分析JavaScript中CSS關(guān)鍵幀的強(qiáng)大功能

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創(chuàng)建一個(gè)簡(jiǎn)單的關(guān)鍵幀Web Animation

要使用Web Animation API為關(guān)鍵幀動(dòng)畫設(shè)置動(dòng)畫，只需animate()在元素上調(diào)用該函數(shù)：

1	`<font style="vertical-align: inherit;"><font style="vertical-align: inherit;">Element.animate（關(guān)鍵幀，關(guān)鍵幀選項(xiàng)）</font></font>`

該函數(shù)接受兩個(gè)參數(shù)：

keyframes：包含所需CSS關(guān)鍵幀的JavaScript表示的文字?jǐn)?shù)組
keyframeOptions：包含動(dòng)畫的其他設(shè)置的文字，例如緩動(dòng)，持續(xù)時(shí)間，填充模式等。

看一下下面的簡(jiǎn)單示例，該示例使用 animate()函數(shù)而不是CSS關(guān)鍵幀來(lái)渲染動(dòng)畫：

var boxframes = [

{

變換：'translateX（0）'，

background:'red',

borderRadius: 0

},

{

transform:'translateX(200px) scale(.5)', 

background:'orange',

borderRadius: 0,

offset: 0.6/* set explicit point (60%) when frame starts */

},

{

transform:'translateX(400px)',

background:'green',

borderRadius:'50%'

}

]

如果我們使用純CSS聲明上面的內(nèi)容，它看起來(lái)像這樣：

@keyframes animatethebox{

0%{

transform: translateX(0);

background: red;

borderRadius: 0;

}

60%{

transform: translateX(200px) scale(.5);

background: orange;

borderRadius: 0;

}

100%{

transform: translateX(400px);

background: green;

borderRadius: 50%;

}

}

正如您所看到的，這兩種語(yǔ)法非常相似，如果您已經(jīng)熟悉CSS關(guān)鍵幀，那么將它移植到JavaScript就沒(méi)有問(wèn)題。與JavaScript版本的一些區(qū)別值得記住：

在JavaScript版本中，屬性字符串值應(yīng)該在引號(hào)（transform: 'translateX(0)'）中
帶連字符的屬性名稱必須轉(zhuǎn)換為camelCase而不是（borderRadius: 0）。
逗號(hào)而不是分號(hào)應(yīng)該跟蹤每個(gè)屬性聲明（最后一個(gè)屬性除外）

默認(rèn)情況下，使用JavaScript設(shè)置的關(guān)鍵幀在播放時(shí)均勻分布，每個(gè)關(guān)鍵幀的時(shí)間相同。但是，通過(guò)offset在關(guān)鍵幀中添加屬性，我們可以設(shè)置該關(guān)鍵幀應(yīng)該開始播放的點(diǎn)，例如60％標(biāo)記的0.6，類似于使用純CSS的關(guān)鍵幀。

keyframeOptions參數(shù)

該animate()方法的第二個(gè)參數(shù)是一個(gè)文字，可以很好地調(diào)整動(dòng)畫的行為。許多選項(xiàng)直接從CSS的animation-*屬性映射，例如“ animation-delay”，“ animation-fill-mode”等。所有屬性都是可選的，如果沒(méi)有設(shè)置，則回退到默認(rèn)值：


property	CSS Property Equivalent	Description
id	none	Option that sets the name of this Animation to refer back to later in your code.
delay	animation-delay	The delay (integer) before the animation starts in milliseconds. Defaults to 0s.
direction	animation-direction	Defines whether the animation should play as normal, in reverse, or alternate between the two. Possible values are: normal: The animation plays forward as normal. After each animation cycle, the animation resets to the beginning state and starts over again. Default value. reverse: Plays the animation in reverse, starting from the end state. After each animation cycle, the animation resets to the end state and starts over again. alternate: The animation alternates between normal and reverse directions. In reverse, the animation starts at the end state and plays backwards. The animation timing function is also reversed. alternate-reverse: The animation alternates between reverse and normal directions, starting in reverse for the first iteration.
duration	animation-delay	The duration (integer) of the animation in milliseconds, such as 1000. Default to 0 (no animation, jumps to last frame).
easing	animation-timing-function	Sets the easing function used to animate the `@keyframe` animation. Valid values include "`ease`", "`ease-in`", "`ease-in-out`","`linear`", "`frames(integer)`" etc. Defaults to "linear".
endDelay	n/a	The number of milliseconds to delay after the end of an animation. This is useful when sequencing multiple animations based on the end time of another animation. Defaults to 0.
fill	animation-fill-mode	Defines how the animation should apply styles to its target when the animation isn't playing anymore. Defaults to "none". Possible values are: none: The animation should not apply any styles to the target when it is not playing. Default value. forwards: The target element will retain the computed styles defined in the last key frame (ie: when key frame is at 100%) when the animation isn't playing. backwards: The target element will retain the computed styles defined in the first key frame (ie: when key frame is at 0%) when the animation isn't playing. both: The target element will retain the computed styles defined in both the first and last key frames when the animation isn't playing.
iterationStart	n/a	Sets the point in the iteration the animation should start. The value should be a positive, floating point number. In an animation with 1 iteration, a `iterationStart` value of 0.5 starts the animation half way through. In an animation with 2 iterations, a i`iterationStart` value of 1.5 starts the animation half way through the 2nd iteration etc. Defaults to 0.0.
iterations	animation-iteration-count	Sets the number of times the animation should run before stopping. A value of `Infinity` means forever. Defaults to 1.

