# transition-timing-function

CSS の `transition-timing-function` プロパティは、トランジション効果の作用を受ける CSS プロパティにおいて、中間状態の値を算出する方法を設定するために使用されます。

このプロパティは、簡単に言えば加速曲線を定義するもので、それによりトランジション実行中の値の変更速度を操作することができます。

この加速曲線は、トランジションが行われるプロパティごとに 1 つの `<timing-function>` を用いて定義されます。

## 構文

```/* キーワード値 */
transition-timing-function: ease;
transition-timing-function: ease-in;
transition-timing-function: ease-out;
transition-timing-function: ease-in-out;
transition-timing-function: linear;
transition-timing-function: step-start;
transition-timing-function: step-end;

/* 関数値 */
transition-timing-function: steps(4, jump-end);
transition-timing-function: cubic-bezier(0.1, 0.7, 1.0, 0.1);

/* 段階式の関数のキーワード */
transition-timing-function: steps(4, jump-start);
transition-timing-function: steps(10, jump-end);
transition-timing-function: steps(20, jump-none);
transition-timing-function: steps(5, jump-both);
transition-timing-function: steps(6, start);
transition-timing-function: steps(8, end);

/* 複数のタイミング関数 */
transition-timing-function: ease, step-start, cubic-bezier(0.1, 0.7, 1.0, 0.1);

/* グローバル値 */
transition-timing-function: inherit;
transition-timing-function: initial;
transition-timing-function: unset;```

### 値

`<timing-function>`

`ease`
Equal to `cubic-bezier(0.25, 0.1, 0.25, 1.0)`, the default value, increases in velocity towards the middle of the transition, slowing back down at the end.
`linear`
Equal to `cubic-bezier(0.0, 0.0, 1.0, 1.0)`, transitions at an even speed.
`ease-in`
Equal to `cubic-bezier(0.42, 0, 1.0, 1.0)`, starts off slowly, with the transition speed increasing until complete.
`ease-out`
Equal to `cubic-bezier(0, 0, 0.58, 1.0)`, starts transitioning quickly, slowing down the transition continues. •
`ease-in-out`
Equal to `cubic-bezier(0.42, 0, 0.58, 1.0)`, starts transitioning slowly, speeds up, and then slows down again.
`cubic-bezier(p1, p2, p3, p4)`
An author defined cubic-Bezier curve, where the p1 and p3 values must be in the range of 0 to 1.
`steps( n, <jumpterm>)`
Displays the transition along n stops along the transition, displaying each stop for equal lengths of time. For example, if n is 5,  there are 5 steps. Whether the transition holds temporarily at 0%, 20%, 40%, 60% and 80%, on the 20%, 40%, 60%, 80% and 100%, or makes 5 stops between the 0% and 100% along the transition, or makes 5 stops including the 0% and 100% marks (on the 0%, 25%, 50%, 75%, and 100%) depends on which of the following jump terms is used:
`jump-start`
Denotes a left-continuous function, so that the first jump happens when the transition begins;
`jump-end`
Denotes a right-continuous function, so that the last jump happens when the animation ends;
`jump-none`
There is no jump on either end. Instead, holding at both the 0% mark and the 100% mark, each for 1/n of the duration
`jump-both`
Includes pauses at both the 0% and 100% marks, effectively adding a step during the transition time.
`start`
Same as `jump-start.`
`end`
Same as `jump-end.`
`step-start`
Equal to `steps(1, jump-start)`
`step-end`
Equal to `steps(1, jump-end)`

### 形式文法

`<easing-function>#ここで`<easing-function> = linear | <cubic-bezier-timing-function> | <step-timing-function>`ここで`<cubic-bezier-timing-function> = ease | ease-in | ease-out | ease-in-out | cubic-bezier([0,1]>, <number>, [0,1]>, <number>)<step-timing-function> = step-start | step-end | steps(<integer>[, <step-position>]?)`ここで`<step-position> = jump-start | jump-end | jump-none | jump-both | start | end``

## 例

### Cubic-Bezier examples

``````.ease {
transition-timing-function: ease;
}
.easein {
transition-timing-function: ease-in;
}
.easeout {
transition-timing-function: ease-out;
}
.easeinout {
transition-timing-function: ease-in-out;
}
.linear {
transition-timing-function: linear;
}
.cb {
transition-timing-function: cubic-bezier(0.2,-2,0.8,2);
}``````

### Step examples

``````.jump-start {
transition-timing-function: steps(5, jump-start);
}
.jump-end {
transition-timing-function: steps(5, jump-end);
}
.jump-none {
transition-timing-function: steps(5, jump-none);
}
.jump-both {
transition-timing-function: steps(5, jump-both);
}
.step-start {
transition-timing-function: step-start;
}
.step-end {
transition-timing-function: step-end;
}``````

## アクセシビリティの考慮事項

Some animations can be helpful such as to guide users to understand what actions are expected, to show relationships within the user interface, and to inform users as to what actions have occurred. Animations can help reduce cognitive load, prevent change blindness, and establish better recall in spatial relationships. However, some animations can be problematic for people with cognitive concerns such as Attention Deficit Hyperactivity Disorder (ADHD) and certain kinds of motion can be a trigger for Vestibular disorders, epilepsy, and migraine and Scotopic sensitivity.

Consider providing a mechanism for pausing or disabling animation, as well as using the Reduced Motion Media Query to create a complimentary experience for users who have expressed a preference for no animated experiences.

## 仕様書

CSS Transitions
transition-timing-function の定義

初期値 `ease` すべての要素、`::before` / `::after` 擬似要素 なし 指定通り 離散値

## ブラウザーの互換性

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