I have few gwt mvp design related questions:
Can we use event bus to switch views from one presenter to other via controller using custom event?
If above is true, can the custom event (say changeViewEvent) contain name of next view, on the basis of which controller can take a decision, which presenter to show?
Is it a good design to make views reusable(as a widget) in an application, though i don't agree with this, but will be happy if someone has any thing to mention in favor of this.
PS: all my views make use of custom widgets and there is no gwt specific widgets(buttons, checkbox etc...) in views.
You can do anything you want, but you have to consider the consequences. For example, if you switch views without creating a history event, a user may be thrown out of your app when a user hits a back button expecting to see the previous view.
I very much like the Activities and Places design pattern. It takes care of all of the issues (history handling, bookmarks, tokens, etc.) You can also extend it to add animation effects when switching views on mobile devices - mgwt does that.
I have few gwt mvp design related questions :
Can we use event bus to switch views from one prsenter to other via controller using custom event ?
This is a bad practice, unless you have real good reasons to do that. Since you're changing the view without having an impact on the url, you won't know what is a the state of a view at a choosen moment, you cannot get back to a previous view in an easy way, and finally, people will have difficulties reading the code, since you're out of the "standard".
The only reference for you should be the url, you cannot assume data is loaded neither the applicatio is in a given state: each and any view may be the start of the navigation story for your users, so if you get information from any other source than the Place, you're probably doing wrong, especially if your source is an event. the only special case here is you don't want users to enter your app in a certain view state, so you 'impose' that another url is called before, and restrict the access to the view through an event starting from a given application state
If above is true, can the custom event (say changeViewEvent) contain name of next view, on the basis of which controller can
take a decission, which prsenter to show ?
as said before, you're reinventing the wheel. It's far better to adapt to the existing mechanism that is well thought and covers the majority of cases. You can put a json formatter to tokenize your url while developing so it's not a nightmare to put variables in. And when you're done, create a nicer regular url format
Is it a good design to make views reusable(as a widget) in an application, though i don't agree with this, but will be happy if
someone has any thing to mention in favor of this.
depends on what you call a View. if it's the activitie's view then youm might need that in very few cases, it's better to inherit a basic view and fork its children for each activity (even if the view does nothing, time will show it will evolve differently): this is better since the base view contains what is common without the specifics of each child activity.
Finally if you mean a composition of widgets when you say, a view, then you should definitely reuse them, it will make you obliged to improove your widgets and compositions to be in a constant improvement, this will heop you for the rest of the project, and maybe for other projects
I'm trying to wrap my head around navigation rules in JSF2.0. If I have a button with an action method, is the purpose of the navigation rule to move me to a 'results' page? This seems too simple I guess, couldn't you just f:ajax and update a result area on the page? What is the real purpose of navigation rules and how can one effectively use them in JSF 2.0?
Bonus points for metaphors that use ponies.
The point of navigation rules is that they allow the server to decide, based on the data in a request, which view to show next - a results page, an error page, the next of several possible following pages in a workflow, whatever.
They're not always useful - most applications only have linear workflows and show errors in-page so that you never really need navigation rules. I've always considered it a bit weird that such an (in my experience) rarely-useful feature became a core concept in Struts and JSF. But it would certainly be useful for a workflow with lots of branching logic.
As for AJAX - that's a different use case. If you're switching to a completely different view, you should do a full update rather than use AJAX.
Why not let your pony jump you on this site and find out the basics from a good tutorial :)
I'm new to JSP and Tiles, as well as Java. We are currently replatforming our site using these, but I am confused as to when something should be put into a view preparer vs coming from the controller.
For example, the current page I am working on will have a tile for pagination, including the content type (X of Y content-type). My original plan was to use a View Preparer to take the data being sent in by the controller (HashMap) and output a couple attributes for the pagination tile, but a co-worker told me this should be done in the controller instead.
If this is the case, what is the point of the View Preparer? I'm just slightly confused. I checked the Tiles docs and they are pretty basic/unusable.
Can someone give me a proper use case for a View Preparer?
The controller is for executing business or transactional logic. That is, in response to a user action the application needs to perform one or more actions and then decide which view to render. Thats why its called a controller because it "controls" the flow of an application. As the controller performs its job the changes to the data will have to become visible to the user. However, preparing that data for display is not the job of the controller. Just making sure that the necessary actions are done and that the data is available.
