settings/widget/DotsPageIndicator.java

/*
 * Copyright (C) 2016 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.android.settings.widget;

import static android.view.animation.AnimationUtils.loadInterpolator;

import android.animation.Animator;
import android.animation.AnimatorListenerAdapter;
import android.animation.AnimatorSet;
import android.animation.ValueAnimator;
import android.content.Context;
import android.content.res.TypedArray;
import android.database.DataSetObserver;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.RectF;
import android.os.Build;
import android.util.AttributeSet;
import android.view.View;
import android.view.animation.Interpolator;

import androidx.viewpager.widget.ViewPager;

import com.android.settings.R;

import java.util.Arrays;

/**
 * Custom pager indicator for use with a {@code ViewPager}.
 */
public class DotsPageIndicator extends View implements ViewPager.OnPageChangeListener {

    public static final String TAG = DotsPageIndicator.class.getSimpleName();

    // defaults
    private static final int DEFAULT_DOT_SIZE = 8;                      // dp
    private static final int DEFAULT_GAP = 12;                          // dp
    private static final int DEFAULT_ANIM_DURATION = 400;               // ms
    private static final int DEFAULT_UNSELECTED_COLOUR = 0x80ffffff;    // 50% white
    private static final int DEFAULT_SELECTED_COLOUR = 0xffffffff;      // 100% white

    // constants
    private static final float INVALID_FRACTION = -1f;
    private static final float MINIMAL_REVEAL = 0.00001f;

    // configurable attributes
    private int dotDiameter;
    private int gap;
    private long animDuration;
    private int unselectedColour;
    private int selectedColour;

    // derived from attributes
    private float dotRadius;
    private float halfDotRadius;
    private long animHalfDuration;
    private float dotTopY;
    private float dotCenterY;
    private float dotBottomY;

    // ViewPager
    private ViewPager viewPager;
    private ViewPager.OnPageChangeListener pageChangeListener;

    // state
    private int pageCount;
    private int currentPage;
    private float selectedDotX;
    private boolean selectedDotInPosition;
    private float[] dotCenterX;
    private float[] joiningFractions;
    private float retreatingJoinX1;
    private float retreatingJoinX2;
    private float[] dotRevealFractions;
    private boolean attachedState;

    // drawing
    private final Paint unselectedPaint;
    private final Paint selectedPaint;
    private final Path combinedUnselectedPath;
    private final Path unselectedDotPath;
    private final Path unselectedDotLeftPath;
    private final Path unselectedDotRightPath;
    private final RectF rectF;

    // animation
    private ValueAnimator moveAnimation;
    private ValueAnimator[] joiningAnimations;
    private AnimatorSet joiningAnimationSet;
    private PendingRetreatAnimator retreatAnimation;
    private PendingRevealAnimator[] revealAnimations;
    private final Interpolator interpolator;

    // working values for beziers
    float endX1;
    float endY1;
    float endX2;
    float endY2;
    float controlX1;
    float controlY1;
    float controlX2;
    float controlY2;

    public DotsPageIndicator(Context context) {
        this(context, null, 0);
    }

    public DotsPageIndicator(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public DotsPageIndicator(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        final int scaledDensity = (int) context.getResources().getDisplayMetrics().scaledDensity;

        // Load attributes
        final TypedArray typedArray = getContext().obtainStyledAttributes(
                attrs, R.styleable.DotsPageIndicator, defStyle, 0);
        dotDiameter = typedArray.getDimensionPixelSize(R.styleable.DotsPageIndicator_dotDiameter,
                DEFAULT_DOT_SIZE * scaledDensity);
        dotRadius = dotDiameter / 2;
        halfDotRadius = dotRadius / 2;
        gap = typedArray.getDimensionPixelSize(R.styleable.DotsPageIndicator_dotGap,
                DEFAULT_GAP * scaledDensity);
        animDuration = (long) typedArray.getInteger(R.styleable.DotsPageIndicator_animationDuration,
                DEFAULT_ANIM_DURATION);
        animHalfDuration = animDuration / 2;
        unselectedColour = typedArray.getColor(R.styleable.DotsPageIndicator_pageIndicatorColor,
                DEFAULT_UNSELECTED_COLOUR);
        selectedColour = typedArray.getColor(R.styleable.DotsPageIndicator_currentPageIndicatorColor,
                DEFAULT_SELECTED_COLOUR);
        typedArray.recycle();
        unselectedPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        unselectedPaint.setColor(unselectedColour);
        selectedPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        selectedPaint.setColor(selectedColour);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            interpolator = loadInterpolator(context, android.R.interpolator.fast_out_slow_in);
        } else {
            interpolator = loadInterpolator(context, android.R.anim.accelerate_decelerate_interpolator);
        }

