I don't have a good understanding of MVVM and have read some articles on other pages.
Does the ViewModel do all the business logic and interact with data model which then send the data to the view for presentation.
On the View, will only have function that related directly UI.
This is my code I have code but not sure is it the correct way to do it
private TestingViewModel Model
{
get
{
return BindingContext as TestingViewModel;
}
}
public TestingPage()
{
InitializeComponent();
BindingContext = new TestingViewModel();
Model.PropertyChanged += TestingPropertyChanged;
NavigationPage.SetHasNavigationBar(this, false);
NavigationPage.SetHasBackButton(this, false);
MediaFactory.ClearAllCachedMedia();
SetupTappedControl();
}
private void TestingPropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName.Equals("ActivePage"))
{
if (Model.ActivePage != null)
{
Device.BeginInvokeOnMainThread(async () =>
{
var path = await MediaFactory.GetImagePathAsync(Model.ActivePage.DocumentId + "_" + Model.ActivePage.PageNumber);
CanvasView.Strokes = Model.ActivePage.Strokes;
CanvasView.LoadBitmapInMemory(path, Model.ActivePage.Width, Model.ActivePage.Height);
});
}
}
}
#region File Button Selected
public void TapFileOpen(object sender, EventArgs e)
{
Task.Run(() =>
{
var pickFileTask = CrossFilePicker.Current.PickFile();
var data = pickFileTask.Result;
Model.OpenDocument(data.FileName, data.DataArray);
});
}
Question 1: Why I have to pass it to a BindingContext if I can just hold TestingViewModel object in the Code-behind class of the View?
The line below :
CanvasView.Strokes = Model.ActivePage.Strokes;
CanvasView.LoadBitmapInMemory(path, Model.ActivePage.Width, Model.ActivePage.Height);
Question 2 : The Strokes is a collection of stroke object. Do Strokes
also a ViewModel.
The Idea of the MVVM is to separate concern between creating Views and business logic.
the ViewModel will manage your business logic including implementation of Properties and Commands or RelyCommands that you will later on bind it into your views, So simple you will not have any code behind in your views on best case.
For better understanding for this pattern, I would suggest you to read this msdn link that will illustrate to you how can you re-structure your code for better use and what are the benefits from using MVVM
Related
I am currently programming using the MVVM pattern.
My view Model Looks like so
class DoorsViewModel
{
ObservableCollection<Door> doorCollection;
};
the door class looks like the following
class Door
{
string name;
bool isOpen;
};
my view is linked to the viewmodel, and simply contains a longlistselector with a picture and the name of the door. I want the picture to be dynamic and change depending on the state of the door( whether it's open or closed ). How would i implement it so that the picture updates dynamically depending on the state of the door? should this be done in the viewmodel? or should it be done within the view?
This logic should be in the ViewModel. All logic that is related to the view or how things are displayed should be in the ViewModel. No logic should be in the view (.xaml.cs).
You typically use the INotifyPropertyChanged interface to notify the View that something has changed. In this case you want a door image to be changed when the state of the door changes. In this case I would try something like this.
class Door: INotifyPropertyChanged
{
private string _name;
private bool _isOpen;
public Uri DoorImage
{
get
{
if (_isOpen) return new Uri("uri_to_open.png");
return new Uri("uri_to_closed.png");
}
}
public bool IsOpen
{
get { return _isOpen; }
set
{
_isOpen = value;
RaisePropertyChanged("IsOpen");
// important, notifies the UI to update the door image
RaisePropertyChanged("DoorImage");
}
}
private void RaisePropertyChanged(string propertyName)
{
var tmp = PropertyChanged;
if (tmp != null) tmp(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
};
Note: I have encapsulated the fields into properties.
If your image is embedded in your assebmly, please check out this link to learn how to write an uri for your image.
I have seen many examples using Entity Framework in MVC3 applications, they are very simple demos which only have one mvc3 web project with edmx inside it.
So, they can use the best practice for open and close connection by "using" statement:
using(var context = new SchoolEntities())
{
// do some query and return View with result.
}
And, It can use lazy load (navigation properties) inside the "using" statment correctly, because the context is not yet
disposed:
foreach(var item in student.Course)
{
// do something with the navigation property Course
}
All things seems to be perfect until it becomes an n-tier application.
