Detecting if a form of a given type is already open in Firemonkey application

Question

I am trying to come up with a way to tell if a given type of form is open in my application, before opening a new one. The reason for this, is that there are certain forms in the application which only make sense to have one instance of them open at a time, but I would rather not make them strictly modal.

To solve this I have implemented the following method, which iterates through Screen->Forms and attempts to cast each form to the type of form provided. I figured if the cast was made, I have discovered that type of form is already open.

template <typename T>
bool __fastcall FormOfTypeExists( T * Form )
{
    for( int i = 0; i < Screen->FormCount; i++ )
    {
        T * CurrentForm = (T*)Screen->Forms[i];

         if( CurrentForm != 0 )
         {
             return true;
         }
    }

    return false;
};

Then I call the method wherever I am attempting to create a form, like so:

TFormA *Form;

if( FormOfTypeExists( Form ) )
{
    return;
}

Form = new TFormA( Owner );

Form->Show();

The problem that I am running into is that it seems I am always able to successfully cast the TCommonCustomForm objects in Screen->Forms to whatever type of form I have passed in.

Is there a way I can alter my logic to successfully detect if a form of a certain type already exists in the application? Is there a better approach to this problem I should be looking at?

Answer 1

You are using a hard cast that does not take the object's real type into account. That is why the cast always succeeds (when it really doesn't). To test if an object is of a particular type, you need to use dynamic_cast instead.

And don't pass a variable as input when you are not actually using it for anything.

Try this instead:

template <typename T>
bool __fastcall FormOfTypeExists()
{
    for( int i = 0; i < Screen->FormCount; i++ )
    {
        T * CurrentForm = dynamic_cast<T*>(Screen->Forms[i]);
        if( CurrentForm != NULL )
        {
            return true;
        }
    }

    return false;
}

if( FormOfTypeExists<TFormA>() )
{
    return;
}

TFormA *Form = new TFormA( Owner );
Form->Show();