Interactive Spin Actionscript Tutorial

I have been thinking of different interactions that are possible with objects. If you’ve read this blog at all you’ll know that I’ve played with physics and gravity and throwing balls and bouncing balls and all sorts. But I hadn’t wrapped my head around an interactive spinner. I know it’d be easy to make a slider or something that would apply a spin to an object, but this just isn’t interactive enough for me.

Circle with slider to rotate and button for random spin:

Get Adobe Flash player

This attempt at spinning is ok. I mean, it spins the object and it even glides to a stop if you press the button for a random spin… But it’s just not intuitive and not fun. But if you want this, here’s how I did it.


[cc lang=”actionscript” tab_size=”2″ lines=”40″]
drag = .96;
speed = 0;

slider.handle.onPress = function() {
spinning = false;
//drag along the line
this.startDrag(true, slider.line._x-slider.handle._width/2, slider.line._y-slider.handle._height/2, slider.line._width-slider.handle._width/2, slider.line._y-slider.handle._height/2);
slider.handle.onRelease = slider.handle.onReleaseOutside = function() {
_root.onEnterFrame = function() {
if (spinning) {
//apply the speed to the rotation
knob._rotation += speed;
//recalculate speed
speed = speed*drag;
//if speed gets unnoticeably tiny just set it to 0
if (Math.pow(speed, 2) < .0001) {
speed = 0;
else {
//set the rotation from the slider position
knob._rotation = slider.line._x + slider.handle._x + slider.handle._width/2;

//spit out feedback continuously
feedbackr.text = knob._rotation;
feedbackaccr.text = speed;
spinner.onRelease = function() {
//find a random speed
speed = (Math.random()* 50) – 25;
spinning = true;

I want to grab it and spin it though. I want to apply the same principles from physics, like acceleration and friction as forces to the object, so I can grab to spin and release to watch it glide gracefully to a stop. I’ve been thinking about this and how I’d have to use trigonometry and stuff to do it. One day I finally had the time and tried it out. It took me a minute but I figured out that what I needed was arctangent. So (with pointers from jbum, thanks Jim!) I came up with this:

Interactive grab-able circle to spin and twirl:

Get Adobe Flash player

This one is much more interactive and intuitive. I really think this is because there are no sliders or buttons, no controls, just an object to interact with. It’s much more like real life!


In order to make a grab and spin object
1. You have to know where you grab. The user clicks on the shape (knob) and you must figure out what degree or rotation point they have started at. (atan2)
2. As the knob is clicked and the mouse moves (dragging), calculate new rotation by mouse position
3. When mouse is released figure out the current speed of rotation and apply it to the knob with friction, so it can be thrown and spun in that way. (Of course this is optional, if you just want to spin it when the mouse is down you’re done at step 2)


[cc lang=”actionscript” tab_size=”2″ lines=”40″]
damp = .96; //friction
r = 0; //rotation
accr = 0; //speed of rotation

knob.onPress = function() {
dragging = true;
//find mouse y coordinate in relation to knob origin
a = _root._ymouse – knob._y;
//find mouse x coordinate in relation to knob origin
b = _root._xmouse – knob._x;
//using arctangent find the spot of rotation (in degrees)
oldr = Math.atan2(a,b)*180/Math.PI;

knob.onRelease = knob.onReleaseOutside = function() {
dragging = false;

knob.onEnterFrame = function() {
if (dragging) {
//find mouse y coordinate in relation to knob origin
a = _root._ymouse-knob._y;
//find mouse x coordinate in relation to knob origin
b = _root._xmouse-knob._x;
//using arctangent find the spot of rotation (in degrees)
r = Math.atan2(a,b)*180/Math.PI;

//use current rotation and previous rotation
//to find acceleration
//averages the acceleration with the
//previous acceleration for smoother spins
accr = ((r – oldr) + accr)/2;
//apply the acceleration to the rotation
knob._rotation += accr;
//remember current rotation as old rotation
oldr = r;
feedbacka.text = a;
feedbackb.text = b;
else {
knob._rotation += accr;
//apply friction to acceleration force
//and if acceleration gets tiny, just set it to zero
if (Math.pow(accr, 2) > .0001 ) {
accr *= damp;
accr = 0;
//spit out feedback continuosly
feedbackr.text = knob._rotation;
feedbackaccr.text = accr;

I commented the code to explain what is happening, if you need more just post a comment. Let me know if you find this useful and what you end up making with it.


spin.fla and interactiveSpin.fla

19 thoughts on “Interactive Spin Actionscript Tutorial

  1. Hi Evan

    Grab the knob and rotate it all the way around for one revolution without letting go.

