I have some properties in ViewModel that are updated/recalculated based on Model Properties updates. I am asking for the best approach to implement this scenario?
I don't like the approach of subscribing to PropertyChanged Event Handler of Model and then updating ViewModel properties. How do you handle this scenario?
Subscribing to events is the right approach, but I agree with you about not wanting to use the PropertyChanged event. I like to leave that event alone and create my own events as needed. Here is my approach:
public class Person : INotifyPropertyChanged
{
//custom events as needed
public event EventHandler NameChanged = delegate { };
//needed for INotifyPropertyChanged
public event PropertyChangedEventHandler PropertyChanged = delegate { };
private string _name;
public string Name
{
get { return _name; }
set
{
if (_name != value)
{
_name = value;
this.NotifyPropertyChanged();
//Fire my custom event
this.NameChanged(this, EventArgs.Empty);
}
}
}
private int _age;
public int Age
{
get { return _age; }
set
{
if (_age != value)
{
_age = value;
this.NotifyPropertyChanged();
//I am not exposing a custom event for this property.
}
}
private void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
In this example, Name and Age are observable for UI purposes, but Name is observable to anything outside of the UI. Now if you remove any PropertyChanged notifications, you don't accidentally cause that runtime error if your ViewModel was subscribed to PropertyChanged and parsing the string.
Since you don’t want to put the dependency on the view model inside the model, listening to model changes in the view model is indeed the right approach to update view model properties that are based on the model.
Related
INotifyPropertyChanged interface implementation (appropriate event) and helper method works fine in my code but I really don't understand how it works. My book does a poor job of explaining it to me in my opinion or I am not very smart.
We have a separate class Car.cs added to my solution with the following code (example from the book) and it should work in TwoWay Binding regards to TextBox control in my WPF application being changed when object's instance is being changed too:
public class Car: INotifyPropertyChanged
{
private string _make;
private string _model;
public event PropertyChangedEventHandler PropertyChanged;
public Car()
{
}
public string Make
{
get { return _make; }
set
{
if (_make != value)
{
_make = value;
OnPropertyChanged("Make");
}
}
}
public string Model
{
get { return _model; }
set
{
if(_model != value)
{
_model = value;
OnPropertyChanged("Model");
}
}
}
private void OnPropertyChanged (string propertyName)
{
if (PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}}
And here is the code which I made myself after learning Microsoft learning materials and it looks and works better imo:
public class Car: INotifyPropertyChanged
{
private string _make;
private string _model;
public event PropertyChangedEventHandler PropertyChanged;
public Car()
{
}
public string Make
{
get { return _make; }
set
{
_make = value;
OnPropertyChanged();
}
}
public string Model
{
get { return _model; }
set
{
_model = value;
OnPropertyChanged();
}
}
private void OnPropertyChanged ()
{
if (PropertyChanged != null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(""));
}
} }
So I have the foolowing question regards to example from the book (the first one):
What this part means? What's the point to check if property not equals to value (never saw anything like it before when during the Set part of the property you check that field of the class checked for being not equalled to value)? Code works even without it. No explanation for it in the book. Here is the code under question:
set
{
if (_make != value)
{
_make = value;
OnPropertyChanged("Make");
}
Why if you'll write the name of the property without brackets in the above-mentioned small piece of code (in subparagraph one) then it will not work? I meant if you'll write Make then it does not work and you should write "Make".
Thanks in advance!
Here is a basic example of how to correctly implement the IPropertyChanged interface, which notifies the view to re-draw on property changing its value.
public class PropertyChangedEventStackOverflow : INotifyPropertyChanged
{
//We have a private property of the same public property. This allows us to store the privous value of the property which we can
//get at any time when we ask.
string name = string.Empty;
//Public property is required to do two-way-binding or one-way-binding from model to the view.
public string Name
{
get => name;
set
{
if (name == value) //Check if the private property is already equals to the exact same value to avoid extra memory
return; //and uneccasary screen re-drawing. So we just return and avoid calling the OnPropertyChanged.
name = value; //If the value is different meaning we want to notify view and tell it to re-draw the screen for us.
OnPropertyChanged(nameof(Name));
}
}
//We create this helper metod to avoid doing something like this every single time we want to notify
string surname = string.Empty;
public string Surname
{
get => surname;
set
{
if (surname == value)
return;
surname = value;
PropertyChanged(this, new PropertyChangedEventArgs(nameof(Surname)));
}
}
//Here is the method to create for OnPropertyChangedEvent
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged([CallerMemberName] string propertyName = "")
{
var changed = PropertyChanged;
if (changed == null)
return;
changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
If you want to go step further and want to have an Action when your property has changed you can do something like this. backingStore type T = private property
public bool SetProperty<T>(ref T backingStore, T value,
[CallerMemberName] string propertyName = "",
Action onChanged = null)
{
if (EqualityComparer<T>.Default.Equals(backingStore, value))
return false;
backingStore = value;
onChanged?.Invoke();
OnPropertyChanged(propertyName);
return true;
}
You can give it an action to execute if once a property has changed.
