I'm not well versed in event-based programming. Basically, I'm still stumbling around with it. I'm trying to get something set up, but even with the tutorials, I can't wrap my head around it. What I would like to do (in words) is the following:
I have a dataobject where a property changes. I notice this in the setter of the property, and want to raise an event that the property has changed.
Elsewhere (in a different class entirely), I want to know that the property on this object has changed, and take some action.
Now I'm sure this is a common enough scenario, but my google-fu is letting me down. I'm simply not understanding http://msdn.microsoft.com/en-us/library/ms743695.aspx.
I have this:
public class ChattyClass {
private int someMember;
public event PropertyChangedEventHandler PropertyChanged;
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value){
someMember = value;
// Raise event/fire handlers. But how?
}
}
}
public class NosyClass{
private List<ChattyClass> myChatters;
public void addChatter(ChattyClass chatter){
myChatters.add(chatter);
// Start listening to property changed events
}
private void listner(){
// I want this to be called when the PropertyChangedEvent is called
Console.WriteLine("Hey! Hey! Listen! A property of a chatter in my list has changed!");
}
}
What do I do to wire this up?
Concerning the comment pointing me back to the link:
In the example I see:
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
What I'm not understanding:
Why isn't this just calling PropertyChanged(this, new PropertyCHangedEventArgs(name))
Where does PropertyChanged get assigned?
What does the assignment look like?
You have to fire the event. In the example on MSDN, they made a protected method OnPropertyChanged to handle this easier (and to avoid duplicate code).
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
What this method does, is look whether there is an event handler assigned or not (if it is not assigned and you just call it, you'll get a NullReferenceException). If there is one assigned, call this event handler. The event handler provided, has to have the signature of the PropertyChangedEventHandler delegate. This signature is:
void MyMethod(object sender, PropertyChangedEventArgs e)
Where the first parameter has to be of the type object and represents the object that fires the event, and the second parameter contains the arguments of this event. In this case, your own class fires the event and thus give this as parameter sender. The second parameter contains the name of the property that has changed.
Now to be able to react upon the firing of the event, you have to assign an event handler to the class. In this case, you'll have to assign this in your addChatter method. Apart from that, you'll have to first define your handler. In your NosyClass you'll have to add a method to do this, for example:
private void chatter_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("A property has changed: " + e.PropertyName);
}
As you can see, this method corresponds to the signature I explained before. In the second parameter, you'll be able to find the information of which parameter has been changed. Last thing to do, is add the event handler. Now in your addChatter method, you'll have to assign this:
public void AddChatter(ChattyClass chatter)
{
myChatters.Add(chatter);
// Assign the event handler
chatter.PropertyChanged += new PropertyChangedEventHandler(chatter_PropertyChanged);
}
I would suggest you to read something about events in .NET / C#: http://msdn.microsoft.com/en-us/library/awbftdfh . I think after reading/learning this, things will be more clear to you.
You can find a console application here on pastebin if you would like to test it quickly (just copy/paste into a new console application).
With newer versions of C#, you can inline the call to the event handler:
// inside your setter
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(MyProperty)));
You could also use something like Fody PropertyChanged to automatically generated the necessary code (visit the link to their GitHub page, with samples).
The link that you looked is for the MVVM pattern and WPF. It is not a general C# implementation. You need something like this:
public event EventHandler PropertyChanged;
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value) {
someMember = value;
if (PropertyChanged != null) { // If someone subscribed to the event
PropertyChanged(this, EventArgs.Empty); // Raise the event
}
}
}
...
public void addChatter(ChattyClass chatter) {
myChatters.add(chatter);
chatter.PropertyChanged += listner; // Subscribe to the event
}
// This will be called on property changed
private void listner(object sender, EventArgs e){
Console.WriteLine("Hey! Hey! Listen! A property of a chatter in my list has changed!");
}
If you want to know what property has changed you need to change your event definition to:
public event PropertyChangedEventHandler PropertyChanged;
And change the calling to:
public int SomeMember {
get {
return this.someMember;
}
set {
if (this.someMember != value){
someMember = value;
if (PropertyChanged != null) { // If someone subscribed to the event
PropertyChanged(this, new PropertyChangedEventArgs("SomeMember")); // Raise the event
}
}
}
private void listner(object sender, PropertyChangedEventArgs e) {
string propertyName = e.PropertyName;
Console.WriteLine(String.Format("Hey! Hey! Listen! a {0} of a chatter in my list has changed!", propertyName));
}
why isn't this just calling PropertyChanged(this, new
PropertyCHangedEventArgs(name))
Because if no one attached an handler to the event, then the PropertyChanged object returns null. So you'll have to ensure it's not null before calling it.
where does PropertyChanged get assigned?
