We are starting a WPF with MVVM project and have to decide on PRISM or MVVM Light (I am new to both these frameworks). I have read through a few posts but still have a few questions. Can someone please throw some light on the following aspects w.r.t. both the frameworks?:
Performance: Will either one framework perform better than the other for any reason?
Communication within the app (viewmodel to viewmodel or between modules etc): I have read that MVVM Light has Messenging Service which appears to be fairly easy as well. But PRISM does not appear to have any equivalent. Is that true? How would PRISM handle interactions?
Unit Testing: Have read that PRISM supports Unit Testing better. Can we still write NUNIT or VSTS tests in MVVM Light also?
I just moved a project from Prism to MvvmLight and it seems to work faster (very subjective).
Both Prism and MvvmLight have Mediator realisation (IEventAggregator in Prism, IMessenger in MvvmLight). But IMessenger has more abilities (for instance, sending messages with tokens) compared to IEventAggregator and is much more convenient to use (see next item).
MvvmLight also has a more powerful ViewModelBase class.
Applications that use MvvmLight are much easier to test than those that use Prism. For instance, IMessenger is easier to mock than IEventAggregator.
PrismViewModel.cs
using System;
using Microsoft.Practices.Prism.Events;
using Microsoft.Practices.Prism.ViewModel;
// An ugly empty event class
public class StringEvent : CompositePresentationEvent<string> { }
public sealed class PrismViewModel : NotificationObject
{
private readonly IEventAggregator _eventAggregator;
private string _name;
public PrismViewModel(IEventAggregator eventAggregator)
{
if (eventAggregator == null)
throw new ArgumentNullException("eventAggregator");
_eventAggregator = eventAggregator;
_eventAggregator.GetEvent<StringEvent>().Subscribe(s => Name = s);
}
public string Name
{
get { return _name; }
set
{
// boiler-plate code
if (value == _name)
return;
_name = value;
RaisePropertyChanged(() => Name);
}
}
public void SendMessage(string message)
{
_eventAggregator.GetEvent<StringEvent>().Publish(message);
}
}
PrismViewModelTestCase.cs
using System;
using FluentAssertions;
using Microsoft.Practices.Prism.Events;
using NSubstitute;
using NUnit.Framework;
public class PrismViewModelTestCase
{
private static PrismViewModel CreateViewModel(IEventAggregator eventAggregator = null)
{
// You can't return Substitute.For<IEventAggregator>()
// because it returns null when PrismViewModel's constructor
// invokes GetEvent<StringEvent>() method which leads to NullReferenceException
return new PrismViewModel(eventAggregator ?? CreateEventAggregatorStub());
}
private static IEventAggregator CreateEventAggregatorStub()
{
var eventAggregatorStub = Substitute.For<IEventAggregator>();
eventAggregatorStub.GetEvent<StringEvent>().Returns(Substitute.For<StringEvent>());
return eventAggregatorStub;
}
[Test]
public void Constructor_WithNonNullEventAggregator_ExpectedSubscribesToStringEvent()
{
// Arrange
var stringEventMock = Substitute.For<StringEvent>();
var eventAggregatorStub = Substitute.For<IEventAggregator>();
eventAggregatorStub.GetEvent<StringEvent>().Returns(stringEventMock);
// Act
CreateViewModel(eventAggregatorStub);
// Assert
// With constrained isolation framework you can only mock virtual members
// CompositePresentationEvent<TPayload> has only one virtual Subscribe overload with four parameters
stringEventMock.Received()
.Subscribe(Arg.Any<Action<string>>(), Arg.Any<ThreadOption>(), Arg.Any<bool>(),
Arg.Any<Predicate<string>>());
}
[Test]
public void Name_ExpectedRaisesPropertyChanged()
{
var sut = CreateViewModel();
sut.MonitorEvents();
sut.Name = "any-value";
sut.ShouldRaisePropertyChangeFor(vm => vm.Name);
}
[Test]
public void SendMessage_ExpectedPublishesStringEventThroughEventAggregator()
{
// Arrange
