I'm trying to accomplish a correct MVVM architecture in WPF.
I have a player, in the "Model" section there is a Boolean property that says if we are "playing" right now.
public bool IsPlaying
{
get
{
return isPlaying;
}
set
{
isPlaying = value;
OnPropertyChanged("IsPlaying");
}
}
(Notice I implemented the "INotifyPropertyChanged" interface, so the OnPropertyChanged function reports the change)
in my ViewModel, I have a ImageSource property called "ToggleButtonIcon":
public ImageSource ToggleButtonIcon
{
get
{
if (Model.IsPlaying)
return pauseIcon;
else
return playIcon;
}
}
Which I bind to a "TogglePlayButton" in the view:
<cc:IconButton x:Name="TogglePlayButton"
Style="{StaticResource playButton}"
ImageSource="{Binding Path=ToggleButtonIcon,
UpdateSourceTrigger=PropertyChanged}"
Command="{Binding Path=TogglePlayCommand}"/>
(It's a custom control, but it's working, I checked)
Of course I want the button to change its image icon according to if it is playing (pause) and if it is not playing (play).
Problem is the ToggleButtonIcon does not notify when it changes, and I can't implement the INotifyValueChanged in the ViewModel section because a. I understood that's not a part of the MVVM architexture, and b. I don't know when it changes since it depends on the IsPlaying property of Model.
I thought about putting the ToggleButtonIcon property on the Model section, but that's not "Business Logic" so I don't think that's the right way.
I also thought about using a converter and bind the IconButton directly to "IsPlaying", which would probably work, but I read here: How can WPF Converters be used in an MVVM pattern? that you should not use converters at all in MVVM because you can do any convertion you want in the "ViewModel" Section.
What's the best way to accomplish this in MVVM architecture?
To me, IsPlaying should be in the ViewModel, with change notification implemented, as it represents an application state of sorts.
To solve the issue I would recommend taking the ToggleButtonIcon out of the ViewModel, and creating a DataTrigger on the IconButton control (via its Style), that binds to the IsPlaying property (on the ViewModel) and alters the ImageSource property based on that.
The model of MVVM should only hold class entities and those entities can on occasion have an INotiftPropertyChanged, but generally they do not.
What your intent though is that it is to convey a status and that should be on your viewmodel.
I would recommend that you have the status of IsPlaying be on the View Model (VM) and bind to that. Then in the command of TogglePlayCommand, it will set that property on the VM.
That way both items update propertly on a change to either. You can still new up your original object in the model and on the Setter of the VM's IsPlaying set the class instance property to its value if needed.
Take a look at my blog post Xaml: ViewModel Main Page Instantiation and Loading Strategy for Easier Binding. Take note on how I use OnPropertyChanged to push change messages on other operations which can all for the flexibility you seek as well as having the viewmodel hold statuses and not the models.
You should put bool on class which implements interface INotifyPropertyChange:
Here an example:
public class Game : INotifyPropertyChanged
{
private bool _isPlaying;
public string IsPlaying
{
get { return _isPlaying; }
set {
_isPlaying = value;
this.NotifyPropertyChangedBool();
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChangedBool()
{
if (PropertyChanged != null)
{
PropertyChanged(this, new
PropertyChangedEventArgs("IsPlaying"));
}
}
Related
Lets assume property Name is bind to TextBox in view like this.
private string name
public string Name
{
get {return name;}
set {
name=value;
OnPropertyChanged("Name");
}
}
View
<TextBox Text="{Binding Name, Mode=TwoWay"/>
When we update the text in text box, it will call the setter in Name property which in turn raise PropertyChanged which suppose to update UI again. I am curious how WPF avoid recursion of update and raise event. is it done by considering the sender of that event?
A standard implementation of a property should look like this:
private string name;
public string Name
{
get { return name; }
set
{
if( name != value )
{
name = value;
OnPropertyChanged("Name");
}
}
}
Note the additional if to make sure the event is only raised if the value of the property actually changed.
There is no recursion as far I understand.
1) TextBox updates the value using viewmodel property.
2) Viewmodel raises update, letting UI know that something changed
3) TextBox now updates himself to match the viewmodel value.
may the answer from here help you out.
if you set a property from ui to viewmodel it goes like this.
setter call started
value set
INotifyPropertyChanged started
INotifyPropertyChanged done
setter done
getter called and done
IDataErrorInfo called and done
but if you set the property in your viewmodel it goes like this
setter call started
value set
INotifyPropertyChanged started
getter called and done
IDataErrorInfo called and done
INotifyPropertyChanged done
setter done
Changing property from UI to ViewModel may lead to deadlock kind of
situation which might run into end less recursive calls in two way
scenarios. In order to block this from happening, when WPF is making
change to model, it will continue to track changes via
INotifyPropertyChanged ,but this change will be queued in dispatcher
queue, and it will be executed after its current update is finished.
Since the change in viewmodel is not initiated by WPF, WPF will not
queue the operation, it will immediately execute the change.
