I would like to subscribe to an event so that when the event fires I can execute a delegate or anonymous function.
Subscribing to events with methods is easy I can just type the method name, this works fine:
UnityEngine.UI.Toggle tgl;
tgl.onValueChanged += myMethod;
But I cannot subscribe a delegate using the same syntax. This will not work:
tgl.onValueChanged += delegate{ Debug.Log("Bang!"); };
I researched this Q&A which suggested I try the following approach but this is also not working:
tgl.onValueChanged += (object sender, EventArgs e) => { Debug.Log("Bang!"); };
How can I make this work?
You need to look at what type onValueChanged is, before trying to subscribe to it. This is its declaration, taken from here.
public UI.Toggle.ToggleEvent onValueChanged;
Its type is a ToggleEvent, which is a class, not an event nor a delegate. Therefore, you cannot use the += operator on it.
You should call AddListener, like the examples did:
tgl.onValueChanged.AddListener(delegate { ... })
Related
(I know the title sounds easy, but hold on—this probably isn't the question you think it is.)
In VB.NET I was able to write custom events. For an example, I had a separate thread that would periodically raise an event and on that event the GUI would need to be updated. I didn't want the busy thread to bother with UI calculations and I didn't want to put Me.Invoke(Sub() ...) in the event handler since it was also called from the GUI thread.
I came up with this very useful bit of code. The GUI thread would set EventSyncInvoke = Me (the main form). The thread could then simply raise the event TestEvent as usual, no special code, and it would be seamlessly executed on the GUI thread:
Private TestEventDelegate As EventHandler
Public EventSyncInvoke As System.ComponentModel.ISynchronizeInvoke
Public Custom Event TestEvent As EventHandler
AddHandler(value As EventHandler)
TestEventDelegate = [Delegate].Combine(TestEventDelegate, value)
End AddHandler
RemoveHandler(value As EventHandler)
TestEventDelegate = [Delegate].Remove(TestEventDelegate, value)
End RemoveHandler
RaiseEvent(sender As Object, e As System.EventArgs)
If EventSyncInvoke IsNot Nothing Then
EventSyncInvoke.Invoke(TestEventDelegate, {sender, e})
Else
TestEventDelegate.Invoke({sender, e})
End If
End RaiseEvent
End Event
Now in C# I can do this much:
public event EventHandler TestEvent
add
{
testEventDelegate = (EventHandler)Delegate.Combine(testEventDelegate, value);
}
remove
{
testEventDelegate = (EventHandler)Delegate.Remove(testEventDelegate, value);
}
}
But where is the ability to do custom raising?
The other answers told me the fact that I couldn't do this directly in C#, but not the rationale behind why I can't and why I wouldn't want to. It took me a while to understand how C# events worked in comparison to VB.NET. So this explanation is for others who don't have a good grasp on this to start thinking along the right lines.
Honestly, I was so used to the boilerplate OnTestEvent format that I didn't quite like the idea of making it different from the rest of the helper methods. :-) But now that I understand the rationale, I see that it is actually the best place to put this stuff.
VB.NET allows you to hide the background details of calling the delegates with the RaiseEvent keyword. RaiseEvent calls either the event delegate or your custom RaiseEvent section for a custom event.
In C#, there is no RaiseEvent. Raising an event is basically no more than calling a delegate. No custom RaiseEvent sections can be seamlessly called when all you're doing to raise it is calling a delegate. So for C#, custom events are like skeletons, implementing add and remove for events but not implementing the ability to raise them. It's like having to replace all your RaiseEvent TestEvent(sender, e) with the code from the custom RaiseEvent section.
For a normal event, raising looks roughly like NormalEvent(sender, e). But as soon as you put in a custom add and remove, you must use whatever variable you used in the add and remove because the compiler isn't doing it anymore. It's like automatic properties in VB.NET: once you put in a getter and setter manually, you have to declare and handle your own local variable. So instead of TestEvent(sender, e), use testEventDelegate(sender, e). That's where you rerouted the event delegates.
I compared moving from VB.NET to C# with having to replace each of your RaiseEvents with your custom RaiseEvent code. A RaiseEvent code section is basically an event and a helper function rolled together. It's actually standard to only have one instance of a RaiseEvent in either VB.NET or C# inside a protected OnTestEvent method and call that method to raise the event. This allows any code with access to the protected (or private or public) OnTestEvent to raise the event. For what you want to do, just putting it in the method is easier, simpler and performs slightly better. This is best practice.
