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;
}
这只适用于现代浏览器，但我发现它更容易使用一个then，所以请先测试，但是:
ES5
function rafAsync() {
return new Promise(resolve => {
requestAnimationFrame(resolve); //faster than set time out
});
}
function checkElement(selector) {
if (document.querySelector(selector) === null) {
return rafAsync().then(() => checkElement(selector));
} else {
return Promise.resolve(true);
}
}
ES6
async function checkElement(selector) {
const querySelector = null;
while (querySelector === null) {
await rafAsync();
querySelector = document.querySelector(selector);
}
return querySelector;
}
使用
checkElement('body') //use whichever selector you want
.then((element) => {
console.info(element);
//Do whatever you want now the element is there
});
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();
在requestAnimationFrame中中继比在setTimeout中更好。这是我在es6模块和使用承诺的解决方案。
Es6、模块和承诺:
// onElementReady.js
const onElementReady = $element => (
new Promise((resolve) => {
const waitForElement = () => {
if ($element) {
resolve($element);
} else {
window.requestAnimationFrame(waitForElement);
}
};
waitForElement();
})
);
export default onElementReady;
// in your app
import onElementReady from './onElementReady';
const $someElement = document.querySelector('.some-className');
onElementReady($someElement)
.then(() => {
// your element is ready
}
纯js和承诺:
var onElementReady = function($element) {
return new Promise((resolve) => {
var waitForElement = function() {
if ($element) {
resolve($element);
} else {
window.requestAnimationFrame(waitForElement);
}
};
waitForElement();
})
};
var $someElement = document.querySelector('.some-className');
onElementReady($someElement)
.then(() => {
// your element is ready
});
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();
}
如果你想要一个通用的MutationObserver解决方案，你可以使用这个函数
// MIT Licensed
// Author: jwilson8767
/**
* Waits for an element satisfying selector to exist, then resolves promise with the element.
* Useful for resolving race conditions.
*
* @param selector
* @returns {Promise}
*/
export function elementReady(selector) {
return new Promise((resolve, reject) => {
const el = document.querySelector(selector);
if (el) {resolve(el);}
new MutationObserver((mutationRecords, observer) => {
// Query for elements matching the specified selector
Array.from(document.querySelectorAll(selector)).forEach((element) => {
resolve(element);
//Once we have resolved we don't need the observer anymore.
observer.disconnect();
});
})
.observe(document.documentElement, {
childList: true,
subtree: true
});
});
}
来源:https://gist.github.com/jwilson8767/db379026efcbd932f64382db4b02853e
如何使用它的例子
elementReady('#someWidget').then((someWidget)=>{someWidget.remove();});
注意:MutationObserver有很好的浏览器支持;https://caniuse.com/#feat=mutationobserver
voila !：）
