Why DataContext for inner object is not updated? - wpf

I have question about DataContext changes and I build example for understand this aspect.
I have MainUserControl on MainWindow. MainUserControl consists of number of User Controls.
One of such User Controls is SubUserControl1.
<Window x:Class="WpfApplicationUcBindingQuestion.MainWindow">
<Grid>
.....
<uc:MainUserControl />
</Grid>
</Window>
<UserControl x:Class="WpfApplicationUcBindingQuestion.MainUserControl">
<Grid>
.....
<uc:SubUserControl1 x:Name="subUserControl1" />
</Grid>
</UserControl>
And in MainWindow I have object of class Info. Class Info consists of a few inner classes.
One of them is, lets say, SubInfo. Both Info and SubInfo classes inherits from INotifyPropertyChanged.
And this is the code of them:
public class Info : INotifyPropertyChanged
{
private SubInfo m_subInfo = new SubInfo();
public Info()
{
}
public SubInfo SubInfo
{
get
{
return m_subInfo;
}
set
{
m_subInfo = value;
OnPropertyChanged("SubInfo");
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public class SubInfo: INotifyPropertyChanged
{
private string m_subString = "subStr";
public SubInfo()
{
}
public string SubString
{
get
{
return m_subString;
}
set
{
m_subString = value;
OnPropertyChanged("SubString");
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
I want to set DataContext for MainUserControl to be object of class Info
and for SubUserControl1 DataContext will be Info.SubInfo.
The following code describes this:
<UserControl x:Class="WpfApplicationUcBindingQuestion.SubUserControl1">
<Grid>
.....
<TextBox Text="{Binding Path=SubString}"/>
</Grid>
</UserControl>
public MainUserControl()
{
InitializeComponent();
MainWindow mainWnd = (MainWindow)Application.Current.MainWindow;
Info info = mainWnd.Info;
this.DataContext = info;
this.subUserControl1.DataContext = info.SubInfo;
}
When new subInfo arrived I update inner object subInfo inside info object:
(This is function of MainWindow)
private void OnUpdateData()
{
SubInfo arrivedSubInfo = new SubInfo();
arrivedSubInfo.SubString = "newString";
m_info.SubInfo = arrivedSubInfo;
}
I want to see that DataContext for subUserControl1 is also changed.
But it doesn't happened and TextBox inside SubUserControl1 is not updated
and doesn't show "newString".
(Note: If I write inside OnUpdateData() function the following:
m_info.SubInfo.SubString = arrivedSubInfo.SubString;
(copy field-field and not whole object) it works,
but I dont 'want copy 50 field...)
Where I'm wrong?
Your help will be really appreciated.

Your problem is the following:
In your constructor, when you do that:
this.DataContext = info;
this.subUserControl1.DataContext = info.SubInfo;
You will set the DataContext ONLY once. Which means it will never change unless you write subUserControl1.DataContext = someNewDataContext somewhere.
What you can do to solve this :
The "proper solution":
Use a binding. In your XAML, just write:
<uc:SubUserControl1 x:Name="subUserControl1" DataContext="{Binding
SubInfo, UpdateSourceTrigger=PropertyChanged}" />
This will work, assuming that your SubInfo property fires the OnPropertyChanged event when it is set.
The "ugly solution":
Explicitly set your UserControl's DataContext in code-behind when you need it. Again, I wouldn't advise that and you are better off applying the first solution!

