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低成本碰撞动画实现

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crash-ball.gif

具体代码

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<html>

<head>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
        html,
        body {
            padding: 0;
            margin: 0;
        }

        .horizontal {
            display: block;
            position: absolute;
            top: 0;
            left: 0;

            -webkit-animation: horizontal 1.4s linear 0s infinite alternate;
            -moz-animation: horizontal 1.4s linear 0s infinite alternate;
            -o-animation: horizontal 1.4s linear 0s infinite alternate;
            animation: horizontal 1.4s linear 0s infinite alternate;
        }

        .vertical {
            display: block;
            position: absolute;
            top: 0;
            left: 0;

            -webkit-animation: vertical 1.1s ease-in 0s infinite alternate;
            -moz-animation: vertical 1.1s ease-in 0s infinite alternate;
            -o-animation: vertical 1.1s ease-in 0s infinite alternate;
            animation: vertical 1.1s ease-in 0s infinite alternate;
        }

        .ball {
            width: 50px;
            height: 50px;
            border-radius: 50%;
            background-color: red;
        }
    </style>
</head>

<body>
    <div id="horizontal" class="">
        <div id="vertical" class="">
            <div class="ball"></div>
        </div>
    </div>
    <script src="https://cdn.bootcss.com/jquery/2.0.3/jquery.js"></script>
    <script>
        $ph = $("body").height();
        $pw = $("body").width();
        $ballR = $(".ball").width();

        $("<style></style>").text(
            "@-webkit-keyframes horizontal {0% {transform: translate(0, 0);}100% {transform: translate(" + ($pw -
                $ballR) + "px, 0);}}" +
            "@keyframes horizontal {0% { transform: translate(0, 0);}100% {transform: translate(" + ($pw - $ballR) +
            "px, 0);}}" +
            "@-webkit-keyframes vertical {0% {transform: translate(0, 0);}100% {transform: translate(0, " + ($ph -
                $ballR) + "px);}}" +
            "@keyframes vertical {0% { transform: translate(0, 0);}100% {transform: translate(0, " + ($ph - $ballR) +
            "px);}}"
        ).appendTo($("head"));

        $('#horizontal').addClass('horizontal');
        $('#vertical').addClass('vertical');
    </script>
</body>

</html>

动画解析

先看 html 结构：

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<div id="horizontal" class="">
  <div id="vertical" class="">
    <div class="ball"></div>
  </div>
</div>

然后看两个动画：

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@-webkit-keyframes horizontal {
    0% {
        transform: translate(0, 0);
    }
    100% {
        transform: translate(290px, 0);
    }
}

@keyframes horizontal {
    0% {
        transform: translate(0, 0);
    }
    100% {
        transform: translate(290px);
    }
}

@-webkit-keyframes vertical {
    0% {
        transform: translate(0, 0);
    }
    100% {
        transform: translate(0, 630px);
    }
}

@keyframes vertical {
    0% {
        transform: translate(0, 0);
    }
    100% {
        transform: translate(0, 630px);
    }
}

显然，包裹 div.ball 的两个元素，div#horizontal 负责 x 轴平移，div#vertical 负责 y 轴平移，x 轴、y 轴两种平移混合，就实现各种斜移。

接下来是动画属性设置：

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.horizontal {
    display: block;
    position: absolute;
    top: 0;
    left: 0;

    -webkit-animation: horizontal 1.4s linear 0s infinite alternate;
    -moz-animation: horizontal 1.4s linear 0s infinite alternate;
    -o-animation: horizontal 1.4s linear 0s infinite alternate;
    animation: horizontal 1.4s linear 0s infinite alternate;
}

.vertical {
    display: block;
    position: absolute;
    top: 0;
    left: 0;

    -webkit-animation: vertical 1.1s ease-in 0s infinite alternate;
    -moz-animation: vertical 1.1s ease-in 0s infinite alternate;
    -o-animation: vertical 1.1s ease-in 0s infinite alternate;
    animation: vertical 1.1s ease-in 0s infinite alternate;
}

对于碰撞运动而言，在 y 轴上，始终有重力的影响。

所以，y 轴动画 animation-timing-function 属性被设置为 ease-in，从开始到结束逐渐加速(从高处到低处逐渐加速)；然后，animation-direction 设置为 alternate，则动画正向播完后，会反向播出，则反向播出时，结束到开始回事逐渐减速(从低处到高处逐渐减速)；最后，animation-iteration-count 设置为 infinite，动画会无限循环。

至于 x 轴，其不受其他力的影响，所以在 x 轴方向上，速度不变，所以 animation-timing-function 属性被设置为 linear，从开始到结束速度不变。其余属性，与 y 轴一样。

另外，x 轴和 y 轴的具体移动距离，这个由 javascript 计算得出:

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$ph = $("body").height();
$pw = $("body").width();
$ballR = $(".ball").width();

$("<style></style>")
  .text(
    "@-webkit-keyframes horizontal {0% {transform: translate(0, 0);}100% {transform: translate(" +
      ($pw - $ballR) +
      "px, 0);}}" +
      "@keyframes horizontal {0% { transform: translate(0, 0);}100% {transform: translate(" +
      ($pw - $ballR) +
      "px, 0);}}" +
      "@-webkit-keyframes vertical {0% {transform: translate(0, 0);}100% {transform: translate(0, " +
      ($ph - $ballR) +
      "px);}}" +
      "@keyframes vertical {0% { transform: translate(0, 0);}100% {transform: translate(0, " +
      ($ph - $ballR) +
      "px);}}"
  )
  .appendTo($("head"));

至此，低成本碰撞动画简述完毕。

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