It looks like the Weak Events or more specifically WeakEventManager or IWeakEventListener are not available in .Net Core as they are part of WindowsBase assembly.
Are there an alternatives to this feature?
Events are often a source of memory leaks in applications and weak references are a great way of dealing with this issue.
I couldn't find any information on this topic in stackoverflow
The library Nito.Mvvm.Core has a WeakCanExecuteChagned class that does weak events using the command class you could use as a starting point for writing your manager backed by a WeakCollection<EventHandler>.
Here is a simple example using a custom class with a event named Foo that takes in a FooEventArgs object.
public class MyClass
{
private readonly WeakCollection<EventHandler<FooEventArgs>> _foo = new WeakCollection<EventHandler<FooEventArgs>>();
public event EventHandler<FooEventArgs> Foo
{
add
{
lock (_foo)
{
_foo.Add(value);
}
}
remove
{
lock (_foo)
{
_foo.Remove(value);
}
}
}
protected virtual void OnFoo(FooEventArgs args)
{
lock (_foo)
{
foreach (var foo in _foo.GetLiveItems())
{
foo(this, args);
}
}
}
}
My library System.Waf.Core provides a WeakEvent implementation that can be used as an alternative to the WeakEventManager.
Example for INotifyPropertyChanged.PropertyChanged:
public class Publisher : INotifyPropertyChanged
{
public event PropertyChangedEventHandler? PropertyChanged;
}
public class Subscriber
{
public void Init(Publisher publisher)
{
// Instead of publisher.PropertyChanged += Handler; use the following statement:
WeakEvent.PropertyChanged.Add(publisher, Handler)
}
public void Handler(object? sender, PropertyChangedEventArgs e) { }
}
More details can be found on this Wiki page Weak Event.
Related
In my application I have a interface IEncoder that is having event EncoderCaller.
public interface IEncoder
{
event EncoderCaller EncoderCalled;
}
public delegate void EncoderCaller(object Source, EventArgs args);
public class Video
{
public string Title { get; set; }
}
public class VideoEventArgs : EventArgs
{
public Video xVideo { get; set; }
}
public class DetectionAction : IEncoder
{
public event EncoderCaller EncoderCalled;
public void Encode(Video video)
{
//some logic to encode video
OnVideoEncoded();
}
protected virtual void OnVideoEncoded()
{
if (EncoderCalled != null)
EncoderCalled(this, EventArgs.Empty);
}
}
public class Client1: IEncoder
{
}
I need some mechanism by which I should be able to share a contract, if that is implemented by any client then that event will trigger event in my class DetectionAction .
Can someone tell me, Am I doing right thing.
How it can be done?
If you have two classes in the same process, you could consider explicitly chain events like this:
public class Client1 : IEncoder
{
public event EncoderCaller EncoderCalled;
public Client1(IEncoder anotherEncoder)
{
// Listen to event raised on another instance and raise event on this instance.
anotherEncoder.EncoderCalled += OnAnotherEncoderCalled;
}
private void OnAnotherEncoderCalled(object source, EventArgs args)
{
if (EncoderCalled != null)
EncoderCalled(this, EventArgs.Empty);
}
}
In this case, for example, anotherEncoder is DetectionAction.
However, if you are seeking solution for sharing events between two different applications running in different processes, you might be looking at inter-process communication, like this post:
Listen for events in another application
And the above example code still works, but the IEncoder in this case is an implementation with IPC support, for example a message queue listener which raises the event on message received.
This question already has answers here:
C# Language Design: explicit interface implementation of an event
(4 answers)
Closed 5 years ago.
I am in the process of creating a system that can receive messages from a variety of different sources.
Using the interface approach, I am adding a custom event which will pass the message back to the calling application.
I've used Vistual Studio's "scaffolding" using Ctrl-. to provide the implementation for the concrete class, but its added the add and remove elements but I dont really know how to wire that bit up.
Interface class
public class MessageEventArgs : EventArgs
{
public Message { get; set; }
}
public interface MessageBroker
{
void Start();
event EventHandler<MessageEventArgs> OnMessageReceived;
}
Implementation class
public class MessageSourceA : MessageBroker
{
event EventHandler<MessageEventArgs> MessageBroker.OnMessageReceived
{
add
{
// What goes here
}
remove
{
// What goes here
}
}
void MessageBroker.Start()
{
}
}
Main Program
static void Main(string[] args)
{
MessageBroker sourceA = new MessageSourceA ();
sourceA.OnMessageReceived += sourceA_OnMessageReceived;
}
private static void sourceA_OnMessageReceived(object sender, MessageEventArgs e)
{
// Do stuff with message
}
Thanks...