I am writing an application based on MVVM architecture. The application has a Wizard like workflow. In couple of pages (views) in my application, I need a button to be auto-clicked when a certain condition is satisfied. The views are tied together using the root Wizard view model which has a ClickNextBtn command that is tied to the Next button in the root Wizard view. So, I need something like in the root Wizard view:
<DataTrigger Binding="{Binding Path=CanAutoClickNext}" Value="True">
<Setter Property="ClickBtn" Value="true" />
</DataTrigger>
The property in the above sample is meaningless, but hopefully it helps convey what I am trying to do.
The CanAutoClickNext bool property is available off of the Wizard view model.
On one of the views where I need the Next button auto-clicked, I tried passing the WizardViewModel as an argument to its corresponding view model's constructor when it is first instantiated in the root wizard view model, and then calling the ClickNextBtn off of it in a method therein later when the view is actually loaded. But that did not work, not surprisingly.
I know how to programmatically click a wpf button, but getting it all tied together in the framework I have is proving to a big challenge. Any feedback is appreciated.
UPDATE:
I ended up rewriting the UI design pattern (still MVVM) so that now instead of having to having to move to a next page automatically, the state within a page changes and a different set of controls become active. Users are then prompted to click next.
Like the comment's on your question stated, this should be a concern of the ViewModel to invoke the Click Handler.
How you could go about implementing this is very similar to something like this Question's answer
Now in MVVM, you should have your Button's connected to an ICommand in the ViewModel(If your using MVVM Light toolkit, it will be RelayCommand/RelayCommand<T>).
Now assuming this ICommand variable in your VM is called NextButtonCommand,
what you can do is
public bool CanAutoClickNext {
get {
return _canAutoClickNext;
}
private set {
if (value == _canAutoClickNext)
return;
_canAutoClickNext = value;
RaisePropertyChanged(() => CanAutoClickNext);
if (_canAutoClickNext)
NextButtonCommand.Execute(null);
}
}
with this, when your property in the VM CanAutoClickNext gets set to "True", the Execute function of the ICommand is automatically invoked by the VM. This seperates all the logic handling to the VM and keeps the View dumb as what is recommended by MVVM when it comes to application / business logic.
Side Note
The property CanAutoClickNext seems a waste if it's not being bound to anything from the View. If this is the case, I'd recommend just getting rid of that property and invoke the ICommand.Execute(null) from the place where the logic holds fit than use a property with INPC just for this case.
I'll follow up from a different angle. Let's say you have any message bus ready (IEventAggregator, IMessenger, doesn't matter). I'll use the Caliburn.Micro's IEventAggregator along with the nomenclature 'cause that's what I'm most familiar with. Now you might have a very simple event:
public class MoveNext
{
}
Then your 'host' viewmodel of the wizard:
public class WizardHost : IHandle<MoveNext>
{
private readonly IEventAggregator messageBus
public WizardHost(IEventAggregator messageBus)
{
this.messageBus = messageBus;
this.messageBus.Subscribe(this);
}
/here you might have the 'real' command method, e.g:
public void GoToNextQuestion()
{
// do stuff
}
public void Handle(MoveNext message)
{
GoToNextQuestion();
}
}
public class WizardPage
{
private readonly IEventAggregator messageBus;
private bool shouldMoveToNext;
public WizardPage(IEventAggregator messageBus)
{
this.messageBus = messageBus;
}
public void DoStuff()
{
//at some point, you might want to switch the flag or do whatever you need/want to do and:
if(shouldMoveToNext)
messageBus.Publish(new MoveNext());
}
}
Now when you DoStuff() in your wizard page, you can publish the event and the 'host' page will react and flip the page.
That's of course all nice if you're using any MVVM framework that's out there. MVVM Light has the Messenger, Caliburn.Micro has - as you might have noticed - IEventAggregator.
Hi I have a viewmodel where i can track the value of a certain item in the constructor. I am opening a dialog window using the MVVM model.
example
private int _myField;
public ClassName(int MyProperty)
{
_myField = MyProperty;
}
public int MyIntProperty
{
get{ return _myField;}
set { _myField = value;}
}
this is all perfect obviously.
but as soon as the window opens the value in the viewmodel changes.
lets say the _myField goes from 1 to 8 with out any interaction. i've walked through the code and there are no other interactions with the field.
also not in the code sample is the bound property.
anyone every came accross this before. it has me stumped.
Edit: included missing property from example
You should either:
1) Implement INotifyPropertyChanged on ClassName. This will allow you to raise the PropertyChanged event when you change MyIntProperty. WPF will listen to this event and update the UI accordingly.
or
2) Make ClassName inherit from DependancyObject and MyIntProperty a dependency property. This will take care of everything for you.
I'm getting confused; I thought I understood INotifyPropertyChanged.
I have a small WPF app with a frontend MainWindow class, a viewmodel in the middle and a model at the back.
The model in my case is the Simulator class.
The SimulatorViewModel is nearly transparent and just interfaces properties between MainWindow and Simulator.