    There is a rather violent movement when the user rotates the knob all the way through x < 0 and when y passes through 0 (positive to negative or negative to positive). It jumps to the other side (180 deg away) and then rapidly comes back again.

    I would suggest not using atan2 and using only sine to determine your rotation.

  2. Thanks for catching that Zac, I hadn’t noticed that hiccup in there…
    It seems to be when it is calculating the acceleration as you drag through the negative x-axis (from the origin of the knob). It’s because I’m using the difference between old rotation and current rotation to find the acceleration (line 32), and when oldr is -179 and new is 180 that makes for quite a bit of acceleration… accr = ((r – oldr) + accr)/2; … it helps I guess that it’s averaged with the old accr, but it doesn’t correct it by any means.
    When I have some time I’ll have to get in and map the accr calculation correctly to see the values as circular, … 178, 179, 180, -179, -178 …

  3. Here’s a fix, albeit not perfect, to the above mentioned problem. Within the onEnterFrame actions, find the line that says:

    accr = ((r-oldr)+accr)/2;

    and change it to this….

    if (r-oldr > 160 || r-oldr < -160){
    accr = ((0)+accr);
    accr = ((r-oldr)+accr)/2;

    I’m in the process of implementing this code into a project. When it’s ready I’ll try to remember to link it.

  4. thanks you guys both. This is perfect for what i am doing. (making tutorials for a gas-fitting school), i am going to try apply this rotation to one of the gas taps. Just have to figure out how to make a ‘maximum’ amount of turns.

  5. Great tutorial. One question, how would I make the knob only spin a certain degree? Say if I wanted this to be a volume knob and would want it to only spin a certain degree as if turning the knob on a speaker amplifier. Thanks!

  6. There are two solutions to the rotation problem. One involves using vector math to find angular displacements by first mapping flash’s piece-wise rotational range of [0, -180][180, 0] to the standard [0, 360] rotational range typically used in math, applying the Vector Dot Product to find the smaller angle between two vectors and finally determining the direction of rotation (clockwise or counter-clockwise). This is the most robust solution since it is based on standard mathematical models of rotation. But most Flash Developers coming from a design background may have trouble grappling with its more technical aspects. The second, more accessible solution involves exploiting the relationships between nested clips. But that’s all I am going to say on the matter. Half the fun is figuring stuff out for oneself.

  7. i have a question!!
    how can i make the object 3d and move it in all directions
    (if i have a footage of anything such as a mobile and i want to make it 3d) how can i modify this script to do what i want??
    thanks in advanced…

  8. Hi,

    I’m currently trying to use the rotator but would like to set it so the circle can only turn to half-way (turn 180 degrees) and no further.
    If there is any chance you could be of assistance it would be a great help.


    1. @von – I have this on my list to convert to as3, it’s only a matter of affording the time to do it and then remembering to post it 😉

      @Daniel- You would just need to set the limits and add a clause around the lines that apply the rotation transformation. Check to see if it would cause the value to go out of bounds and reel it back in if needed. Looks like lines 34 and 41. Best of luck

      1. Hello Evans,
        I need a help from you……..
        I am in the process of creating a rotating knob with push function……
        how to program both rotate and press function in to single object in AS3…….Please help…..

  9. Can you help me to make this dial change the flash timeline? I’m trying to make the position of the dial go to a specific keyframe, or percentage of a movie clip.


  10. Hi Evan-
    nice tutorial, but as some others have asked, I need to limit the rotation to just maybe 30 degrees. Can these scripts be modified to do that?

  11. Excellent! Is there a possibility to rotate the circle 1.) without aceleration 2.) by the 10 degrees steps?

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