I have a Property Student which has Name and SchoolName. I have bound the Student.Name property to a textbox
<TextBox Text="{Binding Student.Name, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}"></TextBox>
My datacontext is
public class MyDataContext : INotifyPropertyChanged
{
private Student _student;
public Student Student
{
get { return _student; }
set
{
if (value != null)
{
_student = value;
NotifyPropertyChanged("Student");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
public class Student
{
public string Name { get; set; }
public string SchoolName { get; set; }
}
When I change the text in the textbox NotifyPropertyChanged event is not firing.
What am I doing wrong here? How can I achieve this?
You need to implement INotifyPropertyChanged on the Student class to get this to work.
The XAML binding framework is monitoring the 'Name' property on the Student class not the Student class.
The event isn't being raised because changing the textbox is not actually modifying the Student Reference. It is modifying the value of a property within the student. The way this is written it would only fire if the entire Student property were replaced with a new instance.
To get the behavior you want you should make Student implement INotifyPropertyChanged and update the properties within it to raise the event similar to the way you raise the event on the context.
An alternate solution would be to add proxy properties to your datacontext for the Student properties that just forward the calls to the Student instance.
Related
Is there a simpeler way to bind many properties?
So if you have a Person class with properties: lastname, firstname, birthday, gender, title, ...
Now I do this for every property on the ViewModel:
public string _LastName;
public string LastName
{
get { return _LastName; }
set { _LastName = value;
RaisePropertyChanged("LastName"); }
}
And on the XAML page this binding:
<TextBlock Text="{Binding FirstName}" />
Now image if Person object has like 20 properties..
So my question is can I do this in a simpeler way?
You only need to raise the PropertyChanged event from the setter of a data-bound property if you actually intend to update the property dynamically at runtime. Otherwise you could use auto-implemented properties without any custom logic:
public FirstName { get; set; }
There is also a NuGet package called Fody that can turn simple public properties into full INotifyPropertyChanged implementations for you automatically: https://github.com/Fody/PropertyChanged
If you use a third party MVVM framework, it also might have a code snippet to create property with INotifyPropertyChanged.
If you use Catel, you can download templates and snippets here:
http://catelproject.com/downloads/general-files/
And here's implementation for Caliburn.Micro:
https://github.com/winterdouglas/propc
The simplest solution is to use the POCO mechanism provided by a free DevExpress MVVM Framework.
POCO will automatically implement INotifyPropertyChanged and raise the PropertyChanged event for all public virtual properties in your view model.
All magic happens when you use the ViewModelSource class to create your view model. You can create your view model in XAML:
<UserControl ...
DataContext="{dxmvvm:ViewModelSource Type=local:MyViewModel}">
Or in code-behind:
this.DataContext = ViewModelSource.Create(() => new MyViewModel());
PREMISE
In a default MVVM scenario, your ViewModel don't have to raise notifications on every property.
Typical case: you get some Person from a database, show them on a View, modify them via TextBoxes and other controls, and click "Save" re-sending them to the database. You can do this by setting the DataContext on the View every time you call the database. This action raises a first update on the bound properties of the control and of every sub-control, so all the getters of the ViewModel's bound properties are called one time and the View get populated with the ViewModel's values. When you modify something on the View, that binding carries the modification to the corresponding ViewModel's property (even a simple plain get-set property).
In this case, you're just fine with something like:
public class Person
{
public string Name { get; set; }
public string Surname { get; set; }
//and so on...
}
You need to raise notifications for the ViewModel's properties only if the View must listen to some property's change. For example, this feature: the Button "Save" is enabled if and only if the Name on the Person is not empty. Here, clearly the Button must be able to see when the Name property changes, so that property setter must raise the PropertyChanged event.
A possible implementation:
Use this as base class for ViewModels:
protected abstract BaseViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
protected void SetAndNotifyIfChanged<T>(
ref T field, T value, [CallerMemberName] string propertyName = null)
{
if (!EqualityComparer<T>.Default.Equals(field, value))
{
field = value;
NotifyPropertyChanged(propertyName);
}
}
}
In a derived class you can write every get-set property like this:
class MyViewModel : BaseViewModel
{
public string MyProp
{
get { return _MyProp; }
set { SetAndNotifyIfChanged(ref _MyProp, value); }
}
private string _MyProp;
}
The type T and the parameter propertyName are automatically inferred.
