ios - 为什么下面的 Core Animation 隐式动画不起作用？

Question

下面的代码是CALayer 的子类。如果我调用 animateVal，则 drawInContext 是动画的。那太棒了。

我试图用 actionForKey 把它变成隐式动画。如果我为 val 属性赋值 (layer.val = 10)，val 会立即更改并且 drawInContext 以相同的 val 值反复调用，这是我刚刚分配的值，一遍又一遍。

我做错了什么？

@interface MyLayer : CALayer
@property (nonatomic, assign) CGFloat val;
- (void)animateVal:(CGFloat)val;
@end

@implementation MyLayer
@dynamic val;

+ (BOOL)needsDisplayForKey:(NSString *)key {
    return [key isEqualToString:@"val"] || [super needsDisplayForKey:key];
}

- (id<CAAction>)actionForKey:(NSString *)event {
    if ([event isEqualToString:@"val"]) {
        CABasicAnimation *anim = [CABasicAnimation animationWithKeyPath:@"val"];
        anim.duration = 3.0;
        return anim;
    }

    return [super actionForKey:event];
}

- (void)animateVal:(CGFloat)val {
    CABasicAnimation *anim = [CABasicAnimation animationWithKeyPath:@"val"];
    anim.toValue = [NSNumber numberWithFloat:val];
    anim.duration = 3.0;
    [self addAnimation:anim forKey:nil];
}

- (void)drawInContext:(CGContextRef)ctx {

    CGContextAddRect(ctx, CGRectMake(10, 10, self.val, self.val));
    CGContextSetFillColorWithColor(ctx, [UIColor greenColor].CGColor);
    CGContextFillPath(ctx);
}

@end

Answer 1

哎呀。

解决方案是在返回操作时设置fromValue。

    anim.fromValue = [[self presentationLayer] valueForKey:event];