Android动画(三)：Interpolator和Evaluator

本系列将介绍以下内容：

Android动画

1 插值器Interpolator

1.1 Interpolator概述

有关动画的变化速率的问题是由Interpolator接口决定的，它是用来指定动画如何变化的变量，即用来控制动画的区间值是如何被计算出来的。

哪些动画可以设置插值器呢？
可以：Tween Animation、ValueAnimator、ObjectAnimator、ViewPropertyAnimator
不可以：Frame Animation
即只有逐帧动画不能设置插值器Interpolator。

View Animation中，补间动画的父类Animation中有具体方法setInterpolator(Interpolator i)，而逐帧动画的实现类AnimationDrawable没有相关方法，即补间动画可以设置插值器，而逐帧动画不能。
Property Animation均可设置插值器，因为ValueAnimator和ObjectAnimator的父类Animator中有抽象方法setInterpolator(TimeInterpolator value)，而ViewPropertyAnimator中单独定义了非抽象方法setInterpolator(TimeInterpolator interpolator)。

Interpolator接口的父接口是TimeInterpolator，而常用的插值器实现类全部继承自抽象类BaseInterpolator。
TimeInterpolator位于包android.animation中，Interpolator、BaseInterpolator及其十个子类全部位于包android.view.animation中。

关于Interpolator接口的UML图（类图Class Diagram）如图所示：

Interpolator类图

package android.animation;

/**
 * A time interpolator defines the rate of change of an animation. This allows animations
 * to have non-linear motion, such as acceleration and deceleration.
 */
public interface TimeInterpolator {
   

    /**
     * Maps a value representing the elapsed fraction of an animation to a value that represents
     * the interpolated fraction. This interpolated value is then multiplied by the change in
     * value of an animation to derive the animated value at the current elapsed animation time.
     *
     * @param input A value between 0 and 1.0 indicating our current point
     *        in the animation where 0 represents the start and 1.0 represents
     *        the end 当前动画的进度。
     *        该参数与任何设定的值无关，只与时间有关，随着时间的推移，动画的进度也自然增加。
     * @return The interpolation value. This value can be more than 1.0 for
     *         interpolators which overshoot their targets, or less than 0 for
     *         interpolators that undershoot their targets. 当前实际想要显示的进度，也即动画的当前数值进度。
     */
    float getInterpolation(float input);
}

package android.view.animation;

import android.animation.TimeInterpolator;

/**
 * An interpolator defines the rate of change of an animation. This allows
 * the basic animation effects (alpha, scale, translate, rotate) to be 
 * accelerated, decelerated, repeated, etc.
 */
public interface Interpolator extends TimeInterpolator {
   
    // A new interface, TimeInterpolator, was introduced for the new android.animation
    // package. This older Interpolator interface extends TimeInterpolator so that users of
    // the new Animator-based animations can use either the old Interpolator implementations or
    // new classes that implement TimeInterpolator directly.
}

package android.view.animation;

import android.content.pm.ActivityInfo.Config;

/**
 * An abstract class which is extended by default interpolators.
 */
abstract public class BaseInterpolator implements Interpolator {
   
    private @Config int mChangingConfiguration;
    /**
     * @hide
     */
    public @Config int getChangingConfiguration() {
   
        return mChangingConfiguration;
    }

    /**
     * @hide
     */
    void setChangingConfiguration(@Config int changingConfiguration) {
   
        mChangingConfiguration = changingConfiguration;
    }
}

1.2 常用插值器

Interpolator Class	Resource ID
AccelerateDecelerateInterpolator	@android:anim/accelerate_decelerate_interpolator
AccelerateInterpolator	@android:anim/accelerate_interpolator
AnticipateInterpolator	@android:anim/anticipate_interpolator
AnticipateOvershootInterpolator	@android:anim/anticipate_overshoot_interpolator
BounceInterpolator	@android:anim/bounce_interpolator
CycleInterpolator	@android:anim/cycle_interpolator
DecelerateInterpolator	@android:anim/decelerate_interpolator
LinearInterpolator	@android:anim/linear_interpolato
OvershootInterpolator	@android:anim/overshoot_interpolator
PathInterpolator	无此用法

常用的Interpolator实现类有十个，全部继承自抽象类BaseInterpolator，且均位于包android.view.animation中：

package android.view.animation;

import android.content.Context;
import android.graphics.animation.HasNativeInterpolator;
import android.graphics.animation.NativeInterpolator;
import android.graphics.animation.NativeInterpolatorFactory;
import android.util.AttributeSet;

/**
 * An interpolator where the rate of change starts and ends slowly but
 * accelerates through the middle.
 */
@HasNativeInterpolator
public class AccelerateDecelerateInterpolator extends BaseInterpolator
        implements NativeInterpolator {
   
    public AccelerateDecelerateInterpolator() {
   
    }

    @SuppressWarnings({
   "UnusedDeclaration"})
    public AccelerateDecelerateInterpolator(Context context, AttributeSet attrs)