Now if you really want to want (or need) somehow to mimic VB.NET's RaiseEvent nitty-gritty-hiding call SomeDelegate(sender, e) and have the magic happen, you can simply hide the nitty-gritty inside a second delegate:
NiceTestEvent = (sender, e) => eventSyncInvoke.Invoke(testEventDelegate, new object[] { sender, e });
Now you can call NiceTestEvent(sender, e). You won't be able to call TestEvent(sender, e) though. TestEvent is only for outside code to add and remove through, as Visual Studio will tell you.
In C#, there isn't any RaiseEvent block. You would do the same thing by creating a method for raising your event.
Here is a working example. In the C# version, you do not even need to use the add and remove block -- you can use the default implementation for that and just create a custom raise method that raises your event.
Below is a working program (the form is just a Windows Forms form with a single button on it).
// Here is your event-raising class
using System;
using System.ComponentModel;
namespace ClassLibrary1
{
public class Class1
{
public ISynchronizeInvoke EventSyncInvoke { get; set; }
public event EventHandler TestEvent;
private void RaiseTestEvent(EventArgs e)
{
// Take a local copy -- this is for thread safety. If an unsubscribe on another thread
// causes TestEvent to become null, this will protect you from a null reference exception.
// (The event will be raised to all subscribers as of the point in time that this line executes.)
EventHandler testEvent = this.TestEvent;
// Check for no subscribers
if (testEvent == null)
return;
if (EventSyncInvoke == null)
testEvent(this, e);
else
EventSyncInvoke.Invoke(testEvent, new object[] {this, e});
}
public void Test()
{
RaiseTestEvent(EventArgs.Empty);
}
}
}
// Here is a form that tests it -- if you run it, you will see that the event is marshalled back to
// the main thread, as desired.
using System;
using System.Threading;
using System.Windows.Forms;
namespace ClassLibrary1
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
this.TestClass = new Class1();
this.TestClass.EventSyncInvoke = this;
this.TestClass.TestEvent += new EventHandler(TestClass_TestEvent);
Thread.CurrentThread.Name = "Main";
}
void TestClass_TestEvent(object sender, EventArgs e)
{
MessageBox.Show(this, string.Format("Event. Thread: {0} Id: {1}", Thread.CurrentThread.Name, Thread.CurrentThread.ManagedThreadId));
}
private Class1 TestClass;
private void button1_Click(object sender, EventArgs e)
{
// You can test with an "old fashioned" thread, or the TPL.
var t = new Thread(() => this.TestClass.Test());
t.Start();
//Task.Factory.StartNew(() => this.TestClass.Test());
}
}
}
You simply can't. But since events can be raised only from inside the type that declares them, you can create a helper method that executes your specific raising code. And then just make sure you don't raise the event directly outside that method.
AFAIK custom event raising as in VB.NET does not exist in C#. However, you could wrap the actual event handler delegates (passed to add as value) in a lambda and subscribe that lambda to the event instead of the original delegate:
add
{
testEventDelegate = Delegate.Combine(testEventDelegate, (s, e) => { ... } )
}
(Above code untested, syntax might be slightly off. I'll fix it as soon as I can test it.)
Crude, but working example:
The following is a concrete example of the above. I am not convinced myself that the following is good, solid code, nor that it would work in all circumstances (such as multi-threading etc.)... nevertheless, here it is:
class Foo
{
public Foo(SynchronizationContext context)
{
this.context = context ?? new SynchronizationContext();
this.someEventHandlers = new Dictionary<EventHandler, EventHandler>();
}
private readonly SynchronizationContext context;
// ^ could also use ISynchronizeInvoke; I chose SynchronizationContext
// for this example because it is independent from, but compatible with,
// Windows Forms.
public event EventHandler SomeEvent
{
add
{
EventHandler wrappedHandler =
(object s, EventArgs e) =>
{
context.Send(delegate { value(s, e); }, null);
// ^ here is where you'd call ISynchronizeInvoke.Invoke().
};
someEvent += wrappedHandler;
someEventHandlers[value] = wrappedHandler;
}
remove
{
if (someEventHandlers.ContainsKey(value))
{
someEvent -= someEventHandlers[value];
someEventHandlers.Remove(value);
}
}
}
private EventHandler someEvent = delegate {};
private Dictionary<EventHandler, EventHandler> someEventHandlers;
public void RaiseSomeEvent()
{
someEvent(this, EventArgs.Empty);
// if this is actually the only place where you'd invoke the event,
// then you'd have far less overhead if you moved the ISynchronize-
// Invoke.Invoke() here and forgot about all the wrapping above...!