The view, in this case your jsp pages, will then take the data and display it. As I understand it the View Preparer helps you factor some aspects of the view out so that a preparation that may have to be done in several diffent views may be done by the Preparer.
So the example in the docs of preparing a menu is a good use case. Menus are sometimes dynamic, in the sense that it depends on the state of the system what exactly is displayed for the user. Let us say that you want to display a login link in the menu when the user is not logged in and remove that link from the menu when the user is logged in. Rather than code that logic into every page that has to display a menu you can use a ViewPreparer that generates the menu implementing any logic that is necessary. That View Preparer can then be associated with several pages.
Think of it more as manipulating the data for viewing rather than business logic.
I think I understand the basic concepts of MVC - the Model contains the data and behaviour of the application, the View is responsible for displaying it to the user and the Controller deals with user input. What I'm uncertain about is exactly what goes in the Controller.
Lets say for example I have a fairly simple application (I'm specifically thinking Java, but I suppose the same principles apply elsewhere). I organise my code into 3 packages called app.model, app.view and app.controller.
Within the app.model package, I have a few classes that reflect the actual behaviour of the application. These extends Observable and use setChanged() and notifyObservers() to trigger the views to update when appropriate.
The app.view package has a class (or several classes for different types of display) that uses javax.swing components to handle the display. Some of these components need to feed back into the Model. If I understand correctly, the View shouldn't have anything to do with the feedback - that should be dealt with by the Controller.
So what do I actually put in the Controller? Do I put the public void actionPerformed(ActionEvent e) in the View with just a call to a method in the Controller? If so, should any validation etc be done in the Controller? If so, how do I feedback error messages back to the View - should that go through the Model again, or should the Controller just send it straight back to View?
If the validation is done in the View, what do I put in the Controller?
Sorry for the long question, I just wanted to document my understanding of the process and hopefully someone can clarify this issue for me!
In the example you suggested, you're right: "user clicked the 'delete this item' button" in the interface should basically just call the controller's "delete" function. The controller, however, has no idea what the view looks like, and so your view must collect some information such as, "which item was clicked?"
In a conversation form:
View: "Hey, controller, the user just told me he wants item 4 deleted."
Controller: "Hmm, having checked his credentials, he is allowed to do that... Hey, model, I want you to get item 4 and do whatever you do to delete it."
Model: "Item 4... got it. It's deleted. Back to you, Controller."
Controller: "Here, I'll collect the new set of data. Back to you, view."
View: "Cool, I'll show the new set to the user now."
In the end of that section, you have an option: either the view can make a separate request, "give me the most recent data set", and thus be more pure, or the controller implicitly returns the new data set with the "delete" operation.
The problem with MVC is that people think the view, the controller, and the model have to be as independent as possible from each other. They do not - a view and controller are often intertwined - think of it as M(VC).
The controller is the input mechanism of the user interface, which is often tangled up in the view, particularly with GUIs. Nevertheless, view is output and controller is input. A view can often work without a corresponding controller, but a controller is usually far less useful without a view. User-friendly controllers use the view to interpret the user's input in a more meaningful, intuitive fashion. This is what it makes it hard separate the controller concept from the view.
Think of an radio-controlled robot on a detection field in a sealed box as the model.
The model is all about state and state transitions with no concept of output (display) or what is triggering the state transitions. I can get the robot's position on the field and the robot knows how to transition position (take a step forward/back/left/right. Easy to envision without a view or a controller, but does nothing useful
Think of a view without a controller, e.g. someone in a another room on the network in another room watching the robot position as (x,y) coordinates streaming down a scrolling console. This view is just displaying the state of the model, but this guy has no controller. Again, easy to envision this view without a controller.
Think of a controller without a view, e.g. someone locked in a closet with the radio controller tuned to the robot's frequency. This controller is sending input and causing state transitions with no idea of what they are doing to the model (if anything). Easy to envision, but not really useful without some sort of feedback from the view.
Most user-friendly UI's coordinate the view with the controller to provide a more intuitive user interface. For example, imagine a view/controller with a touch-screen showing the robot's current position in 2-D and allows the user to touch the point on the screen that just happens to be in front of the robot. The controller needs details about the view, e.g. the position and scale of the viewport, and the pixel position of the spot touched relative to the pixel position of the robot on the screen) to interpret this correctly (unlike the guy locked in the closet with the radio controller).
