How can I improve my Zooming code?

Question

I recently started to work with the EventHandler function to make it possible to Zoom, Pan, etc, with only the use of a mouse. I have found a way to zoom that is somewhat working without a keyboard, but was wondering if anyone had any tips to improve it. It works very slowly, often asking me if I'd like to disable dynamic updating. Obviously, this is unacceptable for any practical purpose. The code so far is as follows:

EventHandler[
Dynamic[Graphics3D[Cylinder[], ViewAngle -> Dynamic[viewval]], 
Initialization :> (mousecoord1 = {0, 0}; mousecoord2 = {0, 0}; 
size = {0, 0}; fracmag = 0; difference = 0; anglediff = 0; 
viewval = 7*Pi/36)], {"MouseDown" :> (mousecoord1 = 
 CurrentValue[MousePosition]; 
size = CurrentValue[
  ImageSize])}, {"MouseDragged" :> (mousecoord2 = 
 CurrentValue[MousePosition]; 
difference = mousecoord2 - mousecoord1; 
fracmag = Norm[difference]/Norm[size]; 
anglediff = fracmag*(Pi - viewval); 
Which[difference[[2]] < 0, 
 Min[viewval = viewval + anglediff, Pi - 0.00001], 
 difference[[2]] > 0, Max[viewval = viewval - anglediff, 0.00001],
  difference[[2]] = 0, 
 viewval = viewval])}, {"MouseUp" :> (anglediff = 0; 
difference = {0, 0}; mousecoord1 = {0, 0}; mousecoord2 = {0, 0})}]

The code above uses a simple 3d Graphic, a cylinder, and the ViewAngle option to track a mouse click and drag which then uses the ratio between the drag and the image size to determine how much to zoom in or out.

Answer 1

@Kuba The mouse defaults to rotation, but I'm trying to use the mouse work for zoom by default, with a checkbox that would make it go back to rotation. I have my zoom function working nicely now, but when I try to put it into a Manipulate function, the checkbox makes it lose all functionality. Any ideas on implementing

DynamicModule[{frac = 0, viewval = 7*Pi/36, drag = {0, 0}}, 
EventHandler[
Dynamic[Show[graphics, 
ViewAngle -> (Which[drag[[2]] < 0, 
   viewval = Min[Pi - 0.0000001, viewval + frac*(Pi - viewval)], 
   drag[[2]] > 0, 
   viewval = Max[viewval - frac*(Pi - viewval), 0.001], 
   drag[[2]] == 0, viewval = viewval]), 
RotationAction -> "Clip"]], {{"MouseDown", 
 2} :> (a = CurrentValue[MousePosition];
 pt = CurrentValue[ImageSize]), {"MouseDragged", 
 2} :> {b = CurrentValue[MousePosition], drag = b - a, 
 frac = Norm[drag]/Norm[pt]}, {"MouseUp", 2} :> {frac = 0, 
 drag = {0, 0}}}]]

work within a Manipulate?

graphics above was simply a cylinder:

graphics = Graphics3D[{Red, Cylinder[]}]