}
}
(Note that I've used the C# 2 anonymous delegate {} syntax for brevity.)
I have a "MyForm" that derives from System.Windows.Form.Form. MyForm handles the FormClosed event whose handler I set up in the constructor. Is it possible for an instantiated MyForm object to successfully remove my MyClass#FormClosed handler using -=? I am using an anonymous method to wire my FormClosed event if that matters.
// In MyForm Constructor, this ALWAYS gets fired...
FormClosed += (_sender, _args) => { m_mutex.Dispose(); m_mutex = null; };
// In Instantiating Class
MyForm form = new MyForm();
form.FormClosed += (_sender, _args) => { };
// I have also tried using "delegate(object sender, EventArgs e)" and neither worked.
Yes, you can remove it - but if you've used an anonymous function, it's slightly trickier.
Basically you'll either need to stop using an anonymous function (i.e. put the behaviour into a method) or you'll need to have a field to store the value of the delegate:
private EventHandler formClosedHandler;
...
// In constructor
formClosedHandler = (sender, args) => { ... };
FormClosed += formClosedHandler;
// Later...
FormClosed -= formClosedHandler;
formClosedHandler = null;
Personally I'd favour moving the logic into a method unless it's really tricky for some reason.
Yes, you can unsubscribe from any of the events, even in the Disposed() method.
I have some code in my project that saves an object to the database, once this is done I want it to call a series of other methods.
I was thinking about allowing other methods to subscribe to this event so I can add more as I need to. The idea I had for this was to create an array of Delegates allowing other methods to register, then when the object had been saved it could loop through the delegates and call each one.
Would this be bad practise or is there a better way of doing this?
Thanks
Event delegates are multicast, which means that they can hold references to more than one event handling method - see the MSDN documentation for Delegate and MulticastDelegate.
The syntax for subscribing to an event gives a clue:
MyEvent += MyHandler; // subscribe to an event
MyEvent -= MyHandler; // unsubscribe from an event
You can subscribe as many delegates as you want to a single event. Under the hood .Net keeps these as an ordered collection anyway.
The standard pattern for this is:
//in your class
public EventHandler<MyEvtArgs> MyEvent;
protected void OnMyEvent ( MyEvtArgs args ) {
if(MyEvent != null) //make sure there is at least 1 subscriber
MyEvent(this, args);
}
Then to call this:
var myInstance = new MyClass();
// all of these can convert to EventHandler<MyEvtArgs> (object, MyEvtArgs)
myInstance.MyEvent += (sender, e) => { /* do something 1 */ };
myInstance.MyEvent += (sender, e) => { /* do something 2 */ };
myInstance.MyEvent += localMethod;
Now when your protected OnMyEvent method is called inside your class all of these events will fire - in order.
You can also remove items from this collection:
myInstance.MyEvent -= localMethod;
You don't need an array. Just let anyone subscribe to one.
You can use a normal event for this. The runtime will handle looping over all fields.
public event EventHandler<EventArgs> WritingToDatabaseFinished;
protected void OnWritingToDatabaseFinished(EventArgs args)
{
EventHandler<EvetnArgs> handler = WritingToDatabaseFinished;
if (handler != null)
{
handler(this, args);
}
}
Your code calls
OnWritingToDatabaseFinished(args);
All methods that want to be informed have to register to the event:
WritingToDatabaseFinished += new EventHandler<EventArgs>(handlermethod);
Every handler that has been registered in the above way will be called when you call OnWritingToDatabaseFinished.
All you need is a multicast delegate. You don't need an array at all. You use += to add a reference to the delegate, and a -= to remove the reference.
I would recommend that you look at using a weak event handler instead. Take a look at this link to see why you'd use a weak event instead of a weak delegate.
Delegates are already multicast, so no need for a delegate array. Now, the recommended way of adding event support to your class is:
Add a public delegate (this will be your event handler)
public delegate void DatabaseEventHandler(object sender, DatabaseEventArgs e);
Add a public event, of your delegate type.
public event DatabaseEventHandler DatabaseDone;
Now, the event should send 2 parameters. The sender (this, usually), and the EventArgs. You should create a new class from System.EventArgs, and send the appropriate information within that class.
I have the following code:
public List<IWFResourceInstance> FindStepsByType(IWFResource res)
{
List<IWFResourceInstance> retval = new List<IWFResourceInstance>();
this.FoundStep += delegate(object sender, WalkerStepEventArgs e)
{
if (e.Step.ResourceType == res) retval.Add(e.Step);
};
this.Start();
return retval;
}
Notice how I register my event member (FoundStep) to local in-place anonymous function.
