com.facebook.keyframes.util.KFPathInterpolator.java Source code

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Here is the source code for com.facebook.keyframes.util.KFPathInterpolator.java

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/*
 * Copyright (C) 2013 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.facebook.keyframes.util;

import android.graphics.Path;
import android.graphics.PathMeasure;
import android.view.animation.Interpolator;

/**
 * This class is directly lifted from the compat library's
 * {@link android.support.v4.view.animation.PathInterpolatorCompat}.  The reason we need this class
 * is to be able to control the precision ourselves for a smaller memory footprint.  The normal
 * PathInterpolator allocates ~1400 floats every time and holds on to this in memory for the entire
 * lifetime of the object, while this one allocates only 10% of that.  We dont need high precision
 * for what we are using this for.
 */
public class KFPathInterpolator implements Interpolator {
    /**
     * Governs the accuracy of the approximation of the {@link Path}.
     */
    private static final float PRECISION = 0.03f;
    private final float[] mX;
    private final float[] mY;

    public KFPathInterpolator(float controlX1, float controlY1, float controlX2, float controlY2) {
        Path path = new Path();
        path.moveTo(0, 0);
        path.cubicTo(controlX1, controlY1, controlX2, controlY2, 1f, 1f);
        final PathMeasure pathMeasure = new PathMeasure(path, false /* forceClosed */);
        final float pathLength = pathMeasure.getLength();
        final int numPoints = (int) (pathLength / PRECISION) + 1;
        mX = new float[numPoints];
        mY = new float[numPoints];
        final float[] position = new float[2];
        for (int i = 0; i < numPoints; ++i) {
            final float distance = (i * pathLength) / (numPoints - 1);
            pathMeasure.getPosTan(distance, position, null /* tangent */);
            mX[i] = position[0];
            mY[i] = position[1];
        }
    }

    @Override
    public float getInterpolation(float t) {
        if (t <= 0.0f) {
            return 0.0f;
        } else if (t >= 1.0f) {
            return 1.0f;
        }
        // Do a binary search for the correct x to interpolate between.
        int startIndex = 0;
        int endIndex = mX.length - 1;
        while (endIndex - startIndex > 1) {
            int midIndex = (startIndex + endIndex) / 2;
            if (t < mX[midIndex]) {
                endIndex = midIndex;
            } else {
                startIndex = midIndex;
            }
        }
        final float xRange = mX[endIndex] - mX[startIndex];
        if (xRange == 0) {
            return mY[startIndex];
        }
        final float tInRange = t - mX[startIndex];
        final float fraction = tInRange / xRange;
        final float startY = mY[startIndex];
        final float endY = mY[endIndex];
        return startY + (fraction * (endY - startY));
    }
}