在android的動畫中，有一個插值器，他的作用是告訴動畫某個屬性（比如顏色的漸變）如何隨時間變化。它將以線性方式變化，還是以指數方式變化？它是否在開始時變化速度很快，然後逐漸變慢？使用方法是在需
要設置插值器的動畫xml文件中增加一個interpoplator屬性，如下：
<alpha xmlns:android="http://schems.android.com/apk/res/android"
android:interpoplator="@android:anim/accelerate_interpoplator"
android:fromAlpha="0.0"
android:toAlpha="1.0"
android:duration="1000" />
這樣就可以使用了，在從完全透明變化爲不透明的過程中，你可以在res/anim/accelerate_interpoplator.xml中定義自己想要的效果，如變化速率等。
下面給出了幾個插值器的源碼，供參考：
AccelerateDecelerateInterpolator();
註解：An interpolator where the rate of change starts and ends slowly but accelerates through the middle.
源碼：public float getInterpolation(float input) {
return (float)(Math.cos((input + 1) * Math.PI) / 2.0f) + 0.5f;
}
AccelerateInterpolator(float mFactor);
註解：An interpolator where the rate of change starts out slowly and and then accelerates.
源碼： public float getInterpolation(float input) {
if (mFactor == 1.0f) {
return input * input;
} else {
return (float)Math.pow(input, mDoubleFactor);
}
}
CycleInterpolator(float cycles);
註解：Repeats the animation for a specified number of cycles. The rate of change follows a sinusoidal pattern.
源碼：public float getInterpolation(float input) {
return (float)(Math.sin(2 * mCycles * Math.PI * input));
}
DecelerateInterpolator();
註解：An interpolator where the rate of change starts out quickly and and then decelerates.
源碼：public float getInterpolation(float input) {
float result;
if (mFactor == 1.0f) {
result = (float)(1.0f - (1.0f - input) * (1.0f - input));
} else {
result = (float)(1.0f - Math.pow((1.0f - input), 2 * mFactor));
}
return result;
}
AnticipateInterpolator();
註解：An interpolator where the change starts backward then flings forward.
源碼：public float getInterpolation(float t) {
// a(t) = t * t * ((tension + 1) * t - tension)
return t * t * ((mTension + 1) * t - mTension);
}
AnticipateOvershootInterpolator();
註解：An interpolator where the change starts backward then flings forward and overshoots the target value and finally goes back to the final value.
源碼：public float getInterpolation(float t) {
// a(t, s) = t * t * ((s + 1) * t - s)
// o(t, s) = t * t * ((s + 1) * t + s)
// f(t) = 0.5 * a(t * 2, tension * extraTension), when t < 0.5
// f(t) = 0.5 * (o(t * 2 - 2, tension * extraTension) + 2), when t <= 1.0
if (t < 0.5f) return 0.5f * a(t * 2.0f, mTension);
else return 0.5f * (o(t * 2.0f - 2.0f, mTension) + 2.0f);
}
private static float a(float t, float s) {
return t * t * ((s + 1) * t - s);
}
private static float o(float t, float s) {
return t * t * ((s + 1) * t + s);
}
public AnticipateOvershootInterpolator(float tension) {
mTension = tension * 1.5f;
}
BounceInterpolator();
註解：An interpolator where the change bounces at the end.
源碼：private static float bounce(float t) {
return t * t * 8.0f;
}
public float getInterpolation(float t) {
// _b(t) = t * t * 8
// bs(t) = _b(t) for t < 0.3535
// bs(t) = _b(t - 0.54719) + 0.7 for t < 0.7408
// bs(t) = _b(t - 0.8526) + 0.9 for t < 0.9644
// bs(t) = _b(t - 1.0435) + 0.95 for t <= 1.0
// b(t) = bs(t * 1.1226)
t *= 1.1226f;
if (t < 0.3535f) return bounce(t);
else if (t < 0.7408f) return bounce(t - 0.54719f) + 0.7f;
else if (t < 0.9644f) return bounce(t - 0.8526f) + 0.9f;
else return bounce(t - 1.0435f) + 0.95f;
}
OvershootInterpolator();
註解：An interpolator where the change flings forward and overshoots the last value then comes back.
源碼：public float getInterpolation(float t) {
// _o(t) = t * t * ((tension + 1) * t + tension)
// o(t) = _o(t - 1) + 1
t -= 1.0f;
return t * t * ((mTension + 1) * t + mTension) + 1.0f;
}
LinearInterpolator();
註解：An interpolator where the rate of change is constant
源碼：public float getInterpolation(float input) {
return input;
}
