A form class. It is intended to create your own descendants where a form components configuration is defined and methods are overridden to handle events from form components and from the form itself. In the visual components libray of the AL-IV, all the visual controls are placed on columns which are placed on a form. More complex nesting is not allowed, to exclude too complex configurations which can not fit into small displays.

--------------------------------------------------------- 'form events'

Override the 'command' method to handle events launching of
another instance of you application by users.
To provide it firing, use {Single_instance} instead of the {Screen} class.
When the 'command' method is called, it is possible to get the secondary application parameters via usual Count_param and Parameter functions from the {System_functions} class.

<p>
Override the 'accept' method to handle drop files event and to return TRUE in case when the drop for a given set of files is acceptable. The files list is available via Count_param and Parameter functions from the {System_functions} class, both in the 'accept' and the 'command' method. Teh 0th parameter is not used as a part of the file list. The 'command' is called to handle the drop files event after the 'accept' was returning TRUE before (when the mouse button is released to perform the allowed drop files operation).

------------------------------------------------------------ 'menu'
							
To create and pop up a menu the method Popup_menu is used. Its single parameter is a string containing item definitions separated by new line characters (#NL). It is convenient to use a multi-line string constant started with the symbol @. In such case for all the lines ending with  a string literal a new line sequence is added. To organyze nested menu items add following the item which is then parent for a nesting menu, the string constant "(" (and to end the nesting level, the string ")"). Each string contains a menu item definition and for real menu items it should start from the item alias in a form "IDENTIFIER:", where a colon separates a menu item alias from the menu item text definition. It is also possible to specify togethewr with an alias some additional properties such as "disabled" or "checked", placing correspondent string constants comma separated. E.g.:
<pre>
  Popup_menu(@
  "COPY,disabled:	&Copy	[Ctrl-C]"
  "CUT,disabled:	C&ut	[Ctrl-X]"
  "PASTE:		&Paste	[Ctrl-V]"
  "-"
  "Mode..."
  "("
    "AUTO,checked:	&Auto"
    "Manual:		&Manual"
  ")"
  "EXIT:		&Exit")
</pre><p class=descr>
A separate string "-" creates a visual group separator. A single symbol "&" in a menu item text creates a shortcut symbol which can be used to select the item by a key combination ALT+ the next character. (But in someenvironments like Java such shortcut has not effect). To use the symbol '&' itself as a part of a menu item text, double it. To handle menu items, it is necessary to redefine menu item event handler:</br>
OVERRIDE menu(STR Alias) : ...
</p>