        // create paths & rect now – reuse & rewind later
        combinedUnselectedPath = new Path();
        unselectedDotPath = new Path();
        unselectedDotLeftPath = new Path();
        unselectedDotRightPath = new Path();
        rectF = new RectF();

        addOnAttachStateChangeListener(new OnAttachStateChangeListener() {
            @Override
            public void onViewAttachedToWindow(View v) {
                attachedState = true;
            }
            @Override
            public void onViewDetachedFromWindow(View v) {
                attachedState = false;
            }
        });
    }

    public void setViewPager(ViewPager viewPager) {
        this.viewPager = viewPager;
        viewPager.setOnPageChangeListener(this);
        setPageCount(viewPager.getAdapter().getCount());
        viewPager.getAdapter().registerDataSetObserver(new DataSetObserver() {
            @Override
            public void onChanged() {
                setPageCount(DotsPageIndicator.this.viewPager.getAdapter().getCount());
            }
        });
        setCurrentPageImmediate();
    }

    /***
     * As this class <b>must</b> act as the {@link ViewPager.OnPageChangeListener} for the ViewPager
     * (as set by {@link #setViewPager(androidx.viewpager.widget.ViewPager)}).  Applications may set a
     * listener here to be notified of the ViewPager events.
     *
     * @param onPageChangeListener
     */
    public void setOnPageChangeListener(ViewPager.OnPageChangeListener onPageChangeListener) {
        pageChangeListener = onPageChangeListener;
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
        // nothing to do – just forward onward to any registered listener
        if (pageChangeListener != null) {
            pageChangeListener.onPageScrolled(position, positionOffset, positionOffsetPixels);
        }
    }

    @Override
    public void onPageSelected(int position) {
        if (attachedState) {
            // this is the main event we're interested in!
            setSelectedPage(position);
        } else {
            // when not attached, don't animate the move, just store immediately
            setCurrentPageImmediate();
        }

        // forward onward to any registered listener
        if (pageChangeListener != null) {
            pageChangeListener.onPageSelected(position);
        }
    }

    @Override
    public void onPageScrollStateChanged(int state) {
        // nothing to do – just forward onward to any registered listener
        if (pageChangeListener != null) {
            pageChangeListener.onPageScrollStateChanged(state);
        }
    }

    private void setPageCount(int pages) {
        pageCount = pages;
        calculateDotPositions();
        resetState();
    }

    private void calculateDotPositions() {
        int left = getPaddingLeft();
        int top = getPaddingTop();
        int right = getWidth() - getPaddingRight();
        int requiredWidth = getRequiredWidth();
        float startLeft = left + ((right - left - requiredWidth) / 2) + dotRadius;
        dotCenterX = new float[pageCount];
        for (int i = 0; i < pageCount; i++) {
            dotCenterX[i] = startLeft + i * (dotDiameter + gap);
        }
        // todo just top aligning for now… should make this smarter
        dotTopY = top;
        dotCenterY = top + dotRadius;
        dotBottomY = top + dotDiameter;
        setCurrentPageImmediate();
    }

    private void setCurrentPageImmediate() {
        if (viewPager != null) {
            currentPage = viewPager.getCurrentItem();
        } else {
            currentPage = 0;
        }

        if (pageCount > 0) {
            selectedDotX = dotCenterX[currentPage];
        }
    }

