My label only seems to get the data from the property it is bound to once. I have the Property raising the Property Changed event in the setter, but when the value of the property gets changed, it raises the event properly (I know this because of the break point I set), but the text in the Label on the window doesn't change. I should maybe also note that the window with the label isn't the main window, but a new one that pops up.
ViewModel:
public class PurchaseVerificationViewModel : ViewModelBase
{
private WindowService.WindowService windowService = new WindowService.WindowService();
private string _verificationQuestion = "Question"; //default so i can check if it changed in the window
public string VerificationQuestion
{
get { return _verificationQuestion; }
set
{
if (_verificationQuestion != value)
{
_verificationQuestion = value;
OnPropertyChanged(nameof(VerificationQuestion));
}
}
}
}
Window:
<Window>
<Window.DataContext>
<viewmodels:PurchaseVerificationViewModel/>
</Window.DataContext>
<Grid>
<Label Content="{Binding VerificationQuestion, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>
</Window>
ViewModelBase:
public class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
There's no problem with your implementation of the INotifyPropertyChanged, since you are correctly invoking the notification that your property has been modified.
So if the problem is not with the one who's raising the notification, might it rather be with what is actively listening to it?
And the problem is that you're defining the DataContext of your Window to the class itself, rather than to the instance which you are utilizing and modifying in the code-behind of your application.
What is actually happening under the hoods, due to the way you defined your DataContext in xaml, is that a new PurchaseVerificationViewModel class is being constructed (is the not the one who are modifying on your logic) and therefore your VerificationQuestion will return it's default value (or rather the private backing field default value, "Question").
In reality the problem is that you have induced your listener to listen to the wrong thing.
Since you want the content of the Label (target) to be update based on a source change, what you have to do, is to set as the DataContextof the Window the specific instance which you are modifying on the logic of your application, and make sure you define it as a property!
public PurchaseVerificationViewModel myViewModel {get;set;}
For instance after InitializeComponent(), on your page constructor, you could initialize the property and set it as the DataContext, like this:
myViewModel = new PurchaseVerificationViewModel();
this.DataContext = myViewModel;
Related
I have a WPF project which has a main window containing a tab control which, in turn, contains several user controls.
I need the main window to display a warning when the value of certain properties have changed on the various user controls. I am using an event handler to populate a TextBlock when these properties change.
The remaining problem I have is that when the application is started, and the different UI properties get assigned an initial value, this assignment seems to be triggering the event. I only want to display this warning when the user has changed something.
Here's what I have:
An abstract class which facilitates the event
public class ViewModelBase {
Boolean isBusy;
protected virtual void OnConfigurationChanged(EventArgs args) {
EventHandler handler = ConfigurationChanged;
handler?.Invoke(this, args);
}
public event EventHandler ConfigurationChanged;
}
User controls which trigger the event when applicable properties change
class MyViewModel : ViewModelBase {
String myTextField;
public MyTextField {
get => myTextField;
set {
myTextField = value;
OnConfigurationChanged(null);
}
}
}
In my main window, I instantiate the user control and subscribe to the event. I also have the unsaved changes property in here
class MainWindowVM : ViewModelBase {
String unSavedChanges;
public MainWindowVM() {
MyViewModel MyVM = new MyViewModel();
MyVM.ConfigurationChanged += onConfigurationChanged;
}
String UnsavedChanges {
get => unSavedChanges;
set {
unSavedChanges = value;
OnPropertyChanged(nameof(UnsavedChanges));
}
}
void onConfigurationChanged(Object sender, EventArgs args) {
UnsavedChanges = "Configuration not saved.";
}
}
A XAML TextBlock bound to UnsavedChanges
<TextBlock Text="{Binding UnsavedChanges}"
Foreground="Red"
Margin="10"/>
I guess this is obvious to most folks, but for anyone else that struggles with this issue:
In order for an initial value to be reflected in a property-bound UI component, the value must be assigned to the underlying field in the object constructor. Changes to the field outside of the constructor will not be seen by the corresponding property and, in turn, will have no effect on the property-bound UI component.
