I plan on writing a WPF app following the MVVM pattern for the first time but something is not quite clear to me. Let's say that the view has a "Save" button and when that is hit I need to save the current state of my data (the model). This will be done by sending a SOAP message to a SOAP service.
Where in my MVVM setup do these SOAP request/response handlers live? Does the view model make the SOAP call itself whenever the save button is hit? Should the view model notify the model to save itself instead? Maybe it's some other way, completely separate from the MVVM?
My thinking was that (at least in this specific case) the view model would handle it since it needs to disable the save button in the view until the current save request has completed.
I typically put a logical client-side application/business layer between the viewmodel and the SOAP/WCF/Webservice layer. This layer is where all the non-view business logic and processing logic lives. Remember, the viewmodel is the model of the view, not the model of the domain. Therefore, you want to hand off control to the next layer down ASAP.
In this scenario, I would have the view trigger a save command on the the viewmodel, which would in turn call into the application layer, which would in turn make any calls to remote services.
The ViewModel, should not do such an operation. It only should trigger it. Therefore the model has to do it (or another intermediate layer that is responsible for the load-and save-operations, but not the ViewModel itself).
The ViewModel can observe the save-operation and may provide state-information about the progress for the View.
I would create a service handler that can be accessed by the ViewModel. Pass this into the constructor of the viewmodel, and call the methods exposed by the service handler.
Related
I am new in angular js. I want to know difference between MVVM and MVC framework and which one is best for AngularJS.
I know about MVC and MVVM pattern but I can not find which one is best and why?
Thanks,
Hitesh
In MVVM, the UI (the View), faces the user and takes user input directly. Within the View, Commands within the ViewModel (which is the DataContext of the View) are triggered by this activity. Control flows to the ViewModel which interprets what the View has sent it and prepares its Models. After control flows back to the View it updates itself according to changes in the Models. If a new View is required, the ViewModel communicates this with the NavigationService (or whatever method of navigation your application uses), which is the purview of the Window or Frame--UI components. You can see that the ViewModel isn't first and last to act; the View plays a much greater role than in MVC.
The architecture of WPF/Silverlight is the reason why things are done this way. The command, binding and navigation infrastructures can't be controlled/replaced by the Controller; they are tightly integrated with the UI. So the Controller must sit below the View and take a more passive role.
AngularJS is MV*(MV what ever) either it can be MVC or MVVC.
Model : Data you want to update.
View : HMLT/Template.
Controller : function which maintains application data and behavior.
Example for MVC is binding data to view using Interpolation {{}}.
You can update view(html) using controller.
Example for MVVC is two way data binding using ngModel.
If you bind a ngModel to input tag.
Value changed in input filed is updated in controller.
Value changed in controller is updated in input field.
Which is the right place to call service in MVVM pattern, View Model or Model? I am planning to invoke service from ViewModel, get the JSON and convert it to corresponding model.
The reason I am not invoking service from Model to keep the Model decoupled from service.
Is this approach right or I should call service from Model?
Typically VM is responsible for making service calls. A sample call stack can be:
UI Event (View) => ICommand Execute (VM) => Service Call (VM).
It's advisable to have a re-usable Service Tier utilising the same domain objects as your app - as it allows the service-calling logic to be shared by multiple VMs (added from comments).
I think that the right place for calling a service is in Model.
The reason I am not invoking service from Model to keep the Model decoupled from service
So in this case you have your ViewModel which should handle presentation logic coupled with Service calls that probably involve data validation or manipulation.
According to
5: Implementing the MVVM Pattern Using the Prism Library 5.0 for WPF
The View Model Class:
It encapsulates the presentation logic required to support a use case or user task in the application. The view model is testable independently of the view and the model.
The view model typically does not directly reference the view. It implements properties and commands to which the view can data bind.
The view model coordinates the view's interaction with the model. It may convert or manipulate data so that it can be easily consumed by the view and may implement additional properties that may not be present on the model.
The Model Class
Model classes are non-visual classes that encapsulate the application's data and business logic. They are responsible for managing the application's data and for ensuring its consistency and validity by encapsulating the required business rules and data validation logic.