    private void resetState() {
        if (pageCount > 0) {
            joiningFractions = new float[pageCount - 1];
            Arrays.fill(joiningFractions, 0f);
            dotRevealFractions = new float[pageCount];
            Arrays.fill(dotRevealFractions, 0f);
            retreatingJoinX1 = INVALID_FRACTION;
            retreatingJoinX2 = INVALID_FRACTION;
            selectedDotInPosition = true;
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int desiredHeight = getDesiredHeight();
        int height;
        switch (MeasureSpec.getMode(heightMeasureSpec)) {
            case MeasureSpec.EXACTLY:
                height = MeasureSpec.getSize(heightMeasureSpec);
                break;
            case MeasureSpec.AT_MOST:
                height = Math.min(desiredHeight, MeasureSpec.getSize(heightMeasureSpec));
                break;
            default: // MeasureSpec.UNSPECIFIED
                height = desiredHeight;
                break;
        }
        int desiredWidth = getDesiredWidth();
        int width;
        switch (MeasureSpec.getMode(widthMeasureSpec)) {
            case MeasureSpec.EXACTLY:
                width = MeasureSpec.getSize(widthMeasureSpec);
                break;
            case MeasureSpec.AT_MOST:
                width = Math.min(desiredWidth, MeasureSpec.getSize(widthMeasureSpec));
                break;
            default: // MeasureSpec.UNSPECIFIED
                width = desiredWidth;
                break;
        }
        setMeasuredDimension(width, height);
        calculateDotPositions();
    }

    @Override
    protected void onSizeChanged(int width, int height, int oldWidth, int oldHeight) {
        setMeasuredDimension(width, height);
        calculateDotPositions();
    }

    @Override
    public void clearAnimation() {
        super.clearAnimation();
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
            cancelRunningAnimations();
        }
    }

    private int getDesiredHeight() {
        return getPaddingTop() + dotDiameter + getPaddingBottom();
    }

    private int getRequiredWidth() {
        return pageCount * dotDiameter + (pageCount - 1) * gap;
    }

    private int getDesiredWidth() {
        return getPaddingLeft() + getRequiredWidth() + getPaddingRight();
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if (viewPager == null || pageCount == 0) {
            return;
        }
        drawUnselected(canvas);
        drawSelected(canvas);
    }

    private void drawUnselected(Canvas canvas) {
        combinedUnselectedPath.rewind();

        // draw any settled, revealing or joining dots
        for (int page = 0; page < pageCount; page++) {
            int nextXIndex = page == pageCount - 1 ? page : page + 1;
            // todo Path.op should be supported in KitKat but causes the app to hang for Nexus 5.
            // For now disabling for all pre-L devices.
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                Path unselectedPath = getUnselectedPath(page,
                        dotCenterX[page],
                        dotCenterX[nextXIndex],
                        page == pageCount - 1 ? INVALID_FRACTION : joiningFractions[page],
                        dotRevealFractions[page]);
                combinedUnselectedPath.op(unselectedPath, Path.Op.UNION);
            } else {
                canvas.drawCircle(dotCenterX[page], dotCenterY, dotRadius, unselectedPaint);
            }
        }

        // draw any retreating joins
        if (retreatingJoinX1 != INVALID_FRACTION) {
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                combinedUnselectedPath.op(getRetreatingJoinPath(), Path.Op.UNION);
            }
        }
        canvas.drawPath(combinedUnselectedPath, unselectedPaint);
    }

    /**
     * Unselected dots can be in 6 states:
     *
     * #1 At rest
     * #2 Joining neighbour, still separate
     * #3 Joining neighbour, combined curved
     * #4 Joining neighbour, combined straight
     * #5 Join retreating
     * #6 Dot re-showing / revealing
     *
     * It can also be in a combination of these states e.g. joining one neighbour while
     * retreating from another.  We therefore create a Path so that we can examine each
     * dot pair separately and later take the union for these cases.
     *
     * This function returns a path for the given dot **and any action to it's right** e.g. joining
     * or retreating from it's neighbour
     *
     * @param page
     */
    private Path getUnselectedPath(int page,
                                   float centerX,
                                   float nextCenterX,
                                   float joiningFraction,
                                   float dotRevealFraction) {
        unselectedDotPath.rewind();

        if ((joiningFraction == 0f || joiningFraction == INVALID_FRACTION)
                && dotRevealFraction == 0f
                && !(page == currentPage && selectedDotInPosition == true)) {
            // case #1 – At rest
            unselectedDotPath.addCircle(dotCenterX[page], dotCenterY, dotRadius, Path.Direction.CW);
        }

        if (joiningFraction > 0f && joiningFraction < 0.5f && retreatingJoinX1 == INVALID_FRACTION) {
            // case #2 – Joining neighbour, still separate
            // start with the left dot
            unselectedDotLeftPath.rewind();

            // start at the bottom center
            unselectedDotLeftPath.moveTo(centerX, dotBottomY);

            // semi circle to the top center
            rectF.set(centerX - dotRadius, dotTopY, centerX + dotRadius, dotBottomY);
            unselectedDotLeftPath.arcTo(rectF, 90, 180, true);

