if you mean add editbox has no UI, you just only need invoke Page.AddAnnotEditBox();
if you mean need integrate into PDFLayoutView,
you shall do like rectangle annotation.
1. implement OnEditboxTouchBegin/OnEditboxTouchMove/OnEditboxTouchEnd same to OnRectTouchBegin/OnRectTouchMove/OnRectTouchEnd, except status check need modified.
2. implement osDrawEditbox same to osDrawRects, except status check need modified.
3. implement vEditboxStart/vEditboxEnd/vEditboxCancel like vRectStart/vRectEnd/vRectCancel. you need 2 change point:
a). status check need modified.
b). in vEditboxEnd method, [page addAnnotRect] shall change to [page addAnnotEditbox];
in this way, editbox added to page not display input keyboard, you shall click on editbox after editbox status ended.
