24 February 2018

Non-bugs (1)– Events in a Form with controls

In the past years quite some (alleged) bugs have been reported. These reports range from editor-problems to compiler-errors to runtime bugs. Many of them have been addressed in a blog post or by a fix in an update in the GfaWin32.exe or GfaWin23.ocx binary. Still, there are issues that need some kind of an answer. In this series I’ll look at some of them.

Alleged Bug: A (single) button on a Form eats the form’s keystrokes.

This problem occurs when old-style GFA-BASIC programming is mixed with OLE Container/Control style of programming. Older GFA-BASIC (16-bit) programs often respond to messages in a message loop using GetEvent/PeekEvent. All messages are retrieved in a single place and dispatched based on a GB specific ID-value in MENU(1) or a Windows message value in MENU(11) or its alias _Mess. This is no problem as long as there are no controls on the window. 

For instance, the well-known OpenW command behaves the same as in older GFA-BASICs, that is until a control is created in its client area. When the window contains a control (OCX) it starts behaving as an OLE Control Container according COM-specifications. In GB32 the concept of simple controls no longer exists. Controls are now OLE controls wrapped in COM objects and behave according strict OLE-rules. One of the consequences is the change in keyboard handling. Now, the focus can no longer be set to the client area of the window, because the client area is now a control container site and has COM defined responsibilities; “An OLE Control Container implements keyboard handling by calling specific methods on a predefined COM interface that the controls supports.” In practice this means that all keyboard messages are send to controls and only controls can have the focus. This makes sense because an OLE control containers must support default and cancel button handling, mnemonic handling, and tab handling, including maintaining tab order.

As an example a snippet that won’t work because the old-style of message handling no longer works. The following is ‘impossible’, because the OpenW 1 is a Form (control container) with a single button OCX. All keyboard messages are (as should be) dispatched to the currently active control, which is - in all circumstances - the single button.

OpenW Center # 1
Ocx Command cmd1 = "Click", 200, 100, 190, 26
  // ~SetFocus(Win(1))  ' This doesn't help
  Select _Mess
    Print "WM_KEYDOWN"
  Case WM_CHAR
    Print "WM_CHAR"
Until Me Is Nothing

Because the button is the only OCX on the Form it will regain focus over and over, how hard you try to shift it back to the window.
If you like a Form to respond to keyboard events don’t use controls or use a separate child Form Ocx next to the other controls.

A Bug: A button on a ChildW form eats the form’s mouse clicks.

In the same situation as above, a window with a button (or a toolbar) the Form’s mouse clicks might be passed on to the button or toolbar. The bug presents itself in ChildW forms in a MDI-application, not in single window.

No comments:

Post a Comment