Question

I know there have been plenty of variations on this question before, but none seem to capture the particular issue I’m having. I’m new to iOS development and Objective-C, so please bear with me if this is a particularly basic oversight on my part.

I’m trying to dynamically change the background color of a view (it’s a single view application.) I can easily change the background color through statements like:

[self.view setBackgroundColor:[UIColor blueColor]];

or

UIColor *newColor = [[UIColor alloc] initWithRed:.777777 green:.777777 blue:.777777 alpha:.777777];
[self.view setBackgroundColor:newColor];

I run into trouble when I try to change one of those RGBA values dynamically (using sliders, in this case.) I have four sliders, with tags 0-3, hooked up via this method:

- (IBAction)sendColor:(id)sender {
    UISlider *slider = (UISlider *)sender;
    CGFloat newValue = (CGFloat)(slider.value);
    int senderTag = [sender tag];
    CGFloat *components = CGColorGetComponents(self.view.backgroundColor.CGColor);
    components[senderTag] = newValue;
    UIColor *newColor = [[UIColor alloc] initWithRed:components[0] green:components[1] blue:components[2] alpha:components[3]];
    [self.view setBackgroundColor:newColor];
    NSString *colorAsString = [NSString stringWithFormat:@"%f, %f, %f, %f", components[0], components[1], components[2], components[3]];
    NSLog(@"%@", colorAsString);
}

When I move a slider, I can see the values updating properly via NSLog:

2011-12-13 20:54:26.468 HelloWorld1[1283:707] 0.739568, 0.865854, 1.000000, 1.000000

…but the background color never changes. I feel like I’m missing something rudimentary here, do you have any suggestions? Thank you!

Answer 1

Don’t call CGColorGetComponents. Rather, create a CGFloat components[4] ivar in your class and set this directly in the init.

Example…

@interface MyView : UIView
{
    CGFloat components[4];
}
@end

…

-(id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self != nil)
    { 
        components[0] = 1;
        components[1] = 1;
        components[2] = 1;
        components[3] = 1;

        self.view.backgrounColor = [UIColor colorWithRed:components[0] green:components[1] blue:components[2] alpha:components[3]];
    }
    return self;
}

Then change only the component that you need to.

- (IBAction)sendColor:(id)sender {
    UISlider *slider = (UISlider *)sender;
    CGFloat newValue = (CGFloat)(slider.value);
    int senderTag = [sender tag];
    components[senderTag] = newValue;
    UIColor *newColor = [[UIColor alloc] initWithRed:components[0] green:components[1] blue:components[2] alpha:components[3]];
    [self.view setBackgroundColor:newColor];
    NSString *colorAsString = [NSString stringWithFormat:@"%f, %f, %f, %f", components[0], components[1], components[2], components[3]];
    NSLog(@"%@", colorAsString);
}