FormUndo 方法

FormUndo is a high-level wrapper around PDFium's FORM_Undo entry point. The PDFium API was already bound at the unit level but had no high-level accessor until v1.23.0; this method exposes per-widget undo to applications.

Each AcroForm widget maintains its own undo / redo stack inside PDFium's form-fill engine. Typing into a text field, deleting characters, paste, autocomplete — every change becomes a new undo entry. FormUndo rolls one step back on the FOCUSED widget; it does NOT undo across multiple widgets at once.

Returns True on success, False when there is nothing to undo (the focused widget's undo stack is empty) or when the view is inactive / has no focused widget. Pair with FormCanUndo to drive an Undo menu item's Enabled state.

• FormUndo is widget-local. To support whole-document undo (form values across multiple widgets, or undo of non-form edits), the application has to track a higher-level edit history.
• FormUndo fires a form-event into the document's JavaScript (if any) just like a regular value change would.
• The undo / redo stack is cleared when the document is reloaded, the widget loses focus, or the page changes.