I've inherited an application which uses the view-first MVVMC patern
In the application I've created 2 step process which creates a person and assigns them to a group. To do this I've created a view and corresponding view model (all views have a 1-2-1 relationship with a view model, view models are injected into the View constructor and are registered with the Unity container using the TransientLifetimeManager) called CreatePersonMaster, the view simply contains a region (shown by the dashed line) which sub-views can be loaded into and the view model subscribes to two loosely coupled events, "PersonCreated" and "GroupSelected". The "PersonCreated" event saves a Person entity in a field and the "GroupSelected" event takes the saved Person, creates a Group association and saves them to a database.
This view/view model doesn't do anything until the events get raised so I load the following sub views into my the region.
These views/view models fire the events which get handled by the master view.
I also have an edit view where I want to re-use the select group view.
I can do this by subscribing to the appropriate events in the EditPersonMaster view model.
My question really is, is this the appropriate way to do this? Because I'm using loosely coupled events I don't get any feedback into the sub-View/ViewModels if there's an error when creating/reassigning? I could probably fire another "ErrorBlah" event for the inner view/model to handle and update the view.
Is there another way to do this? Composite commands don't seem to fit the bill but maybe I don't understand them correctly.
Here is how I understood your question: You have persons stored in a database, which have a property/column group. In your UI, you want to create persons and assign the group property. Also, you want to be able to edit the group property of existing persons in a different region/page/master. You want to reuse the control for assigning the group. Your controls fire an event, which is handled by the Master in a way that it creates the entries in the database (is that correct?). Your question is whether this is a good approach and what alternatives you have?
I see two options:
1. Modularize your business logic:
You could inject a data service into your ViewModels, which could access the database directly. This way you can handle errors etc.. directly where they occur and don't have to pass them through the whole system. If you want to notify other modules of changes on the database (specifying the changed dataset or so), you can do so with a composite event. I think of the PRISM modules more as of complete elements, including UI and business logic and keep communication between them as little as possible.
2. Keep your business logic central and share it:
If you want to stick to your architecture, have singleton Model which is injected into the ViewModels. Manage your state in the model and have the ViewModels pick the pieces they want to provide to their view.
Hope I got he point, though...
Related
I am working on a UI, using Backbone + Marionette.js, which displays the same Widget multiple times on a single page. I am struggling with whats the best way to contain events inside each widget. Lets say each widget displays a selected friend's Facebook information (interests, status feed, mutual friends). If the user changes the selected friend for that specific widget what would be the best way to update the models that are part of that widget?
Here is how I am thinking of solving this ...
Create Setting Model - when user selects a different friend inside a widget the Friend select view updates the Setting model.
Approach 1:
Controller listens to the setting model on "change" events and in turn updates all the relevant models. Each model will never know of the setting model.
Approach 2:
Pass setting model to each Model's options and each model listen's to the setting model and when it changes it does whatever it needs to (reload, etc).
These are the 2 approaches that come to my mind. I feel like I am liking the Approach 1 better, but I would appreciate any feedback from people that have used Backbone in a similar fashion.
Thanks,
Habeel
I would go for approach 2 using a singleton setting model as you suggested. That way the views or models listening to the settings are responsible for doing the updating. Approach 1 has two potential pitfalls. If you move the logic of what actually happens to the controller, it can become convoluted and complex. If the controller just reacts to change events on the settings model and propagate them to other models and views, then there seem to be little reason for it to exist.
Another approach could be to make use of Marionette's event aggregator feature. This would allow you to not even need a settings model. Instead, just have the changed model fire a global event that the other models are listening for and adjust accordingly.
I have a WPF application which follows the MVVM pattern. The application defines two views and view models so far:
LoginView(Model)
ProjectsView(Model)
Both view models need to access several properties from other view models.
Example:
LoginViewModel has a property ProjectList. ProjectsViewModel needs to access this property as well.
This is only a simple example. Later there will be several UserControls which all need to interact with each other.
Would it be a better idea to create one huge view model which all UserControls (views) set as their DataContext? If not, how can all the different view models interact with each other?
Remark:
This question is closely related to this one but with a different approach.
You should definitely not make a huge "main view model" -- this is an anti-pattern not dissimilar to a god object.
The key idea here is that your viewmodels do not need to access several properties from other view models; rather, all of the viewmodels need to access specific pieces of information.
Right now you are most likely injecting this information into each viewmodel at the time of instantiation. Instead of doing this, provide each viewmodel with a reference to a service -- an application module that exposes this information through properties and/or methods. The information can be exposed in the form of scalar values if it's extremely simple in nature, or in the form of models if it's anything more complicated than that.
Schematically this would look like
/--------------\
| ViewModelA |
| | <=======\
| | |
\--------------/ | <--- information is pulled /================\
+=========[model]===[model]====== | Service |
/--------------\ | \================/
| ViewModelB | |
| | <=======/
| |
\--------------/
The service should be injected into the viewmodels upon construction (either manually or by the DI container if you are using one). Each viewmodel should only require a reference to the service and enough information to tell to the service which model it's interested in; it will then request that model from the service and populate its "interesting" properties based on that.