var stringEventMock = Substitute.For<StringEvent>();
var eventAggregatorStub = Substitute.For<IEventAggregator>();
eventAggregatorStub.GetEvent<StringEvent>().Returns(stringEventMock);
var sut = CreateViewModel(eventAggregatorStub);
const string expectedPayload = "any-string-payload";
// Act
sut.SendMessage(expectedPayload);
// Assert
stringEventMock.Received().Publish(expectedPayload);
}
}
MvvmLightViewModel.cs
using System;
using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Messaging;
public sealed class MvvmLightViewModel : ViewModelBase
{
private string _name;
public MvvmLightViewModel(IMessenger messenger)
{
if (messenger == null)
throw new ArgumentNullException("messenger");
// ViewModelBase already have field for IMessenger
MessengerInstance = messenger;
MessengerInstance.Register<string>(this, s => Name = s);
}
public string Name
{
get { return _name; }
set { Set(() => Name, ref _name, value); // Chic! }
}
public void SendMessage(string message)
{
MessengerInstance.Send(message);
}
}
MvvmLightViewModelTestCase.cs
using System;
using FluentAssertions;
using GalaSoft.MvvmLight.Messaging;
using NSubstitute;
using NUnit.Framework;
public class MvvmLightViewModelTestCase
{
private static MvvmLightViewModel CreateViewModel(IMessenger messenger = null)
{
return new MvvmLightViewModel(messenger ?? Substitute.For<IMessenger>());
}
[Test]
public void Constructor_WithNonNullMessenger_ExpectedRegistersToStringMessage()
{
var messengerStub = Substitute.For<IMessenger>();
var sut = CreateViewModel(messengerStub);
messengerStub.Received().Register(sut, Arg.Any<Action<string>>());
}
[Test]
public void Name_ExpectedRaisesPropertyChanged()
{
var sut = CreateViewModel();
sut.MonitorEvents();
sut.Name = "any-value";
sut.ShouldRaisePropertyChangeFor(vm => vm.Name);
}
[Test]
public void SendMessage_ExpectedSendsStringMessageThroughMessenger()
{
var messengerMock = Substitute.For<IMessenger>();
var sut = CreateViewModel(messengerMock);
const string expectedMessage = "message";
sut.SendMessage(expectedMessage);
messengerMock.Received().Send(expectedMessage);
}
}
Disadvantages of Prism:
it's non fully open-source project (official Prism repository is read-only)
2015-10-30: now it's fully open-sourced: https://github.com/PrismLibrary/Prism
it no longer actively developed
2015-10-30: new version of Prism: https://github.com/PrismLibrary/Prism
directly using of its classes leads to boiler-plate and less readable code
I think that any new project should be based on modern solutions and approaches.
IMHO, any modern MVVM-framework (like Catel, Caliburn.Micro, MvvmLight, ReactiveUI) is much better than Prism.
You cannot fully compare Prism and MvvmLight.
Prism is more about application architecture even though Prism has been known as MVVM framework. Actually until Prism 5 it had nothing to do with MVVM and It didn't have BaseViewModel class in Prism 4.1 and in prior.
Prism is not a MVVM framework it is application framework it sits higher than that. Prism 5 introduced some support for MVVM and Prism 6 took it futher.
MVVM is just another aspect of problems that prism provides guidance to solve.
It is like comparing Angular vs. Knockout. AngularJS manages the whole application and defines guidelines on how the application code should be structured, whereas with KnockoutJS the application structure is entirely up to you. It is a similar case between Prism and MvvmLight.
Prism provides an implementation of a collection of design patterns that are helpful in writing well structured and maintainable XAML applications, including MVVM, dependency injection, commanding, event aggregation, and more. Prism's core functionality is a shared code base in a Portable Class Library targeting these platforms; WPF, Windows 10 UWP, and Xamarin Forms.
I would recommend to use Prism if you are developing an enterprise level application using wpf.
