Adds followAnimatedTargetAlongPath, and modifies the expanded controller to use it.

# Physics Animation Layout

## Overview

**PhysicsAnimationLayout** works with an implementation of **PhysicsAnimationController** to construct and maintain physics animations for each of its child views. During the initial construction of the animations, the layout queries the controller for configuration settings such as which properties to animate, which animations to chain together, and what stiffness or bounciness to use. Once the animations are built to the controller’s specifications, the controller can then ask the layout to start, stop and manipulate them arbitrarily to achieve any desired animation effect. The controller is notified whenever children are added or removed from the layout, so that it can animate their entrance or exit, respectively.

**PhysicsAnimationLayout** works with implementations of **PhysicsAnimationController** to configure and run physics-based animations for each of its child views. During the initial construction of the animations, the layout queries the controller for basic configuration settings such as which properties to animate, which animations to chain together, and the default physics parameters to use.

Once the animations are built, the controller can access **PhysicsPropertyAnimator** instances to run them. The animator behaves similarly to the familiar `ViewPropertyAnimator`, with the ability to animate `alpha`, `translation`, and `scale` values. It also supports additional functionality such as `followAnimatedTargetAlongPath` for more advanced motion.

The controller is notified whenever children are added or removed from the layout, so that it can animate their entrance or exit, respectively.

An example usage is Bubbles, which uses a PhysicsAnimationLayout for its stack of bubbles. Bubbles has controller subclasses including StackAnimationController and ExpansionAnimationController. StackAnimationController tells the layout to configure the translation animations to be chained (for the ‘following’ drag effect), and has methods such as ```moveStack(x, y)``` to animate the stack to a given point. ExpansionAnimationController asks for no animations to be chained, and exposes methods like ```expandStack()``` and ```collapseStack()```, which animate the bubbles to positions along the bottom of the screen.

- `translationX/Y/Z(float)`

- `scaleX/Y(float)`

...as well as shortcut methods to reduce the amount of boilerplate code needed for common use cases:

- `position(float, float, Runnable…)`, which starts translationX and translationY animations, and calls the provided callbacks only when both animations have completed.

- `followAnimatedTargetAlongPath(Path, int, TimeInterpolator)`, which animates a ‘target’ point along the given path using a traditional Animator. As the target moves, the translationX/Y physics animations are updated to follow the target, similarly to how they might follow a touch event location. This results in the view roughly following the path, but with natural motion that takes momentum into account. For example, if a path makes a 90 degree turn to the right, the physics animations will cause the view to curve naturally towards the new trajectory.

It also provides the following configuration methods:

- `withStartDelay(int)`, for starting the animation after a given delay.

- `withStartVelocity(float)`, for starting the animation with the given start velocity.

- `withStiffness(float)` and `withDampingRatio(float)`, for overriding the default physics param values returned by the controller’s getSpringForce method.

- `withPositionStartVelocities(float, float)`, for setting specific start velocities for TRANSLATION_X and TRANSLATION_Y, since these typically differ.

- `start(Runnable)`, to start the animation, with an optional end action to call when the animations for every property (including chained animations) have completed.

- Often, animations will set starting values for properties before the animation begins. Property methods like `translationX` have an overloaded variant: `translationX(from, to)`. When `start()` is called, the animation will set the view's translationX property to `from` before beginning the animation to `to`.

- We may want to use different end actions for each property. For example, if we're animating a view to the bottom of the screen, and also fading it out, we might want to perform an action as soon as the fade out is complete. We can use `alpha(to, endAction)`, which will call endAction as soon as the alpha animation is finished. A special case is `position(x, y, endAction)`, where the endAction is called when both translationX and translationY animations have completed.

`PhysicsAnimationController` also provides `animationsForChildrenFromIndex(int, ChildAnimationConfigurator)`. This is a convenience method for starting animations on multiple child views, starting at the given index. The `ChildAnimationConfigurator` is called with a `PhysicsPropertyAnimator` for each child, where calls to methods like `translationX` and `withStartVelocity` can be made. `animationsForChildrenFromIndex` returns a `MultiAnimationStarter` with a single method, `startAll(endAction)`, which starts all of the animations and calls the end action when they have all completed.

- `PhysicsAnimationController` also provides `animationsForChildrenFromIndex(int, ChildAnimationConfigurator)`. This is a convenience method for starting animations on multiple child views, starting at the given index. The `ChildAnimationConfigurator` is called with a `PhysicsPropertyAnimator` for each child, where calls to methods like `translationX` and `withStartVelocity` can be made. `animationsForChildrenFromIndex` returns a `MultiAnimationStarter` with a single method, `startAll(endAction)`, which starts all of the animations and calls the end action when they have all completed.

