I have a class that implements Inotifypropertychanged. I also have this property that I commented out the notification on.
private Color _CoolColor =Colors.Purple;
public Color CoolColor
{
get
{
return _CoolColor;
}
set
{
if (value != _CoolColor)
{
_CoolColor = (Color)value;
//OnPropertyChanged("CoolColor");
}
}
}
a binding in my xaml attaches to this property:
BusTextColor="{Binding Path=CoolColor}"
/// <summary>
/// Color used for the text containing the hex value of the bus
/// </summary>
public Color BusTextColor
{
get
{
return (Color)GetValue(BusTextColorProperty);
}
set
{
SetValue(BusTextColorProperty, value);
}
}
public static readonly DependencyProperty BusTextColorProperty =
DependencyProperty.Register("BusTextColor",
typeof(Color), typeof(SignalGraph),
new FrameworkPropertyMetadata(new Color(), new PropertyChangedCallback(CreateBrushesAndReDraw)));
I only bound this cause I wanted to make sure that I wasn't crazy, but I must be crazy because my BusTextColor is updating when CoolColor changes. Someone please make it stop working.
I was only doing this because another dependency property I have is not binding to my viewmodel properly. I know there is probably some obvious reason for this, but I'm definitely missing it.
edit:
that article was interesting. but in my case I have the Inotifypropertychanged interface implemented, I just don't raise the event OnPropertyChanged. I realized I should have posted that as well.
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
Someone please make it stop working.
Just set the BindingMode to OneTime
Since no one had an obvious answer, I knew I had done something wrong elsewhere and what I was posting should work.
Eventually realized it was a bug I had before. Somewhere I had changed the value of the target manually and broke the binding. Need to start checking for code interference with the binding in these situations rather than assuming I am using incorrect syntax for the binding in xaml
Related
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.
Novice here. I've been trying to wrap my head around databinding, and wanted to do try out two-way binding of a checkbox in the view to a boolean in a separate class that I've called "State". The point is to ensure that they are always in sync.
So I've made a checkbox in the view and bound it to the aforementioned boolean property in the State-class, accompanied by a button that bypasses the checkbox and toggles the boolean property directly (aptly labeled 'Ninja!'). The point was to test that the checkbox' databinding reacts when the property changes. However, I can't for the best of me figure out how the OnPropertyChanged-method is supposed to be invoked when the property changes.
Here's what I have so far:
<CheckBox x:Name="checkBox" Content="CheckBox" HorizontalAlignment="Left" Margin="232,109,0,0" VerticalAlignment="Top" IsChecked="{Binding Checked, Mode=TwoWay}"/>
<Button x:Name="button" Content="Ninja!" HorizontalAlignment="Left" Margin="228,182,0,0" VerticalAlignment="Top" Width="75" Click="button_Click"/>
And the code for the "State"-class I've made:
namespace TestTwoWayBinding
{
class State : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private bool _checked;
public bool Checked {
get
{
return _checked;
}
set
{
_checked = value;
OnPropertyChanged(Checked);
}
}
public void Toggle()
{
if (!Checked)
{
Checked = true;
}
else
{
Checked = false;
}
}
public State(bool c)
{
this.Checked = c;
}
protected virtual void OnPropertyChanged(string propertyName)
{
if(PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(Checked));
}
}
}
}
And the code-behind on the view for initialization and handling the events:
namespace TestTwoWayBinding
{
/// <summary>
/// Interaction logic for MainWindow.xaml
/// </summary>
public partial class MainWindow : Window
{
private State _state;
public MainWindow()
{
InitializeComponent();
_state = new State((bool)checkBox.IsChecked);
}
private void button_Click(object sender, RoutedEventArgs e)
{
_state.Toggle();
}
}
}
From what I gather, OnPropertyChanged expects a String propertyName, but I don't know what that would entail here. When I put in the name of the property (Checked), then that naturally refers to a boolean, not a string. What am I not getting? And what else am I doing wrong, as the checkbox doesn't register the property change when I change it through the button?
The two answers which suggest you pass the string literal "Checked" will work, but IMHO aren't the best way to do it. Instead, I prefer using [CallerMemberName] when implementing the OnPropertyChanged() method. (I have no idea what that third answer is all about…it doesn't appear to have anything to do with this question, and I'd guess it was just copy/pasted from somewhere else).
