<timing-function>

<timing-function> CSS 数据类型表示一个数学函数,它描述了在一个过渡或动画中一维数值的改变速度。这实质上让你可以自己定义一个加速度曲线,以便动画的速度在动画的过程中可以进行改变。这些函数通常被称为缓动函数。

这是一个表示时间输出比率的函数,表示为<number>0, 0 代表初始状态,1, 1 代表终止状态。

输出比可以大于1.0(或者小于0.0)。这是因为在一种反弹效果中,动画是可以比最后的状态走的更远的,然后再回到最终状态。

不过,如果输出值超过了它允许的范围,比如组成一个颜色的值大于了255或者小于了0,这个值会被修改为允许范围内的最接近的值(在颜色值这个例子中分别为255和0)。一些贝塞尔曲线展示了这些性质。

定时函数

CSS 支持两种定时函数:立方贝塞尔曲线的子集和阶梯函数。这些函数中最有用的是一个关键字,可以很容易地引用它们。

cubic-bezier() 定时函数

cubic-bezier() 定义了一条 立方贝塞尔曲线(cubic Bézier curve)。这些曲线是连续的,一般用于动画的平滑变换,也被称为缓动函数(easing functions)。

一条立方贝塞尔曲线需要四个点来定义,P0 、P1 、P2 和 P3。P0 和 P3 是起点和终点,这两个点被作为比例固定在坐标系上,横轴为时间比例,纵轴为完成状态。P0 是 (0, 0),表示初始时间和初始状态。P3 是 (1, 1) ,表示终止时间和终止状态。

并非所有的三次贝塞尔曲线都适合作为计时函数,也并非所有的曲线都是数学函数,即给定横坐标为0或1的曲线。在CSS定义的P0和P3固定的情况下,三次贝塞尔曲线是一个函数,因此,当且仅当P1和P2的横坐标都在[0,1]范围内时,三次贝塞尔曲线是有效的。

Cubic Bézier curves with the P1 or P2 ordinate outside the [0, 1] range may generate bouncing effects.

When you specify an invalid cubic-bezier curve, CSS ignores the whole property.

语法

cubic-bezier(x1, y1, x2, y2)

其中,

x1, y1, x2, y2
Are <number> values representing the abscissas and ordinates of the P1 and P2 points defining the cubic Bézier curve. x1 and x2 must be in the range [0, 1] or the value is invalid.

示例

CSS 中允许使用这些贝塞尔曲线:

cubic-bezier(0.1, 0.7, 1.0, 0.1)   The canonical Bézier curve with four <number> in the [0,1] range.
cubic-bezier(0, 0, 1, 1)           Using <integer> is valid as any <integer> is also a <number>.
cubic-bezier(0.3, -1.9, 2.1, -0.2) Negative values for ordinates are valid, leading to bouncing effects.
cubic-bezier(0.1, 4, 0.6, 2.45)    Values > 1.0 for ordinates are also valid.

These cubic Bézier curves definitions are invalid :

cubic-bezier(0.1, red, 1.0, green) Though the animated output type may be a color, Bézier curves work w/ numerical ratios.
cubic-bezier(-0.2, 0.6, -0.1, 0)   Abscissas must be in the [0, 1] range or the curve is not a function of time.
cubic-bezier(0.3, 2.1)             The two points must be defined, there is no default value.
cubic-bezier(1.1, 0, 4, 0)         Abscissas must be in the [0, 1] range or the curve is not a function of time.

The steps() class of timing-functions

The steps() functional notation defines a step function dividing the domain of output values in equidistant steps. This subclass of step functions are sometimes also called staircase functions.
steps(2, start) steps(4, end)

Syntax

steps(number_of_steps, direction)

where :

number_of_steps
Is a strictly positive <integer> representing the amount of equidistant treads composing the stepping function.
direction
Is a keyword indicating if it the function is left- or right-continuous:
  • start denotes a left-continuous function, so that the first step happens when the animation begins;
  • end denotes a right-continuous function, so that the last step happens when the animation ends.

Examples

These timing functions are valid :

steps(5, end)          There is 5 treads, the last one happens right before the end of the animation.
steps(2, start)        A two-step staircase, the first one happening at the start of the animation.

These timing function are invalid :

steps(2.0, end)        The first parameter must be an <integer> and cannot be a real value, even if it is equal to one.
steps(-3, start)       The amount of steps must be non-negative.
steps(0, end)          There must be at least one step.
steps(2)               The second parameter is not optional.
steps(start, 3)        Though of different types, the order of parameter is important.
step(1, end)           Even if there is one step, the function name is steps, with the plural 's'
steps(3 end)           The two parameters must be separated with a comma; one or several spaces is not enough.

Keywords for common timing-functions

linear

This keyword represents the timing function cubic-bezier(0.0, 0.0, 1.0, 1.0). Using this timing function, the animation goes from its initial state to its final one regularly, with a constant speed.

ease

This keyword represents the timing function cubic-bezier(0.25, 0.1, 0.25, 1.0). This function is similar to ease-in-out, though it accelerates more sharply at the beginning and the acceleration already starts to slow down near the middle of it.

ease-in

This keyword represents the timing function cubic-bezier(0.42, 0.0, 1.0, 1.0). The animation begins slowly, then progressively accelerates until the final state is reached and the animation stops abruptly.

ease-in-out

This keyword represents the timing function cubic-bezier(0.42, 0.0, 0.58, 1.0). With this timing function, the animation starts slowly, accelerates than slows down when approaching to its final state. At the begin, it behaves similarly to the ease-in function, at the end, it is similar to the ease-out function.

ease-out

This keyword represents the timing function cubic-bezier(0.0, 0.0, 0.58, 1.0). The animation starts quickly then slow progressively down when approaching to its final state.

step-start

This keyword represents the timing function steps(1, start). Using this timing function, the animation jumps immediately to the end state and stay in that position until the end of the animation.

step-end

This keyword represents the timing function steps(1, end). Using this timing function, the animation stays in its initial state until the end, where it directly jumps to its final position.

Specifications

Specification Status Comment
CSS Transitions
<timing-function>
Working Draft Defined anonymously
CSS Animations
<timing-function>
Working Draft Defined anonymously, says to see definition in the CSS Transitions Module

浏览器兼容性

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
<timing-function>Chrome Full support 4Edge Full support 12Firefox Full support 4IE Full support 10Opera Full support 10.5Safari Full support 3.1WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 11Safari iOS Full support 2Samsung Internet Android Full support 1.0
cubic-bezier() with ordinate ∉ [0,1]Chrome Full support 16Edge Full support 12Firefox Full support 4IE Full support 10Opera Full support 12.1Safari Full support 6WebView Android Full support 4.4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 14Safari iOS Full support 6Samsung Internet Android Full support 1.0
steps() with start, end or no directionChrome Full support 8Edge Full support 12Firefox Full support 4IE Full support 10Opera Full support 12.1Safari Full support 5.1WebView Android Full support 4Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 14Safari iOS Full support 5Samsung Internet Android Full support 1.0
steps: jump- keywords for steps()
Experimental
Chrome No support NoEdge No support NoFirefox Full support 65IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 65Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.

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