Assigning a value to a property which invokes OnPropertyChanged() on set will always trigger the subscribed event. The constructor is not exempt in this regard.
Can someone explain me why need to use implementation of INotifyPropertyChanged when using binding in wpf?
I can bind properties without implementation of this interface?
For example i have code
public class StudentData : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
string _firstName = null;
public string StudentFirstName
{
get
{
return _firstName;
}
set
{
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
And binding in .xaml
<TextBox Text="{Binding Path=StudentFirstName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
Grid.Row="1"
Grid.Column="2"
VerticalAlignment="Center" />
this code from .xaml.cs
StudentData _studentData = new StudentData { StudentFirstName = "John", StudentGradePointAverage = 3.5};
public MainWindow()
{
InitializeComponent();
this.DataContext = _studentData;
}
why we need to use INotifyPropertyChanged in this case?
It is not my code.
You need INotifyPropertyChanged if you want a wpf form to be automatically updated when a property changes through code. Also some controllers might want to know if edits have been made in order to enable/disable a save-button, for instance. You also might be displaying the same property on different views; in this case INotifyPropertyChanged helps to immediately update the other view when you edit a property.
If you think that your form behaves well without INotifyPropertyChanged, then you can drop it.
Note that binding works even without INotifyPropertyChanged. See: Why does the binding update without implementing INotifyPropertyChanged?
I would implement the properties like this. In some rare cases it can help to avoid endless circular updates. And it is more efficient by the way.
private string _firstName;
public string StudentFirstName
{
get { return _firstName; }
set
{
if (value != _firstName) {
_firstName = value;
OnPropertyChanged("StudentFirstName");
}
}
}
Starting with C#6.0 (VS 2015), you can implement OnPropertyChanged like this:
private void OnPropertyChanged(string propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
When you bind to a property of StudentData such as the StudentFirstName then the binding class tests to see if the StudentData instance provides the INotifyPropertyChanged interface. If so then it will hook into the PropertyChanged event. When the event fires and it fires because of the StudentFirstName property then it knows it needs to recover the source value again because it has changed. This is how the binding is able to monitor changes in the source and reflect them in the user interface.
If you do not provide the INotifyPropertyChanged interface then the binding has no idea when the source value changes. In which case the user interface will not update when the property is changed. You will only see the initial value that was defined when the binding was first used.
It does need to be implemented in order for binding to work but that doesn't mean you always have to do it yourself. There are other options like Castle Dynamic Proxy (which wraps your classes in a proxy and injects INPC into all virtual properties) and Fody (which adds it to the IL in a post-processing step). It's also possible to implement yourself while at the same time reducing code bloat, as demonstrated in my answer to this question.
I have a textblock:
<TextBlock HorizontalAlignment="Left" Name="StatusText" Margin="0,20" TextWrapping="Wrap" Text="{Binding StatusText}">
... Status ...
</TextBlock>
codebehind:
public StatusPage()
{
InitializeComponent();
this.DataContext = new StatusPageViewModel(this);
}
and in viewModel:
private string _statusText;
/// <summary>
/// Status text
/// </summary>
public string StatusText
{
get { return _statusText; }
set { _statusText = value; }
}
and in function in viewModel:
string statusText = Status.GetStatusText();
this.StatusText = statusText;
GetStatusText() returns string like "Work done" etc. Values from that functions are assinged to the this.StatusText but the TextBlock's text property don't change and is showing still placeholder "... Status..."
I'm aware of questions like this -->
CLICK<--- but after reading this I'm still not able to find solution
#Update
After your suggestions i updated my code and now I have this:
public string StatusText
{
get
{
return _statusText;
}
set
{
_statusText = value;
RaisePropertyChanged("StatusText");
}
}
and declaration of viewModel:
public class StatusPageViewModel : ObservableObject, INavigable
where:
ObservableObject class is:
public abstract class ObservableObject : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
/// <summary>
/// Raises the PropertyChange event for the property specified
/// </summary>
/// <param name="propertyName">Property name to update. Is case-sensitive.</param>
public virtual void RaisePropertyChanged(string propertyName)
{
OnPropertyChanged(propertyName);
}
/// <summary>
/// Raised when a property on this object has a new value.
