There is currently little documentation surrounding the limitations of events with the new C#8 default interface implementations (traits). I am particularly confused with the spec proposal. Not only is the example given invalid C# (the "override" event is missing an identifier), but implementing any of these in C#8 (VS2019, .NET Core 3.0) returns a host of compiler exceptions. In addition, the release notes for C#8 don't make any mention of events for interface traits. As I continued to try and track down an answer, I also couldn't gather anything useful from the open issues list.
So the questions are: is this feature implemented and usable? If so, what is the proper syntax?
Default interface members are used for traits, not just versioning, and an INPC trait would make sense.
Unfortunately, it's not possible to use DIMs to raise events right now, and implementing this seems to be a pain - it would require overhauling the events mechanism and break a ton of code, especially library code. We can use DIMs to add or remove handlers, but that's not so useful.
It would be nice to have something like :
interface InpcTrait : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private T Set(T value,String propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
return value;
}
}
class Customer
{
private string _name;
public string Name
{
get=>_name;
set=>_name=Set(value,"Name");
}
}
Unfortunately, this isn't possible. That's because the event keyword in a class generates a backing field that holds the event handler and add/remove accessors. When we raise the event, we call that event handler.
Interfaces can't have state, which means we can't access that event to raise it.
When we specify an event in an interface, we create a virtual event and the compiler only allows adding/removing event handlers to it. Raising the interface still requires access to the backing field.
This Sharplab.io example shows that :
public class DemoCustomer : INotifyPropertyChanged
{
// These fields hold the values for the public properties.
private Guid idValue = Guid.NewGuid();
private string customerNameValue = String.Empty;
private string phoneNumberValue = String.Empty;
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Generates
[CompilerGenerated]
private PropertyChangedEventHandler m_PropertyChanged;
public event PropertyChangedEventHandler PropertyChanged
{
[CompilerGenerated]
add
{
//some code
}
[CompilerGenerated]
remove
{
//some code
}
}
private void NotifyPropertyChanged(string propertyName = "")
{
if (this.m_PropertyChanged != null)
{
this.m_PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
What we can do, is add or remove event handlers, but we can't even check whether the event already has other handlers. We risk adding the same event handler multiple times.
This is valid :
interface INPCtrait:System.ComponentModel.INotifyPropertyChanged
{
private void AddSomeDefaultHandler()
{
PropertyChanged+=Something;
}
private void RemoveDefaultHandler()
{
PropertyChanged-=Something;
}
public void Something(Object sender,System.ComponentModel.PropertyChangedEventArgs args)
{
}
}
But we have no way of knowing whether we need to add that default handler or not.
Related
I have a scenario in wpf +mvvm i.e if my particular property changes in viewmodel1 then i wan to notify viewmodel2 having observable collection that property "A" has bee changed
1)I want to fire it for particular property not for all.
i have tried below code but not working .please let me know how cam i do this.
public class Model1 : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
// Create custom event
public event EventHandler NotifyChange;
private string testProperty;
public string TestProperty
{
get
{
return testProperty;
}
set
{
testProperty = value;
// If changing properties, fire your OnPropertyChanged to update UI
OnPropertyChanged("TestProperty");
}
}
private void OnPropertyChanged(string propName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propName));
// Fire your custom event if a property changed
NotifyChange(this, null);
}
}
}
public class Model2 : INotifyCollectionChanged
{
public event NotifyCollectionChangedEventHandler CollectionChanged;
public Model2()
{
// Assuming there is an accessible instance of model1
Model1 m1Instance = new Model1();
// Hook up your NotifyChange event from model1
m1Instance.NotifyChange += Model1Changed;
}
private void Model1Changed(object sender, EventArgs e)
{
// this will be triggered on change in model1
}
private void OnCollectionChanged(object singleObject)
{
if (CollectionChanged != null)
CollectionChanged(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset, singleObject));
}
}
Use PubSub Events
My suggestion would be to look into PubSub events.
My recommendation of doing this, is to use Prism. Here's some more information: http://www.c-sharpcorner.com/UploadFile/5ffb84/prism-event-aggregator-in-wpf-with-mvvm/
You will be sticking to proper MVVM practices in this case.
Here's MSDN's ever-useful guide: https://msdn.microsoft.com/en-us/library/ff649664.aspx
Have a really good read-up on how this works, and how to use/implement it.