Because raising the PropertyChanged event can be expensive in terms of performance (for example when a complex view has to redraw itself in order to display the latest data), it's a recommended pattern to raise it only when necessary.
It's only necessary when the new value has changed. To update the binding when the new value and the old value are the same is redundant and potentially expensive.
The following example is a very verbose version to highlight what's going on:
private string myProperty;
public string MyProperty
{
get => this.myProperty;
set
{
string newValue = value;
string oldValue = this.MyProperty;
if (newValue != oldValue)
{
this.myProperty = newValue;
OnPropertyChanged(nameof(this.MyProperty));
}
}
}
For reasons of performance, passing an empty string (string.Empty or "") or null to the PropertyChangedEventArgs instance, instead of the property name, will instruct the binding engine to update all bindings of the class that raises the PropertyChanged event.
In other words, it's like raising the PropertyChanged event for all properties at once.
That's why you only do this when this behavior is explicitly desired (for example when you reset and clear all properties):
/* Will raise the PropertyChanged event for ALL properties defined by the declaring type */
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(""));
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(string.Empty));
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(null));
/* Will raise the PropertyChanged event ONLY for the particular property that matches the provided property name */
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("MyProperty"));
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(this.MyProperty)));
"Make" is called a string literal. Every literal enclosed in double-quotes is interpreted as plain text by the compiler (string literal). If it wasn't the double-quotes, the compiler expects the value to be a language object (for example a variable or type).
If you check the signature of the constructor of PropertyChangedEventArgs you will see that it expects the property name as string.
Using Make (without the double-quotes) is a simple value reference (local or member variable) that returns the value of the Make property and not the name of the property (member name).
The property name is required to tell the binding engine which property has changed and therefore which Binding needs to be updated.
The following example assumes that the property Make is set to a value of "BMW":
public string Make
{
get => this.make;
set
{
// For example 'value' is "BMW"
this.make = value;
// Invocation a)
// Pass the property name as argument.
OnPropertyChanged("Make"); // Same as: OnPropertyChanged(nameof(this.Make))
// Invocation b)
// Pass the property value as argument
OnPropertyChanged(Make);
// Verbose version of b)
string modelMake = Make;
OnPropertyChanged(modelMake);
}
}
Version b) won't work, because the binding engine needs the property's member name and not the property value.
See Microsoft Docs: Strings and string literals to learn about strings.
See INotifyPropertyChanged to learn the recommended pattern to implement the interface
Remarks
Checking variable (for example an event delegate) for null using an if statement and the Null-conditional operator ?. or ?[] is redundant.
In high performance code you would want to avoid the redundant double checking.
The following versions are the same:
private void OnPropertyChanged()
{
if (PropertyChanged != null)
{
PropertyChanged.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
private void OnPropertyChanged()
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
private void OnPropertyChanged(string propertyName) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
What's the point to check if property not equals to value
it is just to avoid unnecessary updates. It is not strictly required. Whenever you have events to signal that something is updated it is often a good idea to check if the thing actually changed, since you do not know what is listening to the event, it could potentially trigger some slow computations.
Why if you'll write the name of the property without brackets in the above-mentioned small piece of code (in subparagraph one) then it will not work?
Because the event needs a string. You can let the compiler insert this string for you by writing OnPropertyChanged(nameof(Make)); You can also use CallerMemberName attribute to let the compiler insert this automatically, just call OnPropertyChanged() within your property.
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
Going one step further you you can create a method that does both the comparison, field update, and event, letting you write a one line setter => Set(ref _make, value);
private void Set<T>(ref T field, T value, [CallerMemberName] string caller = "")
{
if (!EqualityComparer<T>.Default.Equals(field, value))
{
field = value;
OnPropertyChanged(caller);
}
}
However, my preference tend to be to wrap all of this inside a separate class:
public class Changeable<T> : INotifyPropertyChanged
{
private T currentValue;
public Changeable(T initialValue) => currentValue = initialValue;
public T Value
{
get => currentValue;
set
{
if (!EqualityComparer<T>.Default.Equals(currentValue, value))
{
this.currentValue = value;
OnPropertyChanged();
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
{
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
}
That way you can declare a property like public Changeable<string> Make {get;} = new (""); and bind to it like {Binding Make.Value}
INotifyPropertyChanged is an interface which only requires a class to implement it's requirements. Which is basically "just" an Event.
public interface INotifyPropertyChanged
{
//
// Summary:
// Occurs when a property value changes.
event PropertyChangedEventHandler? PropertyChanged;
}
The Method OnPropertyChanged is just a "helper" to raise this event. But the important part here, is that the INotifyPropertyChanged implementation by itself is not doing anything.