In the "listener" classes.
For example, you could write in other class:
ChattyClass tmp = new ChattyClass();
tmp.PropertyChanged += (sender, e) =>
{
Console.WriteLine(string.Format("Property {0} has been updated", e.PropertyName));
};
What does the assignment look like?
In C# we use the assignment operators += and -= for events. I recommend reading the following article to understand how to write event handlers using the anonymous method form (example above) and the "old" form.
From taking the original code, and incorporating #Styxxy 's answer, I come out with:
public class ChattyClass : INotifyPropertyChanged
{
private int someMember, otherMember;
public int SomeMember
{
get
{
return this.someMember;
}
set
{
if (this.someMember != value)
{
someMember = value;
OnPropertyChanged("Some Member");
}
}
}
public int OtherMember
{
get
{
return this.otherMember;
}
set
{
if (this.otherMember != value)
{
otherMember = value;
OnPropertyChanged("Other Member");
}
}
}
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
}
public event PropertyChangedEventHandler PropertyChanged;
}
public class NosyClass
{
private List<ChattyClass> myChatters = new List<ChattyClass>();
public void AddChatter(ChattyClass chatter)
{
myChatters.Add(chatter);
chatter.PropertyChanged+=chatter_PropertyChanged;
}
private void chatter_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine("A property has changed: " + e.PropertyName);
}
}
Related
I want to find out when the PropertyChanged event handler is set in my base class.
Both Debug.Print("Is Null") and Debug.Print("not null") get hit. So it must get set somewhere.
How do I find that out?
A search for PropertyChanged does not reveal code that subscribes to the ebvent.
public abstract class NonPersistentObjectBase : INotifyPropertyChanged, IObjectSpaceLink {
public event PropertyChangedEventHandler PropertyChanged; // how do I break here
protected void OnPropertyChanged(string propertyName) {
if(PropertyChanged != null) {
Debug.Print("not null"); // gets hit after I click save
}
else {
Debug.Print("Is Null"); //gets hit
}
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
}
protected void SetPropertyValue<T>(string name, ref T field, T value) {
if(!Equals(field, value)) {
field = value;
OnPropertyChanged(name);
}
}
I added a private event as per Olivier's suggestion but am unsure how to call it.
I tried assigning it in the constructor
private event PropertyChangedEventHandler PropertyChangedAdd {
add => PropertyChanged += value;
remove => PropertyChanged -= value;
}
We cannot add a breakpoint to such a single statement without an assignment.
We can only add on a runtime instruction.
A void declaration as well as a method signature alone is not an real instruction to be executed: no assignment, no call, no loop, no test, no jump, no calculation... just a "static memory reservation" planned and done by the compiler at compile-time.
But we can implement add and remove accessors of a property-event on a real private field, thus we will be able to put breakpoints.
Also, once that done, we can open the call stack window in Visual Studio or go out of the method toward the caller subscriber.
public event PropertyChangedEventHandler PropertyChanged
{
add => _PropertyChanged += value;
remove => _PropertyChanged -= value;
}
private event PropertyChangedEventHandler _PropertyChanged
This question is in reference to another question, mine is similar but I am asking for help beyond what I have read in this answer:
Raise an event whenever a property's value changed?
EDIT: What I am trying to accomplish is a global message "service" within the application such that I can write to the message variable from different places within the application and have the User Interface (winform) pick up on the fact that there was some change to that variable and based upon the event, I can read the message variable and display its content to the user. I hope this makes more sense now.
First, I am new to the world of C# and while I understand the code written as the most accepted answer, where I fail, is to understand the final implementation. If I place this code in a .cs file and I use the namespace in winform file how do I finalize the implementation? In my case, I would want to implement the class in the winform file so I can watch for an event to occur. Once the event occurred I would write some information to the user via the winform interface. I think I would need to use the "get" of the string...but not sure how the implementation would go? I apologize in advance if this doesn't make sense, I am trying to piece this all together. Thanks for any help on this!
