Core Animation深度指南：iOS动画开发

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“CoreAnimationProgrammingGuide，苹果公司的官方文档，涵盖了Core Animation在iOS和iPhone平台上的图形与动画编程指南，发布于2010年9月24日。” Core Animation是苹果开发框架中的一个核心组件，它允许开发者创建复杂的、高性能的动画效果，尤其在iOS和macOS平台上广泛使用。Core Animation的精髓在于它能够将视觉元素的渲染工作优化到操作系统底层，从而实现平滑且高效的动画表现。 1. **基本概念** - **CALayer**: Core Animation的基础是`CALayer`对象，它是屏幕上的可视元素，可以包含内容、边框、阴影等属性，并支持变换、动画等操作。 - **Core Animation图层树**: 层次结构组织`CALayer`，允许嵌套和组合，形成复杂的视图结构。 - **Model Layer与Presentation Layer**: Core Animation区分了模型层（存储实际属性）和呈现层（显示当前动画状态），确保动画过程中的平滑过渡。 2. **动画原理** - **非阻塞UI**: Core Animation通过异步处理动画，保证用户界面的流畅性，即使在执行动画时也不会阻塞主线程。 - **Core Animation事务**: 动画的集合称为事务，可以设置动画的持续时间、速度曲线（如ease-in-ease-out）、填充模式等。 - **关键帧动画**: 允许在指定时间点设定多个属性值，形成平滑过渡的效果。 - **动画组**: 可以将多个动画组合成一个动画组，它们将同步执行。 3. **动画类型** - **位置与尺寸动画**: 改变层的位置、大小或边界，实现移动、缩放效果。 - **透明度动画**: 调整层的不透明度，创建淡入淡出效果。 - **旋转与倾斜动画**: 通过修改变换矩阵，实现旋转和倾斜效果。 - **内容动画**: 动画层的内容，例如图片的渐变、模糊等。 4. **自定义动画** - **基于时间的动画**: 使用`CADisplayLink`或`NSTimer`进行自定义定时更新。 - **基于关键帧的动画**: 使用`CAKeyframeAnimation`类创建复杂路径或属性变化。 - **代理方法**: 通过`CALayer`的代理方法，可以监控动画状态并作出相应反应。 5. **性能优化** - **减少图层数量**: 过多的图层会消耗更多资源，合理合并图层可以提升性能。 - **使用Layer Backing Properties**: 调整图层的渲染质量与性能之间的平衡。 - **避免不必要的图层重绘**: 通过设置`needsDisplayOnBoundsChange`等属性减少不必要的更新。 6. **与其他技术结合** - **与UIKit的集成**: Core Animation可以与UIKit框架无缝配合，如`UIView`类的动画方法实质上是封装了Core Animation。 - **与Metal或OpenGL的结合**: 在需要更高性能的场景下，可以与这些图形库结合使用。 Core Animation是苹果平台上构建动态用户界面的强大工具，理解并熟练运用其原理和技术，可以创造出既美观又高效的交互体验。开发者应深入学习Core Animation，以便在iOS和macOS应用开发中充分利用其潜力。

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Figure 1 Core Animation class hierarchy

QuartzCore.framework

NSObject

CABasicAnimation

CAKeyframeAnimation

CAAnimationGroup

CAPropertyAnimation

CATransition

CAScrollLayer

CAEAGLLayer *

CATiledLayer

CAConstraintLayoutManager

CAConstraint

CAMediaTimingFunction

CARenderer

CATransaction

QuartzComposer.framework

CATextLayer

QTKit.framework

CAOpenGLLayer

QTCaptureLayer

QTMovieLayer

CAAnimation

<CAAction, CAMediaTiming>

CALayer

QCCompositionLayer

* iPhone OS only

Layer Classes

The layer classes are the foundation of Core Animation and provide an abstraction that should be familiar

to developers who have used NSView or UIView. Basic layer functionality is provided by the CALayer class,

which is the parent class for all types of Core Animation layers.

As with an instance of a view class, a CALayer instance has a single parent layer (the superlayer) and a

collection of sublayers, creating a hierarchy of layers that is referred to as the layer tree. Layers are drawn

from back to front just like views and specify their geometry relative to their superlayer, creating a local

coordinate system. However, layers allow a more complex visual display by incorporating transform matrices

that allow you to rotate, skew, scale, and project the layer content. “Layer Geometry and Transforms” (page

21) discusses layer geometry and transforms in more detail.

CALayer diverges from the Application Kit and Cocoa Touch view classes in that it is not necessary to subclass

CALayer in order to display content. The content displayed by a CALayer instance can be provided by:

● Setting the layer’s content property to a Core Graphics image representation directly, or through

delegation.

● Providing a delegate that draws directly into a Core Graphics image context.

Core Animation Classes

What Is Core Animation?