However WinForms and WPF are automatically subscribing in some higher-hirachy classes to such Event. They identify such classes through reflection and add themself to one of the subscribers to the PropertyChanged Event - which your class still needs to raise.
For this purpose you need to override the Setters of your properties:
public string Examle
{
get { return _field; }
set
{
_field = value;
OnPropertyChanged(); // <---
}
}
Otherwise the subscriber will never be notified. However the whole purpose of this is to decouple the handling of changes from the "Model" so your class which act's as container to the data - since the container can be passed to different forms, which might want to handle this container different.
The check if the value actually changed is only something you CAN do - to prevent the notification of the subscriber (let's say your data grid) to re-run all validations you've setup for such property. To avoid unnecessary runs of such validations this if will prevent notifying subscribers of such changes where a value get's re-assigned, but it remains the same.
The auto-wiring of the subscriber however is abstraced for you as developer, which can create difficulties for beginners to understand - however makes your life much more easier on the long run, as you can rely on the Framework as such (less for you to implement, test, etc.) so it's just a "convention" to use INotifyPropertyChange Interface, but the basic building blocks are general C# constructs.
Hope this helped a little :)
I have an ObservableCollection that populates a datagrid in WPF. I need to bind to the total of the "Hours" column, and have that total update when a value in the "Hours" column is changed. I can achieve this by listening to the "LostFocus" event and running a function, but would like to try my hand at binding.
The issue I am running into, is the NotifyPropertyChanged event will not fire when an items property in the collection is changed.
The sortie class NotifyPropertyChanged will fire, but the collection doesn't interpret that as its own property changing. How can I listen to the sortie PropertyChanged from the collection in the missions class?
My Models
public class Mission : INotifyPropertyChanged
{
private ObservableCollection<Sortie> sorties;
public ObservableCollection<Sortie> Sorties
{
get { return this.sorties; }
set
{
if (this.sorties != value)
{
this.sorties = value;
this.NotifyPropertyChanged("Sorties");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged(string propName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
}
public class Sortie : INotifyPropertyChanged
{
private double hours;
public double Hours
{
get {return this.hours;}
set
{
if (this.hours != value)
{
this.hours = value;
this.NotifyPropertyChanged("Hours");
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
public void NotifyPropertyChanged(string propName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
}
I didn't bother posting my XAML or View Model, as I am confident I can solve the issue once I learn how to trigger a PropertyChanged event for the collection and I wanted to spare you having to read through massive amounts of code. If you believe it is needed however, let me know.
Write a readonly property in the parent viewmodel that calculates the value.
public double SortieHours => Sorties.Sum(x => x.Hours);
Parent viewmodel handles PropertyChanged on each item in Sorties, and CollectionChanged on Sorties. In CollectionChanged on Sorties, you have to add/remove PropertyChanged handlers from Sortie instances as they're added and removed. When you get a new Sorties collection (you might want to make that setter private for this reason), you need to toss out all the old handlers and add new ones.
Now, whenever a Sortie is added or removed, or its Hours changes, or somebody hands you a new Sorties collection, raise PropertyChanged:
OnPropertyChanged(nameof(SortieHours));
And bind that property to whatever you like in the XAML.
This looks horrible (because it is), but what else are you going to do?
A lot of people would advise you to give Sortie an HoursChanged event. PropertyChanged is annoying for this type of case because it can get raised for multiple different properties and you have to check which one. And it's a magic string thing.
Above is C#6. For C#5,
public double SortieHours { get { return Sorties.Sum(x => x.Hours); } }
OnPropertyChanged("SortieHours");
I found this link to be a big help. It creates a base class for ObservableCollections to "AutoMagicly" add the PropertyChanged events of the collection items to the CollectionChanged event.
I have a changed event in a class, which is triggered at a variable's value change but I dont know how to send a notification to the main of my C# windows form, for example, show a message box to notify the value changed. I dont wanna set a timer to check every second for a response. There is any better way?
You are probably looking for INotifyPropertyChanged, a common pattern in WPF but it can also be used for Windows Forms. There is a pretty large example on the documentation page:
https://msdn.microsoft.com/en-us/library/system.componentmodel.inotifypropertychanged(v=vs.110).aspx
class SomeClass : INotifyPropertyChanged
{
private int foo;
public event PropertyChangedEventHandler PropertyChanged;
public int Foo
{
get { return foo; }
set
{
if (foo == value)
return;
foo = value;
PropertyChanged("Foo");
}
}
protected void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Please note that much of this becomes safer and much less boilerplate you use newer .NET features such as [CallerMemberName]
As other posted you should impement INotifyPropertyChanged, but in addition I would suggest the following as well:
In the setter of your property check whether the value, that is assigned to your property really changes the property's value. Otherwise you probably fire the notify change to often.
