To implement data binding in WPF, according to MS a "class needs to provide the proper property changed notifications." [ref here]
AFAIK, part of setting this up means taking the following steps, if not already set up in the class (ref this article on MSDN):
When properties are changed, they need call to a method to raise an event.
This means auto-implemented properties must be changed so they use a private backing field, so set can change the property and also call the method to raise an event.
The class needs to implement INotifyPropertyChanged.
The class needs to declare the PropertyChangedEventHandler event.
The event needs to be raised in something like this:
...
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
If I already have many existing classes that do not take any of these steps, what is the best way to change these classes so they change only so far as is needed to make them meet these standards?
If you want the minimum-possible-code-impact solution, then you want the Fody PropertyChanged weaver. This is installed as a NuGet package with Install-Package PropertyChanged.Fody, or through the VS package manager dialogue.
Once installed, you can mark a class with the [ImplementPropertyChanged] attribute, and your job is basically done. The weaver will add the relevant event calls at compile time by manipulating the generated IL, which means that you don't have to explicitly implement the interface or the event calls in code. You can keep your auto-implemented property syntax, too!
The linked docs provide details of more advanced cases but, in my experience, the out-of-box behaviour is sufficient for almost all needs.
I love this implementation with [CallerMemberName] in C# 5 as described here
protected void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
...
}
Just make the parameter optional and decorate it with the CallerMemberName attribute.
Your property now looks like this:
public DateTime Time
{
get { return this.model.DateTime; }
set
{
this.model.DateTime = value;
NotifyPropertyChanged();
}
}
i usually like to use this implementation to notify the changes in my properties
class IWillNotifyYou : INotifyPropertyChanged
{
private int _firstProperty;
public int FirstProperty
{
get { return _firstProperty; }
set
{
if (value != _firstProperty)
{
_firstProperty = value;
OnPropertyChanged("FirstProperty");
}
}
}
private string _secondProperty;
public string SecondProperty
{
get { return _secondProperty; }
set
{
if (value != _secondProperty)
{
_secondProperty = value;
OnPropertyChanged("SecondProperty");
}
}
}
private void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
note that you could create a specialized event with the name of the property like FirstPropertyChanged
You'll need to identify what role your existing classes play now. WPF is most beneficial using MVVM (Model-View-ViewModel). Identifying the role of your current classes will help determine what needs to be changed (if anything).
Assuming your existing classes are Business Objects, it is up to you to decide whether or not you even need to implement INotifyPropertyChanged. WPF is easiest to work with by binding to ViewModel. The ViewModel is what actually handles the data between the View (WPF screen) and the Model.
The Model can implement INotifyPropertyChanged, but it does not have to. The ViewModel will hold whatever properties are needed for that specific screen (or control) and is probably the path you will want to explore so you do not completely 180* your existing business objects.
Add ": INotifyPropertyChanged" after your class definition:
Public class Test : INotifyPropertyChanged
{
}
Accept ReSharpers recommendation to implement the interface for this class (you might have to install ReSharper for this to work).
Then, in the setter for any property, add this:
OnPropertyChanged("PropertyName")
This means that you can now bind properties in the XAML to this property in this class.
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.
I was never sure about the meaning of propertyName when implementing INotifyPropertyChanged. So generally you implement INotifyPropertyChanged as:
public class Data : INotifyPropertyChanged {
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string propertyName = "") {
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
private string itsID;
public string ID {
get { return itsID; }
set {
if (itsID != value) {
itsID = value;
NotifyPropertyChanged("ID");
}
}
}
I was never sure about the propertyName argument to NotifyPropertyChanged(string propertyName).
Can that be any arbitrary string (like "MyID" in the above example)?
Or does .NET use Reflection to align it with a property in a class so it has to match the name of the property exactly?
What if the propertyName doesn't match the name of the Property exactly, does .NET consider the entire object as changed?
It's not .NET Framework itself per se, it's pretty much every PropertyChanged subscriber out there (some of which do indeed happen to be distributed as part of the framework) that assumes you use the interface as intended, by sending the property name. If you send a notification that the property MyID has changed, when another component is looking at the property ID, it will typically see the notification, compare the names, and conclude "this notification isn't for me".
If you want to match the name of the property exactly, you can use the new C#6 feature called nameof. The nameof feature is answers all of your questions because we could say the main advantage of using nameof in one word is refactoring. And "refactoring" is the word that you are looking for based on your questions:
NotifyPropertyChanged(nameof(ID));
As an example renaming ID will change the name of the property too, or it will break compilation but the following doesn't:
NotifyPropertyChanged("ID")
It has to match the name of the property exactly. On a side note, you should check that the value has actually changed before calling PropertyChanged since that is a relatively expensive call (WPF would not know the property has not changed). In VS2015, you can also use the nameof operator.
I'm making a custom control including three buttons. Every button represents a tab. When I click a button to change to the tab corresponding to that tab a variable for the controll is uppdated, this variable is called "selectedIndex". How can I make a custom event that triggers when this index is changed?
I've seen solutions for custom events here but all of them is copies of already existing events such as Click-events. My problem involvs having an event on variable change".
Regards!
One option is to have your object implement the very standard INotifyPropertyChanged interface. There's no rule which states that every property on an object needs to raise the PropertyChanged event, so just raise it for the one property in question. Something like this:
public class MyObject : INotifyPropertyChanged
{
private string _myProperty;
public string MyProperty
{
get { return _myProperty; }
set
{
_myProperty = value;
NotifyPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
If your .NET version doesn't support CallerMemberName then you can remove that and supply the property name manually:
NotifyPropertyChanged("MyProperty");
At this point any calling code can subscribe to the public PropertyChanged event on any instance of this object. This interface is particularly handy because it's commonly used in lots of UI technologies for updating data-bound UI elements. All the interface enforces, really, is a standard name for the event. If you'd prefer, you can forgo the interface and just create a custom public event. And just invoke that event any time your object logically needs to. The structure of exposing an event and invoking it would be the same.
In a WPF application, I need to refresh the UI for all components at a specific time interval.
I would need to know what is the simplest way.
My question: How to rise OnPropertyChanged?
Do you know a better approch?
Please provide a sample of code if possible thanks
namespace MyClient.Common
{
public abstract class BindableBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected bool SetProperty<T>(ref T storage, T value, /*[CallerMemberName]*/ String propertyName = null)
{
if (object.Equals(storage, value)) return false;
storage = value;
this.OnPropertyChanged(propertyName);
return true;
}
protected void OnPropertyChanged(/*[CallerMemberName]*/ string propertyName = null)
{
var eventHandler = this.PropertyChanged;
if (eventHandler != null)
{
eventHandler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
}
The definately best approach would be not to refresh the controls at all :-) As I see that you have implemented the INotifyPropertyChanged interface you are not too far from implementing a simple MVVM pattern. If you do that the user interface will automatically update on events from the view model, and your controls should allways be up to date.
This is one of many references as to how to get started on MVVM: http://www.markwithall.com/programming/2013/03/01/worlds-simplest-csharp-wpf-mvvm-example.html
I've just been through the same jurney myself, and once I got the hang of it, I realized how clean and nice it all is.
In some cases you have to call the OnPropertyChanged manually, if for instance a property is deduced from other members or external factors. Then you just call OnPropertyChanged("PropertyName").
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).