I'm building a WPF app and trying to conform to MVVM best practices. I'm using the MVVM Foundation framework and noticed the Messenger class, which I've read should be used for handling dialogs in WPF. This sounds great, but I'm totally not understanding how to use a Messenger for this purpose. Literally, all I want to do is open a modal About dialog --I don't need to pass any messages back and forth.
Was the intent of the Messenger class to be used for cases where dialogs require a message from its parent, or return a message to its parent? Is it overkill for an About dialog? Would I be better off simply adding code to an event handler to show the dialog?
The idea behind the messaging pattern doesn't specifically have anything to do with showing dialogs. The idea is simply to provide a decoupled way to communicate between ViewModels.
You can leverage this infrastructure to solve your problem but you will have to implement the showing of the dialog yourself.
As Phillip showed above you can send messages between ViewModels. When your ViewModel receives the message it can set it's own internal property, say "ShowDialog", to true.
You can then have a binding that reacts to this property change operation and opens a dialog.
I have also built a simple messaging framework for the MVVM pattern that borrows from Josh's idea (and several other existing frameworks) you can read about it here
Say you have a parent view and a dialog view. In MVVM they would both have a view model. It is good to keep these view models decoupled, i.e. they don't have references to each other. And yet they need to communicate to each other. The Messenger class acts as a go between or Mediator to mediate the communication of information between the two classes. See the code taken from Josh's blog.
Here is Object A. It's call to the mediator's Register method implements: when I receive the message ObjectBSaidSomething, from the mediator, I'll cache it in the member WhatObjectBSays.
Here is Object B, which implements: I'm going to send a message ObjectBSaidSomething. Note, that Object B knows nothing about Object A. There might be nothing listening for ObjectBSaidSomething, or 100 objects listening for ObjectBSaidSomething, but Object B doesn't know and doesn't care. This is good decoupling, and this is why the Mediator pattern is a good idea. And this is the way the MVVM foundation is recommending that information is passed between view models.
Related
Can someone please guide me towards how to implement Custom Dialog for displaying Errors or simple messages to the UI and recieving response back without violating MVVM. I am in badly need of this. Please help me out.
Thanks,
Zafar
Referring to your comment I strongly suggest you take a close look at the Messenger class. It even provides GalaSoft.MvvmLight.Messaging.DialogMessage type for this purpose. But you can provide your custom ones be subclassing GenericMessage<T>
You register for a message like this and define what should happen on arrival of a message:
Messenger.Default.Register<DialogMessage>(this,
dm =>
{
MessageBox.Show(dm.Content, dm.Caption, dm.Button, dm.Icon);
});
Although there is only a WeakReference hold, i tend to unregister if I dont want to be informed anymore or the view is discarded. Always be a good citizen ;)
Messenger.Default.Unregister(this);
Send a message to the receiver and optionally define a callback to get back the dialogresult:
Messenger.Default.Send(new DialogMessage(this, "Content",
result => Console.WriteLine(result.ToString()))
{
Caption = "Caption"
Icon = MessageBoxImage.Asterisk,
Button = MessageBoxButton.OK
});
To handle any kind of dialog stuff within the mvvm pattern, you should go with a kind of Dialog-Service. In this post you will find some hints to go with this approach.
Putting dialog stuff into a service keeps the mvvm pattern untouched. The service takes care of all the creation of the dialogs and can provide the results. The view-model just calls methods and subscribes events provided by a service.
A good way to solve your problem this way is using the User Interaction Patterns.
In terms of the MVVM pattern, the view model is responsible for initiating an interaction with the user and for consuming and processing any response, while the view is responsible for actually managing the interaction with the user using whatever user experience is appropriate. Preserving the separation of concerns between the presentation logic implemented in the view model, and the user experience implemented by the view, helps to improve testability and flexibility.
There are two common approaches to implementing these kinds of user interactions in the MVVM pattern. One approach is to implement a service that can be used by the view model to initiate interaction with the user, thereby preserving its independence on the view's implementation. Another approach uses events raised by the view model to express the intent to interact with the user, along with components in the view that are bound to these events and that manage the visual aspects of the interaction.
