Logic of using UISlider Xcode - iphone

Question

I'm stuck at the moment controlling UISlider in XCode. I've made a horizontal slider in Interface Builder and manage to code the controls in xcode. Right now I'm just not sure how to code the logic. If the user slides the nib to the left, I'd it like it to rotate my image counter-clockwise and if the user slides it to the right, rotate the image clockwise.
Minimum value of slider I've set is 0 and max is 100. Initial value is 50.
For transformation I'm using CGAffineTransformMakeRotation(myAngle). I've set myAngle to float. As for the condition, here's a snippet of code:
mySlider.value = myAngle; // This is for CGAffine
if(myAngle < 50) {
myAngle -= 0.1;
}
if(myAngle > 50) {
myAngle += 0.1;
}
When I slide the nib, it only rotates anti-clockwise. What's the best logic can I use? Hope someone can help. Thanks.
-Hakimo

Answer 1

Note that the Doc says about the angle as,
The angle, in radians, by which this matrix rotates the coordinate system axes. In iOS, a positive value specifies counterclockwise rotation and a negative value specifies clockwise rotation. In Mac OS X, a positive value specifies clockwise rotation and a negative value specifies counterclockwise rotation.
I think something like the following should work. I am not sure though. (Considering that the minimumValue is 0).
float middleValue = slider.maximumValue / 2;
float value = middleValue - slider.value;
float degrees = (middleValue / 180) * value;
float radians = degrees * (3.14/180);

Answer 2

If you want a fixed position on the slider to correspond to a fixed angle of rotation, then you can do something like this:
float degreesToRadians = M_PI / 180.0;
CGFloat angle = (mySlider.value - 50) * degreesToRadians;
myView.transform = CGAffineTransformMakeRotation(angle);
On the other hand, if want more complex behavior, such as that the slider snaps to the middle except when being dragged, and how far it's dragged determines the speed of rotation of your view, then I would suggest using a timer. Set up an NSTimer to call a given selector, and in that selector do something like this:
- (void)timerFired:(NSTimer *)aTimer {
CGFloat angleDelta = (mySlider.value - 50) / 500;
// I'll assume myAngle is an instance variable.
myAngle += angleDelta;
myView.transform = CGAffineTransformMakeRotation(myAngle);
}
And somewhere else you need to write a method to set mySlider.value = 50 when the user stops dragging it, which you can do by adding a target/action for UIControlEventTouchUpInside and ...Outside on the slider.
If you want, you can make the rotation appear very smooth by using UIView animations each time you set the transform, using an animation duration equal to the NSTimer interval. If you do that, then I would actually suggest using the animation callback in place of the timer method, to avoid any possible crossover between the two events.