            // cubic to the right middle
            endX1 = centerX + dotRadius + (joiningFraction * gap);
            endY1 = dotCenterY;
            controlX1 = centerX + halfDotRadius;
            controlY1 = dotTopY;
            controlX2 = endX1;
            controlY2 = endY1 - halfDotRadius;
            unselectedDotLeftPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX1, endY1);

            // cubic back to the bottom center
            endX2 = centerX;
            endY2 = dotBottomY;
            controlX1 = endX1;
            controlY1 = endY1 + halfDotRadius;
            controlX2 = centerX + halfDotRadius;
            controlY2 = dotBottomY;
            unselectedDotLeftPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX2, endY2);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                unselectedDotPath.op(unselectedDotLeftPath, Path.Op.UNION);
            }

            // now do the next dot to the right
            unselectedDotRightPath.rewind();

            // start at the bottom center
            unselectedDotRightPath.moveTo(nextCenterX, dotBottomY);

            // semi circle to the top center
            rectF.set(nextCenterX - dotRadius, dotTopY, nextCenterX + dotRadius, dotBottomY);
            unselectedDotRightPath.arcTo(rectF, 90, -180, true);

            // cubic to the left middle
            endX1 = nextCenterX - dotRadius - (joiningFraction * gap);
            endY1 = dotCenterY;
            controlX1 = nextCenterX - halfDotRadius;
            controlY1 = dotTopY;
            controlX2 = endX1;
            controlY2 = endY1 - halfDotRadius;
            unselectedDotRightPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX1, endY1);

            // cubic back to the bottom center
            endX2 = nextCenterX;
            endY2 = dotBottomY;
            controlX1 = endX1;
            controlY1 = endY1 + halfDotRadius;
            controlX2 = endX2 - halfDotRadius;
            controlY2 = dotBottomY;
            unselectedDotRightPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX2, endY2);
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                unselectedDotPath.op(unselectedDotRightPath, Path.Op.UNION);
            }
        }

        if (joiningFraction > 0.5f && joiningFraction < 1f && retreatingJoinX1 == INVALID_FRACTION) {
            // case #3 – Joining neighbour, combined curved
            // start in the bottom left
            unselectedDotPath.moveTo(centerX, dotBottomY);

            // semi-circle to the top left
            rectF.set(centerX - dotRadius, dotTopY, centerX + dotRadius, dotBottomY);
            unselectedDotPath.arcTo(rectF, 90, 180, true);

            // bezier to the middle top of the join
            endX1 = centerX + dotRadius + (gap / 2);
            endY1 = dotCenterY - (joiningFraction * dotRadius);
            controlX1 = endX1 - (joiningFraction * dotRadius);
            controlY1 = dotTopY;
            controlX2 = endX1 - ((1 - joiningFraction) * dotRadius);
            controlY2 = endY1;
            unselectedDotPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX1, endY1);

            // bezier to the top right of the join
            endX2 = nextCenterX;
            endY2 = dotTopY;
            controlX1 = endX1 + ((1 - joiningFraction) * dotRadius);
            controlY1 = endY1;
            controlX2 = endX1 + (joiningFraction * dotRadius);
            controlY2 = dotTopY;
            unselectedDotPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX2, endY2);

            // semi-circle to the bottom right
            rectF.set(nextCenterX - dotRadius, dotTopY, nextCenterX + dotRadius, dotBottomY);
            unselectedDotPath.arcTo(rectF, 270, 180, true);

            // bezier to the middle bottom of the join
            // endX1 stays the same
            endY1 = dotCenterY + (joiningFraction * dotRadius);
            controlX1 = endX1 + (joiningFraction * dotRadius);
            controlY1 = dotBottomY;
            controlX2 = endX1 + ((1 - joiningFraction) * dotRadius);
            controlY2 = endY1;
            unselectedDotPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX1, endY1);

            // bezier back to the start point in the bottom left
            endX2 = centerX;
            endY2 = dotBottomY;
            controlX1 = endX1 - ((1 - joiningFraction) * dotRadius);
            controlY1 = endY1;
            controlX2 = endX1 - (joiningFraction * dotRadius);
            controlY2 = endY2;
            unselectedDotPath.cubicTo(controlX1, controlY1, controlX2, controlY2, endX2, endY2);
        }

        if (joiningFraction == 1 && retreatingJoinX1 == INVALID_FRACTION) {
            // case #4 Joining neighbour, combined straight
            // technically we could use case 3 for this situation as well
            // but assume that this is an optimization rather than faffing around with beziers
            // just to draw a rounded rect
            rectF.set(centerX - dotRadius, dotTopY, nextCenterX + dotRadius, dotBottomY);
            unselectedDotPath.addRoundRect(rectF, dotRadius, dotRadius, Path.Direction.CW);
        }