You should also implement ISupportInitialize. Usually, you fire the property changed event to inform your main form that data has changed and needs to be saved. But if you load your instances from e.g. a database, the property changed event would also fire, indicating data has changed, which is not true in that sense. With ISupportInitialize you can tell your instances that they are being initialized and that they should not fire the notify property changed event.
Here is some sample code:
class MyTest : INotifyPropertyChanged, ISupportInitialize
{
public event PropertyChangedEventHandler INotifyPropertyChanged.PropertyChanged;
public delegate void PropertyChangedEventHandler(object sender, PropertyChangedEventArgs e);
private bool _IsInitializing;
private bool _MyProperty;
public void BeginInit()
{
_IsInitializing = true;
}
public void EndInit()
{
_IsInitializing = false;
}
public bool MyProperty {
get { return _MyProperty; }
set {
if (_MyProperty == value)
return;
_MyProperty = value;
OnPropertyChanged("MyProperty");
}
}
private void OnPropertyChanged(string propertyName)
{
if (_IsInitializing)
return;
if (PropertyChanged != null) {
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
For initialization you would write (to prevent the property changed event from firering):
MyTest thisTest = new MyTest();
thisTest.BeginInit();
thisTest.MyProperty = true;
thisTest.EndInit();
You should wrap your variable with a "Property" (Getter and Setter).
Then you can implement in the "Setter" either your own event, or you can trigger the property changed event from WPF (if you are using WPF).
Is it possible to trigger some event when something changes in a given class?
E.g. I have a class that has 100 fields and one of them is being modified externally or internally. Now I want to catch this event. How to do this?
The most I wonder if there is a trick to do this quickly for really extended classes.
As a best practice, convert your public fields to manual properties and implement your class with the INotifyPropertyChanged interface in order to raise a change event.
EDIT: Because you mentioned 100 fields I would suggest you to refactor your code like in this great answer: Tools for refactoring C# public fields into properties
Here is an example of it:
private string _customerNameValue = String.Empty;
public string CustomerName
{
get
{
return this._customerNameValue;
}
set
{
if (value != this._customerNameValue)
{
this._customerNameValue = value;
NotifyPropertyChanged();
}
}
}
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Check this out: INotifyPropertyChanged Interface
I have a ListBox on my UI that is bound to a property of ObservableCollection. I set a new instance of the ObservableCollection into the property in the view model's constructor and I can add items to it with a button on the form. These are visible in the list.
All is good.
However, if I reinitialize the property with new in the button callback, it breaks the binding and the UI no longer shows what is in the collection.
I assumed the binding would continue to look up the values of the property, but its presumably linked to a reference which is destroyed by the new.
Have I got this right? Can anyone expand on how this is linked up? Is there a way to rebind it when my view model has no knowledge of the view?
Make sure you are raising a PropertyChangedEvent after you reintialize your collection. Raising this event will allow the view to handle changes to the property with the model having no knowledge of the view.
class Model : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string name)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(name));
}
private ObservableCollection<string> _list = new ObservableCollection<string>();
public ObservableCollection<string> List
{
get { return _list; }
set
{
_list = value;
NotifyPropertyChanged("List");
}
}
public Model()
{
List.Add("why");
List.Add("not");
List.Add("these?");
}
}
I think based on your description that what you need to do is refactor the property that exposes your ObservableCollection so that it raises a PropertyChanged event also when it is assigned a new value. Example:
public ObservableCollection<int> Integers
{
get { return this.integers; }
set {
if (this.integers != value)
{
this.integers = value;
RaisePropertyChanged("Integers");
}
}
}
Supposing you've implemented INotifyPropertyChanged on your ViewModel, you can raise the property changed event on your ObservableCollection whenever you assign a new value to it.
public ObservableCollection<string> MyList { get; set; }
public void SomeMethod()
{
MyList = new ObservableCollection<string>();
RaisePropertyChanged("MyList");
}
Updates to the ObservableCollection are handled by hooking in to the CollectionChanged event so when you create a new ObservableCollection your observer is still looking at the old collection.
Two simple suggestions would be either to implement INotifyPropertyChanged on the class that contains the ObservableCollection and raising the PropertyChanged event in the setter of the collection property (don't forget to unhook from the old one first in your observer if it's your own code).
private ObservableCollection<string> _myCollection = new ObservableCollection<string>();
public ObservableCollection<string> MyCollection
{
get { return _myCollection; }
set
{
if(_myCollection == value)
return;
_myCollection = value;
RaisePropertyChanged("MyCollection");
}
}
A second, and the option I generally prefer is to just clear and repopulate the collection with your new data when it arrives.
public void HandleCollectionData(IEnumerable<string> incomingData)
{
MyCollection.Clear();
foreach(var item in incomingData)
{
MyCollection.Add(item);
}
}