每次用户按下按钮时,我都会在 Canvas 上绘图,但有时图像不会绘制在 Canvas 上。我认为这可能是因为在 context.drawimage 函数运行之前没有及时加载图像,因为有时会绘制一些较小的文件。我已经使用了控制台并检查了资源,所以这是我能想到的唯一问题。
如何避免这个问题?
这是我的 Javascript 代码。
var canvas = document.getElementById("myCanvas");
var context = canvas.getContext("2d");
var questionbg = new Image();
var answerbg = new Image();
//this code is inside a function that is called each time a user presses a button
if(questiontype == "text"){
questionbg.src = "./resources/textquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//if image question
else if(questiontype == "image"){
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//if audio question
else if(questiontype == "audio"){
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
//else it is a video question
else{
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
context.drawImage(questionbg, 0, 0);
}
最佳答案
您应该检查图像是否已加载。如果没有,则监听加载事件。
questionbg.src = "./resources/imageaudiovideoquestionbg.png";
if (questionbg.complete) {
context.drawImage(questionbg, 0, 0);
} else {
questionbg.onload = function () {
context.drawImage(questionbg, 0, 0);
};
}
MDN (Mozilla Doc,顺便说一句,很棒的来源)建议:
function draw() {
var ctx = document.getElementById('canvas').getContext('2d');
var img = new Image();
img.onload = function(){
ctx.drawImage(img,0,0);
ctx.beginPath();
ctx.moveTo(30,96);
ctx.lineTo(70,66);
ctx.lineTo(103,76);
ctx.lineTo(170,15);
ctx.stroke();
};
img.src = '/files/4531/backdrop.png';
}
显然,您不想应用描边或填充。但是,想法是一样的。
关于javascript - HTML5 Canvas - drawImage 并不总是绘制图像,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15504370/