Canvas 绘图回顾

let canvas = document.getElementById("canvas"),
    context = canvas.getContext("2d");

let cacheCanvas = document.createElement("canvas"),
    cacheContext = cacheCanvas.getContext("2d");

context.drawImage(cacheCanvas, 0, 0); 

context.font = 'bold 35px Arial';
context.textAlign = 'center';
context.textBaseline = 'bottom';
context.fillStyle = '#ccc';
context.strokeText("Hello Canvas", 150, 100, 200);
context.fillText("Hello Canvas", 180, 140);

let img = new Image();
img.src = '';
if(img.complete) {
    context.drawImage(img, 0, 0); 
} else {
    img.onload = function(){
     context.drawImage(img, 0, 0); 
   };
   img.onerror = function(){
     alert('加载失败，请重试');
   };
}