MVVM - dynamically create multiple instances of view/viewmodel - wpf

I have a requirement to display a number of graph/chart "thumbnails". Clicking one will show an "expanded view" (in a separate panel) that displays a larger version of the chart, plus controls to view and manipulate the chart.
There will be a number of different charts, each plotting my data in different ways. Also, a given chart type may appear several times, each plotting a different subset of the data. The controls in the "expanded" view will also differ from one chart type to the next, so there is little commonality here.
I'm struggling to get my head around how to model all this in MVVM, especially given the need to dynamically create an unknown number of thumbnails (and in some cases multiple instances of the same type).
Thinking aloud, I guess I need a view/viewmodel that represents a single thumbnail (the view contains the chart component and the VM exposes the data to plot). I guess I also need a V/VM for the "thumbnail list" UI, responsible for creating the thumbnails and exposing them via a collection for binding to the list. But how does it instantiate these? A VM gets injected into its view, suggesting the "thumbnail list" VM would have to dynamically instantiate the thumbnail views - but a VM shouldn't have knowledge of views should it?!
Lastly, when I display the "expanded" view, it would make sense to (somehow) pass it the charting component/view that was used in the thumbnail, to avoid having to render the chart again, but how?
If it's relevant/helps, I'm using Castle Windsor for dependency injection, and the navigation features of Prism.

This is indeed a complex topic,...
I would suggest a VM for the list of icons not necessarily for the icon itself. this can be bound to properities of the IconListViewModels. Then you should think about a ChartViewModelFactory. Which works in conjunction with your DIC.
An important discussion is the VM-V marriage. View first or View Model first... one way could also be ViewResolver if which returns the matching view based on your view model... this can rely on some sort of conventions. So the final steps could be ask the factory for a view model find the matching view glue them together and bind them to a content presenter...
I hope this helps to get you started...

Related

WPF two panel app - reuse or create new instance of panel VM?

I have a WPF application whose main window contains two panels. The layout is pretty standard. The left panel is a a list of objects. The right panel displays information about the object selected in the left panel.
Right now I have the left panel directly in the XAML of the window, although I'm thinking it should be its own UC/View. The right panel is coded as a UC. The ViewModel for the main window has a field that is the VM for the right panel (call it the properties VM). The properties VM encapsulates the data for the selected object from the model.
When a different object is selected in the left panel I see two options:
1) Instantiate a new instance of the properties VM and load the corresponding data from the model
2) Load the corresponding data from the model to the existing properties VM, and issue appropriate change notifications [My current implementation]
Is there an advantage or preferred way of doing things between the two options above? Am I missing a better third option?
* Edit * Maybe a better question is - what are the advantages and disadvantages of the two methods?
Thanks!
You are describing a very standard scenario in the MVVM world. I believe you have a slight architectural issue that when resolved would make this question moot. The 'list' from where you are making your selection should already be a list of view models that wrap the 'object' a.k.a model. The properties view should simply be databound to the selected VM.
Is there an advantage or preferred way of doing things between the two options above?
Not really, but it's probably simpler to just create a new instance and set the data-bound property to this instance instead of trying to re-use and update the state of an existing instance each time the button is clicked.
If you need to cache instances, you could use an IoC container or a custom cache container that resolves the instances for you. You could for example set up the container to always return the same instance of the same class or return a new instance each time depending on your requirements.
But there is "preferred way" really. Your requirements decide what would be the best way to solve your specific issue or use case.

WPF Canvas - Visual Studio like split view

I'm working on an app, that displayes huge (10000+ elements) graphs in a wpf canvas.
I'd like a feature like in Visual Studio, when you can split the view of an editor (so I can view two distant part of the same graph at the same time).
I have some constraints:
data binding (creating the bindings) of graph elements makes the loading of big graphs very slow, so I'm not using MVVM, the "VM" knows about the view and updates it directly when needed
the children of the canvas are frameworkelements, since I use the Tag property
because of the number of graph elements, I don't want to keep two different view for each element for the two part of the split view
So it should be like displaying multiple parts of the same canvas in different places. You can't set two parents for the FrameworkElements in WPF, so the easiest way is out of question :(
What are my options? Should I reconsider my constraints or there is some workaround for this?
Let me know if you need any more details (it's a big application, so I can't give you every information).
Edit: duplicating with visual brush is not an option since I need proper input event notifications, so both view must be editable.
Options:
Bind the same data to two controls.
Use a visual brush and duplicate input on the real control.
Create a custom graphing control that can output two parts of the graph at once.
If binding to two controls is too slow, then I think you need to rethink your application. The very fact that you have so much data displayed at once that you need a dual view to see separate parts is disturbing. That should raise a red flag. The red flag would notify you that, "What I need, and what I have is different." And you should go back to the drawing board and find out what you really need.
Otherwise, it might be best to create a custom control. The graph is rendering in its entirety even though you only need small portions displayed. If you had your own custom control you could speed up the entire app by rendering only visible portions at a time, and splitting within the control.