Please watch this Webinar about MVVM Made Simple with Prism. The presenter is Brian Lagunas:
https://www.youtube.com/watch?v=ZfBy2nfykqY
I don't believe MS has ever promoted PRISM as a "framework" in the same sense that MVVM Light, Caliburn, etc. are "advertised." My understanding is the PRISM was always presented to the "world" as a "practice." Which, I believe, simply means a organized way of building applications. It becomes a bit confusing because of all the code that is supplied with PRISM and one can consider it a "framework" that can be used to build applications. And, it's true, you can use the code that is supplied with PRISM to build applications of your own. But, PRISM, in my mind is much more complicated than the "frameworks" that are available for MVVM and there is a steep learning curve as well as the possibility of "overkill" and making your application more complex than is necessary. If you have the time to learn the latest "framework" or "practice" at the time you are building your application, that's GREAT! My experience has been that I don't have the luxury of factoring in learning some new "practice" or the latest "framework" but have to get the job done. In an ideal world, one would like to be able to use the latest and greatest "framework" or employ the latest "practices" but sometimes you just have to stick with what you already know and get it done.
See the next post. This original one question content has been removed, as doesn't have any sense. Briefly, I asked how to bind XML (which I generated by mistake while parsing DLL assembly) to TreeView using XmlDataProvider in MVVM way. But later I understood that this approach was wrong, and I switched to generation of data entity model (just write classes which represent all the entities I would like to expose in the tree) instead of XML.
So, the result in the next post. Currently from time to time I update this "article", so F5, and
Enjoy reading!
Introduction
The right way I had found reading this article
It's a long story, most of you just can skip it :) But those, who want to understand the problem and solution, must read this all !
I'm QA, and some time ago had become responsible for Automation of the product I clicks. Fortunately, this automaton takes place not in some Testing Tool, but in Visual Studio, so it is maximally close to development.
For our automation we use a framework which consist of MbUnit (Gallio as runner) and of MINT (addition to MbUnit, which is written by the customer we work with). MbUnit gives us Test Fixtures and Tests, and MINT adds additional smaller layer -- Actions inside tests. Example. Fixture is called 'FilteringFixture'. It consist of amount of tests like 'TestingFilteringById', or 'TestingFilteringWithSpecialChars', etc. Each test consist of actions, which are atomic unit of our test. Example of actions are - 'Open app (parameter)', 'OpenFilterDialog', etc.
We already have a lot of tests, which contain a lot of actions, it's a mess. They use internal API of the product we QA. Also, we start investigation a new Automation approach - UI automation via Microsoft UI Automation (sorry for tautology). So the necessity of some "exporter", or "reporter" tool became severe for managers.
Some time ago I have got a task to develop some application, which can parse a DLL (which contains all the fixtures, tests and actions), and export its structure in the human readable format (TXT, HTML, CSV, XML, any other). But, right after that, I went to vacation (2 weeks).
It happens so, that my girlfriend went to her family until vacation (she also got it), and I remained at home so alone. Thinking what me to do all this time (2 weeks), I remember about that "write exporter tool task" and how long I have been planning to start learning WPF. So, I decided to make my task during vacation, and also dress a application to WPF. At that time I heard something about MVVM, and I decided to implement it using pure MVVM.
DLL which can parse DLL with fixrtures etc had been written rather fast (~1-2 days). After that I had started with WPF, and this article will show you how it ended.
I have spent a major part of my vacation (almost 8 days!), trying to sorted it out in my head and code, and finally, it is done (almost). My girlfriend would not believe what I was doing all this time, but I have a proof!
Sharing my solution step by step in pseudo code, to help others avoid similar problems. This answer is more looks like tutorial =) (Really?). If you are interested what were the most complicated things while learning WPF from scratch, I would say -- make it all really MVVM and f*g TreeView binding!
If you want an archived file with solution, I can give it a bit later, just when I have made a decision, that it is worth of that. One limitation, I'm not sure I may share the MINT.dll, which brings Actions, as it has been developed by the customer of our company. But I can just remove it, and share the application, which can display information about Fixtures and Tests only, but not about actions.