I created DAL, BLL, and a MVC3 UI.
The DAL have edmx inside it, and operator classes like SchoolDA.cs:
public class StudentDA()
{
public Student FindStudent(int studentId)
{
using(var context = new SchoolContext())
{
// do query, return a student object.
}
}
}
Then, in the BLL, if I use:
var student = studentDa.FindStudent(103);
then invoke it's navigation property:
student.Course
I will get an error (of course):
The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
So, I have to change StudentDA.cs like this:
public class StudentDA() : IDisposable
{
private SchoolEntites context;
public StudentDA()
{
context = new SchoolEntities();
}
public void Dispose()
{
context.Dispose();
}
public Student FindStudent(int studentId)
{
// do query, return a student object.
}
}
Then, the BLL will change like this:
public Student FindStudent(int id)
{
using(var studentDa = new StudentDA())
{
// this can access navigation properties without error, and close the connection correctly.
return studentDa.FindStudent(id);
}
}
All things seem to be perfect again until it meet Update() method.
Now, if I want to update a student object which is taken from BLL.FindStudent(), the context.SaveChanges() will return 0, because the context is already disposed in the BLL.FindStudent(), and nothing will be updated to database.
var optStudent = new StudentBO();
var student = optStudent.FindStudent(103);
student.Name = "NewValue";
optStudent.Update(student);
Does anyone have idea on how to use EntityFramework in 3 tire application? or how can I manage the context correctly. I will use navigation propertites very often in the web layer, but I can't always remain connection open to consume the server memory.
There are multiple ways to handle EF context's lifetime. In web apps, usually context is unique for an HttpRequest. For example, if you want to handle this manually in a web application and have a per Thread/HttpRequest EF context, you can do so with the following (Code copied from http://www.west-wind.com/weblog/posts/2008/Feb/05/Linq-to-SQL-DataContext-Lifetime-Management):
internal static class DbContextManager
{
public static DbContext Current
{
get
{
var key = "MyDb_" + HttpContext.Current.GetHashCode().ToString("x")
+ Thread.CurrentContext.ContextID.ToString();
var context = HttpContext.Current.Items[key] as MyDbContext;
if (context == null)
{
context = new MyDbContext();
HttpContext.Current.Items[key] = context;
}
return context;
}
}
}
And then you can easily use:
var ctx = DbContextManager.Current
But I suggest you leave the lifetime management to an IoC framework like Autofac, Castle Windsor, or Ninject which automatically handle the creation/disposal of your registered obejcts along with many other features.
Thanks for your answer Kamyar. I came across this whilst looking for a simple strategy to manage the ObjectContext lifetime without having to use an IoC framework, which seems a bit overkill for my needs.
I also came across your other post here, for disposing of the context at the end of the request.
Thought this might be useful for others coming across this, so just posting my implementation of your code here:
Context manager class -
internal static class MyDBContextManager
{
//Unique context key per request and thread
private static string Key
{
get
{
return string.Format("MyDb_{0}{1}", arg0: HttpContext.Current.GetHashCode().ToString("x"),
arg1: Thread.CurrentContext.ContextID);
}
}
//Get and set request context
private static MyDBContext Context
{
get { return HttpContext.Current.Items[Key] as MyDBContext; }
set { HttpContext.Current.Items[Key] = value; }
}
//Context per request
public static MyDBContext Current
{
get
{
//if null, create new context
if (Context == null)
{
Context = new MyDBContext();
HttpContext.Current.Items[Key] = Context;
}
return Context;
}
}
//Dispose any created context at the end of a request - called from Global.asax
public static void Dispose()
{
if (Context != null)
{
Context.Dispose();
}
}
}
Global.asax (MVC) -
public override void Init()
{
base.Init();
EndRequest +=MvcApplication_EndRequest;
}
private void MvcApplication_EndRequest(object sender, EventArgs e)
{
MyDBContextManager.Dispose();
}
I am new to MvvmCross and I have a question.
