iOS 中的动画文本内容 - 相当于 Android ValueAnimator

我正在开发一个 iOS 7+ 应用程序，并且想要以动画方式更改 UILabel 的内容。我do not想要做任何图形动画，例如淡出旧内容/淡入新内容。因此，iOS 提供的所有标准动画功能（例如图层动画或动画块）都无法使用（至少我这么认为）。

假设 UILabel 显示一些仪表值，例如“200 V”，并且该文本应更改为“400 V”。文本不应只是从“200 V”跳转到“400 V”，而应使用某种缓动函数进行计数：“200 V”、“220 V”、“240 V”...“390 V”、“395” V”...“400V”

在 Android 中可以使用 ValueAnimator 轻松解决：

ValueAnimator animation = ValueAnimator.ofFloat(0f, 1f);
animation.setInterpolation(new EaseOutInterpolator());
animation.setDuration(2500);
animation.setStartDelay(500);

animation.addUpdateListener(new AnimatorUpdateListener() {
    @Override
    public void onAnimationUpate(ValueAnimator animator) {
        float currentValue = animator.getAnimatedValue.floatValue();
        label1.setText(String.format("%.2", fromValue1 + ((toValue1 - fromValue1) * currentValue)));
        label2.setText(String.format("%.2", fromValue2 + ((toValue2 - fromValue2) * currentValue)));
    ...
    }
});
animation.start();

iOS 里也有这样的东西吗？我为此找到了不同的解决方案，但它们都相当旧（2010/11），并且最终都使用 NSTimer 和自己的缓动函数手动实现此行为。

毫无疑问，一个人可以自己实现这一点，但这会相当麻烦，而且不太优雅。那么：iOS 中是否有内置的东西可以解决这个问题，或者至少有方便的第三方实现可用？

非常感谢！

由于我没有找到为此量身定制的解决方案，因此我创建了自己的解决方案：一个处理缓动的简单动画器类：

// MyValueAnimation.h
typedef void (^MyAnimationBlock)(double animationValue);

@interface MyValueAnimation : NSObject

- (void)startAnimation:(MyAnimationBlock)animationBlock runtime:(NSUInteger)runtime delay:(NSUInteger)delay;

@end


// MyValueAnimation.m
#import "MyValueAnimation.h"

// Number of seconds between each animation step
#define kStepSize 0.05

@interface MyValueAnimation () {
    NSTimer *timer;            
    NSUInteger totalRunTime;    // Total duration of the animation (delay not included)
    NSUInteger currentRuntime;  // Time the animation is already running
    MyAnimationBlock animationBlock;
}
@end

@implementation MyValueAnimation

- (void)startAnimation:(MyAnimationBlock)block runtime:(NSUInteger)runtime delay:(NSUInteger)delay {
    if (timer != nil)
        [timer invalidate];

    timer = nil;
    totalRunTime = runtime;
    animationBlock = block;
    currentRuntime = 0;

    if (block != nil) {
        if (delay > 0) {
            // Wait to delay the start. Convert delay from millis to seconds
            double delaySeconds = (double)delay / 1000.0;
            timer = [NSTimer scheduledTimerWithTimeInterval:delaySeconds target:self selector:@selector(delayTick:) userInfo:nil repeats:false];
        } else {
            // Run the animation
            timer = [NSTimer scheduledTimerWithTimeInterval:kStepSize target:self selector:@selector(animationTick:) userInfo:nil repeats:true];
        }
    }
}

- (void)delayTick:(NSTimer *)delayTimer {
    // End of delay -> run animation
    [delayTimer invalidate];
    timer = [NSTimer scheduledTimerWithTimeInterval:kStepSize target:self selector:@selector(animationTick:) userInfo:nil repeats:true];
}

- (void)animationTick:(NSTimer *)animationTimer {
    NSUInteger step = 1000 * kStepSize;  // step size/length in milli seconds
    currentRuntime += step;
    double progress = MIN((double)currentRuntime / (double)totalRunTime, 1.0);

    // Progress is a value between 0 and 1. The easing function maps this
    // to the animationValue which is than used inside the animationBlock
    // to calculate the current value of the animiation
    double animationValue = [self customEaseOut:progress];

    if (animationBlock != nil)
        animationBlock(animationValue);

    if (progress >= 1.0) {
        // Animation complete
        [timer invalidate];
        timer = nil;
    }        
}

- (double)customEaseOut:(double)t {
    // Use any easing function you like to animate your values...
    // http://rechneronline.de/function-graphs/
    // http://sol.gfxile.net/interpolation/
    return (1 - pow(1-t, 2));
}

@end 


// =============================================================

// Some code using the animation
- (void)animateValueFrom:(double)fromValue to:(double)toValue {
    if (valueAnimation == nil)
        valueAnimation = [[MyValueAnimation alloc] init];

    MyAnimationBlock animationBlock = ^(double animationValue) {
        double currentValue = fromValue + ((toValue - fromValue) * animationValue);

        someLabel.text = [NSString stringWithFormat:"%dV", currentValue];        
    };

    [valueAnimation startAnimation:animationBlock runtime:1500 delay:500];
}

也许不是最漂亮的解决方案，但它有效:-)