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
/// <summary>
/// Raises this object's PropertyChanged event.
/// </summary>
/// <param name="propertyName">The property that has a new value.</param>
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
#endregion // INotifyPropertyChanged Members
}
But its still not working
You need to implement INotifyPropertyChanged in your ViewModel order to notify the View that the property has changed.
Here's a link to the MSDN page for it: System.ComponentModel.INotifyPropertyChanged
The most important thing to note is that you should raise the PropertyChanged event in your property setter.
Add binding mode two way, because by default Textblock's binding mode is one way
<TextBlock HorizontalAlignment="Left" Name="StatusText" Margin="0,20" TextWrapping="Wrap" Text="{Binding StatusText, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
... Status ...
</TextBlock>
and also, of course you need to implement INotifyPropertyChanged for the purpose, refer to this link for how to implement.
When working with DataModels you have to be sure the model is complete at intial load. So if you do this: this.DataContext = mainViewModel and some parts of you mainViewModel are NOT loaded (=null) then you are not able to bind them. Example, I have a Model within that model an object Program. I bind to Text of a TextBlock to Model.Program.Name. The Program object is not connected at initial load so you will have to rebind to a loaded object after because otherwise no notifications can be send.
Your view model needs to implement INotifyPropertyChanged, and you need to raise it every time one of your property changes (ie in the setter).
Without it WPF has no way of knowing that the property has changed.
I had this problem and here is what I was doing wrong...
Note: I Had INotifyPropertyChanged coded correctly.
In my View, I had
<SomeView.DataContext>
<SomeViewModel/>
<SomeView.DataContext/>
...and this calls the constructor of the VM (creating an instance to point to / to be bound to).
In another class (in the ViewModel code for my program's Main Window) I was also instantiating the ViewModel(s), i.e.:
private SomeAstractBaseViewModel someViewModel = new SomeViewModel();
private SomeAstractBaseViewModel someOtherViewModel = new SomeOtherViewModel();
They both inherited from a superclass and I was switching back and forth between instances showing different Views in a section of the Main Window - as per my intention.
How all that is wired up is another question, but when I remove the problematic xaml (at the top of this answer) that was previously instantiating another "unused" instance of the ViewModel, the View would update as per INotifyPropertyChanged mechanisms.
In my situation, a UserControl was nested in a View and when I tried Binding to a property within the UserControl, the framework was looking for the property in the ViewModel, which does not exist. To solve this, I had to specify ElementName.
MyControl.xaml:
<UserControl
...
x:Name="MyUserControl">
...
<Label Content="{Binding MyProperty, ElementName=MyUserControl}"
...
MyControl.xaml.cs:
public partial class MyControl: UserControl, INotifyPropertyChanged
{
...
private string _myProperty = "";
public string MyProperty
{
get => _myProperty ;
set
{
if (value == _myProperty ) return;
_myProperty = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
...
When you call OnPropertyChanged, make sure you include the property name. That cost me several hours today, this fucking hidden binding magic is so frustrating. Give me an error message, or something instead of just silently failing!
So instead of just calling OnPropertyChanged() without supplying an argument, call it either inside your property setter (so CallerMemberName fills it in for you), or supply it yourself.
E.g. somewhere in xaml you have the binding:
... Value="{Binding Progress}" ...
Then in codebehind you have the DataContext object that inherits from INotifyPropertyChanged, and in a method somewhere after updating your values you call:
OnPropertyChanged("Progress");
In my case the problem was, when I call the OnPropertyChanged. I passed incorrect Member Name. For example:
Incorrect
private Student std = new Student();
public Student Std {
get { return std; }
set { std = value; OnPropertyChanged("Student"); }
}
Correct
private Student std = new Student();
public Student Std {
get { return std; }
set { std = value; OnPropertyChanged("Std"); }
}
Here I have to pass the property name not the class name
I'm using x:Bind and INotifyPropertyChanged to update UI in UWP application. But it behaves like OneTime binding even though I set it to OneWay.