You could normally implement from interface.
public class MessageSourceA : IMessageBroker
{
public void Start();
public event EventHandler<MessageEventArgs> OnMessageReceived;
}
I suggest you to rename MessageBroker to IMessageBroker as its a naming convention. Since "I" helps to differentiate between class and interface when looking at code.
If there is proper reason to implement interface explicitly you need private event handler.
private event EventHandler<MessageEventArgs> _onMessageReceived;
event EventHandler<MessageEventArgs> MessageBroker.OnMessageReceived
{
add
{
_onMessageRecieved += value;
}
remove
{
_onMessageRecieved -= value;
}
}
Lets say I have the below code. What is the difference between assigning the actions directly and subscribing to an event?
//Action directly assigned
public class ClassA
{
public Action<string> OnAdd;
private void SomethingHappened()
{
OnAdd("It Happened");
}
}
public class ClassB
{
public ClassB()
{
var myClass = new ClassA();
myClass.OnAdd = Add;
}
private void Add(string Input)
{
//do something
}
}
//Event handlers
public class ClassA
{
public event Action<string> OnAdd;
private void SomethingHappened()
{
if (OnAdd != null)
OnAdd("It Happened"); //Should it be OnAdd.Invoke("It Happened") ???????
}
}
public class ClassB
{
public ClassB()
{
var myClass = new ClassA();
myClass.OnAdd += Add;
}
private void Add(string Input)
{
//do something
}
}
(As an aside, it's hard to explain things when you've used the same type names twice.)
When you use a public field, clients can not only subscribe to events - they can also completely remove other event handlers by assigning instead of adding:
myClass.OnAdd = Add;
They can also invoke the handler directly:
myClass.OnAdd("foo");
Both of these violate the normal pub/sub pattern, where the various subscribers are isolated from one another. Subscribers don't get to overwrite each other's subscriptions (only add or remove their own) and they don't get to raise the event themselves.
For more on events and delegates, see my article on the topic.
You can assign more than one delegates to one event (thus the += operator).
An Event acts like a wrapper around a Delegate to offer protection from reassigning/removing as John has pointed out. I found this quite a good read.
Is there a way to watch an object graph for changes on any object, and do something based on that change?
Lets say I have the following:
public class Main:INotifyPropertyChanged
{
public ObservableCollection<Foo> FooItems { get; }
public ObservableCollection<Bar> BarItems { get; }
}
public class Foo:INotifyPropertyChanged
public class Bar:INotifyPropertyChanged
{
public ObservableCollection<Other> OtherItems { get; }
}
public class Other:INotifyPropertyChanged
What would be the best way to implement some sort of change notification system across all objects? For example an autosave, where any change would trigger the system to serialize the Main class.
Should I have glue code in the Main class watching the BarItems for changes, hooking up to their PropertyChanged? This seems a bit messy, and error prone to me. Is there a better way?
Rather than objects raising their own property changed events, perhaps they could raise a shared event instead. For example:
public class SharedChangeNotifier
{
public static event EventHandler<DataChangedEventArgs> SharedChangeEvent;
protected void RaiseChangeEvent()
{
if (SharedChangeNotifier.SharedChangeEvent != null)
{
SharedChangeNotifier.SharedChangeEvent(
this, new DataChangedEventArgs());
}
}
}
public class Foo : SharedChangeNotifier
{
public int MyProperty
{
get { ... }
set
{
...
RaiseChangeEvent();
}
}
}
You could then attach an event handler to the static SharedChangeNotifier's SharedChangeEvent to be notified whenever any object deriving from SharedChangeNotifier is changed, like this:
SharedChangeNotifier.SharedChangeEvent += (sender, args) => {
DoWhatever();
};
I just read an interesting blog post on that issue at http://www.lennybacon.com/ReBlinderFleckChangeTracking.aspx
The post is in German, but as it's mostly code, it should be OK.
Hope this helps!
The way I have done it in the past was to create a separate ChangeTracker class with a method to Register objects into it. Inside that method, use reflection to explore the registered object, and hook into events on each of its properties that implements INotifyPropertyChanged.
You can then add methods to the ChangeTracker to interrogate the state, e.g. IsDirty(), or even implement INotifyPropertyChanged on the ChangeTracker.
(Be sure to implement and use IDisposable on the ChangeTracker, and drop all the event handlers at that time).