The model classes are typically used in conjunction with a service or repository that encapsulates data access and caching.
And as the previous answer showed something like this:
UI Event (View) => ICommand Execute (VM) => Service Call (VM).
I think it should be more like this
UI Event (View) => ICommand Execute (VM) => Handle Command/Action (VM) => Execute business/data logic that VM Command should have triggered (M) => Service Call (M).
You can create some kind of service helper that can be called from various models if you want to reuse service access code.
I've created a class named JsData and instantiated it in App.xaml.cs so that I can access it from multiple viewmodels. The JsData has several ObservableCollections, some properties for configuration and some methods which manipulate the process of automatically pulling data from remote server.
Now comes the question. Is it convenient for me to bind the global data to my views with minimum coding? Besides, I'm using Caliburn.Micro. Is it doable and appropriate to notify PropertyChanged events to viewmodels using messaging?
I think the best way to do this is to create a service that your view models can implement. That way on,y the view models that need the data can implement the service, and the service is more flexible because it can be injected in the view model construction. This keep your view models more decoupled and honors the mvvm pattern.
I would not use messaging to not notify changes, that would create unnecessary overhead. You just need to have your view model implement inotifypropertychanged and then get the service in your constructor And then pass the service values to properties in your view model that raise the property changed event.
If your need help defining a service just let me know and I will post a sample
I'm creating my first application in WindowsForms and wondered how I would do to validate the User input logic layer and return to the textbox control such that he filled out incorrectly by ErrorProvider. That is, each would have to return the validation error for each specific control indicating whether the user typed an invalid value.
This is a good practice?
Today, this validation is done on the presentation layer and would like to use my more logical layer so that she is not only acting as an intermediary between the presentation and data access.
If you would like to maintain separation of concerns you should look into implementing the model-view-controller design pattern. This will allow you to keep you UI code cleaner
Overview of MVC
Model
Contains state and implementation
Notifies listeners (views) of changes
View
Displays model state (and responds to model state notifications)
Send all user input to the controller
Controller
Processes all input from the view
Changes model state and calls model methods
Load a different view as needed
Further Reading
http://www.martinfowler.com/eaaDev/uiArchs.html
http://en.wikipedia.org/wiki/Model-view-controller
I am using MVVM patern for developing my WPF application. It working fine for unrelated pages, means how to go in another view from one view.
Eg:
I have one list page in which some records are coming from one ViewModel and another from another ViewModel, means I have two ViewModel form my single View. And now I want to display another View by some event.
I am using IsSelected property for notification of changes. This mechanism works only upto when any action performed on same ViewModel, what should I do for such senario.
MVVM as a pattern is about separating concerns, improving testability of your code, etc.. so your ViewModel should only be concerned with applying business rules and providing data for your View.
You will need to use this in conjunction with some kind of MVC pattern, where the Controller's concern is handling the application navigation/state, etc.
(edit)
For example, imagine your app has a login screen, so you create a LoginView, which contains a username and password; probably an OK button and a Cancel button.
You create a LoginViewModel class to bind this view and handle the logic of the login within this class.
But once the app is logged in, it is not the responsibility of the login ViewModel to know where to go next; or which View to render next.. maybe you want to navigate to the last screen this user was on the previous time they were logged in? Maybe it goes to a default screen, as per the User's profile? This decision is nothing to do with the login function...
So if you create a Controller class, you can: Instantiate an instance of the LoginViewModel class, then depending on the login result, apply business rules as required to remove the LoginViewModel from scope, and create a new ViewModel, (e.g. HomePageViewModel) etc...
Finally, you'll need to let the app know which Views to use for each VM using DataTemplates
There are heaps of other ways to skin this particular cat, of course... this is just one idea...
As long as the core concept remains: Use MVVM to bridge the gap between View and Model in a clean, testable way... don't try and make it the 'one pattern fits all' :)
HTH :)
I agree with IanR to use a Controller for the workflow/navigation.
The ViewModel sample of the WPF Application Framework (WAF) shows how this might be done.