Related

PropertyChanged remains null even if the property has been changed

I am struggling with this for a while and I cannot figure it out. I have a button and a textBox. The textBox is linked to a property named: MessageDisplay. I want to be able to access this property and update the textBox in several places. Sadly, the PropertyChanged is null. The weird thing is that if I copy/paste the MessageDisplayModel class into the *MessageViewModel * class, it works ...
here is my code :
XAMLfile :
<Grid>
<Button Command="{Binding DisplayTextCommand}" Name="DisplayTextCommand" Margin="53,72,544.6,286" Width="Auto">Push</Button>
<TextBox Name="MessageDisplay" Text="{Binding MessageDisplay, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" />
</Grid>
MessageDisplayModel file
public class MessageDisplayModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string _message;
public string MessageDisplay
{
get { return _message; }
set
{
this._message = value;
this.OnPropertyChanged("MessageDisplay");
}
}
public void UpdateTextBox(string output)
{
MessageDisplay = output;
}
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = this.PropertyChanged;
if (handler != null)
{
var e = new PropertyChangedEventArgs(propertyName);
handler(this, e);
}
}
}//class
MessageViewModel file:
public class MessageViewModel
{
private ICommand _testCommand;
public MessageDisplayModel MessageDisplaySmt = new MessageDisplayModel();
public ICommand DisplayTextCommand
{
get
{
return new DelegateCommand(DisplayMessage);
}
set
{
if (_testCommand == value) return;
_testCommand = value;
}
}
public void DisplayMessage()
{
MessageDisplaySmt.UpdateTextBox("Successfuly downloaded");
}
}//class
MainWindow file
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
MessageDisplay.DataContext = new MessageDisplayModel();
DisplayTextCommand.DataContext = new MessageViewModel();
}
}//class
I update the MessageDisplay property by using the method UpdateTextBox(string). I call this method on the click of the button. When debugging the property gets updated but when time comes to notify the UI that the property has changed, PropertyChangedEventHandler PropertyChanged has its value null ... But if I write something in the textBox, the PropertyChangedEventHandler PropertyChanged gets changed and isn't null anymore. All I want is to be able to change the textBox's property whenever I want and from anywhere I want to.
Thank you
You are using two different instances of MessageDisplayModel. You must use a shared instance.
Also the DisplayTextCommand is implemented "wrong". The set method is redundant as the property's get always returns a new instance of the ICommand.
MessageViewModel.cs
public class MessageViewModel
{
pulic MessageViewModel()
{
}
pulic MessageViewModel(MessageDisplayViewModel messageDisplayViewModel)
{
this.MessageDisplaySmt = messageDisplayViewModel;
}
public void DisplayMessage()
{
this.MessageDisplaySmt.UpdateTextBox("Successfuly downloaded");
}
public MessageDisplayViewModel MessageDisplaySmt { get; set; }
public ICommand DisplayTextCommand { get => new DelegateCommand(DisplayMessage); }
}
MainWindow.xaml.cs
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
// Alternatively use XAML to set the DataContext (see MainWindow.xaml). Would require a parameterless constructor.
this.DataContext = new MessageViewModel(new MessageDisplayViewModel());
}
}
MainWindow.xaml
<Window>
<!--
Alternative DataContext declaration using XAML instead of C#.
Requires a parameterless constructor for both view model objects.
-->
<Window.DataContext>
<MessageViewModel>
<MessageViewModel.MessageDisplaySmt>
<MessageDisplayViewModel />
</MessageViewModel.MessageDisplaySmt>
</MessageViewModel>
</Window.DataContext>
<StackPanel>
<Button Command="{Binding DisplayTextCommand}"
Content="Push" />
<TextBox Text="{Binding MessageDisplaySmt.MessageDisplay}" />
</StackPanel>
</Window>

WPF - Binding Textbox Text to a class property

I am doing some changes in my WPF project to make it less deprecated.
One of the things I am trying to do is Binding my Textbox.Text value to a simple Class as shown below.
<TextBox x:Name="txtNCM"
Grid.Column="1"
Margin="5"
MaxLength="8"
Text="{Binding Path=Name}"
</TextBox>
public partial class wCad_NCM : UserControl, INotifyPropertyChanged
{
private string name;
public event PropertyChangedEventHandler PropertyChanged;
public string Name
{
get { return name; }
set
{
name = value;
OnPropertyChanged("Name");
}
}
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
public wCad_NCM()
{
InitializeComponent();
}
}
Everytime I use the Immediate Window to display the Name's value, it is shown as null. I am really new to this, so I had to search for a similar situation to adapt, but I don't know how to make this work :(
You need to set the DataContext and give Name a value.
To do that, change your constructor to include this:
public wCad_NCM()
{
InitializeComponent();
DataContext = this; // Sets the DataContext
Name = "Test";
}
This should make it work, but is typically bad practice. See http://blog.scottlogic.com/2012/02/06/a-simple-pattern-for-creating-re-useable-usercontrols-in-wpf-silverlight.html for more details.
Additionally, I tried running this and ran into a name hiding problem. Try using a variable name other than Name as FrameworkElement already contains it.

Can you have an event setter bound to an event of the data context?