Have I answered your question yet? :-)
The controller is anything that takes input from the user that is used to cause the model to transition state. Try to keep the view and controller a separated, but realize they are often interdependent on each other, so it is okay if the boundary between them is fuzzy, i.e. having the view and controller as separate packages may not be as cleanly separated as you would like, but that is okay. You may have to accept the controller won't be cleanly separated from the view as the view is from the model.
... should any validation etc be
done in the Controller? If so, how do
I feedback error messages back to the
View - should that go through the
Model again, or should the Controller
just send it straight back to View?
If the validation is done in the View,
what do I put in the Controller?
I say a linked view and controller should interact freely without going through the model. The controller take the user's input and should do the validation (perhaps using information from the model and/or the view), but if validation fails, the controller should be able to update its related view directly (e.g. error message).
The acid test for this is to ask yourself is whether an independent view (i.e. the guy in the other room watching the robot position via the network) should see anything or not as a result of someone else's validation error (e.g. the guy in the closet tried to tell the robot to step off the field). Generally, the answer is no - the validation error prevented the state transition. If there was no state tranistion (the robot did not move), there is no need to tell the other views. The guy in the closet just didn't get any feedback that he tried to cause an illegal transition (no view - bad user interface), and no one else needs to know that.
If the guy with the touchscreen tried to send the robot off the field, he got a nice user friendly message asking that he not kill the robot by sending it off the detection field, but again, no one else needs to know this.
If other views do need to know about these errors, then you are effectively saying that the inputs from the user and any resulting errors are part of the model and the whole thing is a little more complicated ...
The MVC pattern merely wants you to separate the presentation (= view) from the business logic (= model). The controller part is there only to cause confusion.
Here is a good article on the basics of MVC.
It states ...
Controller - The controller translates
interactions with the view into
actions to be performed by the model.
In other words, your business logic. The controller responds to actions by the user taken the in the view and responds. You put validation here and select the appropriate view if the validation fails or succeeds (error page, message box, whatever).
There is another good article at Fowler.
Practically speaking, I've never found the controller concept to be a particularly useful one. I use strict model/view separation in my code but there's no clearly-defined controller. It seems to be an unnecessary abstraction.
Personally, full-blown MVC seems like the factory design pattern in that it easily leads to confusing and over-complicated design. Don't be an architecture astronaut.
Controller is really part of the View. Its job is to figure out which service(s) are needed to fulfill the request, unmarshal values from the View into objects that the service interface requires, determine the next View, and marshal the response back into a form that the next View can use. It also handles any exceptions that are thrown and renders them into Views that users can understand.
The service layer is the thing that knows the use cases, units of work, and model objects. The controller will be different for each type of view - you won't have the same controller for desktop, browser-based, Flex, or mobile UIs. So I say it's really part of the UI.
Service-oriented: that's where the work is done.
Based on your question, I get the impression that you're a bit hazy on the role of the Model. The Model is fixated on the data associated with the application; if the app has a database, the Model's job will be to talk to it. It will also handle any simple logic associated with that data; if you have a rule that says that for all cases where TABLE.foo == "Hooray!" and TABLE.bar == "Huzzah!" then set TABLE.field="W00t!", then you want the Model to take care of it.
The Controller is what should be handling the bulk of the application's behavior. So to answer your questions:
Do I put the public void actionPerformed(ActionEvent e) in the View with just a call to a method in the Controller?
I'd say no. I'd say that should live in the Controller; the View should simply feed the data coming from the user interface into the Controller, and let the Controller decide which methods ought to be called in response.
If so, should any validation etc be done in the Controller?
The bulk of your validation really ought to be done by the Controller; it should answer the question of whether or not the data is valid, and if it isn't, feed the appropriate error messages to the View. In practice, you may incorporate some simple sanity checks into the View layer for the sake of improving the user experience. (I'm thinking primarily of web environments, where you might want to have an error message pop up the moment the user hits "Submit" rather than wait for the whole submit -> process -> load page cycle before telling them they screwed up.) Just be careful; you don't want to duplicate effort any more than you have to, and in a lot of environments (again, I'm thinking of the web) you often have to treat any data coming from the user interface as a pack of filthy filthy lies until you've confirmed it's actually legitimate.
If so, how do I feedback error messages back to the View - should that go through the Model again, or should the Controller just send it straight back to View?
