An animated Drawable that expands and reveals

CircleRevealDrawable.java

package com.ifttt.iftttprototype;

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.AnimatorSet;
import android.animation.ValueAnimator;
import android.graphics.Canvas;
import android.graphics.ColorFilter;
import android.graphics.Paint;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.view.animation.Interpolator;

/**
 * A custom {@link Drawable} that plays the Do Button special Recipe run animation.
 * <p/>
 * Currently due to the stroke rendering issue, on devices running lower than SDK 18, no stroke
 * animation is run, instead an expand + fade out animation is used.
 */
public class CircleRevealDrawable extends Drawable {

    private final Paint paint;
    private final int startDiameter;
    private final int maxDiameter;
    private final int centerX;
    private final int centerY;

    private int currentRadius = -1;

    private Interpolator interpolator;

    public CircleRevealDrawable(int color, int startDiameter, int maxDiameter, int cx, int cy) {
        this.startDiameter = startDiameter;
        this.maxDiameter = maxDiameter;

        centerX = cx;
        centerY = cy;

        paint = new Paint();
        paint.setColor(color);
        paint.setAntiAlias(true);
        paint.setStyle(isAboveJellyBeanMR2() ? Paint.Style.STROKE : Paint.Style.FILL);

        setBounds(centerX - startDiameter / 2, centerY - startDiameter / 2,
                centerX + startDiameter / 2, centerY + startDiameter / 2);

        currentRadius = startDiameter / 2;
    }

    public void setInterpolator(Interpolator interpolator) {
        this.interpolator = interpolator;
    }

    @Override
    public void draw(Canvas canvas) {
        canvas.drawCircle(centerX, centerY, currentRadius, paint);
    }

    @Override
    public void setAlpha(int alpha) {
        paint.setAlpha(alpha);
    }

    @Override
    public void setColorFilter(ColorFilter cf) {
        paint.setColorFilter(cf);
    }

    @Override
    public int getOpacity() {
        return paint.getAlpha();
    }

    /**
     * Start playing the expand and reveal animation.
     *
     * @param duration  Duration of the animation.
     * @param listener  AnimatorListener instance for listening to the animation status.
     */
    public void startAnimation(long duration, Animator.AnimatorListener listener) {
        AnimatorSet set = isAboveJellyBeanMR2() ? buildAnimator(listener)
                : buildAnimatorPreJellyBeanMR2(listener);

        if (interpolator != null) {
            set.setInterpolator(interpolator);
        }
        set.setDuration(duration);
        set.start();
    }

    /**
     * Build {@link AnimatorSet} for Jelly Bean MR2, KitKat and Lollipop with smooth
     * stroke animation.
     *
     * @param listener Optional AnimatorListener
     */
    private AnimatorSet buildAnimator(Animator.AnimatorListener listener) {
        final int fullStroke = maxDiameter;

        ValueAnimator expand = ValueAnimator.ofFloat(paint.getStrokeWidth(), fullStroke);
        expand.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                float animatedValue = (float) animation.getAnimatedValue();
                paint.setStrokeWidth(animatedValue);
                invalidateSelf();
            }
        });

        ValueAnimator reveal = ValueAnimator.ofInt(startDiameter / 2, maxDiameter / 2);
        reveal.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                currentRadius = (int) animation.getAnimatedValue();
                paint.setStrokeWidth(fullStroke * (1 - animation.getAnimatedFraction()));

                invalidateSelf();
            }
        });

        AnimatorSet set = new AnimatorSet();
        set.playSequentially(expand, reveal);

        set.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationEnd(Animator animation) {
                currentRadius = startDiameter / 2;
                paint.setStrokeWidth(1);
                invalidateSelf();
            }
        });

        if (listener != null) {
            set.addListener(listener);
        }

        return set;
    }

    /**
     * Build an {@link AnimatorSet} for pre-Jelly Bean MR2 devices, containing
     * full circle expand and alpha animation.
     *
     * @param listener Optional AnimatorListener.
     */
    private AnimatorSet buildAnimatorPreJellyBeanMR2(Animator.AnimatorListener listener) {
        ValueAnimator expand = ValueAnimator.ofInt(startDiameter / 2, maxDiameter / 2);
        expand.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                int value = (int) animation.getAnimatedValue();
                currentRadius = value;
                invalidateSelf();
            }
        });

        ValueAnimator alpha = ValueAnimator.ofInt(255, 0);
        alpha.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator animation) {
                paint.setAlpha((int) animation.getAnimatedValue());
                invalidateSelf();
            }
        });

        AnimatorSet set = new AnimatorSet();
        set.playSequentially(expand, alpha);
        set.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationEnd(Animator animation) {
                currentRadius = 0;
                paint.setAlpha(255);
                invalidateSelf();
            }
        });

        if (listener != null) {
            set.addListener(listener);
        }

        return set;
    }

    private static boolean isAboveJellyBeanMR2() {
        return Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR2;
    }
}