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    timing-function

    概述

    <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。

    这是一个连接时间输出比率的函数,表示为<number>,0,0代表初始状态,1.0代表最终状态。

    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。

    Values

    CSS supports two kinds of timing functions: the subset of the cubic Bézier curves that are functions and staircase functions. The most useful of these functions are given a keyword that allows to easily describe them.

    The cubic-bezier() class of timing-functions

    The cubic-bezier() functional notation defines a cubic Bézier curve. As these curves are continuous, they are often used to smooth down the start and end of the animation and are therefore sometimes called easing functions.

    A cubic Bézier curve is defined by four points P0, P1, P2, and P3. P0 and P3 are the start and the end of the curve and, in CSS these points are fixed as the coordinates are ratios (the abscissa the ratio of time, the ordinate the ratio of the output range). P0 is (0, 0) and represents the initial time and the initial state, P3 is (1, 1) and represents the final time and the final state.

    Not all cubic Bézier curves are suitable as timing functions as not all are mathematical functions, i.e. curves that for a given abscissa have zero or one value. With P0 and P3 fixed as defined by CSS, a cubic Bézier curve is a function, and is therefore valid, if and only if the abscissas of P1 and P2 are both in the [0, 1] range.

    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.

    Syntax

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

    where :

    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.

    Examples

    These cubic Bézier curves are valid for use in CSS :

    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(-0.2, 0.6, -0.1, 0)   Negative values for abscissas are valid, leading to bouncing effects.
    cubic-bezier(1.1, 0, 4, 0)         Values > 1.0 for abscissas 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(0.1, 4,   0.6, 2.45)  Ordinates 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(0.3, -1.9, 2.1, -0.2) Ordinates 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

    Feature Firefox (Gecko) Chrome Internet Explorer Opera Safari
    Basic support 4.0 (2.0) (Yes) 10 10.5 3.1
    cubic-bezier() w/ ordinate ∉ [0,1] 4.0 (2.0) 16.0 ? Not supported Nightly
    steps() 4.0 (2.0) 8.0 ? Not supported Not supported
    Feature Firefox Mobile (Gecko) Android IE Phone Opera Mobile Safari Mobile
    Basic support 4.0 (2.0) (Yes) Not supported 10 2.0
    cubic-bezier() w/ ordinate ∉ [0,1] 4.0 (2.0) Not supported Not supported Not supported Not supported
    steps() 4.0 (2.0) Not supported Not supported Not supported 5.0

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