You should have some protocol set up where the View doesn't necessarily know what happens next until the Controller tells it. What screen do you show them after the user whacks that button? The View might not know, and the Controller might not know either until it looks at the data it just got. It could be "Go to this other screen, as expected" or "Stay on this screen, and display this error message".
In my experience, direct communication between the Model and the View should be very, very limited, and the View should not directly alter any of the Model's data; that should be the Controller's job.
If the validation is done in the View, what do I put in the Controller?
See above; the real validation should be in the Controller. And hopefully you have some idea of what should be put in the Controller by now. :-)
It's worth noting that it can all get a little blurry around the edges; as with most anything as complex as software engineering, judgment calls will abound. Just use your best judgment, try to stay consistent within this app, and try to apply the lessons you learn to the next project.
Here is a rule of thumb that I use: if it is a procedure that I will be using specifically for an action on this page, it belongs in the controller, not the model. The model should provide only a coherent abstraction to the data storage.
I've come up with this after working with a large-ish webapp written by developers who thought they were understood MVC but really didn't. Their "controllers" are reduced to eight lines of calling static class methods that are usuall called nowhere else :-/ making their models little more than ways of creating namespaces. Refactoring this properly does three things: shifts all the SQL into the data access layer (aka model), makes the controller code a bit more verbose but a lot more understandable, and reduces the old "model" files to nothing. :-)
The controller is primarily for co-ordination between the view and the model.
Unfortunately, it sometimes ends up being mingled together with the view - in small apps though this isn't too bad.
I suggest you put the:
public void actionPerformed(ActionEvent e)
in the controller. Then your action listener in your view should delegate to the controller.
As for the validation part, you can put it in the view or the controller, I personally think it belongs in the controller.
I would definitely recommend taking a look at Passive View and Supervising Presenter (which is essentially what Model View Presenter is split into - at least by Fowler). See:
http://www.martinfowler.com/eaaDev/PassiveScreen.html
http://www.martinfowler.com/eaaDev/SupervisingPresenter.html
also note that each Swing widget is can be considered to contain the three MVC components: each has a Model (ie ButtonModel), a View (BasicButtonUI), and a Control (JButton itself).
You are essentially right about what you put in the controller. It is the only way the Model should interact with the View. The actionperformed can be placed in the View, but the actual functionality can be placed in another class which would act as the Controller. If you're going to do this, I recommend looking into the Command pattern, which is a way of abstracting all of the commands that have the same receiver. Sorry for the digression.
Anyway, a proper MVC implementation will have the following interactions only:
Model -> View
View -> Controller
Controller -> View
The only place where there may be another interaction is if you use an observer to update the View, then the View will need to ask the Controller for the information it needs.
As I understand it, the Controller translates from user-interface actions to application-level actions. For instance, in a video game the Controller might translate "moved the mouse so many pixels" into "wants to look in such and such a direction. In a CRUD app, the translation might be "clicked on such and such a button" to "print this thing", but the concept is the same.
We do it thusly, using Controllers mainly to handle and react to user-driven input/actions (and _Logic for everything else, except view, data and obvious _Model stuff):
(1) (response, reaction - what the webapp "does" in response to user)
Blog_Controller
->main()
->handleSubmit_AddNewCustomer()
->verifyUser_HasProperAuth()
(2) ("business" logic, what and how the webapp "thinks")
Blog_Logic
->sanityCheck_AddNewCustomer()
->handleUsernameChange()
->sendEmail_NotifyRequestedUpdate()
(3) (views, portals, how the webapp "appears")
Blog_View
->genWelcome()
->genForm_AddNewBlogEntry()
->genPage_DataEntryForm()
(4) (data object only, acquired in _construct() of each Blog* class, used to keep all webapp/inmemory data together as one object)
Blog_Meta
(5) (basic data layer, reads/writes to DBs)
Blog_Model
->saveDataToMemcache()
->saveDataToMongo()
->saveDataToSql()
->loadData()
Sometimes we get a little confused on where to put a method, in the C or the L. But the Model is rock solid, crystal clear, and since all in-memory data resides in the _Meta, it's a no-brainer there, too. Our biggest leap forward was adopting the _Meta use, by the way, as this cleared out all the crud from the various _C, _L and _Model objects, made it all mentally easy to manage, plus, in one swoop, it gave us what's being called "Dependency Injection", or a way to pass around an entire environment along with all data (whose bonus is easy creation of "test" environment).