vue+canvas踩坑之旅

               let img=new Image();
               if(img.complete) {  
                   console.log('dd');  
               }  

               img.src="http://localhost:8888/static/img/logo11.6d2b322.png"

               img.onload = function() {
 console.log('ff')  
               }

首先，一张图片在定义 img.src="http://localhost:8888/static/img/logo11.6d2b322.png"的时候，img就已经结束了它的渲染，那么自然而然不会进到下面的onload的函数。

好的，现在把图片往下调就可以

let img=new Image();
               if(img.complete) {  
                   console.log('dd');  
               }  
            
               img.onload = function() {
                    console.log('ff')  
               }

   img.src="http://localhost:8888/static/img/logo11.6d2b322.png"

那么一次完整的vue+canvas绘制图片的过程就如下所示：

              let cans=document.getElementById("myCanvas");
              let ctx=cans.getContext("2d");
              let img=new Image();

               img.onload = function(){
           
                ctx.drawImage(img,50,50,100,100);
                ctx.strokeStyle = "#0695FF";        //定义矩形的颜色
                ctx.strokeRect(50,50,100,100);
                ctx.strokeRect(50,170,100,100);
              };
                img.src="http://localhost:8888/static/img/logo11.6d2b322.png"
                ctx.drawImage(img, 10, 10);   

但是这个毕竟还是太扑街了，最后成功的demo如下

<template>
    <div>
        <h2>
            TestCanvas
        </h2>

        <img
            v-show="false"
            ref="myImg"
            src="../assets/logo.png"
        >
        <canvas
            ref="myCanvas"
            width="200"
            height="100"
            
        >
            您的浏览器不支持 HTML5 canvas 标签。
        </canvas>
        <button
            @click="downloadCanvas">
            下载
        </button>
    </div>
</template>

<script>
export default {
    name: "TestCanvas" ,
    data () {
        return {

        }
    } ,
    methods: {
        alert : function () {
            alert("emem") ;
        } ,
        downloadCanvas : function () {
            // 内部函数1(可忽略细节)
            const saveFile = function(data, fileName){
                let save_link = document.createElement('a');
                save_link.href = data;
                save_link.download = fileName;
        
                let event = document.createEvent('MouseEvents');
                event.initEvent("click", true, false);
                save_link.dispatchEvent(event);
            };
            // 内部函数2(可忽略细节)
            const imgType = function (ty) {
                let type = ty.toLowerCase().replace(/jpg/i, 'jpeg');
                var r = type.match(/png|jpeg|bmp|gif/)[0];
                return 'image/' + r;
            } ;

            const type = 'png';   //设置下载图片的格式
            const cans = this.$refs.myCanvas ; //canvas实例子
            const img_png_src = cans.toDataURL("image/png");  //将canvas保存为图片
            const imgData = img_png_src.replace(imgType(type),'image/octet-stream');
            const filename = 'canvas' + '.' + type; //下载图片的文件名
    
            // shoot
            saveFile(imgData,filename);

        }
    } ,
    mounted: function () {
        console.log("挂载成功") ;
        var c=this.$refs.myCanvas ;
        var ctx=c.getContext("2d");
        var img=this.$refs.myImg;
        // 贴图
        ctx.drawImage(img,0,0);
        ctx.font="10px Arial";
        // 打水印
        ctx.fillText("Kingdee",160,90);
    }
}
</script>