This is a MVVM pattern for doing DialogServices etc. so it will fit your requirement, too.
I am using the MVVM pattern in my WPF application. In one of my Views I have a button that when clicked uses Commands to talk to its ViewModel. The problem I have is that I need the ViewModel to then talk to other ViewModels to call some of their public methods. I use IOC (Unity) and inject the container into the first ViewModel, so could access the others by using this. I’m not sure if this fits in with the MVVM concept.
Is it possible for all my ViewModels to somehow subscribe to the one button click?
Are any of these the recommended way of solving this problem or is there a better way?
To explain a bit more about my application, each view is a tab control with several textboxes. On the first tab there is also a button and combobox. The user is free to enter their own data or select an option from the combo. In this instance, if the button is then clicked I need all the tabs to load their textboxes based on the selected item in the combo from the first tab. I somehow need to wire this button click in such a way that the value from the combo is passed to all the related viewmodels.
You can use the EventAggregator. Have the command publish an event that the other ViewModels can subscribe to.
When the event is raised they'll all get the event, without needing for one VM to know the other VMs
Another option is to use Composite Commands instead of a regular command.
Make the command the button uses a composite command, and have the other viewmodels register to that Composite command.
You could go a few ways with this one:
use some kind of eventing framework to notify all subscribers if something happens: eg Prism EventAggregator. For this to work you'll need to set up Prism obviously. There are other (MVVM) frameworks out there which support some kind of event/message system like Caliburn.Micro or MVVMLight
Create a MasterViewModel that contains all the child viewmodels for all the tabs. This way the 'master' can subscribe to the PropertyChanged events from its children and execute the appropriate actions. Or the Master can even contain the commands which you are binding to.
I would recommend using some form of "Messenger" service. By this, I mean a class that implements the "Subscription Pattern."
I'm not sure which MVVM library you might be using, but if you look at the "MVVM Light Toolkit" - which is available on CodePlex - you will find a very light implementation of a Messenger there.
Basically each ViewModel will subscribe to receive a specific notification and your ViewModel with the combo box and button will publish the message when the button is clicked. It is really quite flexible in how you send the messages and your ViewModels don't need to know anything about each other.
If you are using the MVVM lite toolkit from GalaSoft you have access to the Messenger which would allow you to send a message that you can subscribe to in each of your view models.
// Send message from command handler
Messenger.Default.Send<MyMessage>(new MyMessage());
// Register for message in view models.
Messenger.Default.Register<MyMessage>(this, MyMessageReceived);
// Method to do work
private void MyMessageReceived(MyMessage myMessage)
{
// Do Work
}
i know some Mvvm Frameworks that introduced in this thread
please describe or give me link for that what are them useful for?
not information about MVVM about MVVM Framework.
thanks :)
i want to know :
What Is MVVM Framework?
I think your question is not really precise. As far as I understand, you ask for the features of each framework?!
You can find detailed information here and here. However, at least one of these links has already been given in the thread you mentioned...
EDIT:
Basically, an MVVM framework is a collection of classes which are commonly used in applications utilising the MVVM (Model-View-ViewModel) pattern. This may include messaging systems to communicate between independent parts of a software, dependency injection techniques, base classes for ViewModels, project/class templates, validation mechanisms, commonly used commands, techniques for displaying dialog boxes, and so on...
To completely understand such a framework, you will have to understand the MVVM pattern first. Because only then (or even only after you did your first MVVM project) you will have an understanding of the problems and/or challenges of this pattern.
To use Mvvm framework just simply follow below steps:
You have a model and a view-model with the same name.
View-models are not supposed to be wrappers around models. The job of a view-model is to broker requests for external services such as the loading and saving of data. The data itself, as well as validation and most of the business logic, should be in the models.