Simulator implements INotifyPropertyChanged and each property setter in Simulator calls the RaisePropertyChanged method:
private string serialNumber;
public string SerialNumber
{
get { return serialNumber; }
set
{
serialNumber = value;
RaisePropertyChanged("SerialNumber");
}
}
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged(string propName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
In the xaml, I have a TextBox with binding like this:
Text="{Binding Path=SerialNumber, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
and the DataContext is the SimulatorViewModel (but see my comment about changing the DataContext to the model)
The ViewModel just passes the properties through:
public string SerialNumber
{
get { return Simulator.SerialNumber; }
set { Simulator.SerialNumber = value; }
}
Programmatic updates to the property SerialNumber in Simulator are not propagating up to the UI although curiously the initial value, set in the Simulator constructor, is getting there.
If I break on the SerialNumber setter in the Simulator and follow into RaisePropertyChanged, I find that PropertyChanged is null and so the event is not propagating upwards to the GUI.
Which leads me to several questions:
What exactly should be hooking into the PropertyChanged event? I'm looking to be more specific than just "WPF". What's the link between that event and the Binding statement in the xaml?
Why is the initial property value getting up to the UI at startup, but not subsequently?
Am I right to have Simulator (the model) implementing INotifyPropertyChanged, or should it be the ViewModel doing that? If the ViewModel does it, then programmatic changes in the model don't trigger PropertyChanged, so I'm unclear on the correct pattern. I realise my ViewModel is virtually redundant, but that's due to the simplicity of the project; a more complex one would make the ViewModel concept work harder. My understanding is that the ViewModel is the place to interface my unit tests.
The problem is that you're raising PropertyChanged on your model, but your view is bound to ViewModel. So your View subscribes only to ViewModel events (not Model's) - that is why textbox is not updated - because it never receives PropertyChanged event. One of possible solutions is to listen in ViewModel for Model PropertyChanged events and raise same event on ViewModel accordingly.
Initial value is being propagated because your setters/getters in ViewModel are correct, the problem is in events.
Yes, you are correct (see my #1), you should raise PropertyChanged on your ViewModel because View is bound to it. These events will not be triggered automatically after changes in Model, so you should listen for Model.PropertyChanged in ViewModel.
Simplest dirty fix to understand the idea:
SimulatorViewModel(Simulator model)
{
// this will re-raise Model's events on ViewModel
// VM should implement INotifyPropertyChanged
// method OnPropertyChanged should raise INPC for a given property
model.PropertyChanged += (sender, args) => this.OnPropertyChanged(args.PropertyName);
}
Always make sure that the instance of the class which has been set as Data-context should implement INotifyPropertyChanged
In your case Simulator implements INotifyPropertyChanged but SimulatorViewModel is set as DataContext, in this case the UI will come to know only if there are changes in the SimulatorViewModel but not in the Simulator.
What you can do is that:
You can expose some events or delegates in the Simulator class and hook to some methods in SimulatorViewModel class. Then when ever values in Simulator class changes, invoke the event or the delegate that will get Execute on SimulatorViewModel, now you can get the updated values from the simulator and update the properties in SimulatorViewModel. Make sure that you use different properties in both the class
i`m trying to design a dialog window which will display different messages depending on what case will be true.
so for example when i have a
<CustomDialog Content="{Binding Path=Name}"/>
is there a possibility to change the Binding Path=Name to Path=Whatever or how do you implement something like that ? When the Control should use other resources on runtime.
--------------edit
I ok i`ll try to describe my problem better ;)
I have an ResourceDictionary with strings for example
<System:String x:Key="Message1">Message1</System:String>
<System:String x:Key="Message2">Message2</System:String>
<System:String x:Key="Message3">Message3</System:String>
So when I now call my UserControl
Doing it customdialog.visibility = true; for example
<CustomDialog Text=”” />
I want to define which key from the resourcedictionary is taken when the dialog popups up.
something like customdialog.text = Message1; but Loaded from the ResourceDictionary
is that possible or is there an better way of doing something like this ?
You may provide another content to the same property Name at runtime in code-behind. Suppose you have Initialize (or may be Show) method in your CustomDialog and the last one implements INotifyPropertyChanged:
public class CustomDialog : INotifyPropertyChanged
{
//Your implementation of class goes here
public void Initialize(string message)
{
Name = message;
Visibility = Visibility.Visible;
}
public string Name
{
get {return _name;}
set
{
if (_name != value)
{
_name = value;
raiseOnPropertyChanged("Name");
}
}
}
//Your implementation of class goes here
}
In method Initialize there will be updated Name property and your control will be shown. When there will be setting of Name property must be raise PropertyChanged event which will tell presentation that binded value has updated and to reflect it in the UI.
The easiest way I can think of would be to bind to the parent item, not to a child property, and then use a DataTemplateSelector to select a different template at run-time, depending on some condition involving the bound object or its properties.
Alternatively, if the Content has well defined types, you only need to define DataTemplates with specific data types, and they will be automatically used to display objects of those types.
Not knowing more about the context I can't be much more specific, but if you search for more information on DataTemplates and DataTemplateSelectors you should be fine - you can find a lot of useful information here.