        // case #5 is handled by #getRetreatingJoinPath()
        // this is done separately so that we can have a single retreating path spanning
        // multiple dots and therefore animate it's movement smoothly
        if (dotRevealFraction > MINIMAL_REVEAL) {
            // case #6 – previously hidden dot revealing
            unselectedDotPath.addCircle(centerX, dotCenterY, dotRevealFraction * dotRadius,
                    Path.Direction.CW);
        }

        return unselectedDotPath;
    }

    private Path getRetreatingJoinPath() {
        unselectedDotPath.rewind();
        rectF.set(retreatingJoinX1, dotTopY, retreatingJoinX2, dotBottomY);
        unselectedDotPath.addRoundRect(rectF, dotRadius, dotRadius, Path.Direction.CW);
        return unselectedDotPath;
    }

    private void drawSelected(Canvas canvas) {
        canvas.drawCircle(selectedDotX, dotCenterY, dotRadius, selectedPaint);
    }

    private void setSelectedPage(int now) {
        if (now == currentPage || pageCount == 0) {
            return;
        }

        int was = currentPage;
        currentPage = now;

        // These animations are not supported in pre-JB versions.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
            cancelRunningAnimations();

            // create the anim to move the selected dot – this animator will kick off
            // retreat animations when it has moved 75% of the way.
            // The retreat animation in turn will kick of reveal anims when the
            // retreat has passed any dots to be revealed
            final int steps = Math.abs(now - was);
            moveAnimation = createMoveSelectedAnimator(dotCenterX[now], was, now, steps);

            // create animators for joining the dots.  This runs independently of the above and relies
            // on good timing.  Like comedy.
            // if joining multiple dots, each dot after the first is delayed by 1/8 of the duration
            joiningAnimations = new ValueAnimator[steps];
            for (int i = 0; i < steps; i++) {
                joiningAnimations[i] = createJoiningAnimator(now > was ? was + i : was - 1 - i,
                        i * (animDuration / 8L));
            }
            moveAnimation.start();
            startJoiningAnimations();
        } else {
            setCurrentPageImmediate();
            invalidate();
        }
    }

    private ValueAnimator createMoveSelectedAnimator(final float moveTo, int was, int now,
                                                     int steps) {
        // create the actual move animator
        ValueAnimator moveSelected = ValueAnimator.ofFloat(selectedDotX, moveTo);

        // also set up a pending retreat anim – this starts when the move is 75% complete
        retreatAnimation = new PendingRetreatAnimator(was, now, steps,
                now > was
                        ? new RightwardStartPredicate(moveTo - ((moveTo - selectedDotX) * 0.25f))
                        : new LeftwardStartPredicate(moveTo + ((selectedDotX - moveTo) * 0.25f)));

        moveSelected.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                // todo avoid autoboxing
                selectedDotX = (Float) valueAnimator.getAnimatedValue();
                retreatAnimation.startIfNecessary(selectedDotX);
                postInvalidateOnAnimation();
            }
        });

        moveSelected.addListener(new AnimatorListenerAdapter() {
            @Override
            public void onAnimationStart(Animator animation) {
                // set a flag so that we continue to draw the unselected dot in the target position
                // until the selected dot has finished moving into place
                selectedDotInPosition = false;
            }
            @Override
            public void onAnimationEnd(Animator animation) {
                // set a flag when anim finishes so that we don't draw both selected & unselected
                // page dots
                selectedDotInPosition = true;
            }
        });