Alternatively
This will work, but I would still defer to using PubSub events if possible.
You could try this:
public class Model1 : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private string _property;
public string Property
{
get { return _property; }
set
{
_property = value;
OnPropertyChanged("Property");
}
}
private void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
}
public class Model2
{
public Model2()
{
// You might be storing your Model1 as a property in the Model2?
// I don't know, but I've put it in the constructor just for example.
var model1 = new Model1();
model1.PropertyChanged += OnModel1PropertyChanged;
}
private void OnModel1PropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "Property")
{
// Do stuff here when the right property has changed in model 1
}
}
}
I have only new'd up a Model1 in the constructor of Model2 as an example - you might be assigning it and storing as a field or property elsewhere in the Model2 ViewModel.
This might be particularly useful if you have ViewModels within ViewModels (parent VM > child VMs).
I use parent > child VMs quite regularly, and I don't think it's against MVVM best practices, but I still use the EventAggregator, instead of events.
As a side note, if you are using C#6:
Use nameof(Property) instead of "magic strings" (e.g. "Property"). This makes for much easier refactoring and compiler can tell you about errors - but essentially does the same job. Use this in the OnPropertyChanged() call in your setter
You can also use the nameof keyword when checking the property name, with the same principle as above. Like this: if (e.PropertyName == nameof(Model1.Property)) ...
Use null propagation: PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));. Changes the method body of your OnPropertyChanged to a nice one-liner, whilst still doing the same job
I digress
I would always look at sticking to proper MVVM practices, where possible.
I use Prism's EventAggregator on a daily basis and will swear by it.
Have a good read up on PubSub Events (you can use any type of event aggregator, but I would say to use Prism's, preferably)
Hope this helps! :)
If this is just to notify Model2 I think you dont need all these implementations. You can do something like
public string TestProperty
{
get
{
return testProperty;
}
set
{
testProperty = value;
// If changing properties, fire your OnPropertyChanged to update UI
OnPropertyChanged("TestProperty");
//Here you can call a method of Model2 sating that its changed
Model2 m2Instance = new Model2();
m2Instance.ValueChanged();
}
}
Add the method ValueChanged in you model 2.
That's because you're not registering to PropertyChanged. You're registering your event handler on NotifyChange so that PropertyChanged in Model1 is NULL and so NotifyChange is not fired.
So, you need to implement your OnPropertyChanged as the following:
private void OnPropertyChanged(string propName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propName));
if (NotifyChange != null)
NotifyChange(this, null);
}
Finally, if you want your NotifyChange to be fired for a specific property, then adjust the code above to check for the propName before you fire the event.
I know that the proper course of action is to create a class, make an event in said class, then use said class in another part of the program where the variable would be changed (e.g. Use said class in the equal button of a calculator, so that an event handler knows that a variable has been changed because an event would be fired). But, trying to streamline my code, I'm looking for a way to monitor a variable directly without an infinite loop/timer and raise an event when it changes. Is there such a thing? If not, are there any other alternatives aside for the one I mentioned?
Here is what I'm trying to mention:
Code that changes a variable -> Another piece of code (not a loop) watching for changes then throws an event if there are changes -> Event handler
You can't do it with fields but with properties:
class SomeClass : INotifyPropertyChanged
{
private string someProperty;
public string SomeProperty
{
get { return someProperty; }
set { someProperty = value; OnPropertyChanged(); }
}
private void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged = delegate {};
}
Edit (.net 4.0)
class SomeClass : INotifyPropertyChanged
{
private string someProperty;
public string SomeProperty
{
get { return someProperty; }
set { someProperty = value; OnPropertyChanged("SomeProperty"); }
}
private void OnPropertyChanged(string propertyName)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged = delegate {};
}
Edit (Winforms example)
public partial class Form1 : Form
{
private SomeClass theObject = new SomeClass(); //keep a reference of the object.
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
//here we do the binding... we want the 'Text' Property of the control to change if the 'SomeProperty' changes OnPropertyChanged
textBox1.DataBindings.Add("Text",theObject,"SomeProperty",false,DataSourceUpdateMode.OnPropertyChanged);
}
private void button1_Click(object sender, EventArgs e)
{
theObject.SomeProperty = "This works!"; //just a test button that changes the property...