##### Examples

Spring the stack of bubbles (whose animations are chained) to the bottom of the screen, shrinking them to 50% size. Once the first bubble is done shrinking, begin fading them out, and then remove them all from the parent once all bubbles have faded out:

    .startAll(removeFirstView);

```

## PhysicsAnimationLayout

The layout itself is a FrameLayout descendant with a few extra methods:

Move a view up along the left side of the screen, and then to the top right of the screen (assume a view that is currently halfway down the left side of the screen). When the translation animations have finished following the target, call a callback:

```setController(PhysicsAnimationController controller)```

Attaches the layout to the controller, so that the controller can access the layout’s protected methods. It also constructs or reconfigures the physics animations according to the new controller’s configuration methods.

```

Path path = new Path();

path.moveTo(view.getTranslationX(), view.getTranslationY());

path.lineTo(view.getTranslationX(), 0);

path.lineTo(mScreenWidth, 0);

animationForChild(view)

    .followAnimatedTargetAlongPath(path, 100, new LinearInterpolator())

    .start(callbackAfterFollowingFinished);

```

```setEndListenerForProperty(ViewProperty property, AnimationEndListener endListener)```

Sets an end listener that is called when all animations on the given property have ended.

## PhysicsAnimationLayout

The layout itself is a FrameLayout descendant with a few extra methods:

```setMaxRenderedChildren(int max)```

Child views beyond this limit will be set to GONE, and won't be animated, for performance reasons. Defaults to **5**.

```setActiveController(PhysicsAnimationController controller)```

Sets the given controller as the active controller for the layout. This causes the layout to construct or reconfigure the physics animations according to the new controller’s configuration methods, and halt any in-progress animations.

It has one protected method, ```animateValueForChildAtIndex(ViewProperty property, int index, float value)```, which is visible to PhysicsAnimationController descendants. This method dispatches the given value to the appropriate animation.

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Only the currently active controller is allowed to start animations. If a different controller is set as the active controller, the previous controller will no longer be able to start animations. Attempts to do so will have no effect. This is to ensure that multiple controllers aren’t updating animations at the same time, which can cause undefined behavior.

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    <item type="id" name="scale_x_dynamicanimation_tag"/>

    <item type="id" name="scale_y_dynamicanimation_tag"/>

    <item type="id" name="physics_animator_tag"/>

    <item type="id" name="target_animator_tag" />

    <!-- Global Actions Menu -->

    <item type="id" name="global_actions_view" />

import android.content.res.Configuration;

import android.content.res.Resources;

import android.graphics.Path;

import android.graphics.Point;

import android.graphics.PointF;

import android.view.View;

import androidx.dynamicanimation.animation.DynamicAnimation;

import androidx.dynamicanimation.animation.SpringForce;

import com.android.systemui.Interpolators;

import com.android.systemui.R;

import com.google.android.collect.Sets;

     */

    private static final int ANIMATE_TRANSLATION_FACTOR = 4;

    /** How much to scale down bubbles when they're animating in/out. */

    private static final float ANIMATE_SCALE_PERCENT = 0.5f;

    /** Duration of the expand/collapse target path animation. */

    private static final int EXPAND_COLLAPSE_TARGET_ANIM_DURATION = 175;

    /** The stack position to collapse back to in {@link #collapseBackToStack}. */

    private PointF mCollapseToPoint;

    /** Stiffness for the expand/collapse path-following animation. */

    private static final int EXPAND_COLLAPSE_ANIM_STIFFNESS = 1000;

    /** Horizontal offset between bubbles, which we need to know to re-stack them. */

    private float mStackOffsetPx;

        mAnimatingExpand = true;

        mAfterExpand = after;

        startOrUpdateExpandAnimation();

        startOrUpdatePathAnimation(true /* expanding */);

    }

    /** Animate collapsing the bubbles back to their stacked position. */

        mAfterCollapse = after;

        mCollapsePoint = collapsePoint;

        startOrUpdateCollapseAnimation();

        startOrUpdatePathAnimation(false /* expanding */);

    }

    /**

        mScreenWidth = mDisplaySize.x;

    }

    private void startOrUpdateExpandAnimation() {

        animationsForChildrenFromIndex(

                0, /* startIndex */

                (index, animation) -> animation.position(getBubbleLeft(index), getExpandedY()))

                .startAll(() -> {

    /**

     * Animates the bubbles along a curved path, either to expand them along the top or collapse

     * them back into a stack.

     */

    private void startOrUpdatePathAnimation(boolean expanding) {

        Runnable after;

        if (expanding) {

            after = () -> {

                mAnimatingExpand = false;

                if (mAfterExpand != null) {

                }

                mAfterExpand = null;

                });

    }

    private void startOrUpdateCollapseAnimation() {

        // Stack to the left if we're going to the left, or right if not.

        final float sideMultiplier = mLayout.isFirstChildXLeftOfCenter(mCollapsePoint.x) ? -1 : 1;

        animationsForChildrenFromIndex(

                0, /* startIndex */

                (index, animation) -> {

                    animation.position(

                            mCollapsePoint.x + (sideMultiplier * index * mStackOffsetPx),

                            mCollapsePoint.y);

                })

                .startAll(() -> {

            };

        } else {

            after = () -> {

                mAnimatingCollapse = false;

                if (mAfterCollapse != null) {

                }

                mAfterCollapse = null;

                });

            };

        }

        // Animate each bubble individually, since each path will end in a different spot.

        animationsForChildrenFromIndex(0, (index, animation) -> {

            final View bubble = mLayout.getChildAt(index);

            // Start a path at the bubble's current position.

            final Path path = new Path();

            path.moveTo(bubble.getTranslationX(), bubble.getTranslationY());

            final float expandedY = getExpandedY();

            if (expanding) {

                // If we're expanding, first draw a line from the bubble's current position to the

                // top of the screen.

                path.lineTo(bubble.getTranslationX(), expandedY);

                // Then, draw a line across the screen to the bubble's resting position.

                path.lineTo(getBubbleLeft(index), expandedY);

            } else {

                final float sideMultiplier =

                        mLayout.isFirstChildXLeftOfCenter(mCollapsePoint.x) ? -1 : 1;

                final float stackedX = mCollapsePoint.x + (sideMultiplier * index * mStackOffsetPx);

                // If we're collapsing, draw a line from the bubble's current position to the side

                // of the screen where the bubble will be stacked.

                path.lineTo(stackedX, expandedY);

                // Then, draw a line down to the stack position.

                path.lineTo(stackedX, mCollapsePoint.y);

            }

            // The lead bubble should be the bubble with the longest distance to travel when we're

            // expanding, and the bubble with the shortest distance to travel when we're collapsing.

            // During expansion from the left side, the last bubble has to travel to the far right

            // side, so we have it lead and 'pull' the rest of the bubbles into place. From the

            // right side, the first bubble is traveling to the top left, so it leads. During

            // collapse to the left, the first bubble has the shortest travel time back to the stack

            // position, so it leads (and vice versa).

            final boolean firstBubbleLeads =

                    (expanding && !mLayout.isFirstChildXLeftOfCenter(bubble.getTranslationX()))

                            || (!expanding && mLayout.isFirstChildXLeftOfCenter(mCollapsePoint.x));

            final int startDelay = firstBubbleLeads

                    ? (index * 10)

                    : ((mLayout.getChildCount() - index) * 10);

            animation

                    .followAnimatedTargetAlongPath(

                            path,

                            EXPAND_COLLAPSE_TARGET_ANIM_DURATION /* targetAnimDuration */,

                            Interpolators.LINEAR /* targetAnimInterpolator */)

                    .withStartDelay(startDelay)

                    .withStiffness(EXPAND_COLLAPSE_ANIM_STIFFNESS);

        }).startAll(after);

    }

    /** Prepares the given bubble to be dragged out. */

        // If a bubble is added while the expand/collapse animations are playing, update the

        // animation to include the new bubble.

        if (mAnimatingExpand) {

            startOrUpdateExpandAnimation();

            startOrUpdatePathAnimation(true /* expanding */);

        } else if (mAnimatingCollapse) {

            startOrUpdateCollapseAnimation();

            startOrUpdatePathAnimation(false /* expanding */);

        } else {

            child.setTranslationX(getBubbleLeft(index));

            animationForChild(child)

                        .setDampingRatio(SPRING_AFTER_FLING_DAMPING_RATIO),

                /* destination */ null);

        mLayout.setEndActionForMultipleProperties(

        setEndActionForMultipleProperties(

                () -> {

                    mRestingStackPosition = new PointF();

                    mRestingStackPosition.set(mStackPosition);

                    mLayout.removeEndActionForProperty(DynamicAnimation.TRANSLATION_X);

                    mLayout.removeEndActionForProperty(DynamicAnimation.TRANSLATION_Y);

                    removeEndActionForProperty(DynamicAnimation.TRANSLATION_X);

                    removeEndActionForProperty(DynamicAnimation.TRANSLATION_Y);

                },

                DynamicAnimation.TRANSLATION_X, DynamicAnimation.TRANSLATION_Y);

        cancelStackPositionAnimation(DynamicAnimation.TRANSLATION_X);

        cancelStackPositionAnimation(DynamicAnimation.TRANSLATION_Y);

        mLayout.removeEndActionForProperty(DynamicAnimation.TRANSLATION_X);

        mLayout.removeEndActionForProperty(DynamicAnimation.TRANSLATION_Y);

        removeEndActionForProperty(DynamicAnimation.TRANSLATION_X);

        removeEndActionForProperty(DynamicAnimation.TRANSLATION_Y);

    }

    /** Save the current IME height so that we know where the stack bounds should be. */