Here's an example of how I'd write your State class:
class State : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private bool _checked;
public bool Checked
{
get { return _checked; }
set { _checked = value; OnPropertyChanged(); }
}
public void Toggle()
{
Checked = !Checked;
}
public State(bool c)
{
this.Checked = c;
}
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
The key here is that the parameter marked with [CallerMemberName] will automatically be filled in with the correct value from the caller, simply by not passing any value. The default value of null is there just so the compiler will allow the caller to not pass a value.
Note that I also simplified the Toggle() method. There's no need to use an if statement to transform one bool value into another; that's what the Boolean operators are there for.
I also changed the OnPropertyChanged() method so that it's thread-safe, i.e. won't crash if some code unsubscribes the last handler from the PropertyChanged event between the time the event field is compared to null and the time the event is actually raised. Typically, this is a non-issue as these properties are nearly always accessed only from a single thread, but it's easy enough to protect against and is a good habit to get into.
Note that in C# 6, you have the option of just writing PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName)); for the method body. Not everyone is using the new compiler 100% of the time yet, so I just mention that as an optional choice for you.
Naturally, you also need to set the DataContext correctly, as shown in one of the other answers:
public MainWindow()
{
InitializeComponent();
_state = new State((bool)checkBox.IsChecked);
this.DataContext = _state;
}
Though, personally, I'm not sure I'd bother with the constructor. You appear to have no other code that would set checkBox.IsChecked, so it seems to me that you're always going to get the default value anyway. Besides, you can't create your view model class in XAML if it doesn't have a parameterized constructor; in the future, you may prefer to configure your DataContext like that. E.g.:
<Window.DataContext>
<l:State Checked="True"/>
</Window.DataContext>
And in the window's constructor:
public MainWindow()
{
InitializeComponent();
_state = (State)this.DataContext;
}
See also the related Q&A Automatically INotifyPropertyChanged. The question there is really about something different — they want to implement the interface without having to explicitly write anything in the property setter — but for better or worse, the answers they got are really more about your scenario, where it's just a question of simplifying the property setter implementation rather than making it completely automatic.
I have to admit, I would've thought there would have been another question already with which to mark yours as a duplicate. And I did find lots of related questions. But nothing that focuses directly on just "how do I implement and use a view model that implements INotifyPropertyChanged?", which is really what your question seems to be about.
Addendum:
I did some more searching, and while none of these seem like they would be considered exact duplicates per se, they all have good information that help address the question about implementing INotifyPropertyChanged:
Use of Attributes… INotifyPropertyChanged
INotifyPropertyChanged for model and viewmodel
BindableBase vs INotifyChanged
How to write “ViewModelBase” in MVVM (WPF)
You are real close. You need to make 2 small changes and your test works:
Assign the DataContext of your Window to the _state variable.
Put the string "Checked" into the OnPropertyChanged and pass propertyName to the PropertyChangedEventArgs in the OnPropertyChanged method.
So your MainWindow ctor becomes:
public MainWindow()
{
InitializeComponent();
_state = new State((bool)checkBox.IsChecked);
this.DataContext = _state;
}
and the State class file looks like:
class State : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private bool _checked;
public bool Checked
{
get
{
return _checked;
}
set
{
_checked = value;
OnPropertyChanged("Checked");
}
}
public void Toggle()
{
if (!Checked)
{
Checked = true;
}
else
{
Checked = false;
}
}
public State(bool c)
{
this.Checked = c;
}
protected virtual void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
As a novice, I recommend you learn more about Model-View-ViewModel MVVM Design Pattern. It is a common pattern with WPF and helps encourage separation of concerns (keeping your business logic out of your user interface logic)
The OnPropertyChanged method expects the Checked property's name as argument - at the moment, you're passing its value!