}
}
Though I know that it is an often undesired practice here at Stack Overflow, you may find my project NMF Expressions interesting: http://nmfexpressions.codeplex.com/
Basically, the project aims to allow you to write such as follows:
var myObservedVariable = Observable.Expression(() => whatever you want)
In this scenario, myObservedVariable will be of INotifyValue<T> which provides a ValueChanged event. Alternatively, you can use the query syntax. Alternatively, you may have a look at other similar frameworks like Obtics, BindableLINQ or ContinuousLINQ. A comparison of the latter was done in Bindable Linq vs. Continuous Linq.
However, this only works under pretty strong assumptions like all the object models that you are working with completely support INotifyPropertyChanged and INotifyCollectionChanged.
In addition to #Florian's answer, you can inject an implementation of the INotifyPropertyChanged interface at compile time using Fody.PropertyChanged.
I am currently working on a project where we would like to use the Fody PropertyChanged IL weaver. It appears to work fantastically, but it is doing strange things to our FxCop analysis built into visual studio.
We have CA1062 and CA2214 enabled: The former validates arguments of public methods and the latter checks for calls to overridable methods in constructors.
The second one, we have figured out is due to the fact that we are also using Caliburn.Micro which supplies a method called NotifyOfPropertyChanged which is virtual. This is a simple fix of changing the method to not be virtual.
The first issue is much harder and seemingly random. We have been hard core about validating our arguments from the get go and did not have any FxCop analysis errors before Fody, but when I added it through NuGet FxCop was seeing changes in places that shouldn't have any weaved code for property changed. I even looked at a diff of the de-compiled source before and after Fody, and there are FxCop conflicts in places where methods weren't changed at all.
So my question is whether I can somehow postpone Fody weaving till after the analysis is finished, or if there is some other way to get Fody and FxCop to interact nicely?
Thanks in advance
Example:
public class Base : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public class Descendent : Base
{
public Descendent()
{
DetermineDefaultMyPropertyValue();
}
public string MyProperty { get; set; }
private void DetermineDefaultMyPropertyValue()
{
MyProperty = "Default value";
}
protected override void OnPropertyChanged(string propertyName)
{
// some property changed logic
}
}
It could be solved by changing the auto implemented property to use a backing field.
( Only the ones that are set while constructing the object. )
public class Descendent : Base
{
public Descendent()
{
DetermineDefaultMyPropertyValue();
}
private string _myProperty;
public string MyProperty {
get { return _myProperty; }
set { _myProperty = value; }
}
private void DetermineDefaultMyPropertyValue()
{
_myProperty = "Default value";
}
protected override void OnPropertyChanged(string propertyName)
{
// some property changed logic
}
}
In my mvvm ViewModel I have such field
public int Delta { get; private set; }
However when I update it like that:
Delta = newValue;
UI is not refreshed.
I was thinking that databinding will do that for me. For example I can declare collection as ObservableCollection and then databinding will work.
However there are no ObservableInt, how to say View that it need to be refreshed then?
Probably I should raise some event "notify property changed" or something?
You have two choices:
Implement the INotifyPropertyChanged interface on your class.
Inherit from DependencyObject and implement Delta as a DependencyProperty.
The simplest option is #1. You can implement the INotifyPropertyChanged interface on your class quite easily:
public class YourClass : INotifyPropertyChanged
{
private int _delta;
public int Delta
{
get { return _delta; }
set { _delta = value; PropertyChanged?.Invoke(nameof(Delta)); }
}
public event PropertyChangedEventHandler PropertyChanged;
}
You can read more about using and implementing dependency properties on MSDN.
While we're at it with improving the answer, some of the other new additions of c# 6.0 and 7.0 help make it ever more compact:
public class Prop<T> : INotifyPropertyChanged
{
private T _value;
public T Value
{
get => _value;
set { _value = value; NotifyPropertyChanged(nameof(Value)); }
}
public event PropertyChangedEventHandler PropertyChanged;
internal void NotifyPropertyChanged(string propertyName) =>
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
This way, you aren't using any "embedded values" (i.e - the property's name) and are keeping the code refactor-safe.