        // slightly delay the start to give the joins a chance to run
        // unless dot isn't in position yet – then don't delay!
        moveSelected.setStartDelay(selectedDotInPosition ? animDuration / 4L : 0L);
        moveSelected.setDuration(animDuration * 3L / 4L);
        moveSelected.setInterpolator(interpolator);
        return moveSelected;
    }

    private ValueAnimator createJoiningAnimator(final int leftJoiningDot, final long startDelay) {
        // animate the joining fraction for the given dot
        ValueAnimator joining = ValueAnimator.ofFloat(0f, 1.0f);
        joining.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
            @Override
            public void onAnimationUpdate(ValueAnimator valueAnimator) {
                setJoiningFraction(leftJoiningDot, valueAnimator.getAnimatedFraction());
            }
        });
        joining.setDuration(animHalfDuration);
        joining.setStartDelay(startDelay);
        joining.setInterpolator(interpolator);
        return joining;
    }

    private void setJoiningFraction(int leftDot, float fraction) {
        joiningFractions[leftDot] = fraction;
        postInvalidateOnAnimation();
    }

    private void clearJoiningFractions() {
        Arrays.fill(joiningFractions, 0f);
        postInvalidateOnAnimation();
    }

    private void setDotRevealFraction(int dot, float fraction) {
        dotRevealFractions[dot] = fraction;
        postInvalidateOnAnimation();
    }

    private void cancelRunningAnimations() {
        cancelMoveAnimation();
        cancelJoiningAnimations();
        cancelRetreatAnimation();
        cancelRevealAnimations();
        resetState();
    }

    private void cancelMoveAnimation() {
        if (moveAnimation != null && moveAnimation.isRunning()) {
            moveAnimation.cancel();
        }
    }

    private void startJoiningAnimations() {
        joiningAnimationSet = new AnimatorSet();
        joiningAnimationSet.playTogether(joiningAnimations);
        joiningAnimationSet.start();
    }

    private void cancelJoiningAnimations() {
        if (joiningAnimationSet != null && joiningAnimationSet.isRunning()) {
            joiningAnimationSet.cancel();
        }
    }

    private void cancelRetreatAnimation() {
        if (retreatAnimation != null && retreatAnimation.isRunning()) {
            retreatAnimation.cancel();
        }
    }

    private void cancelRevealAnimations() {
        if (revealAnimations != null) {
            for (PendingRevealAnimator reveal : revealAnimations) {
                reveal.cancel();
            }
        }
    }

    int getUnselectedColour() {
        return unselectedColour;
    }

    int getSelectedColour() {
        return selectedColour;
    }

    float getDotCenterY() {
        return dotCenterY;
    }

    float getDotCenterX(int page) {
        return dotCenterX[page];
    }

    float getSelectedDotX() {
        return selectedDotX;
    }

    int getCurrentPage() {
        return currentPage;
    }

    /**
     * A {@link android.animation.ValueAnimator} that starts once a given predicate returns true.
     */
    public abstract class PendingStartAnimator extends ValueAnimator {

        protected boolean hasStarted;
        protected StartPredicate predicate;

        public PendingStartAnimator(StartPredicate predicate) {
            super();
            this.predicate = predicate;
            hasStarted = false;
        }

        public void startIfNecessary(float currentValue) {
            if (!hasStarted && predicate.shouldStart(currentValue)) {
                start();
                hasStarted = true;
            }
        }
    }

    /**
     * An Animator that shows and then shrinks a retreating join between the previous and newly
     * selected pages.  This also sets up some pending dot reveals – to be started when the retreat
     * has passed the dot to be revealed.
     */
    public class PendingRetreatAnimator extends PendingStartAnimator {

        public PendingRetreatAnimator(int was, int now, int steps, StartPredicate predicate) {
            super(predicate);
            setDuration(animHalfDuration);
            setInterpolator(interpolator);

            // work out the start/end values of the retreating join from the direction we're
            // travelling in.  Also look at the current selected dot position, i.e. we're moving on
            // before a prior anim has finished.
            final float initialX1 = now > was ? Math.min(dotCenterX[was], selectedDotX) - dotRadius
                    : dotCenterX[now] - dotRadius;
            final float finalX1 = now > was ? dotCenterX[now] - dotRadius
                    : dotCenterX[now] - dotRadius;
            final float initialX2 = now > was ? dotCenterX[now] + dotRadius
                    : Math.max(dotCenterX[was], selectedDotX) + dotRadius;
            final float finalX2 = now > was ? dotCenterX[now] + dotRadius
                    : dotCenterX[now] + dotRadius;
            revealAnimations = new PendingRevealAnimator[steps];