This means, change the Checked property declaration to:
public bool Checked {
get
{
return _checked;
}
set
{
_checked = value;
OnPropertyChanged("Checked");
}
}
I'm currently using the following code to be notified when a DependencyProperty's Value has changed:
DependencyPropertyDescriptor propDescriptor = DependencyPropertyDescriptor.FromProperty(property, control.GetType());
propDescriptor.AddValueChanged(control, controlChangedHandler);
This works great and is quite simple, but what I really need now is to be notified when a DependencyProperty's Value is about to change. I thought there would be a DependencyPropertyDescriptor.AddValueChanging() method, but it doesn't seem to exist. Any ideas how I can create this functionality?
I need to be able to cancel the change, fire off some asynchronous backend logic, and only have the control's property really change if the backend logic succeeds.
I solved the problem at hand by implementing wrapping my IODevice in an INotifyPropertyChanged implementation and binding it to the DependencyProperty.
The magic is in the fact that IODeviceWrapper.Value's setter doesn't actually set the value, but rather does the IO. It turns out that when the setter is called by the DependencyProperty it's bound to, the change hasn't yet been committed to the DependencyProperty's value. Hence, IODeviceWrapper.Value's setter gets called in by the DependencyProperty's sudo-, non-existent ValueChanging event.
At this time, if the DependencyProperty reads from the Value's getter it will get the old value until the IO is complete. When the IO is complete IODeviceWrapper.Value's PropertyChanged event gets fired, and the DependencyProperty then reads the new value.
My flawed design is now working flawlessly. Here's the code in case anyone else is interested. Ignore the naysayers.
public class IODeviceWrapper : INotifyPropertyChanged
{
public IODeviceWrapper(IODevice ioDevice)
{
_ioDevice = ioDevice;
_ioDevice.ValueChanged += ValueChanged;
}
private IODevice _ioDevice;
public event PropertyChangedEventHandler PropertyChanged;
private void ValueChanged()
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs("Value"));
}
}
public int Value
{
get { return _ioDevice.Value; }
set
{
//Do ansynchronous IO
Task task = new Task(() => _ioDevice.DoIO(value));
task.Start();
}
}
}
I have a simple usercontrol (WinForms) with some public properties. When I use this control, I want to databind to those properties with the DataSourceUpdateMode set to OnPropertyChanged. The datasource is a class which implements INotifyPropertyChanged.
I'm aware of the need to create bindings against the properties and I'm doing that.
I assumed that my usercontrol would have to implement an interface, or the properties would need to be decorated with some attribute, or something along those lines.But my research has come up blank.
How should this be accomplished? At the moment I'm doing it by calling OnValidating() in my usercontrol whenever a property changes, but that doesn't seem right.
I can get validation to happen if I set the CausesValidation to true on the usercontrol, but that's not very useful to me. I need to validate each child property as it changes.
Note this is a WinForms situation.
EDIT: Evidently I have no talent for explanation so hopefully this will clarify what I'm doing. This is an abbreviated example:
// I have a user control
public class MyControl : UserControl
{
// I'm binding to this property
public string ControlProperty { get; set; }
public void DoSomething()
{
// when the property value changes, the change should immediately be applied
// to the bound datasource
ControlProperty = "new value";
// This is how I make it work, but it seems wrong
OnValidating();
}
}
// the class being bound to the usercontrol
public class MyDataSource : INotifyPropertyChanged
{
private string sourceProperty;
public string SourceProperty
{
get { return sourceProperty; }
set
{
if (value != sourceProperty)
{
sourceProperty = value;
NotifyPropertyChanged("SourceProperty");
}
}
}
// boilerplate stuff
public event PropertyChangedEventHandler PropertyChanged;
protected void NotifyPropertyChanged(string info)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
public class MyForm : Form
{
private MyControl myControl;
public MyForm()
{
// create the datasource
var dataSource = new MyDataSource() { SourceProperty = "test" };
// bind a property of the datasource to a property of the usercontrol
myControl.DataBindings.Add("ControlProperty", dataSource, "SourceProperty",
false, DataSourceUpdateMode.OnPropertyChanged); // note the update mode
}
}
(I have tried this using a BindingSource, but the result was the same.)