這是我在上面的例子中使用的keyframeOptions參數(shù)：

var boxref = document.getElementById("box")

boxref.animate(boxframes, {

duration: 1000,

fill:'forwards',

easing:'ease-in'

})

如果我們想使用動(dòng)畫速記屬性在CSS中定義相同的選項(xiàng)，它將如下所示：

1	`animation: animatethebox 1s ease-in` `forwards;`

控制動(dòng)畫（播放，暫停等）

使用Animation API創(chuàng)建關(guān)鍵幀動(dòng)畫的部分優(yōu)點(diǎn)是可以按需操作結(jié)果，例如暫停，跳過(guò)或掛鉤到動(dòng)畫的事件處理程序。執(zhí)行所有這些操作的第一步是在調(diào)用animate()方法時(shí)將動(dòng)畫分配給變量：

var myanimation = Element.animate(keyframes, keyframeOptions)

這將創(chuàng)建對(duì)Animation對(duì)象實(shí)例的引用，以允許我們通過(guò)各種公開的屬性和方法來(lái)操作動(dòng)畫：

var myanimation = Element.animate(/* .. */)

myanimation.pause()// immediately pause animation to control it manually

myanimation.curentTime = 1000// jump to 1 second from start of animation

myanimation.play()// play animation

這是修改后使用控件進(jìn)行回放的原始示例：

請(qǐng)注意，在此示例中，我animate()立即調(diào)用目標(biāo)元素，這將導(dǎo)致動(dòng)畫立即運(yùn)行。為了防止這種情況，我pause()隨后調(diào)用了該方法。這是您希望手動(dòng)控制動(dòng)畫時(shí)使用的常用模式：

var boxanimation = boxref.animate(boxframes, {

duration: 1000,

fill:'both',

easing:'ease-in'

})

boxanimation.pause()

動(dòng)畫對(duì)象實(shí)例屬性和方法

下面列出了動(dòng)畫對(duì)象實(shí)例的屬性，方法和事件處理程序，如上所述，在為animate()方法分配引用時(shí)創(chuàng)建：

屬性

currentTime: Gets or sets the current time value of the animation in milliseconds.
effect: Gets or sets the target effect of an animation. Support for this property is currently limited in all browsers.
finished: A promise object that's resolved when the animation has completed. Support for this property is currently limited in all browsers.
id: Gets or sets a string used to identify the animation.
playbackRate: Integer that gets or sets a playback rate of the animation. For example, 1=normal, 0 = pause, 2 = double, -1 = backwards etc.
playState: Read-only property that returns the current state of the animation: "idle", "pending", "running", "paused", or "finished".
ready: A promise object that's resolved when the animation is ready to be played. Support for this property is currently limited in all browsers.
startTime: Floating point number that gets or sets the current time (in milliseconds) of the animation.
timeline: Gets or sets the current timeline of the animation. Defaults to the document timeline (document.timeline). Support for this property is currently limited in all browsers.

方法

cancel(): Cancels the animation.
finish(): Immediately completes an animation.
pause(): Pauses an animation.
play(): Plays an animation.
reverse(): Reverses the current direction of the animation and plays it.

事件處理程序

oncancel: Triggered when the animation is canceled, such as by calling the cancel() method.
onfinish: Triggered when the animation is finished, such as by calling the finish() method.

使用Web Animation API創(chuàng)建簡(jiǎn)單的scrubber

通過(guò)操作currentTime屬性，下面為我們看到的基本動(dòng)畫添加了一個(gè)簡(jiǎn)單的擦除器：

我創(chuàng)建了一個(gè)HTML5 Range Slider來(lái)用作scrubber。當(dāng)動(dòng)畫首次運(yùn)行（自動(dòng)）時(shí)，動(dòng)畫的currentTime 屬性值將持續(xù)傳送到滑塊，以使兩者同步。目前沒(méi)有“onprogress”事件處理程序或類似的東西（據(jù)我所知）只在WAAPI動(dòng)畫運(yùn)行時(shí)運(yùn)行代碼，所以我用它來(lái)監(jiān)視動(dòng)畫的進(jìn)度。動(dòng)畫結(jié)束后，我利用WAAPI事件調(diào)用并不必要地停止更新滑塊。 requestAnimationFrame()onfinishcancelAnimationFrame()