This way of doing things is more involved than simply constructing a viewmodel object and setting its properties externally, but it will allow your application to grow in complexity without becoming unmanageable.
For example, if there are lots of views that bind on different viewmodels and these viewmodels depend in complex ways on a number of models, a sane manner to work would be:
A view/viewmodel pair is constructed
The viewmodel requests any models it needs to know about from the service; it subscribes to an event that the service exposes to let subscribers know that a model has changed
Changes are performed to the models through databound controls
The view invokes a "save" command on the viewmodel
In response to that, the viewmodel invokes a "save this model" method on the service
The service persists the information and publishes a "model changed" event (see step 2)
Other viewmodels interested in the same model know that the model has changed and can query the service for its new state
This is possible if everything is routed through the service. Imagine what it would take to keep everything synchronized otherwise -- the only way to cope would be to put all the information into a giant viewmodel and reference that from everything else. Ugly.
Usually I do one of 4 things:
Make my ViewModels reference each other, and pass the property around. For example, LoginViewModel might set ProjectsViewModel.ProjectList on successful login. How you implement this depends on how your ViewModels are related to each other, and where logical points of connection would be.
Make an ApplicationViewModel which manages things like which page is current, and application-wide objects like the current user or current project list. It would handle transferring shared data to the ViewModels that needs it.
Use some kind of Event system to broadcast a message anytime an application-wide property changes, and have any ViewModel that's interested subscribe to receive those messages. The message typically contains the new object as well, so anyone subscribed to receive that message type has access to the new object. (I have a brief summary of Event Systems in my blog article Communication between ViewModels with MVVM if you're interested)
Depending on if any of the other options work better, I may consider creating a singleton to hold application-wide data instead. For example, if the User is set on Login and is accessed from many of my ViewModels, I may create a singleton to set the user on first login, and then all my ViewModels can access it.
The one thing I would not do is make one big ViewModel holding all available data. ViewModels should only contain the data specific to it.
If you are using some sort of dependency injection tooling, you could inject a class that provides those values, for example, an IProjectService, that can return the list of projects on each of those view models.
The idea of one giant view model does not sound appealing, nor does strong coupling between view models by accessing each other's public properties. And if you want interaction between the view models, like when a project gets added, use a publish / subscribe model for each of the events you expect to occur.
Your Model should contain things like a project list, or give you the ability to access it. The ViewModel is simply meant to be a layer between the Model and View, giving you the chance to bring different Model objects together and mold them into a shape which the View can readily display and to handle commands from the View (modifying the Model in response).
Create one Base View Model keep all common Properties in that, Create singleton to BaseViewModel and add that singleton property in all View Models. All Viewmodels will get synced at one time.
This nice article recommends against keeping track of the views belonging to a model inside the model. What about the opposite? It is recommended for views to keep track of the model(s) they are based on?
It seems difficult to imagine to do without. Maybe the recommended way is to use events, or something?
Views always keep a reference to the model. It is accessible through myView.model or myView.collection.models.
Due to the nature of views, I cannot imagine a case where you would want the view to not know about the model. Event binding happens in the view with a reference to a model. (Think about the collection.add event. Wouldn't be possible if you didn't bind to a reference to the collection)
Most of the time the view should know its model cause he is the visualization of the model. So the model can be there without a view, but a view without a model doesn't make much sense.
But as always there are cases where the view should not know its model directly. Think about a basket where a user can add products and maybe he can configure that products. You have different views that visualize the model, like a table with the products, a basket ico with the count of products and a view to show the total amount. All share the same model. So when ever the user creates a new model cause he delete the old one or order something and there after something new, you have to create a new model and pass it to your views. Sure you can to this by fire an event. But you can also pass a proxy for your model to your views, so the views always comunicate with your proxy and they never know that sometimes a new model was created.
Currently I'm building an application using latest Prism for Silverlight 4.
I've a module and in that module I've two views with view models. Also I've a module view with two regions for each view.
In module initialization I'm registering my views and view models in Unity container and also register views with corresponding regions.
The problem is that views should display something similar to table-detail information - first view shows available entities ans the second view shows detail of selected entity.
I need a way how to pass them initial selected entity. Newly created first view doesn't have any selected entity and newly created second view doesn't show any details.
Currently I'm doing that this way:
In module I create two view models and register them as instances in Unity container and then I register views as types for corresponding regions. Each view subscribes to EntitySelectedEvent from EventAggregator. Module initializer publish this event after initialization and this way two views are selecting the same entity.
I know this looks ugly - I tried publishing this event from one of view models but the problems is that EventAggregator in Prism doesn't support durable subscribers - this means that if the second view model didn't subscribe to event before the first view model fired it, it won't receive and event. I know this is a normal behavior of EventAggregator, but I'm looking for a solution when view models can fire events without depending on initialization order of them - that is the first model can fire event before the second model was created and the second model will receive this 'queued' event after subscribing to it.
