控制动画的节奏。可以使用系统提供的，也可以自定义。
/* CoreAnimation - CAMediaTimingFunction.h
Copyright (c) 2006-2016, Apple Inc.
All rights reserved. */
#import <QuartzCore/CAMediaTiming.h>
#import <Foundation/NSObject.h>
@class NSArray, NSString;
NS_ASSUME_NONNULL_BEGIN
/* Represents one segment of a function describing a timing curve. The
* function maps an input time normalized to the range [0,1] to an
* output time also in the range [0,1]. E.g. these functions are used
* to define the pacing of an animation over its duration (or over the
* duration of one keyframe). */
CA_CLASS_AVAILABLE (10.5, 2.0, 9.0, 2.0)
@interface CAMediaTimingFunction : NSObject <NSCoding>
{
@private
struct CAMediaTimingFunctionPrivate *_priv;
}
// 可以通过以下方法去自定义
/* A convenience method for creating common timing functions. The
* currently supported names are `linear', `easeIn', `easeOut' and
* `easeInEaseOut' and `default' (the curve used by implicit animations
* created by Core Animation). */
+ (instancetype)functionWithName:(NSString *)name;
/* Creates a timing function modelled on a cubic Bezier curve. The end
* points of the curve are at (0,0) and (1,1), the two points 'c1' and
* 'c2' defined by the class instance are the control points. Thus the
* points defining the Bezier curve are: '[(0,0), c1, c2, (1,1)]' */
+ (instancetype)functionWithControlPoints:(float)c1x :(float)c1y :(float)c2x :(float)c2y;
- (instancetype)initWithControlPoints:(float)c1x :(float)c1y :(float)c2x :(float)c2y;
/* 'idx' is a value from 0 to 3 inclusive. */
- (void)getControlPointAtIndex:(size_t)idx values:(float[2])ptr;
@end
// 系统提供的，当然也可以自定义
/** Timing function names. **/
// 匀速，线性
CA_EXTERN NSString * const kCAMediaTimingFunctionLinear
CA_AVAILABLE_STARTING (10.5, 2.0, 9.0, 2.0);
// 慢进快出，渐进
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseIn
CA_AVAILABLE_STARTING (10.5, 2.0, 9.0, 2.0);
// 快进慢出，渐出
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseOut
CA_AVAILABLE_STARTING (10.5, 2.0, 9.0, 2.0);
// 慢进慢出，中间加速，渐进渐出。这个是默认的动画行为。
CA_EXTERN NSString * const kCAMediaTimingFunctionEaseInEaseOut
CA_AVAILABLE_STARTING (10.5, 2.0, 9.0, 2.0);
// 默认 kCAMediaTimingFunctionEaseInEaseOut
CA_EXTERN NSString * const kCAMediaTimingFunctionDefault
CA_AVAILABLE_STARTING (10.6, 3.0, 9.0, 2.0);
NS_ASSUME_NONNULL_END