I noticed that the following binding code works in one way only:
{ this, "{'CurrentIndex':{'Path':'CurrentIndex','Mode':'TwoWay'}}" }
CurrentIndex is an Int Property in the View
CurrentIndex is also an Int Property in the ViewModel
This way works!
ViewModel => View
But not this way!
View => ViewModel
I have a collection of ViewControllers and my goal was to call a DeleteCommand for the CurrentIndex in the viewModel.
However,
"Android and Touch 2 way bindings are incomplete"
Reference: MvvmCross experiences, hindsight, limitations?
My guess is the TwoWay mode only works for Controls (UILabel, UITextfield, ...) but not for Properties.
So, is there a good way to make it works in both ways? Or Are there any alternatives to my problem?
Patrick
In order for a binding to transfer any value between a View to a ViewModel, then it needs to hook into some event when the value changes.
In the ViewModel, this event is always the event in the INotifyProperty interface.
In the View/Activity, there is one single pattern employed - so each binding has to hook into a separate event. For example, the Text on EditText is hooked up using the TextChanged event (see MvxEditTextTextTargetBinding.cs) while the value in a SeekBar is hooked up using a Listener object rather than an event (see MvxSeekBarProgressTargetBinging.cs).
So if you wanted to implement this two-way binding for your activity, then you could do this by:
declaring an event - CurrentIndexChanged - in your activity (MyActivity) which is fired whenever CurrentIndex changes
declare a custom binding for your MyActivity which programmatically links CurrentIndex and CurrentIndexChanged
adding the custom binding to the binding registry during Setup
For example, your activity might include:
public event EventHandler CurrentIndexChanged;
private int _currentIndex;
public int CurrentIndex
{
get { return _currentIndex; }
set { _currentIndex = value; if (CurrentIndexChanged != null) CurrentIndexChanged(this, EventArgs.Empty); }
}
And you might then declare a binding class like:
public class MyBinding : MvxPropertyInfoTargetBinding<MyActivity>
{
public MyBinding (object target, PropertyInfo targetPropertyInfo)
: base(target, targetPropertyInfo)
{
View.CurrentIndexChanged += OnCurrentIndexChanged;
}
public override MvxBindingMode DefaultMode
{
get
{
return MvxBindingMode.TwoWay;
}
}
private void OnCurrentIndexChanged(object sender, EventArgs ignored)
{
FireValueChanged(View.CurrentIndex);
}
protected override void Dispose(bool isDisposing)
{
base.Dispose(isDisposing);
if (isDisposing)
{
View.CurrentIndexChanged -= OnCurrentIndexChanged;
}
}
}
And you'd need to tell the binding system about this binding in setup like:
registry.RegisterFactory(new MvxSimplePropertyInfoTargetBindingFactory(typeof(MyBinding), typeof(MyActivity), "CurrentIndex"));
However... at a practical level, if you are operating in C# rather than in XML, then you might be better off in this case using C# to simply update the ViewModel rather than using declarative binding in this case.
To be clear... in this case, I would most probably just write the Activity property as:
public int CurrentIndex
{
get { return _currentIndex; }
set { _currentIndex = value; ViewModel.CurrentIndex = value; }
}
Or... I'd consider not having this property in the Activity at all.
If it helps, there's some more information on custom bindings in:
MonoTouch MVVMCross binding to instance variables
In MvvmCross how do I do custom bind properties
Hope this helps! IMHO the bindings are there to help you when you're working in XML - you don't have to use them...