Im trying to create something like this -
I have an observable collection of points. Each point has a position and a colour. When any points position or colour changes(they implement notification change), I want to "repaint" the background gradient. Currently I have an itemscontrol where I have the sliders bound to the points position and the gradient is initially drawn. Now, I want to know how I can call a function in the code behind of my view when the propertychanged event on a 'point' fires, so that I can repaint the gradient. Im wondering if an event setter can somehow be used?
Whilst I could do the propertychanged event subscribing in code behind, I'd like to do it in XAML?
PLease note : I specifically want to take this approach of manually repainting in code behind for other reasons, so if I could get answers to the specific problem above rather than alternative solutions please.
I guess you can create an attached property to subscribe to PropertyChanged events of the value of the DataContext property.
public static class Props
{
public static DependencyProperty OnPropertyChangedProperty = DependencyProperty.RegisterAttached(
"OnPropertyChanged", typeof(PropertyChangedEventHandler), typeof(Props),
new PropertyMetadata(OnPropertyChangedPropertyChanged));
public static PropertyChangedEventHandler GetOnPropertyChanged (DependencyObject d)
{
return (PropertyChangedEventHandler)d.GetValue(OnPropertyChangedProperty);
}
public static void SetOnPropertyChanged (DependencyObject d, PropertyChangedEventHandler value)
{
d.SetValue(OnPropertyChangedProperty, value);
}
private static void OnPropertyChangedPropertyChanged (DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var inpc = (INotifyPropertyChanged)((FrameworkElement)d).DataContext;
if (inpc == null)
throw new ArgumentException("DataContext of the framework element must not be null.");
var oldChanged = (PropertyChangedEventHandler)e.OldValue;
if (oldChanged != null)
inpc.PropertyChanged -= oldChanged;
var newChanged = (PropertyChangedEventHandler)e.NewValue;
if (newChanged != null)
inpc.PropertyChanged += newChanged;
}
}
Usage:
<Window x:Class="So17382721PropertyChangedXaml.MainWindow" x:Name="root"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:So17382721PropertyChangedXaml"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<DataTemplate DataType="{x:Type local:Foo}">
<!-- Here, we subscribe to DataContext.PropertyChanged;
handler is defined in the MainWindow class -->
<Grid local:Props.OnPropertyChanged="{Binding FooPropertyChanged, ElementName=root}">
<TextBox Text="{Binding Bar, UpdateSourceTrigger=PropertyChanged}"/>
</Grid>
</DataTemplate>
</Window.Resources>
<Grid>
<ItemsControl ItemsSource="{Binding Foos, ElementName=root}"/>
</Grid>
</Window>
Code-behind:
using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
namespace So17382721PropertyChangedXaml
{
public partial class MainWindow
{
public ObservableCollection<Foo> Foos { get; private set; }
public MainWindow ()
{
Foos = new ObservableCollection<Foo> {
new Foo { Bar = "1" },
new Foo { Bar = "2" },
new Foo { Bar = "3" },
};
InitializeComponent();
}
private void OnFooPropertyChanged (object sender, PropertyChangedEventArgs e)
{
MessageBox.Show(this, string.Format("{0} of {1} changed.", e.PropertyName, sender));
}
// Subscribing to non-RoutedEvents in XAML is not straightforward, but we can define a property
public PropertyChangedEventHandler FooPropertyChanged
{
get { return OnFooPropertyChanged; }
}
}
public class Foo : INotifyPropertyChanged
{
private string _bar;
public string Bar
{
get { return _bar; }
set
{
_bar = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged ([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Note: the attached property Props.OnPropertyChanged expects that DataContext is not changed during lifetime and is already specified. Handling DataContextChanged events is left as an exircize, if you need it.

