Change color based on property value

Question

I have got a custom UIView which is a circle and I want to change the color of the circle based on the value of a property. The value is in the range of 0 to 100 where 100 should be red and 0 should be yellow. Is there a way to calculate the current color based on the property?

Franz

Answer 1

In RGB red is:

{ 1.0, 0.0, 0.0 }

and yellow is:

{ 1.0, 1.0, 0.0 }

so we can see that it's the green channel that needs to be calculated from the input of 0 to 100, however that range needs to be inverted as red is 100 and yellow is 0.

So (untested):

- (void)setCircleColor:(int)value
{
    if (value < 0)
        value = 0;
    else if (value > 100)
        value = 100;

    CGFloat green 1.0 - ((CGFloat)value / 100.0);
    UIColor *color = [UIColor colorWithRed:1.0
                                     green:green
                                      blue:0.0
                                     alpha:1.0];
    self.circle.color = color;
}

Note this assumes the property is called circleColor and that your circle object is accessible from the circle property and itself has a color property.