cc/animation provides animation support - generating output values (usually visual properties) based on a predefined function and changing input values. Currently the main clients of cc/animation are Blink and ui/, targeting composited layers, but the code is intended to be agnostic of the client it is supporting. Aspirationally we could eventually merge cc/animation and Blink animation and have only a single animation system for all of Chromium.
This document covers two main topics. The first section explains how cc/animation actually works: how animations are ticked, what animation curves are, what the ownership model is, etc. Later sections document how other parts of Chromium interact with cc/animation, most prominently Blink and ui/.
Animation A cc::Animation is responsible for managing animating properties for a set of targets. Each target is represented by a KeyframeEffect and can be animating multiple properties on that target; see KeyframeEffect below.
A particular Animation may not own all the KeyframeEffects for a given target. Animation is only a grouping mechanism for related effects, and the grouping relationship is defined by the client. It is also the client's responsibility to deal with any conflicts that arise from animating the same property of the same target across multiple Animations.
Each Animation has a copy on the impl thread, and will take care of synchronizing to/from the impl thread when requested.
SingleKeyframeEffectAnimation It is a sub-class of Animation that serves as a bridge between the cc animation clients and cc because currently only a single keyframe effect per animation is supported.
There is a 1:1 relationship between SingleKeyframeEffectAnimation and the KeyframeEffect.
NOTE: SingleKeyframeEffectAnimation is being deprecated.
Keyframe model KeyframeModels contain the state necessary to ‘play’ (interpolate values from) an
animation curve, which is a function that returns a value given an input time. Aside from the animation curve itself, a keyframe model's state includes the run state (playing, paused, etc), the start time, the current direction (forwards, reverse), etc. An animation does not know or care what property is being animated, and holds only an opaque identifier for the property to allow clients to map output values to the correct properties.
[Keyframe effect] A KeyframeEffect owns a group of KeyframeModels for a single target (identified by PropertyToElementIdMap). It is responsible for managing the KeyframeModels' running states (starting, running, paused, etc), as well as ticking the KeyframeModels when it is requested to produce new outputs for a given time.
Note that a single KeyframeEffect may not own all the KeyframeModels for a given target. KeyframeEffect is only a grouping mechanism for related KeyframeModels. The commonality between keyframe models on the same target is found via ElementAnimations - there is only one ElementAnimations for a given target.
Group: KeyframeModels that must be run together are called ‘grouped’ and have the same group id. Grouped KeyframeModels are guaranteed to start at the same time and no other KeyframeModels may animate any of the group‘s target properties until all KeyframeModels in the group have finished animating. It’s also guaranteed that no two keyframe models within a keyframe effect that have both the same group id and property id.
In general, KeyframeModels are grouped together based on their animation target (the entity whose property is being animated) and each such group is owned by an animation. Note that there may be multiple animations with the same target (each with a set of KeyframeModels for that target); the ElementAnimations class wraps the multiple animations and has a 1:1 relationship with target entities.
TODO(smcgruer): Why are ElementAnimations and Animations separate?
In order to play an animation, input time values must be provided to the animation curve and output values fed back into the animating entity. This is called ‘ticking’ an animation and is the responsibility of the animation host. The animation host has a list of currently ticking animations (i.e. those that have any non-deleted animations), which it iterates through whenever it receives a tick call from the client (along with a corresponding input time). The animations then call into their non-deleted animations, retrieving the value from the animation curve. As they are computed, output values are sent to the target which is responsible for passing them to the client entity that is being animated.
As noted above, an animation curve is simply a function which converts an input time value into some output value. Animation curves are categorized based on their output type, and each such category can have multiple implementations that provide different conversion functions. There are many categories of animation curve, but some common ones are
The most common implementation of the various animation curve categories are the keyframed animation curves. These curves each have a set of keyframes which map a specific time to a specific output value. Producing an output value for a given input time is then a matter of identifying the two keyframes the time lies between, and interpolating between the keyframe output values. (Or simply using a keyframe output value directly, if the input time happens to line up exactly.) Exact details of how each animation curve category is interpolated can be found in the implementations.
There is one category of animation curve that stands somewhat apart, the scroll offset animation curve. This curve converts the input time into a scroll offset, interpolating between an initial scroll offset and an updateable target scroll offset. It has logic to handle different types of scrolling such as programmatic, keyboard, and mouse wheel scrolls.
cc/animation has a concept of an animation timeline. This should not be confused with the identically named Blink concept. In cc/animation, animation timelines are an implementation detail - they hold the animations and are responsible for syncing them to the impl thread (see below), but do not participate in the ticking process in any way.
One part of cc/animation that is not client agnostic is its support for the Chromium compositor thread. Most of the cc/animation classes have a
PushPropertiesTo method, in which they synchronize necessary state from the main thread to the impl thread. It is feasible that such support could be abstracted if necessary, but so far it has not been required.
As noted above, the main clients of cc/animation are currently Blink for accelerated web animations, and ui/ for accelerated user interface animations. Both of these clients utilize cc::Layer as their animation entity and interact with cc/animation via the MutatorHostClient interface (which is implemented by cc::LayerTreeHost and cc::LayerTreeHostImpl). Recently a third client, chrome/browser/vr/, has started using cc/animations as well. The vr/ client does not use cc::Layer as its animation entity.
TODO(smcgruer): Summarize how vr/ uses cc/animation.
As cc::Layers are just textures which are reused for performance, clients that use composited layers as their animation entities are limited to animating properties that do not cause content to be redrawn. For example, a composited layer's opacity can be animated as promoted layers are aware of the content behind them. On the other hand we cannot animate layer width as changing the width could modify layout - which then requires redrawing.
Blink is able to move compatible animations off the main thread by promoting the animating element into a layer. The Blink Lifetime of a compositor animation document describes how composited animations are created in blink. Once a compositor animation is created it is pushed through the commit cycle.
The lifetime of a newly started cc::Animation is roughly the following:
element_idif one does not already exist. This ElementAnimations instance is shared by all animations with the same target and tracks the existence of the target.
element_idis created on the main thread LayerTreeHost's
property_trees_. This begins ticking the attached KeyframeEffects and tracks that the element exists in the active layer / property tree.
element_idis pushed by cc::KeyframeEffect::PushPropertiesTo cc::Animation::AttachElementForKeyframeEffect creates a compositor side cc::ElementAnimations instance to track the existence of the element on the compositor. Since animations are pushed after the layer and property trees, the element should already exist on the pending tree. This will result in the animation being added to the ticking animations list.
TODO(flackr): Document finishing / cancel / abort.
TODO(smcgruer): Write this.
The Compositor Property Trees talk slides includes discussion on compositor animations.
Smooth scrolling is implemented via animations. See also references to “scroll offset” animations in the cc code base. Smooth Scrolling in Chromium provides an overview of smooth scrolling. There is further class header documentation in Blink's platform/scroll directory.