每當(dāng)用戶與Ranger Slider交互時(shí)，我都會(huì)更新WAAPI動(dòng)畫以與滑塊同步：

scrubber.addEventListener('mousedown', ()=>{

boxanimation.pause()

updateScrubber()

})

scrubber.addEventListener('mouseup', ()=>{

boxanimation.play()

})

scrubber.addEventListener('input', ()=>{

boxanimation.currentTime = scrubber.value * animationlength

})

當(dāng)用戶按下滑塊時(shí)，我暫停動(dòng)畫并更新滑塊的值以與動(dòng)畫的currentTime 屬性同步。當(dāng)用戶拖動(dòng)滑塊時(shí)，反過(guò)來(lái)發(fā)生 - 我同步currentTime屬性以反映滑塊的值，因此前者依賴于后者。最后，當(dāng)用戶將鼠標(biāo)放在滑塊上時(shí)，我恢復(fù)動(dòng)畫的自動(dòng)播放。

一次動(dòng)畫多個(gè)元素

在下一個(gè)示例中，我將使用WAAPI一次演示動(dòng)畫多個(gè)元素，并在它們?nèi)拷Y(jié)束后執(zhí)行動(dòng)作。

注意：在撰寫本文時(shí)，即使在Chrome和FF中，對(duì)WAAPI 承諾的原生支持也很不穩(wěn)定。我不得不使用 Web Animation Next Polyfill來(lái)使這個(gè)功能在瀏覽器中運(yùn)行。

這里沒(méi)有什么太復(fù)雜的事了。基本上我循環(huán)并調(diào)用animate() 標(biāo)題中的每個(gè)字母，并將每個(gè)Animation對(duì)象實(shí)例存儲(chǔ)在數(shù)組中。有了這個(gè)數(shù)組，我可以根據(jù)需要循環(huán)播放一系列動(dòng)畫。每個(gè)動(dòng)畫的finished屬性都返回一個(gè) Promise，該動(dòng)畫在動(dòng)畫完成播放時(shí)被解析，我利用 Promise.all（）在所有動(dòng)畫完成播放時(shí)重置整個(gè)動(dòng)畫。

使用`Animation()`構(gòu)造函數(shù)創(chuàng)建動(dòng)畫

到目前為止，我只使用animate()對(duì)象直接在一個(gè)元素上創(chuàng)建了WAAPI動(dòng)畫，該元素返回一個(gè)Animation對(duì)象實(shí)例。我會(huì)失職但更不用說(shuō)你也可以使用Animation() 構(gòu)造函數(shù)來(lái)完成同樣的事情。

注意：Animation()在撰寫本文時(shí)，即使在Chrome和FF中，本機(jī)支持也是不穩(wěn)定的。我不得不使用 Web Animation Next Polyfill來(lái)使這個(gè)功能在瀏覽器中運(yùn)行。

有了警告，這里是如何調(diào)用 Animation() 構(gòu)造函數(shù)：

1	`var` `myanimation =new` `Animation([effect][, timeline]);`

該函數(shù)接受兩個(gè)參數(shù)：

effect：動(dòng)畫效果。在撰寫本文時(shí)，僅keyframeEffect支持該對(duì)象。
timeline：動(dòng)畫時(shí)間軸。在撰寫本文時(shí)，僅document.timeline支持。

讓我們看一下如何使用一個(gè)簡(jiǎn)單的例子：

這是JavaScript代碼：

var boxref = document.getElementById("box")

var boxkeyFrames =new KeyframeEffect( 

boxref,// element to animate

[

{ transform:'translateX(0) rotate(0deg)', background:'red' },// keyframe 

{ transform:'translateX(90vw) rotate(180deg)', background:'blue' }

], 

{ duration: 2000, fill:'forwards', iterations: 5 }// keyframe options

);

var boxanimation =new Animation(boxkeyFrames, document.timeline)

boxanimation.play()

新KeyframeEffect()對(duì)象是一個(gè)一體化對(duì)象，它包含一個(gè)位置的動(dòng)畫的所有設(shè)置，從目標(biāo)元素，要使用的關(guān)鍵幀到關(guān)鍵幀選項(xiàng)。

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分析JavaScript中CSS關(guān)鍵幀的強(qiáng)大功能

創(chuàng)建一個(gè)簡(jiǎn)單的關(guān)鍵幀Web Animation

keyframeOptions參數(shù)

控制動(dòng)畫（播放，暫停等）

動(dòng)畫對(duì)象實(shí)例屬性和方法

使用Web Animation API創(chuàng)建簡(jiǎn)單的scrubber

一次動(dòng)畫多個(gè)元素

使用Animation()構(gòu)造函數(shù)創(chuàng)建動(dòng)畫

使用`Animation()`構(gòu)造函數(shù)創(chuàng)建動(dòng)畫