Bindings.Update() works, but I want to know why INotifyPropertyChanged fails.
XAML
<TextBlock Text="{x:Bind staffVM.Name, Mode=OneWay}"/>
Code-behind:
private StaffViewModel staffVM;
private void ListView_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
// I want to change staffVM according to ListView's selection.
staffVM = staffListView.SelectedItem as StaffViewModel;
staffVM.Update(); // If change this to Bindings.Update(), It works.
}
ViewModel:
public class StaffViewModel: INotifyPropertyChanged
{
private Character character;
public string Name => character.name == string.Empty ? null : character.name;
public void Update()
{
RaisePropertyChanged(string.Empty);
}
public event PropertyChangedEventHandler PropertyChanged;
public void RaisePropertyChanged([CallerMemberName]string propName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propName));
}
}
Firstly, you need to specify the name of the variable that you want to update:
public void Update()
{
RaisePropertyChanged(nameof(Name));
}
Documentation and sample: https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.data.inotifypropertychanged.propertychanged
Secondly, by default x:Bind is OneTime
To fix it, add Mode="OneWay"
Mode Specifies the binding mode, as one of these strings: "OneTime", "OneWay", or "TwoWay". The default is "OneTime". Note that this differs from the default for {Binding}, which is "OneWay" in most cases.
Please read documentation
https://learn.microsoft.com/en-us/windows/uwp/xaml-platform/x-bind-markup-extension
The problem here is not on the level of the StaffViewModel class, but on the level of the page. When you do:
staffVM = staffListView.SelectedItem as StaffViewModel;
The UI has no notification about the fact that the staffVM field has changed. So the binding is still pointing to the old instance of StaffViewModel. Hence when you do staffVM.Update(), it does notify about changes, but the UI is not listening to that instance - it is still listening to notifications on the first selected item. Bindings.Update() fixes this because it completely re-evaluates all bindings so it will "get" the new value of staffVM field.
Solution would be to implement INotifyPropertyChanged on the Page and encapsulate the staffVM in a property which raises PropertyChanged event.
Ideally I would however suggest creating a "root" view model, which you will set only once and will not change and which will contain the selected item as its property. This way you don't have to implement INotifyPropertyChanged in the Page and its code-behind will be simpler. As a result you will have something like the following in the code-behind:
public RootViewModel VM {get;} = new RootViewModel();
And in XAML:
<TextBlock Text="{x:Bind VM.SelectedStaff.Name, Mode=OneWay}"/>
So here is the problem I'm beating my head against: I have a custom user control that exposes two dependency properties that are bound to my ViewModel. In my ViewModel I have an instance of a class that holds multiple properties that express values that relate to the user control as well as to items that control manipulates. Here's a bit of sample code to explain it visually so here is a simple sample of my control, it's a Slider that is combined with a checkbox that allows the user to lock the slider.
<custom:SliderControl IsLocked="{Binding Path=CustomClass.IsLocked, Mode=TwoWay}" SliderValue="{Binding Path=CustomClass.Value, Mode=TwoWay}" />
IsLocked and SliderValue are dependency properties that effectively manipulate the checkbox and slider that are contained in the custom control. All of the control functions work as intended, except for the bindings to the class I've defined. If I create individual properties, as in one int property and one bool property the bindings work as intended. However I have five sliders, and each slider in my actual code has five properties that tie in to them. I'm trying to eliminate code duplication by creating a class to hold these properties in a reusable object shrinking my 25 properties down to 5 class instances.
