Force Custom Event Handler First

Question

When I create custom handlers like:

Public Class MyCustomClass
    Public Sub AddHandlers()
        AddHandler Form1.MouseMove, AddressOf MoveMouse
    End Sub
    Private Sub MoveMouse(sender As Object, e As MouseEventArgs)
        MsgBox("Needs to happen first.")
    End Sub
End Class

I need MoveMouse in this class to fire before any other event when the user moves their mouse over Form1.

Private Sub Form1_MouseMove(sender As Object, e As MouseEventArgs) Handles Me.MouseMove
    MsgBox("Needs to happen second.")
End Sub

While writing this, I realized I could create yet another custom event handler in Form1's class, but is there any other way to ensure that MoveMouse (regardless of what class it is in) happens before Form1_MouseMove?

Thanks- ~Nic

You are adding a handler to the same event - 2 handlers for the same event which is pointless - just add code to the existing event. Note that `AddHandler` adds a handler (!) so the one already there will fire first. — Ňɏssa Pøngjǣrdenlarp, Jul 14 '17 at 21:05
I know. This is assuming there is a reason to have 2 handlers. I'm planning to make an application extension file which will handle certain events from the form but require them to fire first. — , Jul 14 '17 at 21:09
I have no idea what an 'application extension' is but events are private to the form other parts of the same app are not even aware of them: `Private Sub MoveMouse(...)` — Ňɏssa Pøngjǣrdenlarp, Jul 14 '17 at 21:12
I want to create a DLL file which will customize the way the user interacts with the form. It's merely a practice project using a few things I've learned and to hopefully learn more things on the way. — , Jul 14 '17 at 21:20
`Sub MoveMouse` would not exist in `Class Form1`, it would be written in the other project I am working on. This is example code but I thought my question was fairly clear. — , Jul 14 '17 at 21:21

score 0 · Accepted Answer · answered Jul 15 '17 at 02:47

Events are fired in the order in which they are declared:

So if you want your custom class to raise MouseMove on Form1 before Form1 raises the event you need to make your custom class add the handler first:

Public Class CustomClass
    Public Sub OnMouseMoved(sender As Object, e As MouseEventArgs)
        Console.WriteLine("Custom mouse moved")
    End Sub
End Class

Public Class Form1
    Public Custom As CustomClass
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Custom = New CustomClass
        AddHandler MouseMove, AddressOf Custom.OnMouseMoved
        AddHandler MouseMove, AddressOf OnMouseMoved
 End Sub

    Private Sub OnMouseMoved(sender As Object, e As MouseEventArgs)
        Console.WriteLine("Form1 mouse moved")
    End Sub
End Class

This is more or less what I'm looking for, but more just reassurance that there's no other factors at play than simply the order of `AddHandler` you use. — , Jul 15 '17 at 04:20
@NinjaNic : Not really, no. It more or less [**combines**](https://msdn.microsoft.com/en-us/library/30cyx32c(v=vs.110).aspx) the delegates (event handlers) as you add them, thus making the first one you added getting called first. — Visual Vincent, Jul 15 '17 at 07:33

Force Custom Event Handler First

1 Answers1