text: (window.webpackJsonp=window.webpackJsonp||[]).push([[41],{451:function(t,s,a){"use strict";a.r(s);var n=a(25),p=Object(n.a)({},(function(){var t=this,s=t.$createElement,a=t._self._c||s;return a("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[a("h1",{attrs:{id:"微信小程序canvas"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#微信小程序canvas"}},[t._v("#")]),t._v(" 微信小程序canvas")]),t._v(" "),a("p",[t._v("近期，电商直播业务热火朝天，直播间有一个很重要的互动：点赞。")]),t._v(" "),a("p",[t._v("为了烘托直播间的氛围，直播相对于普通视频或者文本内容，点赞通常有两个特殊需求：")]),t._v(" "),a("p",[t._v("点赞动作次数不限制，引导用户疯狂点赞\n直播间的所有疯狂点赞，都需要在所有用户界面都动画展现出来\n我们先来看点赞效果图：\n"),a("img",{attrs:{src:"https://img-blog.csdnimg.cn/20210427231549948.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L20wXzQ2MTgwNzY0,size_16,color_FFFFFF,t_70",alt:"在这里插入图片描述"}})]),t._v(" "),a("p",[t._v("从效果图上我们还看到有几点重要信息：")]),t._v(" "),a("ul",[a("li",[t._v("点赞动画图片大小不一，运动轨迹也是随机的。")]),t._v(" "),a("li",[t._v("点赞动画图片都是先放大再匀速运动。")]),t._v(" "),a("li",[t._v("快到顶部的时候，逐渐缩小并消失。")]),t._v(" "),a("li",[t._v("收到大量的点赞请求的时候，点赞动画不扎堆，井然有序持续出现。")]),t._v(" "),a("li",[t._v("刚接到这个需求的时候，考虑过CSS 动画实现，但是 CSS 需要手动清除节点来防止节点过多而造成的性能问题。同时 CSS 动画在部分 iphone 机型上会偶现“闪烁”现象。综合考虑，选择用canvas实现。")])]),t._v(" "),a("p",[t._v("下面介绍实现原理和踩过的那些坑。")]),t._v(" "),a("h2",{attrs:{id:"_1-canvas-绘图实现原理"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#_1-canvas-绘图实现原理"}},[t._v("#")]),t._v(" 1 Canvas 绘图实现原理")]),t._v(" "),a("h3",{attrs:{id:"_1-1-初始化"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#_1-1-初始化"}},[t._v("#")]),t._v(" 1.1 初始化")]),t._v(" "),a("p",[t._v("页面元素上新建 canvas 标签，初始化 canvas。")]),t._v(" "),a("p",[t._v("canvas 上可以设置 width 和 height 属性，也可以在 style 属性里面设置 width 和 height。")]),t._v(" "),a("p",[t._v("style 中的 width 和 height 分别代表 canvas 这个元素在界面上所占据的宽高，即样式上的宽高。attribute 中的 width 和 height 则代表 canvas 实际像素的宽高。\n使用 Canvas 2D 可以使微信小程序环境中的 Canvas 与 W3C 标准 Canvas 接口更为接近，因而可以解决之前接口实现不一致引起的 bug。并且，Canvas 2D 的同层渲染可以解决图表与其他原生组件覆盖层级的问题。 想要进一步了解同层渲染的原理，可以参考这篇文章——《小程序同层渲染原理剖析》。")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),t._v("canvas id"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"likestar"')]),t._v(" type"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"2d"')]),t._v(" width"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"{{realWidth}}"')]),t._v(" height"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"{{realHeight}}"')]),t._v(" style"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"width: {{ width }}rpx; height: {{ height }}rpx;"')]),t._v(" "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("class")]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),a("span",{pre:!0,attrs:{class:"token string"}},[t._v('"like-fx wr-class"')]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("<")]),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("/")]),t._v("canvas"),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v(">")]),t._v("\n")])])]),a("h3",{attrs:{id:"_1-2-提前加载图片资源"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#_1-2-提前加载图片资源"}},[t._v("#")]),t._v(" 1.2 提前加载图片资源")]),t._v(" "),a("p",[t._v("将需要随机渲染的点赞图片，先预加载，获得图片的宽高，如果有下载失败的，则不显示该随机图片即可。\n"),a("img",{attrs:{src:"https://img-blog.csdnimg.cn/20210427231634638.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L20wXzQ2MTgwNzY0,size_16,color_FFFFFF,t_70",alt:"在这里插入图片描述"}})]),t._v(" "),a("p",[t._v("由于 canvas 不是矢量图，而是像图片一样是位图模式的。高 dpi 显示设备意味着每平方英寸有更多的像素。也就是说二倍屏，浏览器就会以 2 个像素点的宽度来渲染一个像素，该 canvas 在 Retina 屏幕下相当于占据了 2 倍的空间，相当于图片被放大了一倍，因此绘制出来的图片文字等会变模糊。")]),t._v(" "),a("p",[t._v("因此，要做 Retina 屏适配，关键是知道当前屏幕的设备像素比，将 canvas 放大到该设备像素比来绘制，然后将 canvas 压缩到一倍来展示。")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[t._v("    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("// 创建canvas上下文")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("this")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("createSelectorQuery")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\n      "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(".")]),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("select")]),a("span",{pre:!0,attrs:{class:"token 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parameter"}},[t._v("res")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=>")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("if")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("res"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("]")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("{")]),t._v("\n          "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("const")]),t._v(" canvas "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" res"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("[")]),a("span",{pre:!0,attrs:{class:"token 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-10px，那显然，动画就是忽左忽右左右闪烁了。 理想的轨迹是上一个位置是 10px,接下来是 9px，然后一直平滑到 -10px，这样的坐标点就是连贯的，看起来动画就是平滑运行。")]),t._v(" "),a("p",[a("img",{attrs:{src:"https://img-blog.csdnimg.cn/20210427231652904.png",alt:"在这里插入图片描述"}})]),t._v(" "),a("p",[t._v("canvas 动画中常使用贝塞尔曲线来平滑路径，本次动画应用的是四阶贝塞尔曲线，如上图所示。贝塞尔曲线原理可参考下面两篇文章：")]),t._v(" "),a("ul",[a("li",[t._v("深入理解贝塞尔曲线")]),t._v(" "),a("li",[t._v("如何理解并应用贝塞尔曲线\n"),a("img",{attrs:{src:"https://img-blog.csdnimg.cn/20210427231701279.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L20wXzQ2MTgwNzY0,size_16,color_FFFFFF,t_70",alt:"在这里插入图片描述"}})])]),t._v(" "),a("p",[t._v("首先，随机生成四个点，作为气泡的运动路径，为了保证气泡的随机性，使气泡不扎堆，将每个点的位置范围都设置为一个区间。")]),t._v(" "),a("div",{staticClass:"language-js extra-class"},[a("pre",{pre:!0,attrs:{class:"language-js"}},[a("code",[t._v("    "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("generatePathData")]),a("span",{pre:!0,attrs:{class:"token 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