For reference, I have start with the answer provide and altered it for my purposes:
public class Messaging : INotifyPropertyChanged
{
private string dLMessage;
protected void OnPropertyChanged(PropertyChangedEventArgs e)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, e);
}
protected void OnPropertyChanged(string propertyName)
{
OnPropertyChanged(new PropertyChangedEventArgs(propertyName));
}
public string DLMessage
{
get { return dLMessage; }
set
{
if (value != dLMessage)
{
dLMessage = value;
OnPropertyChanged("DLMessage");
OnDLMessageChanged(EventArgs.Empty);
}
}
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnDLMessageChanged(EventArgs e)
{
EventHandler handler = DLMessageChanged;
if (handler != null)
handler(this, e);
}
public event EventHandler DLMessageChanged;
}
Edit
According to your edited question, there are many different ways. One of those is making DLMesaage property and its change event static:
public class Messaging
{
private static string dLMessage;
public static string DLMessage
{
get { return dLMessage; }
set
{
if (value != dLMessage)
{
dLMessage = value;
OnDLMessageChanged(EventArgs.Empty);
}
}
}
protected static void OnDLMessageChanged(EventArgs e)
{
EventHandler handler = DLMessageChanged;
if (handler != null)
handler(null, e);
}
public static event EventHandler DLMessageChanged;
}
and then subscribe for event this way in all your different forms (for example in form load event)
Messaging.DLMessageChanged += msg_DLMessageChanged;
having this function in that form:
void msg_DLMessageChanged(object sender, EventArgs e)
{
MessageBox.Show("at last changed!");
}
You can unsubscribe for event this way:
Messaging.DLMessageChanged -= msg_DLMessageChanged;
For example if you subscribed for event in some forms, you can put unsubscribe code in Dispose override:
protected override void Dispose(bool disposing)
{
if (disposing && (components != null))
{
Messaging.DLMessageChanged -= msg_DLMessageChanged;
components.Dispose();
}
base.Dispose(disposing);
}
This way if you close the form, the event never be handled in that form.
Please note that I am keeping things simple in order to get your job done with minimum changes.
Original
Put this some where you instantiate Messageing instance for example in your form's constructor or load event handler:
Messaging msg = new Messaging();
msg.DLMessageChanged += msg_DLMessageChanged;
Add this to the form:
void msg_DLMessageChanged(object sender, EventArgs e)
{
MessageBox.Show("at last changed!");
//You can access the new value using Messaging.DLMessage
}
And also it seems you don't need to implement INotifyPropertyChanged if you only want DLMessageChanged. Now you are raising both events.
Or in case you want to use PropertyChanged event, put this some where you instantiate Messageing instance for example in your form's constructor or load event handler:
Messaging msg = new Messaging();
msg.PropertyChanged+= msg_PropertyChanged;
Add this to the form:
void msg_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "DLMessage")
MessageBox.Show("at last changed!");
}
I have implemented WPF data binding with INotifyPropertyChanged.
public class ExportNode : INotifyPropertyChanged
{
public uint Handle { get; set; }
public String Text { get; set; }
private bool _ischecked;
public bool IsChecked
{
get
{
return _ischecked;
}
set
{
_ischecked = value;
OnPropertyChanged("IsChecked");
}
}
public event PropertyChangedEventHandler PropertyChanged;
// Create the OnPropertyChanged method to raise the event
protected void OnPropertyChanged(string name)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
}
And than subscribing to event form my code, so whenever I change property in UI, it fires callback.
But now I'm trying to figure out the best way to change property from code, and than not fire callback, just update UI.
void newNode_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
if (e.PropertyName == "IsChecked")
{
}
}
For now I just thought about implementing some "blocker" member property in ExportNode
protected void OnPropertyChanged(string name)
{
if (Blocked)
return;
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
}
or delete event form instance before change.
newNode.PropertyChanged -= newNode_PropertyChanged;
newNode.IsChecked = true;
newNode.PropertyChanged += newNode_PropertyChanged;
But is there any better way? I just don't understand some basics? :-)
Thank you very much
Roman
You've got this a little backwards.
INotifyPropertyChanged, and thus the PropertyChanged event, is what makes the UI update, in fact, its what makes the whole binding system work.
So to update the UI, you have to raise that event. Now, from the code side, you almost never subscribe to that event, because you could just invoke a method from the setter. Something like:
set
{
_ischecked = value;
OnPropertyChanged("IsChecked");
if (!Blocked)
MyOtherMethod();
}
Note that if you are dealing with threads, that Blocked condition is a major synchronization hazard.