Stuart
UPDATE If you are going to do lots of these and follow the same name pattern - using property named X with changed EventHandler event named XChanged then something like this might work - it uses reflection to find the event automagically:
public class MyBinding<T> : MvxPropertyInfoTargetBinding<T>
where T : class
{
private readonly PropertyInfo _propertyInfo;
private readonly EventInfo _eventInfo;
public MyBinding(object target, PropertyInfo targetPropertyInfo)
: base(target, targetPropertyInfo)
{
_propertyInfo = targetPropertyInfo;
var eventName = _propertyInfo.Name + "Changed";
_eventInfo = View.GetType().GetEvent(eventName);
if (_eventInfo == null)
{
throw new MvxException("Event missing " + eventName);
}
if (_eventInfo.EventHandlerType != typeof(EventHandler))
{
throw new MvxException("Event type mismatch for " + eventName);
}
var addMethod = _eventInfo.GetAddMethod();
addMethod.Invoke(View, new object[] { new EventHandler(OnChanged) });
}
public override MvxBindingMode DefaultMode
{
get
{
return MvxBindingMode.TwoWay;
}
}
private void OnChanged(object sender, EventArgs ignored)
{
var value = _propertyInfo.GetValue(View, null);
FireValueChanged(value);
}
protected override void Dispose(bool isDisposing)
{
base.Dispose(isDisposing);
if (isDisposing)
{
var removeMethod = _eventInfo.GetRemoveMethod();
removeMethod.Invoke(View, new object[] { new EventHandler(OnChanged) });
}
}
}
The Navigation framework in Windows Phone 7 is a cut down version of what is in Silverlight. You can only navigate to a Uri and not pass in a view. Since the NavigationService is tied to the View, how do people get this to fit into MVVM. For example:
public class ViewModel : IViewModel
{
private IUnityContainer container;
private IView view;
public ViewModel(IUnityContainer container, IView view)
{
this.container = container;
this.view = view;
}
public ICommand GoToNextPageCommand { get { ... } }
public IView { get { return this.view; } }
public void GoToNextPage()
{
// What do I put here.
}
}
public class View : PhoneApplicationPage, IView
{
...
public void SetModel(IViewModel model) { ... }
}
I am using the Unity IOC container. I have to resolve my view model first and then use the View property to get hold of the view and then show it. However using the NavigationService, I have to pass in a view Uri. There is no way for me to create the view model first. Is there a way to get around this.
Instead of passing the view through the constructor. You could construct the view first via the NavigationService and pass it into the view-model. Like so:
public class ViewModel : IViewModel
{
private IUnityContainer container;
private IView view;
public ViewModel(IUnityContainer container)
{
this.container = container;
}
public ICommand GoToNextPageCommand { get { ... } }
public IView
{
get { return this.view; }
set { this.view = value; this.view.SetModel(this); }
}
public void GoToNextPage()
{
// What do I put here.
}
}
PhoneApplicationFrame frame = Application.Current.RootVisual;
bool success = frame.Navigate(new Uri("View Uri"));
if (success)
{
// I'm not sure if the frame's Content property will give you the current view.
IView view = (IView)frame.Content;
IViewModel viewModel = this.unityContainer.Resolve<IViewModel>();
viewModel.View = view;
}
If you are using Mvvm Light you could try:
Windows Phone 7 — Navigation between pages using MVVM Light Messaging
(See similar post: Silverlight Navigation using Mvvm-light(oobe)+MEF?)
My opinion is that the view-model should be created and registered at application startup. By placing it inside the root DataContext all pages will automatically get a reference to it without any code-behind or IoC tricks.
// Code to execute when the application is launching (eg, from Start)
// This code will not execute when the application is reactivated
private void Application_Launching(object sender, LaunchingEventArgs e)
{
m_ViewModel = new PrimaryViewModel(RootFrame) ;
RootFrame.DataContext = m_ViewModel;
}
// Code to execute when the application is activated (brought to foreground)
// This code will not execute when the application is first launched
private void Application_Activated(object sender, ActivatedEventArgs e)
{
m_ViewModel = new PrimaryViewModel(RootFrame) ;
m_ViewModel.Activated(PhoneApplicationService.Current.State);
RootFrame.DataContext = m_ViewModel;
}
If you are using MVVM architecture,then you can pass navigationPage after registering using Messenger. Create a model class (say NavigateToPageMessage) with a string(say PageName) variable. You want to pass string from homepage.xaml to newpage.xaml,then in Homepage viewmodel just send the message like this under the command you binded (say HomeNavigationCommand)
private void HomeNavigationCommandHandler()
{
Messenger.Default.Send(new NavigateToPageMessage {PageName = "newpage"});
}
In the newpage Viewmodel,you should register the messenger like this,
Messenger.Default.Register<NavigateToPageMessage>(this, (action) => ReceiveMessage(action));
private object ReceiveMessage(NavigateToPageMessage action)
{
var page = string.Format("/Views/{0}.xaml", action.PageName);
NavigationService.Navigate(new System.Uri(page,System.UriKind.Relative));
return null;
}
//Assuming your views are in View Folder
My application has a menu option to allow the creation of a new account. The menu option's command is bound to a command (NewAccountCommand) in my ViewModel. When the user clicks the option to create a new account, the app displays a "New Account" dialog where the user can enter such data as Name, Address, etc... and then clicks "Ok" to close the dialog and create the new account.