My question is: when the function 'FindStepByType' will end - will the anonymous function be removed automatically from the delegate list of the event or I have to manually remove it before steping out the function? (and how do I do that?)
I hope my question was clear.
Your code has a few problems (some you and others have identified):
The anonymous delegate cannot be removed from the event as coded.
The anonymous delegate will live longer than the life of the method calling it because you've added it to FoundStep which is a member of this.
Every entry into FindStepsByType adds another anonymous delegate to FoundStep.
The anonymous delegate is a closure and effectively extends the lifetime of retval, so even if you stop referencing retval elsewhere in your code, it's still held by the anonymous delegate.
To fix this, and still use an anonymous delegate, assign it to a local variable, and then remove the handler inside a finally block (necessary in case the handler throws an exception):
public List<IWFResourceInstance> FindStepsByType(IWFResource res)
{
List<IWFResourceInstance> retval = new List<IWFResourceInstance>();
EventHandler<WalkerStepEventArgs> handler = (sender, e) =>
{
if (e.Step.ResourceType == res) retval.Add(e.Step);
};
this.FoundStep += handler;
try
{
this.Start();
}
finally
{
this.FoundStep -= handler;
}
return retval;
}
With C# 7.0+ you can replace the anonymous delegate with a local function, achieving the same effect:
public List<IWFResourceInstance> FindStepsByType(IWFResource res)
{
var retval = new List<IWFResourceInstance>();
void Handler(object sender, WalkerStepEventArgs e)
{
if (e.Step.ResourceType == res) retval.Add(e.Step);
}
FoundStep += Handler;
try
{
this.Start();
}
finally
{
FoundStep -= Handler;
}
return retval;
}
Below is approach about how unsubscribe event in anonymous method:
DispatcherTimer _timer = new DispatcherTimer();
_timer.Interval = TimeSpan.FromMilliseconds(1000);
EventHandler handler = null;
int i = 0;
_timer.Tick += handler = new EventHandler(delegate(object s, EventArgs ev)
{
i++;
if(i==10)
_timer.Tick -= handler;
});
_timer.Start();
No, it will not be removed automatically. In this sense, there's not a difference between an anonymous method and a "normal" method. If you want, you should manually unsubscribe from the event.
Actually, it'll capture other variables (e.g. res in your example) and keep them alive (prevents garbage collector from collecting them) too.
When using an anonymous delegate (or a lambda expression) to subscribe to an event does not allow you to easily unsubscribe from that event later. An event handler is never automatically unsubscribed.
If you look at your code, even though you declare and subscribe to the event in a function, the event you are subscribing to is on the class, so once subscribed it will always be subscribed even after the function exits. The other important thing to realize is that each time this function is called, it will subscribe to the event again. This is perfectly legal since events are essentially multicast delegates and allow multiple subscribers. (This may or may not be what you intend.)
In order to unsubscribe from the delegate before you exit the function, you would need to store the anonymous delegate in a delegate variable and add the delegate to the event. You should then be able to remove the delegate from the event before the function exits.
For these reasons, if you will have to unsubscribe from the event at some later point it is not recommended to use anonymous delegates. See How to: Subscribe to and Unsubscribe from Events (C# Programming Guide) (specifically the section titled "To subscribe to events by using an anonymous method").
C# 2.0 has a neat feature called anonymous functions. This is intended to be used mostly with events:
Button.Click += delegate(System.Object o, System.EventArgs e)
{ System.Windows.Forms.MessageBox.Show("Click!"); };
Now, suppose that Button is a static member, then adding delegates to it would count as unmanaged resources. Normally, I would have to deregister the handler before regestring it again. This is a pretty common use case for GUI programming.
What are the guidelines with anonymous functions? Does the framework deregrister it automatically? If so, when?
No, anonymous functions will not get deregistered automatically. You should make sure to do it yourself, if the event should not be hooked up for the whole lifetime of your application.
To do this, of course, you would have to store the delegate reference, to be able to de-register it.
Something like:
EventHandler handler = delegate(System.Object o, System.EventArgs e)
{ System.Windows.Forms.MessageBox.Show("Click!"); };
Button.Click += handler;
// ... program code
Button.Click -= handler;
Also, see this question.
If I recall correctly (and I can recall where I read this) inline anonymous delegates cannot be removed.
You would need to assign to a (static) delegate field.
private static EventHandler<EventArgs> myHandler = (a,b) => { ... }
myButton.Click += myhandler;
...
myButton.Click -= myHandler;