I can’t emphasize this enough. Whenever you create a view-model that wraps a model by delegation you introduce a huge hole in your API. Specially, anything with a direct reference to the model can change a property in such a way that the view-model and thus the UI are never notified. Likewise, any changes to calculated fields in the model won’t be propagated back to the view-model.
You have a view and a view-model with the same name.
Ideally view-models are agnostic to the screens they are used by. This is especially true in a WPF application where multiple windows may be sharing the same instance of a view-model.
For smaller applications such you may only need a single view-model for the whole application. For larger applications you may need one for the main functionality and one for each secondary aspect such as configuration management.
You have no code behind.
In absolute terms code behind is neither a good nor a bad thing. It is merely a place to put logic that is specific to a single view or control. So when I see a view with no code-behind at all I immediately check for the following mistakes:
Does the view-model touch specific controls by name?
Is the view-model being given access to controls via a command parameter?
Is EventToCommand or another leaky behavior being used in place of simple event handler?
EventToCommand from MVVM Light is especially bad because it will prevent controls from being garbage collected after they are removed from the screen.
View-models are listening to property changed notifications
If a model has a longer life-span then the view-model that listens to its events then you probably have a memory leak. Unlike views which have an unloaded event, view-models don’t have a good story for life-cycle management. So if they attach an event to a model that may out-last them then the view-model will be leaked.
I am trying to get into MVVM and away from the code behind approach in Silverlight, and I want to know the best practices around how to invoke view logic.
I have a very basic page where I have bound a listbox to a collection of domain objects, this is all using MVVM, so when I recieve my data back from the services, I want to fire off an animation and view changes on the screen.
Where/How is the best way to doing this? Silverlight (version 3, BTW) doesnt have triggers does it? I have seen blogs where people seem to be using them, but i think they must be rolling their own? Not sure... anyways, any thoughts ideas here is greatly appreciated
First of all, I think code behind is just fine as long as it only works with the view, i.e. it is concerned only with UI concerns. Don't struggle for no-code-behind when the easier way out is just as correct.
Secondly, of course sometime you need some sort of disconnected communication between your view and view-model (for example, getting multiple selected items from your view into your view-model). For these purposes you could use an aggregator like MVVMLight's Messenger, which is both simple and expresses the concept nicely. You can send a message from the view-model and have the view listen for it; also you can send messages from your view (when some events occur) and broadcast them.
MVVMLight also includes some utility classes which make it easy to bind events directly to Commands in your view-model, so that's the easier option in most cases I think.
I am currently working with the Microsoft MVVM template and find the lack of detailed examples frustrating. The included ContactBook example shows very little Command handling and the only other example I've found is from an MSDN Magazine article where the concepts are similar but uses a slightly different approach and still lack in any complexity. Are there any decent MVVM examples that at least show basic CRUD operations and dialog/content switching?
Everyone's suggestions were really useful and I will start compiling a list of good resources
Frameworks/Templates
WPF Model-View-ViewModel Toolkit
MVVM Light Toolkit
Prism
Caliburn
Cinch
Useful Articles
WPF Apps With The Model-View-ViewModel Design Pattern
Data Validation in .NET 3.5
Using a ViewModel to Provide Meaningful Validation Error Messages
Action based ViewModel and Model validation
Dialogs
Command Bindings in MVVM
More than just MVC for WPF
MVVM + Mediator Example
Application
Screencasts
Jason Dolinger on Model-View-ViewModel
Additional Libraries
WPF Disciples' improved Mediator Pattern implementation(I highly recommend this for applications that have more complex navigation)
MVVM Light Toolkit Messenger
Unfortunately there is no one great MVVM example app that does everything, and there are a lot of different approaches to doing things. First, you might want to get familiar with one of the app frameworks out there (Prism is a decent choice), because they provide you with convenient tools like dependency injection, commanding, event aggregation, etc to easily try out different patterns that suit you.
The prism release:
http://www.codeplex.com/CompositeWPF
It includes a pretty decent example app (the stock trader) along with a lot of smaller examples and how to's. At the very least it's a good demonstration of several common sub-patterns people use to make MVVM actually work. They have examples for both CRUD and dialogs, I believe.