And there's also no need for redundant code blocks due to c# 6.0 and 7.0's new Expression body features
Using #LBushKin's Answer, i modified it to
public class Prop<T> : INotifyPropertyChanged
{
private T _value;
public T Value
{
get { return _value; }
set { _value = value; NotifyPropertyChanged("Value"); }
}
public event PropertyChangedEventHandler PropertyChanged;
internal void NotifyPropertyChanged(String propertyName)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
and to set it up:
class MainWindow ...
// a bool with initial value of true
public static Prop<bool> optionBool { get; set; } = new Prop<bool>{ Value = true };
private void Window_Loaded(object sender, RoutedEventArgs e)
{
// connect UI to be able to use the Prop
DataContext = this;
}
and to use it:
<Grid ...
<CheckBox Content="Da Check" ... IsChecked="{Binding optionBool.Value}"/>
There is also a Collection and 2-Properties version here:
Utils.ObservableProperties.cs (this repo contains several related classes)
Just implement INotifyPropertyChanged Interface in your class and use it to raise a PropertyChanged for your Property and then UI will update. If you are using an MVVM project template then there is a good chance you already have a helper method implemented you only need to use it.
MSDN INotifyPropertyChanged
GalaSoft MVVM Light Toolkit
The ObservableCollection raises events automatically but for your own properties you have to raise the events yourself.
A good example is here: http://www.codeproject.com/Tips/228352/Naming-Properties-in-MVVM?display=Print
I'd suggest using mvvm light: http://mvvmlight.codeplex.com, I used it in silverlight and wpf applications. Very easy to use and provides a messageing system between model, view model and view.
Adding on to https://stackoverflow.com/a/8316100/5725669, there is a new and easy way to do this without remembering to keep track of PropertyChanged?.Invoke(nameof(Delta)); in every location
public class YourClass : INotifyPropertyChanged
{
private int _delta;
public int Delta
{
get { return _delta; }
set {
_delta = value;
// Call OnPropertyChanged whenever the property is updated
OnPropertyChanged();
}
}
// Declare the event
public event PropertyChangedEventHandler PropertyChanged;
public YourClass()
{
}
// Create the OnPropertyChanged method to raise the event
// The calling member's name will be used as the parameter.
protected void OnPropertyChanged([CallerMemberName] string name = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
}
}
It makes use of CallerMemberName for skipping manual entries for property name. More details on this MSDN Doc
This is just something I was thinking as I was learning on Attributes and I was using the INotifyPropertyChanged too much, is just and Idea and I would like to hear some opinios about it.( I know this would require some work on the compiler and not on the cosumer side)
Since INotifyPropertyChanged is used with the same Pattern most of the time .. just like calling the method that fire ups the event with the name of the property ,could it be designed as and Attribute and using Auto-Properties? So that the compiler knows it need to add the call to the PropertyChanged event?
So if we have the class....
public class DemoCustomer : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(String info)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(info));
}
}
private string companyNameValue = String.Empty;
...
}
Instead of declaring the property
public string CompanyName
{
get
{
return this.companyNameValue;
}
set
{
if (value != this.companyNameValue)
{
this.companyNameValue = value;
NotifyPropertyChanged("CompanyName");
}
}
}
we could do something like this if we can indicate to the compiler by this attribute that it needs to generate a call to the PropertyChanged with the name of the property if the new value is different from the previous
[NotifyPropertyChanged]
public string CompanyName
{
get;set;
}
We could still keep coding in the old way for some custom behaviours when no using the Attribute..
In case anyone happens across this thread and is using C# 5 (VS 2012+, .NET 4.5+). This case be done "more easily" now with CallerMemberNameAttribute. This attribute is applied to a string parameter and causes the compiler to pass in the name of the calling method/property when the default value us used (i.e. when and argument is not passed). Making implementing INotifyPropertyChanged less tiresome. For example:
public class MyClass
{
private string myProperty;
public string MyProperty
{
get { return myProperty; }
set
{
myProperty = value;
OnPropertyChanged();
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
var handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
}
So, you just need OnPropertyChanged() in each setter to send out the event and don't have to deal with a hard-coded string for the property name.
This style of thinking is called Aspect Oriented Programming (or AOP). You can achieve the end result by adding a post build action using Mono's Cecil to go through properties with that attribute and modify the behavior of the property and spit out the newly compiled assembly with the appropriate behavior. You can look at Jason Bock's Dimecast on "Leveraging Cecil to inject code into your Assemblies"