Boastful words. With just a little C# / WinForms / HTML background and no practice I have been able to implement this version of the application in almost 1 week (and write this article). So, impossible is possible! Just take a vacation like me, and spend it to WPF learning!
Step by step tutorial (w/o attached files yet)
Short repetition of the task:
Some time ago I have got a task to develop an application, which can parse a DLL (which contains test fixtures, test methods and actions - units of our unit testing based automation framework), and export its structure in the human readable format (TXT, HTML, CSV, XML, any other). I decided to implement it using WPF and pure MVVM (both were absolutely new things for me). The 2 most difficult problems for me became MVVM approach itself, and then MVVM binding to TreeView control. I skip the part about MVVM division, it's a theme for separate article. The steps below are about binding to TreeView in MVVM way.
Not so important: Create DLL which can open DLL with unit tests and finds fixtures, test methods and actions (more smaller level of unit test, written in our company) using reflection. If you are interested in how it had been done, look here: Parsing function / method content using Reflection
DLL: Separated classes are created for both fixtures, tests and actions (data model, entity model?).We'll use them for binding. You should think by yourself, what will be an entity model for your tree. Main idea - each level of tree should be exposed by appropriate class, with those properties, which help you to represent the model in the tree (and, ideally, will take right place in your MVVM, as model or part of the model). In my case, I was interested in entity name, list of children and ordinal number. Ordinal number is a number, which represents order of an entity in the code inside DLL. It helps me show ordinal number in the TreeView, still not sure it's right approach, but it works!
public class MintFixutre : IMintEntity
{
private readonly string _name;
private readonly int _ordinalNumber;
private readonly List<MintTest> _tests = new List<MintTest>();
public MintFixutre(string fixtureName, int ordinalNumber)
{
_name = fixtureName;
if (ordinalNumber <= 0)
throw new ArgumentException("Ordinal number must begin from 1");
_ordinalNumber = ordinalNumber;
}
public List<MintTest> Tests
{
get { return _tests; }
}
public string Name { get { return _name; }}
public bool IsParent { get { return true; } }
public int OrdinalNumber { get { return _ordinalNumber; } }
}
public class MintTest : IMintEntity
{
private readonly string _name;
private readonly int _ordinalNumber;
private readonly List<MintAction> _actions = new List<MintAction>();
public MintTest(string testName, int ordinalNumber)
{
if (string.IsNullOrWhiteSpace(testName))
throw new ArgumentException("Test name cannot be null or space filled");
_name = testName;
if (ordinalNumber <= 0)
throw new ArgumentException("OrdinalNumber must begin from 1");
_ordinalNumber = ordinalNumber;
}
public List<MintAction> Actions
{
get { return _actions; }
}
public string Name { get { return _name; } }
public bool IsParent { get { return true; } }
public int OrdinalNumber { get { return _ordinalNumber; } }
}
public class MintAction : IMintEntity
{
private readonly string _name;
private readonly int _ordinalNumber;
public MintAction(string actionName, int ordinalNumber)
{
_name = actionName;
if (ordinalNumber <= 0)
throw new ArgumentException("Ordinal numbers must begins from 1");
_ordinalNumber = ordinalNumber;
}
public string Name { get { return _name; } }
public bool IsParent { get { return false; } }
public int OrdinalNumber { get { return _ordinalNumber; } }
}
BTW, I also created an interface below, which implement all the entities. Such interface can help you in the future. Still not sure, should I was also add there Childrens property of List<IMintEntity> type, or something like that?
public interface IMintEntity
{
string Name { get; }
bool IsParent { get; }
int OrdinalNumber { get; }
}
DLL - building data model: DLL has a method which opens DLL with unit tests and enumerating data. During enumeration, it builds a data model like below. Real method example is given, reflection core + Mono.Reflection.dll are used, don't be confused with complexity. All that you need - look how the method fills _fixtures list with entities.
