Proper way to change contents of a VCL component

Question

Quite often when I make VCL programs, I run into a scenario like this:

I have a number of components on the form, that the users are allowed to fiddle with. Most commonly a bunch of edit boxes.
The contents of these edit boxes need to be verified by the OnChange event when the user types in stuff manually.
Somewhere else on the form, there's some component that the user can click on to get some default values loaded into the edit boxes (in TEdit::Text).

Now what I want is that whenever the user is typing something in the TEdit::Text, the OnChange event must process the user input. But when my program is setting the TEdit::Text to a default value, this isn't necessary, because then I know that the value is correct.

Unfortunately, writing code like myedit->Text = "Default"; triggers the OnChange event.

I tend to solve this with what I think is a rather ugly approach: by creating a bool variable is_user_input, which TEdit::OnChange checks. If it is true, the TEdit::Text will get validated, otherwise it will get ignored. But of course, this doesn't prevent the program from launching TEdit::OnChange when it is unnecessary.

Is there a better or cleaner way to achieve this?

Is there a way for OnChange to check who called it? Or I suppose, a way of disabling the OnChange event temporarily would be even better. TEdit::Enabled doesn't seem to affect whether OnChange gets triggered or not.

Don't you consider it good that the verification is applied on your default values as well? What if you decide to change a default value at some point, and by mistake set it outside the legal range? — barak manos
@barakmanos Normally, yes. But often there is a scenario where multiple components are related to the same set of private variables. So I let all of the components call the same "change" setter function from their individual OnChange. Now suppose I also want to change this variable internally - then I must update all components to reflect the change. And if I do, they automatically trigger their OnChange and launch the "change" function, over and over. So with such a design, I end up with weird, circular dependencies. Plus pointless execution delays. — Lundin

Remy Lebeau Remy Lebeau · Accepted Answer · 2014-02-10T21:35:53

You could simply unassign the OnChange event handler temporarily:

template <typename T>
void SetControlTextNoChange(T *Control, const String &S)
{
    TNotifyEvent event = Control->OnChange;
    Control->OnChange = NULL;
    try {
        Control->Text = S;
    }
    __finally {
        Control->OnChange = event;
    }
 }

SetControlTextNoChange(myedit, "Default");

Alternatively, RAII is good for this kind of thing:

template <typename T>
class DisableChangeEvent
{
private:
    T *m_control;
    TNotifyEvent m_event;
public:
    DisableChangeEvent(T *control);
    {
        m_control = control;
        m_event = control->OnChange;
        control->OnChange = NULL;
     }

    ~DisableChangeEvent();
    {
        m_control->OnChange = m_event;
    }

    T* operator->() { return m_control; }
};

DisableChangeEvent(myedit)->Text = "Default";

Proper way to change contents of a VCL component

2 Answers