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.
Related
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"));
}
}
I'm struggling with an issue where the following binding seems to have no impact on the UI:
<Toolbar MaxWidth="{Binding AllowedHorizontalSpace}" />
Property and field:
private int allowedHorizontalSpace;
public int AllowedHorizontalSpace {
get { return this.allowedHorizontalSpace; }
set {
this.allowedHorizontalSpace = value;
this.OnPropertyChanged(nameof(this.AllowedHorizontalSpace));
}
}
In the function that listens for monitor size changes:
this.AllowedHorizontalSpace = (int) (monitorWidth * 0.4)
What am I missing? The size of the control just does not want to change! The same error persists for MaxHeight as well.
I have tried manually changing this value using the tool snoop. The change is reflected in the control when I do this.
In comments, it comes out that when the monitor size changes, the setter for AllowedHorizontalSpace is called but not the getter.
The INotifyPropertyChanged implementation looks right from here, and the whole thing works correctly when AllowedHorizontalSpace is set via Snoop, so it is right, and the DataContext must be an instance of your viewmodel.
This kind of thing is commonly caused by there being a redundant viewmodel instance. Often, one is created in the XAML, and another is created in the codebehind constructor. The second instance assigned to DataContext is the one the property will be bound to, but you may be setting the property on the first. In that case the setter would execute on the first, but since no bindings are using that instance as a source, nothing would call the getter.
And you mention that the viewmodel's Initialize() method is unexpectedly being called twice, which is what I'd expect if the above were the case.
The problem is that AllowedHorizontalSpace is not a dependency property yet you are using the dependency property change notification.
The thing to do is to implement INotifyPropertyChanged on the view model. Change the property implementation to the following and everything will start working.
public int AllowedHorizontalSpace
{
get { return this.allowedHorizontalSpace; }
set
{
this.allowedHorizontalSpace = value;
// this.OnPropertyChanged(nameof(this.AllowedHorizontalSpace));
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("AllowedHorizontalSpace"));
}
}
I have an class, Loans and I have an instance of it in my ViewModel.
I bind to multiple properties in Loans, like
loan.amount;
loan.name;
Etc. I only raise INotifyPropertyChanged on loan itself, and the rest of the properties don't raise it. IE
ViewModel{
loan { get; set { notifypropertychanged("loan")}
Everything I read says this shouldn't work, but it does...in fact, all of loans properties are binding properly and updating property. I always thought you have to raise notifyproperty changed on each and every property, and can't just do it at the main object. Am I missing something?
This is working the way it should be. When you raise the property change, binding framework goes and get the updated value for the property.
Here when you are raising the property change on property of custom type (loan) in your VM, binding framework will get the updated value of loan and will hit getter of each and every property of 'loan' bound on the UI.
No, that will work fine. You're specifying the full path to the property so anything that changes along that path will cause the binding to refresh. It also works if you do something like this:
<StackPanel DataContext="{Binding loan}">
<TextBlock Text="{Binding amount}" />
</StackPanel>
In this case you're not specifying the full path in the TextBlock binding but its parent is bound to a property that does change notification and again it will inform its children if it sees there's been a change. Where you don't get change notification is if you do something like the above and keep loan the same but change amount.
Bear in mind there are different times when change notification is applied. If your UpdateSourceTrigger is set to "PropertyChanged" and you want updates to be applied the instant a value changes then yes, you have to apply it to each and every child member. If it's LostFocus then they'll all get done in one go once the focus is lost irrespective of whether or not they support change notification.
The change notification only works when setting the loan object, not on its properties.
loan.amount = 123;
This uses the [loan] Getter and then then [amount] Setter.
If you need to listen form the parent object then you could set the loan as a private variable and expose the loan properties from the ViewModel
ViewModel
{
private Loan _loan
public decimal loanAmmont
{
get { return _loan.amount; }
set
{
_loan.amount = value;
notifypropertychanged("loan")
}
}
}
Or add the change notification to the loan properties and register to their change from the parent object
ViewModel
{
private Loan _loan;
public Loan loan;
{
get { return _loan; }
set
{
_loan = value;
_loan.PropertyChanged += NotifiyLoanChanged;
}
}
void NotifyLoanChanged(object sender, EventArgs e)
{
// invoke my events to ViewModel listners
notifypropertychanged(e.PropertyName);
}
}
http://msdn.microsoft.com/en-us/library/ms743695(v=vs.110).aspx
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`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.