private void ParseDllToEntityModel()
{
_fixutres = new List<MintFixutre>();
// enumerating Fixtures
int f = 1;
foreach (Type fixture in AssemblyTests.GetTypes().Where(t => t.GetCustomAttributes(typeof(TestFixtureAttribute), false).Length > 0))
{
var tempFixture = new MintFixutre(fixture.Name, f);
// enumerating Test Methods
int t = 1;
foreach (var testMethod in fixture.GetMethods().Where(m => m.GetCustomAttributes(typeof(TestAttribute), false).Length > 0))
{
// filtering Actions
var instructions = testMethod.GetInstructions().Where(
i => i.OpCode.Name.Equals("newobj") && ((ConstructorInfo)i.Operand).DeclaringType.IsSubclassOf(typeof(BaseAction))).ToList();
var tempTest = new MintTest(testMethod.Name, t);
// enumerating Actions
for ( int a = 1; a <= instructions.Count; a++ )
{
Instruction action = instructions[a-1];
string actionName = (action.Operand as ConstructorInfo).DeclaringType.Name;
var tempAction = new MintAction(actionName, a);
tempTest.Actions.Add(tempAction);
}
tempFixture.Tests.Add(tempTest);
t++;
}
_fixutres.Add(tempFixture);
f++;
}
}
DLL: Public property Fixtures of the List<MintFixutre> type is created to return just created data model ( List of Fixtures, which contain lists of tests, which contains lists of Actions ). This will be our binding source for TreeView.
public List<MintFixutre> Fixtures
{
get { return _fixtures; }
}
ViewModel of MainWindow (with TreeView inside): Contains object / class from DLL which can parse unit tests DLLs. Also exposes Fixtures public property from the DLL of List<MintFixutre> type. We will bind to it from XAML of MainWindow. Something like that (simplified):
var _exporter = MySuperDllReaderExporterClass ();
// public property of ViewModel for TreeView, which returns property from #4
public List<MintFixture> Fixtures { get { return _exporter.Fixtures; }}
// Initializing exporter class, ParseDllToEntityModel() is called inside getter
// (from step #3). Cool, we have entity model for binding.
_exporter.PathToDll = #"open file dialog can help";
// Notifying all those how are bound to the Fixtures property, there are work for them, TreeView, are u listening?
// will be faced later in this article, anticipating events
OnPropertyChanged("Fixtures");
XAML of MainWindow - Setup data templates: Inside a Grid, which contains TreeView, we create <Grid.Resources> section, which contains a set of templates for our TreeViewItems. HierarchicalDataTemplate (Fixtures and Tests) is used for those who have child items, and DataTemplate is used for "leaf" items (Actions). For each template, we specify which its Content (text, TreeViewItem image, etc.), ItemsSource (in case of this item has children, e.g. for Fixtures it is {Binding Path=Tests}), and ItemTemplate (again, only in case this item has children, here we set linkage between templates - FixtureTemplate uses TestTemplate for its children, TestTemplate uses ActionTemplate for its children, Action template does not use anything, it is a leaf!). IMPORTANT: Don't forget, that in order to "link" "one" template to "another", the "another" template must be defined in XAML above the "one"! (just enumerating my own blunders :) )
XAML - TreeView linkage: We setup TreeView with: linking with data model from ViewModel (remember public property?) and with just prepared templates, which represent content, appearance, data sources and nesting of tree items! One more important note. Don't define your ViewModel as "static" resource inside XAML, like <Window.Resources><MyViewModel x:Key="DontUseMeForThat" /></Window.Resources>. If you do so, then you won't be able to notify it on property changed. Why? Static resource is static resource, it initializes ones, and after that remains immutable. I might be wrong here, but it was one of my blunders. So for TreeView use ItemsSource="{Binding Fixtures}" instead of ItemsSource="{StaticResource myStaticViewModel}"
ViewModel - ViewModelBase - Property Changed: Almost all. Stop! When user opens an application, then initially TreeView is empty of course, as user hasn't opened any DLL yet! We must wait until user opens a DLL, and only then perform binding. It is done via OnPropertyChanged event. To make life easier, all my ViewModels are inherited from ViewModelBase, which right exposes this functionality to all my ViewModel.
