binding value of text box to an object's property on canvas

binding value of text box to an object's property on canvas

I will appreciate it if experts could give me direction on following
scenario?
Consider a set of entries or we can just focus on one edit box for sake of
simplicity which its input has to be bound to a canvas. let's say user has
to be able to modify the angle of a line in the edit box using keyboard
while this edit box having two arrows (up/down arrow to increase or
decrease a value in the edit box) so mouse would be another option to
change the value and also losing focus should be the third element of
affecting line on canvas. this works almost but my major challenge here is
that sometimes user might remove the value from text box and re-enter a
new value. this is affecting line (or arc or any object) to disappear. how
can this issue be addressed and resolved?
Any advice will be appreciated.
Thanks.