Where should I draw the line between view and controller in a stateful application?

In a MVC application that lives on the web, it's easy to draw the line between view and controller - views display data to the user, while controllers gather input and route the user to views based on the input provided. But in a stateful (e.g. desktop) application, it's a bit harder to know where to draw the line, because the boundaries between operations are not as clear-cut as with a web application. So where do I draw the line?
For instance, I have a view in my application that lets the user place items on a grid. Right now I have the "place item on grid" code within the view itself (accessing the grid model to do the actual placement). But I could also put it in a controller, and pass the necessary information from the view to the controller. Neither seems like a clear-cut choice - putting the code in the view will eventually bloat the view, since there will be a large number of operations that the user can do, not just place items; but putting it in a controller just seems like an extra level of indirection, making the code harder to follow.
Or am I going about this all wrong? This is a WinForms app; my "view" is actually a form, and forms by their nature can both receive input and display information. So is the distinction meaningless? Or should I have the view be separate from the form? Or is the MVC pattern incompatible with WinForms altogether?
MVC is a separation of concerns. It is designed to help the programmer keep details of the two areas separate. The MODEL and VIEW are collections/groupings of objects.
Each object within the VIEW may be part of the class sets BUTTON, WINDOW, TEXTAREA etc.
Each object within the MODEL will have a type corresponding to your domain such as CUSTOMER_DETAILS, ADDRESS, POSTCODE etc
The CONTROLLER object provides an interface into your MODEL objects, which your VIEW objects will use to communicate to your system. The CONTROLLER may also contain functionality that can not possibly be stored in your MODEL objects.
Traditional MVC also allows the VIEW objects to bypass the CONTROLLER and talk directly to MODEL objects, but is not recommended.
All objects conform to OO principles, and are complete and fully functional objects.

VB WPF MVC (Model + View + ?)

I have an old VB6 application. I want to recreate it in VB.Net using WPF. But I am a bit confused about the "Model View Controller"-pattern. I have two books about design patterns (GoF and J.Bishop) afair this pattern is indeed not mentioned inside one of the two books. I have also searched the internet I found some java-examples. But I have still no clue how I should use MVC-Pattern (should I?) in my new WPF-application.
Let's say for example my model (in fact it is more complex) is only a wheel rim (circle) with the properties Manufacturer, Diameter and Depth. The user should be able to modify the properties using textboxes and ComboBoxes.
Could somebody create a small example that explaines the MVC-Pattern with WPF?
Of course I like reusable classes to have a feasible concept throughout the whole application.
thanks in advance
Oops
Here's a "brief" description of what the MVC pattern is and how I would apply it to a WPF application.
(I might have a few details slightly off since I've mainy hacked in Silverlight but the concept is similar enough.)
Basically, the idea is to separate concerns and define interfaces between the different parts of an application, with the goal of keeping the code structured and maintainable.
The Model in your example would be pretty much exactly as you described the wheel rim - a WheelRim class with the various properties defined in suitable data types. I would put the model i an separate assembly to keep it apart from the other code, but you can settle for just keeping the model classes in a "Models" folder. The model would also have a "twin" in a database, the model classes being pretty much one-to-one-mapped to tables.
(You might wanna have a look at Linq2SQL or Entity Framework, if the database is defined you can pretty much get the model for free along with suitable database access code.)
The View would be the actual WPF xaml files - Defining the Grid or Canvas or what have you. On the WheelRimView there would be labels and textboxes for displaying or accessing the different properties, perhaps along with product images and the like. The code behind for the view would have all the relevant event handlers (start, button_click and so on) for getting the data from the fields and passing them to the controllers.
The Controller would be any "handler code" that you would use to manipulate the data. We're talking the basic CRUD operations here, along with validation and the like. Also, the controller layer would be responsible for compiling the data in a format that can go seamlessly into the View. The WheelRimController hence would have methods like "GetWheelRimList", "GetWheelRim", "AddWheelRim", "ModifyWheelRim" and "DeleteWheelRim". The methods take the values as in parameters and manipulate the model objects accordingliy. the
I would recommend keeping the code-behind of the xaml files free from any "controller"-ish code like validation, aggregation and the like - the code behind should basically only take the values from the textboxes, listboxes and such and send them on "as is" to the controller methods for processing. Also, you should keep any data formatting code to a minimum when getting data for presentation (i.e., no filtering or translating in the view).
A typical use case of "User opens a wheel rim and edits the diameter" would play out thus in code:
User clicks "Edit" on a list page. The WheelRimView page loads.
The WheelRimView.Load() method (or corresponding) calls WheelRimController.GetWheelRim(wheelRimId).
WheelRimController.GetWheelRim(wheelRimId) gets the corresonding data from a database table and populates the properties of a WheelRim object, which is returned to the WheelRimView.
The WheelRimView inserts the property values into the labels and textboxes.
The user changes the diameter value and clicks the "Save button.
The WheelRimView.Save() method calls the WheelRimController.ModifyWheelRimDiameter(wheelRimId, diameter) method.
The WheelRimController.ModifyWheelRimDiameter(wheelRimId, diameter) method parses the diameter (if it is a string) and loads the model object. It applies the modified value to the model object and saves it into the database.
The WheelRimController.ModifyWheelRimDiameter(wheelRimId, diameter) returns a status code to the WheelRimView (for instance a predefined numeric stating any validation errors) to report the success of the save.
The WheelRimView displays a result message (hopfully "saved") to the user.
I hope that clears a few bits up.
Bevcause of the rich binding support available, WPF (and Silverlight) are well suited to MVVM (Model-View-ViewModel). MVVM is an extension of MVC that uses a view model to bind the current state of a view, instead of manipulating the view directly.
There are a bunch of MVVM frameworks available, as well as Microsoft's own Prism framework (which is arguably more useful if you have a larger, modular application).
WPF is probably more well suited to MVVM (Model-View-ViewModel). I'd recommend reading this MSDN article on MVVM and, perhaps, following their advice. There's also a nice collection of links I found on the Bryant Likes blog.