If you really need to register for PropertyChanged from code, then your best bet is to just unregister with -=. That way the UI still gets its event, but you don't.
I use INotifyPropertyChanged to notify class when there is any change in a variable of a particular object within it.
Below is the class:
public class MyClass
{
public SecClass MyObj { get; set; }
//A few more variables
}
SecClass:
public class SecClass:INotifyPropertyChanged
{
private bool _noti= false;
public bool Noti
{
get { return _noti; }
set
{
_noti= value;
NotifyPropertyChanged("Noti");
}
}
//A few more variables
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged(string name)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(name));
}
}
}
Here my function that makes the event registration:
public void Register()
{
MyObj.PropertyChanged += MyObj_PropertyChanged;
}
Function works and the registration is done, but when it comes to change it displays the Property Change as null (I guess that somewhere registration deleted, before happens change, how can I check this?)
I hooked this together with:
static class Program
{
static void Main()
{
var c = new MyClass();
c.MyObj = new SecClass();
c.Register();
c.MyObj.Noti = !c.MyObj.Noti;
}
}
adding (for illustration):
private void MyObj_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
Console.WriteLine(e.PropertyName);
}
to MyClass, and:
public event PropertyChangedEventHandler PropertyChanged;
to SecClass (to get them to compile), and it works fine - printing "Noti" at runtime. There is a theoretical thread-race, but it is very unlikely in any sane usage, but recommended usage is:
var handler = PropertyChanged;
if (handler != null)
{
handler(this, new PropertyChangedEventArgs(name));
}
Also, for info: if you add [CallerMemberName] to that, you don't need to specify the property explicitly:
private void NotifyPropertyChanged([CallerMemberName] string name = null) {...}
with:
NotifyPropertyChanged(); // the compiler adds the "Noti" itself
But fundamentally: "cannot reproduce" - it works fine. I wonder if maybe it relates to your PropertyChanged implementation, since you don't actually show that. In particular, I wonder if you actually have two events: one explicitly implemented. That would mean that it is getting treated differently by your cast.
I'm making custom events for C# and sometimes it isn't working.
This is how I'm making the event happen:
private bool isDoorOpen;
public bool IsDoorOpen {
get { return isDoorOpen;}
private set { isDoorOpen = value; DoorsChangeState(this, null);}
}
And these are the event declarations:
//events
public delegate void ChangedEventHandler(Elevator sender, EventArgs e);
public event ChangedEventHandler PositionChanged;
public event ChangedEventHandler DirectionChanged;
public event ChangedEventHandler BreaksChangeState;
public event ChangedEventHandler DoorsChangeState;
This works as long as there are methods attached to the events, but if there isn't, it throws a null ref exception. What am I doing wrong?
The recommended way to call an event is
var handler = this.DoorsChangeState;
if (handler != null)
handler(this, null);
The reason for copying the handler locally is incase the event handler changes on another thread while you're checking for null.
EDIT: Found the article talking about race conditions.
http://blogs.msdn.com/ericlippert/archive/2009/04/29/events-and-races.aspx
I know this question has been discussed (and answered) several times here on SO.
Also somewhere here i got the following extension methods to make this pattern more easy to use:
public static class EventHandlerExtensions
{
public static void FireEvent<T>(this EventHandler<T> handler, object sender, T args) where T : EventArgs
{
var temp = handler;
if (temp != null)
{
temp(sender, args);
}
}
public static void FireEvent(this EventHandler handler, object sender)
{
var temp = handler;
if (temp != null)
{
temp(sender, EventArgs.Empty);
}
}
}
So in your code you can say:
public bool IsDoorOpen
{
get { return isDoorOpen;}
private set
{
isDoorOpen = value;
DoorsChangeState.FireEvent(this);
}
}
If a event isn't subscribed to when it fires, a NullReferenceException will be thrown. This is correct behaviour, not something you've done wrong.
You should check:
if(DoorsChangeState != null)
{
DoorsChangeState(this, null); // Only fire if subscribed to
}
Before invoking an event you must check if the event is null:
if (DoorsChangeState != null)
DoorsChangeState(this, null);
When DoorsChangeState is null that means there are no listeners on that event.
You need to check to see if the event has been subscribed to.
I use this standard form for throwing all of my events.
var temp = EventName;
if(EventName!= null)
temp(this, null);