Prism isn't necessarily for every project, but it's a good thing to get familiar with.
CRUD:
This part is pretty easy, WPF two way bindings make it really easy to edit most data. The real trick is to provide a model that makes it easy to set up the UI. At the very least you want to make sure that your ViewModel (or business object) implements INotifyPropertyChanged to support binding and you can bind properties straight to UI controls, but you may also want to implement IDataErrorInfo for validation. Typically, if you use some sort of an ORM solution setting up CRUD is a snap.
This article demonstrates simple crud operations:
http://dotnetslackers.com/articles/wpf/WPFDataBindingWithLINQ.aspx
It is built on LinqToSql, but that is irrelevant to the example - all that is important is that your business objects implement INotifyPropertyChanged (which classes generated by LinqToSql do). MVVM is not the point of that example, but I don't think it matters in this case.
This article demonstrates data validation
http://blogs.msdn.com/wpfsdk/archive/2007/10/02/data-validation-in-3-5.aspx
Again, most ORM solutions generate classes that already implement IDataErrorInfo and typically provide a mechanism to make it easy to add custom validation rules.
Most of the time you can take an object(model) created by some ORM and wrap it in a ViewModel that holds it and commands for save/delete - and you're ready to bind UI straight to the model's properties.
The view would look like something like this (ViewModel has a property Item that holds the model, like a class created in the ORM):
<StackPanel>
<StackPanel DataContext=Item>
<TextBox Text="{Binding FirstName, Mode=TwoWay, ValidatesOnDataErrors=True}" />
<TextBox Text="{Binding LastName, Mode=TwoWay, ValidatesOnDataErrors=True}" />
</StackPanel>
<Button Command="{Binding SaveCommand}" />
<Button Command="{Binding CancelCommand}" />
</StackPanel>
Dialogs:
Dialogs and MVVM are a bit tricky. I prefer to use a flavor of the Mediator approach with dialogs, you can read a little more about it in this StackOverflow question:
WPF MVVM dialog example
My usual approach, which is not quite classic MVVM, can be summarized as follows:
A base class for a dialog ViewModel that exposes commands for commit and cancel actions, an event to lets the view know that a dialog is ready to be closed, and whatever else you will need in all of your dialogs.
A generic view for your dialog - this can be a window, or a custom "modal" overlay type control. At its heart it is a content presenter that we dump the viewmodel into, and it handles the wiring for closing the window - for example on data context change you can check if the new ViewModel is inherited from your base class, and if it is, subscribe to the relevant close event (the handler will assign the dialog result). If you provide alternative universal close functionality (the X button, for instance), you should make sure to run the relevant close command on the ViewModel as well.
Somewhere you need to provide data templates for your ViewModels, they can be very simple especially since you probably have a view for each dialog encapsulated in a separate control. The default data template for a ViewModel would then look something like this:
<DataTemplate DataType="{x:Type vmodels:AddressEditViewModel}">
<views:AddressEditView DataContext="{Binding}" />
</DataTemplate>
The dialog view needs to have access to these, because otherwise it won't know how to show the ViewModel, aside from the shared dialog UI its contents are basically this:
<ContentControl Content="{Binding}" />
The implicit data template will map the view to the model, but who launches it?
This is the not-so-mvvm part. One way to do it is to use a global event. What I think is a better thing to do is to use an event aggregator type setup, provided through dependency injection - this way the event is global to a container, not the whole app. Prism uses the unity framework for container semantics and dependency injection, and overall I like Unity quite a bit.
Usually, it makes sense for the root window to subscribe to this event - it can open the dialog and set its data context to the ViewModel that gets passed in with a raised event.
Setting this up in this way lets ViewModels ask the application to open a dialog and respond to user actions there without knowing anything about the UI so for the most part the MVVM-ness remains complete.