Where do I instantiate my Objects in CRUD n-Tiered WinForm App?

Say I have a WinForm CRUD(like) application. I want to follow best practices on this so I try and make it follow OOP and a n-Tiered design.
Unfortunately I am familar with the words but not the practice of them. So lets go with the following example: My CaseNote program. I have a tabbed application where you go to the search tab to find a member and then one of the options is to go to the CaseNote tab to create a new case note, look at existing CaseNotes, or create a follow up CaseNote to a Parent Note. All of this is for the member you selected from the search tab.
So if I am creating objects and passing them around to be used by different forms where should I be instantiating them? I had thought I would have 3 layers; UI, Object, DAL. Where I fall flat is where I instance tho objects. I can't instance them in the object layer, can I? Also, if I instance them on the form how do I pass them from form to form in a quite large app?
CaseNotes Screen Shot
If you want to look at some more words around this problem you want to look at MVP and MVC. (These stand for Model View Controller and Model View Presenter). Some people will shoot me down for saying this but they are quite similar in concept.
The aim of MVP and MVC is to allow you to design your application logic without even having to think about your application apperance. It also allows you to define your user interactions without implementing an actual GUI. Esentially your model is your application logic, your data, your classes which actually do stuff like talk to your database. Your presenter or controller is what interacts with your model and what controls your user interface and reacts to user operations on the interface. Finally your View is your winforms design or your web page.
I'm sure you will be able to find plenty of material on the web about this but to give you some concrete help with this problem should serve to inform and illustrate your reading.
The first thing you need to do is start creating your objects that represent your data. So you will have a CaseNote object which is contains the casenote data. You will have a case note data container of some sort such as a case note database. You can define the logical operations and properties of these as if they where real items.
Then you would move on to define your presenter or controller which will define the operations that you want to support from the GUI. At the same time you should define an Interface that will define for the presenter/controller what operations is can perform on the GUI. So for instance your presenter may expose a method called SearchForCaseNote which takes a string parameter. Your view Interface will expose a method called DisplayCaseNote. When a user clicks on the search button the view will pass through the command to the presenter which will then call the model to get the data. The presenter may format the data at this point, i.e. convert DateTime object to a string and then pass the data back to the view through the interface define method called DisplayCaseNote.
You don't have to use the View interface, you could call into the view directly, but having the interface means you can have many different view implementations.
One last thing i need to mention is where you create these different parts of your application. My view is everything thing should fall out from the presenter/controller. So when you application starts it creates the presenter/controller object which then create and displays your view passing itself as a variable to the view. The presenter/controller can then either create the initial models by loading them from disk or ideally discover them through a dependency injection container like unity. In fact using unity to discover the view implementation is probably a better idea again as it gives you true seperation between view and presenter/controller. When you come to move to another view, (i.e. open another window), your presenter/controller should expose a method such as DisplayDetailPage which the view calls when a button is clicked. This would create the presenter/controller for the next view which would in turn create the view and get a reference to the model.
Hope this helps.
I think you should use the RocketFramework

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