Binding is not working with dynamic data updates

I'm new in WPF and I have the following problem.
I have the following class with many properties , but here is only one property for example:
public class StatusData : INotifyPropertyChanged
{
private string m_statusText = String.Empty;
public StatusData()
{
m_statusText = "1234";
}
public string StatusText
{
get
{
return m_statusText;
}
set
{
if (m_statusText != value)
{
m_statusText = value;
NotifyPropertyChanged("StatusText");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Another component of the project changes StatusData and calls Update() function in MainWindow.
So, m_statusData of this MainWindow has changed and I want update the textbox with m_statusText accordingly.
public class MainWindow
{
private StatusData m_statusData = new StatusData();
public MainWindow()
{
InitializeComponent();
}
private void Window_Loaded(object sender, RoutedEventArgs e)
{
grid1.DataContext = m_statusData ;
}
public void Update(StatusData newStatusData)
{
m_statusData = newStatusData;
}
}
Xaml code:
<Window x:Class="WpfApplicationUpdateTextBox.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Name="myWin"
xmlns:local="clr-namespace:WpfApplicationUpdateTextBox"
Title="MainWindow" Height="350" Width="525" Loaded="Window_Loaded" >
<Grid Name="grid1">
<TextBox Text="{Binding Path=StatusText}" Name="textBox1" />
</Grid>
</Window>
The question is : why the textBox is not updated withnewStatusData.StatusText?
Here, you are assigning the grid's DataContext to m_statusData:
grid1.DataContext = m_statusData ;
And here, you are reassigning m_statusData to something else:
m_statusData = newStatusData;
The problem is that this has no effect on grid1.DataContext, which was set to the previous instance of m_statusData.
In this case, doing grid1.DataContext = newStatusData should solve your problem. However, a better solution would be to create a StatusData property which returns m_statusData. You can then do a RaisePropertyChanged() on it when m_statusData changes.
private void Update(StatusData newStatusData)
{
StatusData = newStatusData;
}
public StatusData StatusData
{
get
{
return m_statusData;
}
set
{
m_statusData = value;
RaisePropertyChanged("StatusData");
}
}
... and then in your XAML, bind your Grid's DataContext to the StatusData property
Edit:
To bind the grid's data context to the StatusData property, you can do this in your XAML:
<Grid Name="grid1" DataContext="{Binding StatusData}">
<TextBox Text="{Binding Path=StatusText}" Name="textBox1" />
</Grid>
You will also need to set the initial DataContext of your window, to make all of the other databindings work (this is a little strange and non-standard, but it will do the trick):
this.DataContext = this;
Your class StatusData has only 1 property: StatusText. If this property is the only thing you want to change with this code:
m_statusData = newStatusData;
You can change it to this:
m_statusData.StatusText = newStatusData.StatusText;
This code will fire the PropertyChangedEvent of StatusData class and that will change TextBox value.

Update text in StatusBar in wpf using C#

I have a TextBox in StatusBar in wpf which I want to update.
I have a list of files in ListBox. On each file I would be doing some operation by calling say method ProcessFile(). So whenever the file processing is completed I want to show that file's name in the StatusBar text.
I have tried something like this:
private void button_Click(object sender, RoutedEventArgs e)
{
statusBar.Visibility = Visibility.Visible;
DispatcherFrame frame = new DispatcherFrame();
Dispatcher.CurrentDispatcher.BeginInvoke(DispatcherPriority.Background, new DispatcherOperationCallback(TimeConsumingMethod), frame);
Dispatcher.PushFrame(frame);
statusBar.Visibility = Visibility.Collapsed;
}
public object TimeConsumingMethod(Object arg)
{
((DispatcherFrame)arg).Continue = false;
foreach (string fileName in destinationFilesList.Items)
{
txtStatus.Text = fileName.ToString();
//Assume that each process takes some time to complete
System.Threading.Thread.Sleep(1000);
}
return null;
}
But I can only see the last file's name in the StatusBar. What's wrong with the code? How can I correct it?
There's more ways to do this.
Set content directly from code
You need give name to the TextBox so that you can access it's content:
XAML
<TextBox x:Name="myTextBox" />
C#
...
ProcessFile(someFileName);
myTextBox.Text = someFileName;
Use data binding
You need to create some object and set it as DataContext to the TextBox or some WPF element that contain that text box (status bar, window, ...).
XAML:
<TextBox Text="{Binding Path=ProcessedFileName}" />
C#
public MyClass : INotifyPropertyChanged
{
public string ProcessedFileName {get; set;}
public void ProcessFile(string someFileName)
{
// Processing file code here
// When done processing, set file name to property
ProcessedFileName = someFileName;
OnPropertyChanged("ProcessedFileName");
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
For more information on data binding see Data Binding Overview
When you are using a ViewModel, i would define a Property "ProcessedFile" in your ViewModel and bind the Textbox of your StatusBar to the Property.
Every time you processed a file i would set the Property "ProcessedFile" to the name of the file.
HereĀ“s some code for the ViewModel.
public class ViewModel : INotifyPropertyChanged {
private string _processedFile;
public string ProcessedFile {
get {
return _processedFile;
}
set {
if (_processedFile != value) {
_processedFile = value;
if (PropertyChanged != null) {
PropertyChanged(this, new PropertyChangedEventArgs("ProcessedFile"));
}
}
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
public void ProcessFile() {
// Process the file
ProcessedFile = //Set the Property to the processed file
}
}
Heres the XAML to bind the TextBox to the Property. (I assume that the ViewModel is set as DataContext for the TextBox)
<TextBox Text="{Binding ProcessedFile, Mode=OneWay}"/>

Resources