the normal implement shall be:
when click on a button, invoke PDFGLLayoutView.PDFSetSelect()
click this button again, invoke PDFGLLayoutView.PDFSetSelect() again to cancel select.
after first call of PDFGLLayoutView.PDFSetSelect(), you can touch and drag any position on screen.
when your finger touch up, ILayoutView.PDFLayoutListener.OnPDFSelectEnd will callback.
you can implement same to PDFGLViewAct.OnPDFSelectEnd(), which implement highlight/underline/strikeout/squiggly and clipboard operations.
other some similar operations:
PDFSetLine to add line annotation
PDFSetNote to add note annotation
PDFSetRect to add rectangle operation
PDFSetEllipse to add oval operation
...