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
}
protected virtual void OnPropertyChanged(PropertyChangedEventArgs args)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, args);
}
}
XAML - OnPropertyChanged and commanding. User clicks a button to opens DLL which contains unit tests data. As we using MVVM, then click is handled via commanding. At the end of the OpenDllExecuted handler OnPropertyChanged("Fixtures") is executed, notifying the Tree, that the property, to which it is bind to has been changed, and that now is time to refresh itself. RelayCommand helper class can be taken for example from there). BTW, as I know, there are some helper libraries and toolkits exist Something like that happens in the XAML:
And ViewModel - Commanding
private ICommand _openDllCommand;
//...
public ICommand OpenDllCommand
{
get { return _openDllCommand ?? (_openDllCommand = new RelayCommand(OpenDllExecuted, OpenDllCanExecute)); }
}
//...
// decides, when the <OpenDll> button is enabled or not
private bool OpenDllCanExecute(object obj)
{
return true; // always true for Open DLL button
}
//...
// in fact, handler
private void OpenDllExecuted(object obj)
{
var openDlg = new OpenFileDialog { ... };
_pathToDll = openDlg.FileName;
_exporter.PathToDll = _pathToDll;
// Notifying TreeView via binding that the property <Fixtures> has been changed,
// thereby forcing the tree to refresh itself
OnPropertyChanged("Fixtures");
}
Final UI (but not final for me, a lot of things should be done!). Extended WPF toolkit was used somewhere: http://wpftoolkit.codeplex.com/
i have a solution combined with several projects.
this is my App.xaml.cs
public partial class App : Application
{
private SiteManager _siteManager = new SiteManager();
public SiteManager SiteManager
{
get { return _siteManager; }
set { _siteManager = value; }
}
}
during my run i call another project in the same solution
SiteDll.MainWindow siteManagerDialog = new SiteDll.MainWindow();
siteManagerDialog.Show();
but i dont know how to pass all vars in App.xaml.cs to SiteDll.MainWindow siteManagerDialog.
i tried:
SiteDll.MainWindow siteManagerDialog = new SiteDll.MainWindow((App)Application.Current);
siteManagerDialog.Show();
and cast it in SiteDll.MainWindow constructor:
public MainWindow(object me)
{
Application app = ((App)me);
InitializeComponent();
}
but i get casting error...
what is the correct way to do it ?
You are casting YourApplication1.App type to YourApplication2.App which are different types. That's why you get a invalid cast exception.
What you can do is, you can put your common variables in a separate assembly and reference this assembly from both of your applications.
Edit
I would also suggest you to add a 'WPF User Control Library' and then reference it when you are using wpf windows/pages/user controls from other assemblies, to prevent such confusions.
Why the methods of DomainService which have parameters of complex types (all types except int, bool, float,...) are not recognizable in Model?
in build and even in intelisense! :(
Update
CODE
namespace KhorasanMIS.Web.Services
{
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Data;
using System.Linq;
using System.ServiceModel.DomainServices.EntityFramework;
using System.ServiceModel.DomainServices.Hosting;
using System.ServiceModel.DomainServices.Server;
using KhorasanMIS.Web.Entities;
// Implements application logic using the KhorasanMISEntities context.
// TODO: Add your application logic to these methods or in additional methods.
// TODO: Wire up authentication (Windows/ASP.NET Forms) and uncomment the following to disable anonymous access
// Also consider adding roles to restrict access as appropriate.
// [RequiresAuthentication]
[EnableClientAccess()]
public class DSCountry : LinqToEntitiesDomainService<KhorasanMISEntities>
{
// TODO:
// Consider constraining the results of your query method. If you need additional input you can
// add parameters to this method or create additional query methods with different names.