● Setting any of the number of visual style properties that all layer types have in common, for example,

background colors, opacity, and masking. Mac OS X applications also have access to visual properties

that make use of Core Image filters.

● Subclassing CALayer and implementing any of the above techniques in a more encapsulated manner.

“Providing Layer Content” (page 33) describes the available techniques for providing the content for a layer.

The visual style properties and the order in which they are applied to the content of a layer is discussed in

“Layer Style Properties” (page 59).

In addition to the CALayer class, the Core Animation class collection provides additional classes that allow

applications to display other types of content. The available classes differ slightly between Mac OS X and

iOS. The following classes are available on both Mac OS X and iOS:

● CAScrollLayer class is a subclass of CALayer that simplifies displaying a portion of a layer. The extent

of the scrollable area of a CAScrollLayer object is defined by the layout of its sublayers. CAScrollLayer

does not provide keyboard or mouse event-handling, nor does it provide visible scrollers.

● CATextLayer is a convenience class that creates a layer's content from a string or attributed string.

● CATiledLayer allows the display of large and complex images in incremental stages.

Mac OS X provides these additional classes:

● CAOpenGLLayer provides an OpenGL rendering environment. You must subclass this class to provide

content using OpenGL. The content can be static or can be updated over time.

● QCCompositionLayer (provided by the Quartz Composer framework) animates a Quartz Composer

composition as its content.

● QTMovieLayer and QTCaptureLayer (provided by the QTKit framework) provides playback of QuickTime

movies and live video.

iOS adds the following class:

● CAEAGLLayer provides an OpenGLES rendering environment.

The CALayer class introduces the concept of a key-value coding compliant container class–that is, a class

that can store arbitrary values, using key-value coding compliant methods, without having to create a subclass.

CALayer also extends the NSKeyValueCoding informal protocol, adding support for default key values and

automatic object wrapping for the additional structure types (CGPoint, CGSize, CGRect,

CGAffineTransform and CATransform3D) and provides access to many of the fields of those structures

by key path.

CALayer also manages the animations and actions that are associated with a layer. Layers receive action

triggers in response to layers being inserted and removed from the layer tree, modifications being made to

layer properties, or explicit developer requests. These actions typically result in an animation occurring. See

“Animation” (page 39) and “Layer Actions” (page 43) for more information.

Core Animation Classes 15

What Is Core Animation?

Animation and Timing Classes

Many of the visual properties of a layer are implicitly animatable. By simply changing the value of an animatable

property the layer will automatically animate from the current value to the new value. For example, setting

a layer's hidden property to YES triggers an animation that causes the layer to gradually fade away. Most

animatable properties have an associated default animation which you can easily customize and replace. A

complete list of the animatable properties and their default animations are listed in “Animatable

Properties” (page 79).

Animatable properties can also be explicitly animated. To explicitly animate a property you create an instance

of one of Core Animation’s animation classes and specify the required visual effects. An explicit animation

doesn’t change the value of the property in the layer, it simply animates it in the display.

Core Animation provides animation classes that can animate the entire contents of a layer or selected

attributes using both basic animation and key-frame animation. All Core Animation's animation classes

descend from the abstract class CAAnimation. CAAnimation adopts the CAMediaTiming protocol which

provides the simple duration, speed, and repeat count for an animation. CAAnimation also adopts the

CAAction protocol. This protocol provides a standardized means for starting an animation in response to

an action triggered by a layer.

The animation classes also define a timing function that describes the pacing of the animation as a simple

Bezier curve. For example, a linear timing function specifies that the animation's pace is even across its

duration, while an ease-out timing function causes an animation to slow down as it nears its duration.

Core Animation provides a number of additional abstract and concrete animation classes:

● CATransition provides a transition effect that affects the entire layer's content. It fades, pushes, or

reveals layer content when animating. The stock transition effects can be extended by providing your

own custom Core Image filters.

● CAAnimationGroup allows an array of animation objects to be grouped together and run concurrently.

● CAPropertyAnimation is an abstract subclass that provides support for animating a layer property

specified by a key path.

● CABasicAnimation provides simple interpolation for a layer property.

● CAKeyframeAnimation provides support for key frame animation. You specify the key path of the layer

property to be animated, an array of values that represent the value at each stage of the animation, as

well as arrays of key frame times and timing functions. As the animation runs, each value is set in turn

using the specified interpolation.

These animation classes are used by both Core Animation and Cocoa Animation proxies. “Animation” (page

39) describes the classes as they pertain to Core Animation, Animation Types and Timing Programming Guide

contains a more in-depth exploration of their capabilities.

Layout Manager Classes

Application Kit view classes provide the classic "struts and springs" model of positioning layers relative to

their superlayer. While layers support this model, Core Animation on Mac OS X also provides a more flexible

layout manager mechanism that allows developers to write their own layout managers.

Core Animation Classes

What Is Core Animation?

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