This is the shortest piece of code you could write, and is not so different from a normal full-property:
public string NormalProp
{
get { return _ NormalProp; }
set { _NormalProp = value; }
}
private string _MyProp;
If you don't want to write all this code every time, use a code snippet.
I'm sure this is a very basic question but I don't even know the technical term / jargon to Google and self-educate on.
I have created a simple model implementing INotifyPropertyChanged.
public class PushNotes : INotifyPropertyChanged
{
public string CompletePushNotes { get; set; }
}
Binding in cs:
evt_pushNotes = new PushNotes()
{
CompletePushNotes = "HelloThere"
};
this.DataContext = evt_pushNotes;
//snip later in code
Helpers.UpdateCompletePushNotes();
In XAML:
<xctk:RichTextBox x:Name="PushEmail" Text="{Binding Path=CompletePushNotes, Mode=OneWay}" ScrollViewer.VerticalScrollBarVisibility="Auto" Margin="40,398,40,40">
<xctk:RichTextBox.TextFormatter>
<xctk:PlainTextFormatter />
</xctk:RichTextBox.TextFormatter>
</xctk:RichTextBox>
Helper:
internal static class Helpers
{
internal static void UpdateCompletePushNotes()
{
//duhhhh what do I do now??
//If I create a new PushNotes it will be a different instantiation....???
}
}
Now this is all fine but I have a method in a helper class that needs to change the CompletePushNotes.
Again I know this is a simplistic / newbie question but I don't know what I need to learn.
So do I make my PushNotes class static, or singleton. Is there some global binding "tree" I can walk to find my instantiated and bound PushNotes class that is attached to the UI element?
Not looking for an a handout just need to know what it is I'm looking for.
TIA
Your PushNotes class does not implement the INotifyPropertyChanged interface. Once you have implemented it, you need to modify your CompletePushNotes property to have a backing field and in the setter of the property you can raise the PropertyChanged event to notify the UI of the source property update.
public class PushNotes : INotifyPropertyChanged
{
string completePushNotes;
public string CompletePushNotes
{
get
{
return completePushNotes;
}
set
{
completePushNotes = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
Making the PushNotes class static will not help you. You seem to have a variable of some sort to the PushNotes instance (evt_pushNotes), so just do:
evt_pushNotes.CompletePushNotes = something;
If you have a helper class that does something, call the method in the helper class and get the value back or pass the PushNotes instance into the helper class as a parameter.
internal static class Helpers
{
internal static void UpdateCompletePushNotes(PushNotes pushNotes)
{
pushNotes.CompletePushNotes = something;
}
}
I have my XAML code (inside a Page of the standard blank page template) as
<Grid>
<TextBlock x:Name="tbBindingBlock">
</Grid>
And I have an int named iTestBindingin the code-behind. Is there a simple (since I'm only binding one variable and not a collection) way of binding the two together so that the latest value of iTestBinding (which keeps getting changed from the code-behind) is always displayed inside tbBindingBlock ?
Edit : The code behind is pretty short :
public sealed partial class MainPage : Page
{
public int iTestBinding=0;
you can bind your value directly using the below code
<TextBlock x:Name="tbBindingBlock" Text="{Binding iTestBinding}">
keep your value to bind as a property in code behind like this
public int iTestBinding{ get; set; }
and set the datacontext in page loaded event like this
this.DataContext = this;
if you want to update the value on button click and to be reflected in UI, you need to implement the PropertyChangedEventHandler.
public event PropertyChangedEventHandler PropertyChanged;
private void RaisePropertyChanged(string propertyName)
{
// take a copy to prevent thread issues
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
call RaisePropertyChanged("changed property name") where-ever you updated the value of the property.
Its better to keep a custom get set for the property so that we can call this method from the setter.for example
private string myText;
public string MyText {
get {
return myText;
}
set {
myText = value;
RaisePropertyChanged("MyText");
}
}
I am having a problem with the textbox text updating in my view. I have seen some other threads but cannot find one that is close to my setup.