You could have the same handler for all items that implement INotifyPropertyChanged events:
foreach (INotifyPropertyChanged obj in FooItems)
obj.PropertyChanged+= this.modified;
// likewise for bar items, and when items are added
private void modified(object sender, EventArgs args)
{
this.Save();
}
edit> To do the same when an item is added:
private void addToList<T>(ref List<T> l, T item) where T : INotifyPropertyChanged
{
item.PropertyChanged += this.modified;
l.Add(item);
}
call it using:
Foo item = new Foo();
List<Foo> fooItems = new List<Foo>();
addToList<Foo>(ref fooItems, item);
In following code, I want to extend the behaviour of a class by deriving/subclassing it, and make use of an event of the base class:
public class A
{
public event EventHandler SomeEvent;
public void someMethod()
{
if(SomeEvent != null) SomeEvent(this, someArgs);
}
}
public class B : A
{
public void someOtherMethod()
{
if(SomeEvent != null) SomeEvent(this, someArgs); // << why is this not possible?
//Error: The event 'SomeEvent' can only appear on the left hand side of += or -=
//(except when used from within the type 'A')
}
}
Why isn't it possible?
And what is the common solution for this kind of situation?
Others have explained how to get round the issue, but not why it's coming up.
When you declare a public field-like event, the compiler creates a public event, and a private field. Within the same class (or nested classes) you can get at the field directly, e.g. to invoke all the handlers. From other classes, you only see the event, which only allows subscription and unsubscription.
The standard practice here is to have a protected virtual method OnSomeEvent on your base class, then call that method in derived classes. Also, for threading reasons you will want to keep a reference to the handler before checking null and calling it.
For an explanation of the why read Jon Skeet's answer or the C# specification which describes how the compiler automatically creates a private field.
Here is one possible work around.
public class A
{
public event EventHandler SomeEvent;
public void someMethod()
{
OnSomeEvent();
}
protected void OnSomeEvent()
{
EventHandler handler = SomeEvent;
if(handler != null)
handler(this, someArgs);
}
}
public class B : A
{
public void someOtherMethod()
{
OnSomeEvent();
}
}
Edit: Updated code based upon Framework Design Guidelines section 5.4 and reminders by others.
Todd's answer is correct. Often you will see this implemented throughout the .NET framework as OnXXX(EventArgs) methods:
public class Foo
{
public event EventHandler Click;
protected virtual void OnClick(EventArgs e)
{
var click = Click;
if (click != null)
click(this, e);
}
}
I strongly encourage you to consider the EventArgs<T>/EventHandler<T> pattern before you find yourself making all manner of CustomEventArgs/CustomEventHandler for raising events.
The reason the original code doesn't work is because you need to have access to the event's delegate in order to raise it, and C# keeps this delegate private.
Events in C# are represented publicly by a pair of methods, add_SomeEvent and remove_SomeEvent, which is why you can subscribe to an event from outside the class, but not raise it.
My answer would be that you shouldn't have to do this.
C# nicely enforces Only the type declaring/publishing the event should fire/raise it.
If the base class trusted derivations to have the capability to raise its events, the creator would expose protected methods to do that. If they don't exist, its a good hint that you probably shouldn't do this.
My contrived example as to how different the world would be if derived types were allowed to raise events in their ancestors. Note: this is not valid C# code.. (yet..)
public class GoodVigilante
{
public event EventHandler LaunchMissiles;
public void Evaluate()
{
Action a = DetermineCourseOfAction(); // method that evaluates every possible
// non-violent solution before resorting to 'Unleashing the fury'
if (null != a)
{ a.Do(); }
else
{ if (null != LaunchMissiles) LaunchMissiles(this, EventArgs.Empty); }
}
virtual protected string WhatsTheTime()
{ return DateTime.Now.ToString(); }
....
}
public class TriggerHappy : GoodVigilante
{
protected override string WhatsTheTime()
{
if (null != LaunchMissiles) LaunchMissiles(this, EventArgs.Empty);
}
}
// client code
GoodVigilante a = new GoodVigilante();
a.LaunchMissiles += new EventHandler(FireAway);
GoodVigilante b = new TriggerHappy(); // rogue/imposter
b.LaunchMissiles += new EventHandler(FireAway);
private void FireAway(object sender, EventArgs e)
{
// nuke 'em
}
Wrap it with a protected virtual On... method:
public class BaseClass
{
public event EventHandler<MyArgs> SomeEvent;
protected virtual void OnSomeEvent()
{
if(SomeEvent!= null)
SomeEvent(this, new MyArgs(...) );
}
}
Then override this in a derived class
public class DerivedClass : BaseClass
{
protected override void OnSomeEvent()
{
//do something
base.OnSomeEvent();
}
}
You'll set this pattern all over .Net - all form and web controls follow it.
Do not use the prefix Raise... - this is not consistent with MS's standards and can cause confusion elsewhere.