// To support paging you will need to add ordering to the 'Countries' query.
public IQueryable<Country> GetCountries()
{
return this.ObjectContext.Countries;
}
public IEnumerable<Country> GetCountryByCode(string pCode)
{
return this.ObjectContext.Countries.Where(a => a.ISOCode.Equals(pCode));
}
public bool IsValidEdit(string pCode)
{
List<string> message = new List<string>();
IEnumerable<Country> list = GetCountryByCode(pCode);
if (list.Count() > 0)
{
return false;
}
return true;
}
//***********************************************************
public bool Update(Country currentItem)
{
this.ObjectContext.Countries.AttachAsModified(currentItem, this.ChangeSet.GetOriginal(currentItem));
return true;
}
//***********************************************************
}
}
The method in the stars-comment can not be used in a model but if I change its parameter to int, it will become usable.
Check out the documentation of DomainServices. Insert, update and delete operation are handled by the EntitySet and the SubmitChanges method on the client side.
When you expose a domain service, an EntitySet object is generated in the domain context with properties that indicate which operations (insert, update, or delete) are permitted from the client. You execute data modifications by modifying the entity collection and then calling the SubmitChanges method.
I'm trying to write a ParentAdapter implementation; I'm interested in providing design-time support for some WPF controls I'm writing and this is how you manage custom logic for reparenting items to different container controls. I started small, with the notion of creating a StackPanel-derived class that would only allow Button elements to be parented at design-time (yes, I'm aware the panel itself needs code to support this as well.) I started with what I figured would be the simplest the ParentAdapter could be:
using System;
using System.Windows;
using System.Windows.Controls;
using Microsoft.Windows.Design.Interaction;
using Microsoft.Windows.Design.Model;
namespace ControlLibrary.Design
{
internal class SimplePanelParentAdapter : ParentAdapter
{
public override bool CanParent(ModelItem parent, Type childType)
{
return (childType == typeof(Button));
}
// moves the child item into the target panel; in this case a SimplePanel
public override void Parent(ModelItem newParent, ModelItem child)
{
using (ModelEditingScope undoContext = newParent.BeginEdit())
{
// is this correct?
//child.Content.SetValue("I'm in a custom panel!");
SimplePanel pnl = newParent.GetCurrentValue() as SimplePanel;
pnl.Children.Add(child.GetCurrentValue() as UIElement);
undoContext.Complete();
}
}
public override void RemoveParent(ModelItem currentParent, ModelItem newParent, ModelItem child)
{
// No special things need to be done, right?
child.Content.SetValue("I was in a custom panel.");
}
}
}
When I work with this at design-time, as soon as I drag a button over my custom panel, a NullReferenceException is thrown from deep within the VS code. My code is not throwing the exception, because I can step all the way through my method; the call stack indicates that code in Microsoft.Windows.Design.Developer.dll is throwing the exception.
Obviously I'm doing something incorrectly, but the documentation provides no examples and my search-fu seems to indicate that either no one is trying this or anyone who is trying it isn't talking about it. Does anyone have suggestions?
I found the answer to my question myself. The problem is caused by editing the model instead of the ModelItem wrapper. What I should have done (and does work) is something like this:
using System;
using System.Windows.Controls;
using Microsoft.Windows.Design.Interaction;
using Microsoft.Windows.Design.Model;
namespace ControlLibrary.Design
{
internal class SimplePanelParentAdapter : ParentAdapter
{
public override bool CanParent(ModelItem parent, Type childType)
{
return (childType == typeof(Button));
}
// moves the child item into the target panel; in this case a SimplePanel
public override void Parent(ModelItem newParent, ModelItem child)
{
using (ModelEditingScope undoContext = newParent.BeginEdit())
{
ModelProperty prop = newParent.Properties["Children"];
ModelItemCollection items = (ModelItemCollection)prop.Value;
items.Add(child);
undoContext.Complete();
}
}
public override void RemoveParent(ModelItem currentParent, ModelItem newParent, ModelItem child)
{
using (ModelEditingScope scope = child.BeginEdit())
{
ModelProperty prop = currentParent.Properties["Children"];
((ModelItemCollection)prop.Value).Remove(child);
scope.Complete();
}
}
}
}
I was confused when I wrote the first code and unsure how I was supposed to call Add() on the Children property; it looks like ModelProperty.Value wraps collections with ModelItemCollection, so unless you go out of your way to make your class use an obtuse interface this should work.