绕圆旋转

Question

I need to implement the following application in objective c >> http://bit.ly/rHzVkF

How can I rotate a circle by draging like in the app ? Do you know some similar examples ?

Answer 1

In your touches began method you need to check the x and y and see if it's in the boundaries of the two spinning tabs.

Now on your touches moved you need to detect the change (delta) of your x and y location and use ATAN2(dx,dy) I believe to convert to radians for revolution. If I'm wrong you can figure it out on paper, it's simple trigonometry (Don't forget to import <Math.h> )

With this new variable for revolutions apply it to the object you want to spin using CGAffineTransformRotate(); (I believe this is part of Core Graphics so make sure you've imported that library as well).

Answer 2

@ihkawiss do you know how to draw a circle with cocoa-touch? – vikingosegundo

@ihkawiss, I guess, you dont want to answer me. no problem. Your taks needs to be divided in smaller tasks.

1. draw small circles on given default position (the handles)
2. draw big circle
3. get touch coordinates and check, if it is inside a handle, proceed to next point.
4. calculate angle between circle center, touch position and a fixed point (ie.: top center).
5. with the angle, fixed point and radius of the circle calculate the new positions of the handles
6. re-draw everything as long as a touch is triggered.

you should be able to get enough information for each of this steps in the net and on stack overflow. If you get stuck (what would be quite normal and to be expected), come back, show what you get so far and explain, where you got stuck.

Answer 3

I will attempt to put my two cents into this as I just had to solve a similar scenario.

you have to implement the touches methods in the view that you want to handle all of the movement

• (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
• (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
• (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event

Albert is right in writing that you need to detect the change of your x and y location. You can get the location from the NSSet touches that is sent into the method on trigger of the method.

In the methods you will need to implement some variation of this code in order to work with the touches data:

for(UITouch *t in touches){
    if([t tapCount] > 1){
        return;
    }
    CGPoint loc = [t locationInView:self.view];

Once you have the x and y of where a user touched, in this case the CGPoint loc, you can run the atan2 method from the math.h library and get back the radian of the circle. You will need to #include < math.h > in order to use atan2()

atan2() needs two arguments in order to work, the difference between two y points, and two x points. In my case I used the current (touch y point, - center of the circle y), and the current (touch x point, - center of the circle x).

    double rads = atan2((loc.y - (self.frame.size.height/2)), (loc.x - (self.frame.size.width/2))); 

from there you can do any number of things with that information. To calculate the new x and y position around the circle you need to utilize the cosine and sine functions to calculate the proper coordinates.

   newXposition = CenterX + cos(rads) * radiusOfCircle
   newYposition = CenterY + sin(rads) * radiusOfCircle

You can get your new xPosition by adding the center X position of your circle to the result of the cos(inputting the radians we just got from before) multiplied by the radius of your circle And the new yposition is obtained by doing the same thing just with sin()instead.

So my code looked something like this....

    float radius = 61;
    float newXPos = 140 + (cos(rads) * radius);
    float newYPos = 165 + (sin(rads) * radius);

Good luck, hope this helps you get started. This stuff can be tricky