Are there any other messaging implementations for WPF/SL which do support such behavior or using a mediator (in my example it's a module itself) isn't such a bad idea after all? One big problem with mediator is that models must be created right away in initialize and they can't be registered as types in container because this leads again to missing subscribers.
Create a Model that will be shared by ViewModels of each of the views.
When a row gets selected in View 1 its ViewModel (via a CurrentEntity property bound to a selected row) will update the Model.
ViewModel of View 2 would subscribe to changes of Model's CurrentEntity and will properly update it's own CurrentEntity property that would cause it's View to update.
My concept for MVVM in WPF is that we have a ViewModel for every Model in your application. This means that if we have Customer class (entity) then we will have CustomerViewModel. The CustomerViewModel will have all the properties which are necessary to represent a customer. The CustomerView usercontrol will be responsible for creating the UI for the Customer model.
Now, let's say that we are adding a new customer. So, we have a form which consists of FirstName, LastName etc. Do we need a ViewModel for this scenario. I mean all I have to do is to take all the input values from TextBox and create a Customer object and then save it to the database. Why should I bother creating a ViewModel for this scenario?
First of all, that is not the main purpose of MVVM, to "mirror" everything. The View should provide the means for a user input, and certainly not process calls to any of the database layers. The ViewModel should be a GUI-agnostic application backbone, and it definetly should handle the creating of customers.
That said, what you should do, is have a ViewModel which represents a workspace for handling customers, and not just a customer ViewModel. If you really want to save on a few objects being created, add to that workspace the possibility to create and add a new customer (not CustomerViewModel). That way, you can have a View of the workspace which has elements for each relevant/required property of the customer, and by invoking some command added to that workspace ViewModel, you could get the current values filled in those (data bound to ViewModel) View elements directly to the customer model.
Consider if you could probably drop the specific customer (and other Model) ViewModels if you refactor things a bit, it would be good practice to keep things from adhering blindly to a certain pattern without explicit cause.
Let's pretend for a second that there is no business model. The only thing you have is a view. If you were to model just that view, without any knowledge of what the data means elsewhere in the system, that is a ViewModel.
The goal of a ViewModel is to, well, model the view it backs. This is a different goal than modeling the idea of a customer in your business domain. To say you will have have one ViewModel per business entity, then, is to say you will have one view per business entity, which leads to run-of-the-mill data-focused UI.
In your particular case, think about the customer edit view. It has fields that correspond to a customer's properties, and thus seems like a natural fit for binding to a customer directly. However, where on the customer object is the "Submit" action modeled? Where is the "Cancel" action modeled? Where is it modeled that field X is an enumerated value selected from a list?
How about the password? If persisted as a binary hashed value, does the view know how to hash its text? What if the system has password strength requirements?
The ViewModel is the mortar between the business model and the UI. It takes concerns from one and translates them into terms of the other. It is the point at which all the above issues are addressed. To say a ViewModel isn't necessary is to ignore its necessity.
You don't need to have a separate ViewModel for Add, you only need a single ViewModel which should do Edit and Add scenarios. If you can delete the record from the edit page than that ViewModel should also have the ability to delete. Your ViewModel should reflect the functionality your View exposes regardless of data.
I think you should reconsider having a single ViewModel for each model. I like to think of ViewModels as normalizing behavior inserted of data. By having a ViewModel for each Model class you will run into architecture issues sooner or later. I look at the application from top down overview, what is my UI trying to accomplish and from there I will get to the ViewModel and eventually I will get to my DataFactory and how the ViewModel maps down to data is almost always not 1 to 1 except for the most simplistic Views. If you try to map 1 to 1 you will have bad UI or your data will not be normalized very well.
The Stack we have here is:
View
ViewModel (Controls everything the user can do in the view, wraps properties from our POCO's)
DataFactory (Maps our POCO's to Entity Framework objects and CRUD)
POCO's (Business Logic, all rules and validation)
Entity Framework (Model, Data Access)
One note here is that ViewModel contains properties from multiple POCO's!
We inject the DataFactory through StructureMap and Unit test with xUnit along with Moq.
To answer you second question I would create a separate view only view to drop in as a user control. But you should still have a CRUD ViewModel in you app that encapsulate all that functionality in a user friendly way.
Thanks.
One reason for this VM abstraction is for testability. Another reason why you want a ViewModel is because it's basically a Data Transfer Object that might be a combination of fields from multiple models in a single container that is more relevant to your current view. Yet another reason to have VM is to take advantage of WPF two ways binding capabilities.
Using your regular model (plain POCO), you can update the View when your model change, but since your model does not implement dependency properties (most likely), you won't be able to take advantage of updating your model when the value in WPF control changes. Which mean you have to manual add a handler and do the copy this value from this control back to the model kind of thing.