            // hold on to the indexes of the dots that will be hidden by the retreat so that
            // we can initialize their revealFraction's i.e. make sure they're hidden while the
            // reveal animation runs
            final int[] dotsToHide = new int[steps];
            if (initialX1 != finalX1) { // rightward retreat
                setFloatValues(initialX1, finalX1);
                // create the reveal animations that will run when the retreat passes them
                for (int i = 0; i < steps; i++) {
                    revealAnimations[i] = new PendingRevealAnimator(was + i,
                            new RightwardStartPredicate(dotCenterX[was + i]));
                    dotsToHide[i] = was + i;
                }
                addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
                    @Override
                    public void onAnimationUpdate(ValueAnimator valueAnimator) {
                        // todo avoid autoboxing
                        retreatingJoinX1 = (Float) valueAnimator.getAnimatedValue();
                        postInvalidateOnAnimation();
                        // start any reveal animations if we've passed them
                        for (PendingRevealAnimator pendingReveal : revealAnimations) {
                            pendingReveal.startIfNecessary(retreatingJoinX1);
                        }
                    }
                });
            } else { // (initialX2 != finalX2) leftward retreat
                setFloatValues(initialX2, finalX2);
                // create the reveal animations that will run when the retreat passes them
                for (int i = 0; i < steps; i++) {
                    revealAnimations[i] = new PendingRevealAnimator(was - i,
                            new LeftwardStartPredicate(dotCenterX[was - i]));
                    dotsToHide[i] = was - i;
                }
                addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
                    @Override
                    public void onAnimationUpdate(ValueAnimator valueAnimator) {
                        // todo avoid autoboxing
                        retreatingJoinX2 = (Float) valueAnimator.getAnimatedValue();
                        postInvalidateOnAnimation();
                        // start any reveal animations if we've passed them
                        for (PendingRevealAnimator pendingReveal : revealAnimations) {
                            pendingReveal.startIfNecessary(retreatingJoinX2);
                        }
                    }
                });
            }

            addListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationStart(Animator animation) {
                    cancelJoiningAnimations();
                    clearJoiningFractions();
                    // we need to set this so that the dots are hidden until the reveal anim runs
                    for (int dot : dotsToHide) {
                        setDotRevealFraction(dot, MINIMAL_REVEAL);
                    }
                    retreatingJoinX1 = initialX1;
                    retreatingJoinX2 = initialX2;
                    postInvalidateOnAnimation();
                }
                @Override
                public void onAnimationEnd(Animator animation) {
                    retreatingJoinX1 = INVALID_FRACTION;
                    retreatingJoinX2 = INVALID_FRACTION;
                    postInvalidateOnAnimation();
                }
            });
        }
    }

    /**
     * An Animator that animates a given dot's revealFraction i.e. scales it up
     */
    public class PendingRevealAnimator extends PendingStartAnimator {

        private final int dot;

        public PendingRevealAnimator(int dot, StartPredicate predicate) {
            super(predicate);
            this.dot = dot;
            setFloatValues(MINIMAL_REVEAL, 1f);
            setDuration(animHalfDuration);
            setInterpolator(interpolator);

            addUpdateListener(new AnimatorUpdateListener() {
                @Override
                public void onAnimationUpdate(ValueAnimator valueAnimator) {
                    // todo avoid autoboxing
                    setDotRevealFraction(PendingRevealAnimator.this.dot,
                            (Float) valueAnimator.getAnimatedValue());
                }
            });

            addListener(new AnimatorListenerAdapter() {
                @Override
                public void onAnimationEnd(Animator animation) {
                    setDotRevealFraction(PendingRevealAnimator.this.dot, 0f);
                    postInvalidateOnAnimation();
                }
            });
        }
    }

    /**
     * A predicate used to start an animation when a test passes
     */
    public abstract class StartPredicate {

        protected float thresholdValue;

        public StartPredicate(float thresholdValue) {
            this.thresholdValue = thresholdValue;
        }

        abstract boolean shouldStart(float currentValue);
    }

    /**
     * A predicate used to start an animation when a given value is greater than a threshold
     */
    public class RightwardStartPredicate extends StartPredicate {

        public RightwardStartPredicate(float thresholdValue) {
            super(thresholdValue);
        }

        boolean shouldStart(float currentValue) {
            return currentValue > thresholdValue;
        }
    }

    /**
     * A predicate used to start an animation then a given value is less than a threshold
     */
    public class LeftwardStartPredicate extends StartPredicate {

        public LeftwardStartPredicate(float thresholdValue) {
            super(thresholdValue);
        }

        boolean shouldStart(float currentValue) {
            return currentValue < thresholdValue;
        }
    }
}