpinnerStateChanged, which in turn calls the controller and then updates the appropriate property in the model. With a traditional MVC design, the view would still contain the previous value, and the change in the model would update the view to the current value. However, there is no need to update the Swing component because it has already reset itself to the correct value - it did so before it even passed an event to the controller. How do you get around this?
One way is to write a mechanism that tells the model or the controller not to propagate a change notification under these circumstances, but this is not a good idea. Remember that more than one view may be listening for changes on the model. If you shut down the change notification for the model, no other listeners, including other views, will be notified of the change. In addition, other components in the same view may rely on the property change notification, with a slider and spinner combination, for example. Ideally, each Swing component would be aware of its current value and the value that the view is trying to set it to.
If they match, no change notifications will be sent. However, some Swing components include this logic, and others do not. One possible solution is to check the incoming changed value of the model against the current value stored in the Swing component. If they are identical, there is no need to reset the value of the Swing component. Code Sample 8 shows an update of the modelPropertyChange method to demonstrate this approach. Code Sample 8. You can download the example source code for this article.
The original MVC implementation is described in the now-canonical paper., also known as The Swing Tutorial, shows you how to get started using the Java Swing components. gives the inside story on how Swing uses a variant of MVC. The page provides a good background on the problem space and various solutions, even though its target audience is developers using the Java EE platform. ' ' is an excellent article by Scott Stanchfield that discusses some of common problems and solutions in using MVC with Java technology. A follow-up article, ',' also points out some best practices to follow when populating models with data from underlying domain sources. If you are curious as to how the textual portion of the Java 2D libraries work, take a look at.
Rate and Review Tell us what you think of the content of this page. Excellent Good Fair Poor Comments: Your email address (no reply is possible without an address): Note: We are not able to respond to all submitted comments.
@JDPekham, why do you say 'You cannot instantiate an activity without talking to your layout/view'? Instantiating an activity does not require talking to views, in fact talking to views is by no means a part of the Activity instantiation. You CAN (but do not have to) call various Activity methods that interact with your views when and if you see fit.
Second question: Assuming Activity is intended to take the 'controller' role (I believe many Android devs see it that way) why not talk to your views from the Activity? – user1545072 Oct 1 '13 at 7:19.
There is no universally unique MVC pattern. MVC is a concept rather than a solid programming framework.
You can implement your own MVC on any platform. As long as you stick to the following basic idea, you are implementing MVC:. Model: What to render. View: How to render. Controller: Events, user input Also think about it this way: When you program your model, the model should not need to worry about the rendering (or platform specific code). The model would say to the view, I don't care if your rendering is Android or iOS or Windows Phone, this is what I need you to render. The view would only handle the platform-specific rendering code.
This is particularly useful when you use to share the model in order to develop cross-platform applications. After some searching, the most reasonable answer is the following: MVC is already implemented in Android as:.
View = layout, resources and built-in classes like Button derived from android.view.View. Controller = Activity. Model = the classes that implement the application logic (This by the way implies no application domain logic in the activity.) The most reasonable thing for a small developer is to follow this pattern and not to try to do what Google decided not to do. PS Note that Activity is sometimes restarted, so it's no place for model data (the easiest way to cause a restart is to omit android:configChanges='keyboardHidden orientation' from the XML and turn your device).
EDIT We may be talking about MVC, but it will be so to say FMVC, Framework-Model-View-Controller. The Framework (the Android OS) imposes its idea of component life cycle and related events, and in practice the Controller ( Activity/ Service/ BroadcastReceiver) is first of all responsible for coping with these Framework-imposed events (such as onCreate). Should user input be processed separately? Even if it should, you cannot separate it, user input events also come from Android. Anyway, the less code that is not Android-specific you put into your Activity/ Service/ BroadcastReceiver, the better. Ever heard of observers?
The best separation Iv found so far is when 1. Controller has only model instance, 2. Model has no knowladge of controller or view but view can register as model observer (so model kinda know about view but he doesnt know who it is and he doesnt care) - when model is done with loading data, he notifies all observers (usually 1) and 3. View has only model instance to pull data out of it.
This way there are only 2 dependecies for all MVC framework. I think 2 is minimum so it should be the best layout.
– Apr 22 '15 at 12:35. There is no single MVC pattern you could obey to. MVC just states more or less that you should not mingle data and view, so that e.g.
Views are responsible for holding data or classes which are processing data are directly affecting the view. But nevertheless, the way Android deals with classes and resources, you're sometimes even forced to follow the MVC pattern.
More complicated in my opinion are the activities which are responsible sometimes for the view, but nevertheless act as an controller in the same time. If you define your views and layouts in the XML files, load your resources from the res folder, and if you avoid more or less to mingle these things in your code, then you're anyway following an MVC pattern.
MVC- Architecture on Android Its Better to Follow Any MVP instead MVC in android. But still according to the answer to the question this can be solution Description and Guidelines Controller - Activity can play the role. Use an application class to write the global methods and define, and avoid static variables in the controller label Model - Entity like - user, Product, and Customer class. View - XML layout files. ViewModel - Class with like CartItem and owner models with multiple class properties Service - DataService- All the tables which have logic to get the data to bind the models - UserTable, CustomerTable NetworkService - Service logic binds the logic with network call - Login Service Helpers - StringHelper, ValidationHelper static methods for helping format and validation code. SharedView - fragmets or shared views from the code can be separated here AppConstant - Use the Values folder XML files for constant app level NOTE 1: Now here is the piece of magic you can do. Once you have classified the piece of code, write a base interface class like, IEntity and IService.
Declare common methods. Now create the abstract class BaseService and declare your own set of methods and have separation of code. NOTE 2: If your activity is presenting multiple models then rather than writing the code/logic in activity, it is better to divide the views in fragments. Then it's better. So in the future if any more model is needed to show up in the view, add one more fragment. NOTE 3: Separation of code is very important.
Every component in the architecture should be independent not having dependent logic. If by chance if you have something dependent logic, then write a mapping logic class in between. This will help you in the future.
Android's MVC pattern is (kind-of) implemented with their classes. They replace a controller with an 'adapter.' The description for the adapter states: An Adapter object acts as a bridge between an AdapterView and the underlying data for that view. I'm just looking into this for an Android application that reads from a database, so I don't know how well it works yet.
However, it seems a little like Qt's Model-View-Delegate architecture, which they claim is a step up from a traditional MVC pattern. At least on the PC, Qt's pattern works fairly well.