I know my code in the ViewModel is not correct because it creates the "New Account" dialog and calls ShowDialog(). Here is a snippet from the VM:
var modelResult = newAccountDialog.ShowDialog();
if (modelResult == true)
{
//Create the new account
}
how do i avoid creating and showing the dialog from within my VM so I can unit test the VM?
I like the approach explained in this codeproject article:
http://www.codeproject.com/KB/WPF/XAMLDialog.aspx
It basically creates a WPF Dialog control that can be embedded in the visual tree of another window or usercontrol.
It then uses a style trigger that causes the dialog to open up whenever there is content in the dialog.
so in you xaml all you have to do is this(where DialogViewModel is a property in you ViewModel):
<MyControls:Dialog Content = {Binding DialogViewModel}/>
and in you ViewModel you just have to do the following:
DialogViewModel = new MyDialogViewModel();
so in unit testing all you have to do is:
MyViewModel model = new MyViewModel();
model.DialogViewModel = new MyDialogViewModel();
model.DialogViewModel.InputProperty = "Here's my input";
//Assert whatever you want...
I personally create a ICommand property in my ViewModel that sets the DialogViewModel property, so that the user can push a button to get the dialog to open up.
So my ViewModel never calls a dialog it just instantiates a property. The view interprets that and display a dialog box. The beauty behind this is that if you decide to change your view at all and maybe not display a dialog, your ViewModel does not have to change one bit. It pushes all the User interaction code where it should be...in the view. And creating a wpf control allows me to re-use it whenever I need to...
There are many ways to do this, this is one I found to be good for me. :)
In scenarios like this, I typically use events. The model can raise an event to ask for information and anybody can respond to it. The view would listen for the event and display the dialog.
public class MyModel
{
public void DoSomething()
{
var e = new SomeQuestionEventArgs();
OnSomeQuestion(e);
if (e.Handled)
mTheAnswer = e.TheAnswer;
}
private string mTheAnswer;
public string TheAnswer
{
get { return mTheAnswer; }
}
public delegate void SomeQuestionHandler(object sender, SomeQuestionEventArgs e);
public event SomeQuestionHandler SomeQuestion;
protected virtual void OnSomeQuestion(SomeQuestionEventArgs e)
{
if (SomeQuestion == null) return;
SomeQuestion(this, e);
}
}
public class SomeQuestionEventArgs
: EventArgs
{
private bool mHandled = false;
public bool Handled
{
get { return mHandled; }
set { mHandled = value; }
}
private string mTheAnswer;
public string TheAnswer
{
get { return mTheAnswer; }
set { mTheAnswer = value; }
}
}
public class MyView
{
private MyModel mModel;
public MyModel Model
{
get { return mModel; }
set
{
if (mModel != null)
mModel.SomeQuestion -= new MyModel.SomeQuestionHandler(mModel_SomeQuestion);
mModel = value;
if (mModel != null)
mModel.SomeQuestion += new MyModel.SomeQuestionHandler(mModel_SomeQuestion);
}
}
void mModel_SomeQuestion(object sender, SomeQuestionEventArgs e)
{
var dlg = new MyDlg();
if (dlg.ShowDialog() != DialogResult.OK) return;
e.Handled = true;
e.TheAnswer = dlg.TheAnswer;
}
}
The WPF Application Framework (WAF) shows a concrete example how to accomplish this.
The ViewModel sample application shows an Email Client in which you can open the “Email Account Settings” dialog. It uses dependency injection (MEF) and so you are still able to unit test the ViewModel.
Hope this helps.
jbe
There are different approaches to this. One common approach is to use some form of dependency injection to inject a dialog service, and use the service.
This allows any implementation of that service (ie: a different view) to be plugged in at runtime, and does give you some decoupling from the ViewModel to View.