Now what I want to happen is that when the value of MyControl.ControlProperty changes, the change is immediately propagated to the datasource (the MyDataSource instance). To achieve this I call OnValidating() in the usercontrol after changing the property. If I don't do that, I have to wait until validation gets triggered by a focus change, which is the equivalent of the "OnValidation" update mode, rather than the desired "OnPropertyUpdate" validation mode. I just don't feel like calling OnValidating() after altering a property value is the right thing to do, even if it (kind of) works.
Am I right in assuming the calling OnValidating() is not the right way to do this? If so, how do I notify the datasource of the ControlProperty change?
I think I've got this figured out. I didn't understand how change notifications were sent from control to bound datasource.
Yes, calling OnValidating() is the wrong way.
From what I've pieced together, there are two ways a control can notify the datasource that a property has changed.
One way is for the control to implement INotifyPropertyChanged. I had never done this from the control side before, and I thought only the datasource side of the binding had to implement it.
When I implemented INotifyPropertyChanged on my user control, and raised the PropertyChanged event at the appropriate time, it worked.
The second way is for the control to raise a specific change event for each property. The event must follow the naming convention: <propertyname>Changed
e.g. for my example it would be
public event EventHandler ControlPropertyChanged
If my property was called Foo, it would be FooChanged.
I failed to notice the relavent part of the MSDN documentation, where it says:
For change notification to occur in a
binding between a bound client and a
data source, your bound type should
either:
Implement the INotifyPropertyChanged
interface (preferred).
Provide a change event for each
property of the bound type.
This second way is how all existing WinForms controls work, so this is how I'm doing it now. I use INotifyPropertyChanged on my datasource, but I raise the Changed events on my control. This seems to be the conventional way.
Implementing the INotifyPropertyChanged interface is very simple. Here is a sample that shows an object with a single public field...
public class Demo : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
private string _demoField;
public string DemoField
{
get {return demoField; }
set
{
if (value != demoField)
{
demoField = value;
NotifyPropertyChanged("DemoField");
}
}
}
}
Then you would create a Binding instance to bind a control property to a property (DemoField) on your source instance (instance of Demo).
I feel like I'm missing something here, but I have this datagrid which when the datasource changes, automatically redraws it self with no logical reason for it doing so.
I have the datagrid bound to a DataView property which implements INotifyPropertyChanged and I want to do some other stuff when that event is fired before calling Refresh().
So here is the datasource.
public class MainScreenDataView : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
DataView _dataview;
public DataView GetDataView
{
get { return _dataview; }
set
{
_dataview = value;
OnPropertyChanged("GetDataView");
}
}
public MainScreenDataView()
{
}
}
And the binding (I call this in the constructor of the window)
public void MakeData()
{
MiddleMan midman = MiddleMan.Instance;
midman.PropertyChanged += new System.ComponentModel.PropertyChangedEventHandler(midman_PropertyChanged); //unrelated event for error messages
midman.InstantiateAll();
Binding bind = new Binding();
bind.Source = midman.GetDict["contact"].GetDataView; //GetDict is a dictionary that holds instances of MainScreenDataView
bind.UpdateSourceTrigger = UpdateSourceTrigger.Explicit;
DG_Contacts.SetBinding(BetterDataGrid.ItemsSourceProperty, bind);
}
The class that updates the DataView with data from a database has access to that same instance of MainScreenDataView as the window. The instance is kept in a dictionary in a singleton.
Now I see no reason why the datagrid would refresh it self, I even tried removing the INotifyPropertyChanged stuff from MainScreenDataview and yet it keeps the same behavior.
Guess there's something I'm missing here. Default behavior somewhere that needs to be overridden or something?
You've got target and source swapped. Done it myself. The UpdateSourceTrigger.Explicit setting affects how the binding updates the source which is the MainScreenDataView.GetDataView property not the DataGrid.ItemSource. The DataGrid.ItemSource is the target.
Removing INotifyPropertyChanged from MainScreenDataView will have no effect on a singleton because the instance doesn't change, only the values inside the instance. In other words, GetDataView is a "set it and forget it" property.
As long as the binding is in effect, there is no way to prevent changes made to the collection from being propagated by the binding system unless you suppress DataView.CollectionChanged events from firing or block so that the binding subsystem simply doesn't run.
If you really want this you can disconnect the binding and set it again when you are ready or create an entirely new DataView and overwrite the binding when you are ready.