I have a model with a class of
interface IPerson : INotifyPropertyChanged
{
string FirstName { get; set;}
}
public class Person
{
public event PropertyChangedEventHandler PropertyChanged = delegate { };
private string _firstName;
public string FirstName
{
get {return _firstName;}
set
{
_firstName = value;
PropertyChanged(this , new PropertChangedEventArgs("FirstName"));
}
}
}
This is what my viewModel Looks Like
interface IViewModel : INotifyPropertyChanged
{
string FirstName { get; set;}
IPerson Person { get; set; }
}
public class viewModel : IViewModel
{
public event PropertyChangedEventHandler PropertyChanged = delegate { };
private IPerson _person;
public Person Person
{
get {return _person;}
set
{
_person = value;
PropertyChanged(this , new PropertyChangedEventArgs("Person"));
}
}
public string FirstName
{
get {return Person.FirstName;}
Set
{
Person.FirstName = value;
PropertyChanged(this , new PropertChangedEventArgs("FirstName"));
}
}
}
Xaml Binding Setup
// If i use this binding my model will get updated but my textbox text will never show what the value is in the model
Text="{Binding Path=FirstName ,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,diag:PresentationTraceSources.TraceLevel=High}"
// This binding works fine both ways
Text="{Binding Path=Person.FirstName ,UpdateSourceTrigger=PropertyChanged,Mode=TwoWay,diag:PresentationTraceSources.TraceLevel=High}"
So if i bind to the Person.First Name everything works great. If I bind to FirstName then it will update my model fine but will not get the data for The TextBox Text. This becomes apparent when i select an item in my list.
Does anyone have an idea why this would happen. I want to be able to bind to the ViewModel Firstname and just have it pass that to my person object.
If your model changes the FirsName value it triggers an PropertyChanged event but that is not extended (relayed) by the ViewModel.
To make the first binding work your ViewModel should subscribe to Person.PropertyChanged and then raise its own event when the nested FirstName changes.
But of course the Path=Person.FirstName binding is preferable anyway.
With this binding "Binding Path=Person.FirstName" the View is listening to your viewModel for the PropertyChanged event. The View is not listening to your model (i.e. Person).
If you are setting the DataContext of your View to the viewModel, then in order for the View to know about changes to your model (i.e. person) you are going to have to have your viewModel listen for the PropertyChanged event of your model. Then, raise the PropertyChanged event from within your viewModel.
I am a beginner to use MVVM in WPF and found that it seem impossible to change the value of a textbox or a label. Here is an example.
In Xaml:
The original value of Name is "Peter".
But after I press a button which invoke a command in the ViewModel and change the value of Name to be
"John". So, suppose the value of the text box will be changed to John as well. However, it doesn't change.
I have found a lot of example in the net and found that none of them implemented this kind of functions. What I have learnt from them is to use Command and ItemsSource of ListView.
The value of ListView will change when I use button to raise command to change the ItemsSource of the view. Its value will change automatically when the Binding to ItemsSource changed.
However, I cannot make the value of TextBox or Label change even the value of the bindings to them are changed already.
Actually, I am really quite young in MVVM. I think I still have so much that I don't know.
Could you give me an example of how exactly I should do to make change to textbox after a button click? By the way, I am not quite sure how to make command for button. It seem to involve so much codes that I found in the sample from the net. Is there any simplier way?
Thank you very much.
Your ViewModel needs to implement INotifyPropertyChanged .
Documentation see here
public class Bar : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string foo;
public string Foo
{
get { return this.foo; }
set
{
if(value==this.foo)
return;
this.foo = value;
this.OnPropertyChanged("Foo");
}
}
private void OnPropertyChanged(string propertyName)
{
if(this.PropertyChanged!=null)
this.PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
}
}
Your view model should implement INotifyPropertyChanged so that WPF knows that you've altered a value of a property.
Here is an example from
// This is a simple customer class that
// implements the IPropertyChange interface.
public class DemoCustomer : INotifyPropertyChanged
{
// These fields hold the values for the public properties.
private string customerNameValue = String.Empty;
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
var listeners = PropertyChanged;
if (listeners != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
public string CustomerName
{
get
{
return this.customerNameValue;
}
set
{
if (value != this.customerNameValue)
{
this.customerNameValue = value;
NotifyPropertyChanged("CustomerName");
}
}
}
}