我试图在另一个画布上添加一个画布-我如何使这个函数等待开始，直到第一个画布被创建?
function PaintObject(brush) {
this.started = false;
// get handle of the main canvas, as a DOM object, not as a jQuery Object. Context is unfortunately not yet
// available in jquery canvas wrapper object.
var mainCanvas = $("#" + brush).get(0);
// Check if everything is ok
if (!mainCanvas) {alert("canvas undefined, does not seem to be supported by your browser");}
if (!mainCanvas.getContext) {alert('Error: canvas.getContext() undefined !');}
// Get the context for drawing in the canvas
var mainContext = mainCanvas.getContext('2d');
if (!mainContext) {alert("could not get the context for the main canvas");}
this.getMainCanvas = function () {
return mainCanvas;
}
this.getMainContext = function () {
return mainContext;
}
// Prepare a second canvas on top of the previous one, kind of second "layer" that we will use
// in order to draw elastic objects like a line, a rectangle or an ellipse we adjust using the mouse
// and that follows mouse movements
var frontCanvas = document.createElement('canvas');
frontCanvas.id = 'canvasFront';
// Add the temporary canvas as a second child of the mainCanvas parent.
mainCanvas.parentNode.appendChild(frontCanvas);
if (!frontCanvas) {
alert("frontCanvas null");
}
if (!frontCanvas.getContext) {
alert('Error: no frontCanvas.getContext!');
}
var frontContext = frontCanvas.getContext('2d');
if (!frontContext) {
alert("no TempContext null");
}
this.getFrontCanvas = function () {
return frontCanvas;
}
this.getFrontContext = function () {
return frontContext;
}
这是一种纯粹的基于承诺的JavaScript方法，您可以知道需要等待多少毫秒。
const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
return new Promise((resolve) => {
var waited = 0;
var el = null;
var wi = setInterval(function() {
el = document.querySelector(query);
if (waited >= ms || el) {
clearInterval(wi);
if(el) {
resolve(el);
} else {
resolve(null);
}
}
waited += 10;
}, 10);
});
}
要使用该函数，只需在异步函数中使用以下代码。
var element = await waitElementFor('#elementID');
代码片段:
const waitElementFor = function(query, ms = 3000) { // 3000 === 3 seconds
return new Promise((resolve) => {
var waited = 0;
var el = null;
var wi = setInterval(function() {
el = document.querySelector(query);
if (waited >= ms || el) {
clearInterval(wi);
if(el) {
resolve(el);
} else {
resolve(null);
}
}
waited += 10;
}, 10);
});
}
async function snippetTestAyncFunction(){
var element = await waitElementFor('#elementID');
console.log(element);
}
snippetTestAyncFunction();
根据您需要支持的浏览器，可以选择MutationObserver。
编辑:现在所有主流浏览器都支持MutationObserver。
以下内容应该可以达到目的:
// callback executed when canvas was found
function handleCanvas(canvas) { ... }
// set up the mutation observer
var observer = new MutationObserver(function (mutations, me) {
// `mutations` is an array of mutations that occurred
// `me` is the MutationObserver instance
var canvas = document.getElementById('my-canvas');
if (canvas) {
handleCanvas(canvas);
me.disconnect(); // stop observing
return;
}
});
// start observing
observer.observe(document, {
childList: true,
subtree: true
});
注意:我自己还没有测试过这段代码，但这是大概的想法。
您可以轻松地将其扩展为只搜索DOM中发生更改的部分。为此，使用突变参数，它是MutationRecord对象的数组。
只需使用递归的setTimeOut:
waitUntilElementIsPresent(callback: () => void): void {
if (!this.methodToCheckIfElementIsPresent()) {
setTimeout(() => this.waitUntilElementIsPresent(callback), 500);
return;
}
callback();
}
用法:
this.waitUntilElementIsPresent(() => console.log('Element is present!'));
您可以限制尝试次数，因此当元素在限制之后不存在时将抛出错误:
waitUntilElementIsPresent(callback: () => void, attempt: number = 0): void {
const maxAttempts = 10;
if (!this.methodToCheckIfElementIsPresent()) {
attempt++;
setTimeout(() => this.waitUntilElementIsPresent(callback, attempt), 500);
return;
} else if (attempt >= maxAttempts) {
return;
}
callback();
}