There are times, however, where the UI has to raise the dialogs, which can make things a bit trickier. Consider for example, if the dialog position depends on the location of the button that opens it. In this case you need to have some UI specific info when you request a dialog open. I generally create a separate class that holds a ViewModel and some relevant UI info. Unfortunately some coupling seems unavoidable there.
Pseudo code of a button handler that raises a dialog which needs element position data:
ButtonClickHandler(sender, args){
var vm = DataContext as ISomeDialogProvider; // check for null
var ui_vm = new ViewModelContainer();
// assign margin, width, or anything else that your custom dialog might require
...
ui_vm.ViewModel = vm.SomeDialogViewModel; // or .GetSomeDialogViewModel()
// raise the dialog show event
}
The dialog view will bind to position data, and pass the contained ViewModel to the inner ContentControl. The ViewModel itself still doesn't know anything about the UI.
In general I don't make use of the DialogResult return property of the ShowDialog() method or expect the thread to block until the dialog is closed. A non-standard modal dialog doesn't always work like that, and in a composite environment you often don't really want an event handler to block like that anyhow. I prefer to let the ViewModels deal with this - the creator of a ViewModel can subscribe to its relevant events, set commit/cancel methods, etc, so there is no need to rely on this UI mechanism.
So instead of this flow:
// in code behind
var result = somedialog.ShowDialog();
if (result == ...
I use:
// in view model
var vm = new SomeDialogViewModel(); // child view model
vm.CommitAction = delegate { this.DoSomething(vm); } // what happens on commit
vm.CancelAction = delegate { this.DoNothing(vm); } // what happens on cancel/close (optional)
// raise dialog request event on the container
I prefer it this way because most of my dialogs are non-blocking pseudo-modal controls and doing it this way seems more straightforward than working around it. Easy to unit test as well.
Jason Dolinger made a good screencast of MVVM. Like Egor mentioned there is no one good example. They are all over. Most are good MVVM examples, but not when you get into complex issues. Everyone has their own way. Laurent Bugnion has a good way to communicate between viewmodels as well. http://blog.galasoft.ch/archive/2009/09/27/mvvm-light-toolkit-messenger-v2-beta.aspx Cinch is also a good example. Paul Stovel has a good post that explains a lot too with his Magellan framework.
Have you looked at Caliburn? The ContactManager sample has a lot of good stuff in it. The generic WPF samples also provide a good overview of commands. The documentation is fairly good and the forums are active. Recommended!
Found this one useful. Has code too.
http://msdn.microsoft.com/en-us/magazine/dd419663.aspx
I have written a simple MVVM example from scratch on code project here is the link MVVM WPF step by step.
It starts from a simple 3 layer architecture and graduates you to use some framework like PRISM.
The sample project in the Cinch framework shows basic CRUD and navigation tools. It's a fairly good example of using MVVM, and includes a multi-part article explaining its usage and motivations.
I also shared in your frustration. I'm writing an application and I had these 3 requirements:
Extensible
WPF with MVVM
GPL compatible examples
All I found were bits and pieces, so I just started writing it the best I could. After I got into it a bit, I realized there might be other people (like yourself) who could use a reference application, so I refactored the generic stuff out into a WPF/MVVM application framework and released it under the LGPL. I named it SoapBox Core. If you go to the downloads page, you'll see it comes with a small demo application, and the source code for that demo application is also available for download. Hope you find that helpful. Also, email me at scott {at} soapboxautomation.com if you want more info.
EDIT: Also posted a CodeProject article explaining how it works.
Here I am adding link of a WPF(Inventory Management App) application which using MVVM architecture designed by me .
Its UI is awesome.
https://github.com/shivam01990/InventoryManagement
Even I shared the frustration until I took the matter into my hands. I started IncEditor.
IncEditor (http://inceditor.codeplex.com) is an editor that tries to introduce developers to WPF, MVVM & MEF. I started it and managed to get some functionality like 'theme' support. I am no expert in WPF or MVVM or MEF so I can't put a lot of functionality in it. I make a sincere request to you guys to make it better so that nutters like me can understand it better.