我正在Chrome中开发一个扩展,我想知道:当一个元素出现时,最好的方法是什么?使用纯javascript,间隔检查,直到一个元素存在,或jQuery有一些简单的方法来做到这一点?
当前回答
这是一个纯Javascript函数,它允许你等待任何事情。设置更长的间隔,以占用更少的CPU资源。
/**
* @brief Wait for something to be ready before triggering a timeout
* @param {callback} isready Function which returns true when the thing we're waiting for has happened
* @param {callback} success Function to call when the thing is ready
* @param {callback} error Function to call if we time out before the event becomes ready
* @param {int} count Number of times to retry the timeout (default 300 or 6s)
* @param {int} interval Number of milliseconds to wait between attempts (default 20ms)
*/
function waitUntil(isready, success, error, count, interval){
if (count === undefined) {
count = 300;
}
if (interval === undefined) {
interval = 20;
}
if (isready()) {
success();
return;
}
// The call back isn't ready. We need to wait for it
setTimeout(function(){
if (!count) {
// We have run out of retries
if (error !== undefined) {
error();
}
} else {
// Try again
waitUntil(isready, success, error, count -1, interval);
}
}, interval);
}
要调用它,例如在jQuery中,使用如下代码:
waitUntil(function(){
return $('#myelement').length > 0;
}, function(){
alert("myelement now exists");
}, function(){
alert("I'm bored. I give up.");
});
其他回答
更新
下面是一个更新的版本,可以使用承诺。如果达到特定的尝试次数,它也会“停止”。
function _waitForElement(selector, delay = 50, tries = 100) {
const element = document.querySelector(selector);
if (!window[`__${selector}`]) {
window[`__${selector}`] = 0;
window[`__${selector}__delay`] = delay;
window[`__${selector}__tries`] = tries;
}
function _search() {
return new Promise((resolve) => {
window[`__${selector}`]++;
setTimeout(resolve, window[`__${selector}__delay`]);
});
}
if (element === null) {
if (window[`__${selector}`] >= window[`__${selector}__tries`]) {
window[`__${selector}`] = 0;
return Promise.resolve(null);
}
return _search().then(() => _waitForElement(selector));
} else {
return Promise.resolve(element);
}
}
用法很简单,用await使用它只是确保你在一个 异步功能:
const start = (async () => {
const $el = await _waitForElement(`.my-selector`);
console.log($el);
})();
过时的版本
只需添加所需的选择器。一旦找到元素,就可以在回调函数中访问它。
const waitUntilElementExists = (selector, callback) => {
const el = document.querySelector(selector);
if (el){
return callback(el);
}
setTimeout(() => waitUntilElementExists(selector, callback), 500);
}
waitUntilElementExists('.wait-for-me', (el) => console.log(el));
对于一个使用jQuery的简单方法,我发现这工作得很好:
// Wait for element to exist.
function elementLoaded(el, cb) {
if ($(el).length) {
// Element is now loaded.
cb($(el));
} else {
// Repeat every 500ms.
setTimeout(function() {
elementLoaded(el, cb)
}, 500);
}
};
elementLoaded('.element-selector', function(el) {
// Element is ready to use.
el.click(function() {
alert("You just clicked a dynamically inserted element");
});
});
在这里,我们只需每500毫秒检查一次元素是否加载,当加载成功时,我们就可以使用它。
这对于向已动态添加到文档中的元素添加单击处理程序特别有用。
我认为仍然没有任何答案在这里与简单易读的工作实例。使用MutationObserver接口来检测DOM的变化,如下所示:
var observer = new MutationObserver(function(mutations) { if ($("p").length) { console.log("Exist, lets do something"); observer.disconnect(); //We can disconnect observer once the element exist if we dont want observe more changes in the DOM } }); // Start observing observer.observe(document.body, { //document.body is node target to observe childList: true, //This is a must have for the observer with subtree subtree: true //Set to true if changes must also be observed in descendants. }); $(document).ready(function() { $("button").on("click", function() { $("p").remove(); setTimeout(function() { $("#newContent").append("<p>New element</p>"); }, 2000); }); }); <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <button>New content</button> <div id="newContent"></div>
注意:关于MutationObserver的西班牙语Mozilla文档更多 详情,如果你想了解更多信息。
我也有同样的问题,所以我继续写了一个插件。
$(选择).waitUntilExists(函数);
代码:
;(function ($, window) {
var intervals = {};
var removeListener = function(selector) {
if (intervals[selector]) {
window.clearInterval(intervals[selector]);
intervals[selector] = null;
}
};
var found = 'waitUntilExists.found';
/**
* @function
* @property {object} jQuery plugin which runs handler function once specified
* element is inserted into the DOM
* @param {function|string} handler
* A function to execute at the time when the element is inserted or
* string "remove" to remove the listener from the given selector
* @param {bool} shouldRunHandlerOnce
* Optional: if true, handler is unbound after its first invocation
* @example jQuery(selector).waitUntilExists(function);
*/
$.fn.waitUntilExists = function(handler, shouldRunHandlerOnce, isChild) {
var selector = this.selector;
var $this = $(selector);
var $elements = $this.not(function() { return $(this).data(found); });
if (handler === 'remove') {
// Hijack and remove interval immediately if the code requests
removeListener(selector);
}
else {
// Run the handler on all found elements and mark as found
$elements.each(handler).data(found, true);
if (shouldRunHandlerOnce && $this.length) {
// Element was found, implying the handler already ran for all
// matched elements
removeListener(selector);
}
else if (!isChild) {
// If this is a recurring search or if the target has not yet been
// found, create an interval to continue searching for the target
intervals[selector] = window.setInterval(function () {
$this.waitUntilExists(handler, shouldRunHandlerOnce, true);
}, 500);
}
}
return $this;
};
}(jQuery, window));
这是写在王勇答案(最高分答案)上面的一个更好的版本。
增加的特性:您可以等待一个元素特定的时间,精确定位,以提高性能。
async function waitForElement(selector, timeout = null, location = document.body) {
return new Promise((resolve) => {
let element = document.querySelector(selector);
if (element) {
return resolve(element);
}
const observer = new MutationObserver(async () => {
let element = document.querySelector(selector);
if (element) {
resolve(element);
observer.disconnect();
} else {
if (timeout) {
async function timeOver() {
return new Promise((resolve) => {
setTimeout(() => {
observer.disconnect();
resolve(false);
}, timeout);
});
}
resolve(await timeOver());
}
}
});
observer.observe(location, {
childList: true,
subtree: true,
});
});
}
用法:
await waitForElement(".nav-alt", 500, ".main-body")
奖励:等待一个元素从DOM中消失。
async function waitForElementDeath(selector, location = document.body) {
return new Promise((resolve) => {
const observer = new MutationObserver(async () => {
if (!document.querySelector(selector)) {
resolve(true);
observer.disconnect();
}
});
observer.observe(location, {
childList: true,
subtree: true,
});
});
}
用法:
await waitForElementDeath(".Popup-div", "Popup-Container")
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