动画NSMutableAttributedString颜色-字符串消失

Question

I have a layer and in its drawInContext: I draw an attributed string with drawInRect: . Here's how I initialise the layer.

+ (id)layer
{
    Character *layer = [[self alloc] init];
    if (layer) {
        NSDictionary *attributes = @{NSFontAttributeName: [UIFont fontWithName:@"Fabada" size:72]};
        layer.symbol = [[NSMutableAttributedString alloc] initWithString:@"X" attributes:attributes];
    }
    return layer;
}

I want to animate the string's color, so I have three dynamics properties.

@interface Character : CALayer

@property (nonatomic, strong) NSMutableAttributedString *symbol;
@property (nonatomic) int red;
@property (nonatomic) int green;
@property (nonatomic) int blue;

@end

@implementation Character

@dynamic red;
@dynamic green;
@dynamic blue;

/* ... */

In the draw method, I set the foreground color.

/* Set symbol color */
NSRange range = {0, [self.symbol length]};
UIColor *foreground = [UIColor colorWithRed:self.red    / 255.0
                                      green:self.green  / 255.0
                                       blue:self.blue   / 255.0
                                      alpha:1];
[self.symbol addAttribute:NSForegroundColorAttributeName value:foreground range:range];
CGRect symbolRect; /* The frame */
[self.symbol drawInRect:symbolRect];

The character appears on screen as desired. But once I add a CABasicAnimation to the layer, the symbol just disappears.

CABasicAnimation *animation = [CABasicAnimation animation];
animation.duration = 10.0;
animation.fillMode = kCAFillModeForwards;
animation.removedOnCompletion = NO;
animation.delegate = self;
animation.fromValue = [NSNumber numberWithInt:0];
animation.toValue   = [NSNumber numberWithInt:255];
[self.character addAnimation:animation forKey:@"blue"];

What does this symptom imply? How do I animate NSAttributedString attributes?

Answer 1

This was a simple question of implementing the - (id)initWithLayer:(id)layer method, because it gets called once the animation begins.

- (id)initWithLayer:(id)layer
{
    Character *temp = (Character *)layer;
    self = [super initWithLayer:layer];
    if (self) {
        self.symbol = temp.symbol;
        self.red    = temp.red;
        self.green  = temp.green;
        self.blue   = temp.blue;
    }
    return self;
}

Now the properties carry over and the results are as expected.