Enable a button after closing a form - c++

Question

I want to Enable a button from the main form whenever a second form has been closed. I've read something about invokes, but didn't understood much.
How could I achieve this?

Answer 1

When you close the second form, its FormClosed event is automatically raised. Before an event is raised, you can register an event handler to events. This enables you to write code, which is automatically executed, when the event occurrs.
Registering an event handler in .NET is realised by adding a delegate instance to the event. A delegate is a type that describes a method signature. If you instantiate a delegate with gcnew you associate it with a function in your code. You can call the delegate by yourself (which is not needed here) or you can pass it to some other code, which then can invoke it. The latter one is used for events.
For your case that means:
Look at the FormClosed event's delegate type. It is the FormClosedEventHandler which is defined as delegate void FormClosedEventHandler(Object^ sender, FormClosedEventArgs^ e)
This means you must implement a method returning nothing (void) and accepting two arguments: a System::Object and a System::Windows::Forms::FormClosedEventArgs
Instantiate a FormClosedEventHandler delegate and associate it with your method
Register to the FormClosed event on the second form and enable the button in the event handler.
An example:
ref class MainForm
{
...
// event handler function (compatible to the FormClosedEventHandler delegate)
void OnSecondFormClosed(Object^ sender, FormClosedEventArgs^ e)
{
myButton->Enabled = true;
}
void DoSomethingWithSecondForm(Form^ secondForm)
{
// get a disabled Button
myButton->Enabled = false;
// create an event handler by instantiating a delegate
FormClosedEventHandler^ handler = gcnew FormClosedEventHandler(this, &MainForm::OnSecondFormClosed);
// register event handler
secondForm->FormClosed += handler;
}
...
}
(I did not compile the code, but this is how it works in general)
When both involved forms are created from within the same thread, there is no need to do some additional Invoke. Otherwise you must put changes to controls into the same thread that created the control. You can achieve this by passing a delegate to Control::Invoke or Control::BeginInvoke.
// event handler function (compatible to the FormClosedEventHandler delegate)
void OnSecondFormClosed(Object^ sender, FormClosedEventArgs^ e)
{
if (myButton->InvokeRequired)
{
// create a delegate to call the same event handler again
FormClosedEventHandler^ handler = gcnew FormClosedEventHandler(this, &MainForm::OnSecondFormClosed);
// BeginInvoke causes the delegate to be called asynchronously from the UI thread
myButton->BeginInvoke(handler, sender, e);
// nothing to be done here, the actual work happens when the delegate is actually called
return;
}
myButton->Enabled = true;
}