My CustomClass inherits ObservableObject and has a bool property and int property named IsLocked and SliderValue respectively. For more visual aids here is what it looks like:
public class CustomClass : ObservableObject
{
public const string SliderValuePropertyName = "SliderValue";
private int _sliderValue= 0;
public int SliderValue
{
get
{
return _sliderValue;
}
set
{
if (_sliderValue== value)
{
return;
}
_sliderValue= value;
RaisePropertyChanged(SliderValuePropertyName );
}
}
public const string IsCheckedPropertyName = "IsChecked";
private bool _isChecked = false;
public bool IsChecked
{
get
{
return _isChecked;
}
set
{
if (_isChecked == value)
{
return;
}
_isChecked = value;
RaisePropertyChanged(IsCheckedPropertyName);
}
}
The ViewModel property is very similar and looks like this, an new instance of the class is created when the ViewModel loads:
public const string SliderOnePropertyName = "SliderOne";
private CustomClass _sliderOne;
public CustomClass SliderOne
{
get
{
return _sliderOne;
}
set
{
if (_sliderOne== value)
{
return;
}
_sliderOne= value;
RaisePropertyChanged(SliderOnePropertyName );
}
}
Why won't the updating of the dependency property that is bound to the property in the class update properly? Is it because you can't properly update the class instance property by itself and instead have to update the entire class instance whenever changes occur? Or do I need to further customize the setter in this ViewModel property? As it sits now changing the slider value or checkbox never hits the bound property at all and nothing errors out when debugging.
EDIT: I've also surrounded the control in a Border and set the Border UIElement's DataContext to that of the class and then subsequently applied the more simple path binding to the underlying custom control. This however did not have any effect on my problem.
I'm a homegrown programmer so I often miss things when putting code together and I'm guessing this is the case here, unless what I'm trying just won't work.
Any help would be greatly appreciated.
EDIT: So I've been toying around with using a custom event that will let me know when the specific property of the custom control changes and then having that event wired up in my ViewModel to update the existing class. This works but still creates code duplication as now I have to have 10 events, 2 events per control, one to check for when the value of the slider changes and the other to detect when the checkbox IsChecked value changes. This code duplication exists since you can't route multiple command parameters (like a simple string identifier for which slider is being manipulated as well as the value you want to use in the code). This limitation means I can't just use 2 events that differentiate between which control is undergoing changes within the defined method as exposing the physical control to the ViewModel breaks the MVVM pattern. Using a class as the datacontext for the user control made it so I didn't care what control was being manipulated as they each had their own class instance. Using events this unravels the MVVM pattern as now I need to know which of the five controls is being manipulated by the user.
It can't be this hard to use a class in property bindings. I have to be missing something remedial.
here is a full example:
public partial class MainPage : UserControl
{
public MainPage()
{
InitializeComponent();
this.DataContext = new ViewModel();
}
}
public class ViewModel
{
public SliderValues slv { get; private set; }
public ViewModel()
{
slv = new SliderValues();
}
}
public class SliderValues : INotifyPropertyChanged
{
bool _isLocked = false;
public bool IsLocked
{
get { return _isLocked; }
set
{
_isLocked = value;
OnPropertyChanged("IsLocked");
}
}
int _theValue = 5;
public int TheValue
{
get { return _theValue; }
set
{
_theValue = value;
OnPropertyChanged("TheValue");
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void OnPropertyChanged(string prop)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(prop));
}
}
Now the xaml:
<UserControl x:Class="TestBindings.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
d:DesignHeight="300" d:DesignWidth="400">
<Grid x:Name="LayoutRoot" Background="White">
<Slider Height="23" HorizontalAlignment="Left" Margin="114,138,0,0" Name="slider1" VerticalAlignment="Top" Width="100"
DataContext="{Binding slv}" Value="{Binding TheValue, Mode=TwoWay}"/>
</Grid>
</UserControl>
May be there is just a syntactical error. Try this
{Binding Path=CustomClass.IsLocked, Mode=TwoWay}
Try this...<custom:SliderControl DataContext="{Binding CustomClass}" IsLocked="{Binding IsLocked, Mode=TwoWay}" SliderValue="{Binding Value, Mode=TwoWay}" />