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
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。
其他回答
下面的observe函数将允许您通过选择器监听元素。
在下面的例子中,2秒过后,.greeting将被插入到.container中。因为我们正在监听这个元素的插入,所以我们可以有一个在插入时触发的回调。
const observe = (selector, callback, targetNode = document.body) => new MutationObserver(mutations => [...mutations] .flatMap((mutation) => [...mutation.addedNodes]) .filter((node) => node.matches && node.matches(selector)) .forEach(callback)) .observe(targetNode, { childList: true, subtree: true }); const createGreeting = () => { const el = document.createElement('DIV'); el.textContent = 'Hello World'; el.classList.add('greeting'); return el; }; const container = document.querySelector('.container'); observe('.greeting', el => console.log('I have arrived!', el), container); new Promise(res => setTimeout(() => res(createGreeting()), 2000)) .then(el => container.appendChild(el)); html, body { width: 100%; height: 100%; margin: 0; padding: 0; } body { display: flex; } .container { display: flex; flex: 1; align-items: center; justify-content: center; } .greeting { font-weight: bold; font-size: 2em; } <div class="container"></div>
更新
下面是一个实验性的async/await示例。
const sleep = (ms) => new Promise((res) => setTimeout(res, ms)); const observe = (selector, targetNode = document.body) => new Promise(res => { new MutationObserver(mutations => res([...mutations] .flatMap((mutation) => [...mutation.addedNodes]) .find((node) => node.matches && node.matches(selector)))) .observe(targetNode, { childList: true, subtree: true }); }); const createGreeting = () => { const el = document.createElement('DIV'); el.textContent = 'Hello World'; el.classList.add('greeting'); return el; }; const container = document.querySelector('.container'); observe('.greeting', container) .then(el => console.log('I have arrived!', el)); (async () => { await sleep(2000); container.appendChild(createGreeting()); })(); html, body { width: 100%; height: 100%; margin: 0; padding: 0; } body { display: flex; } .container { display: flex; flex: 1; align-items: center; justify-content: center; } .greeting { font-weight: bold; font-size: 2em; } <div class="container"></div>
我使用这种方法等待一个元素出现,这样我就可以在那之后执行其他函数。
让我们说doTheRestOfTheStuff(参数)函数应该只在ID为the_Element_ID的元素出现或完成加载后调用,我们可以使用,
var existCondition = setInterval(function() {
if ($('#the_Element_ID').length) {
console.log("Exists!");
clearInterval(existCondition);
doTheRestOfTheStuff(parameters);
}
}, 100); // check every 100ms
下面是一个函数,充当MutationObserver的薄包装器。唯一的要求是浏览器支持MutationObserver;不依赖于JQuery。运行下面的代码片段以查看一个工作示例。
function waitForMutation(parentNode, isMatchFunc, handlerFunc, observeSubtree, disconnectAfterMatch) { var defaultIfUndefined = function(val, defaultVal) { return (typeof val === "undefined") ? defaultVal : val; }; observeSubtree = defaultIfUndefined(observeSubtree, false); disconnectAfterMatch = defaultIfUndefined(disconnectAfterMatch, false); var observer = new MutationObserver(function(mutations) { mutations.forEach(function(mutation) { if (mutation.addedNodes) { for (var i = 0; i < mutation.addedNodes.length; i++) { var node = mutation.addedNodes[i]; if (isMatchFunc(node)) { handlerFunc(node); if (disconnectAfterMatch) observer.disconnect(); }; } } }); }); observer.observe(parentNode, { childList: true, attributes: false, characterData: false, subtree: observeSubtree }); } // Example waitForMutation( // parentNode: Root node to observe. If the mutation you're looking for // might not occur directly below parentNode, pass 'true' to the // observeSubtree parameter. document.getElementById("outerContent"), // isMatchFunc: Function to identify a match. If it returns true, // handlerFunc will run. // MutationObserver only fires once per mutation, not once for every node // inside the mutation. If the element we're looking for is a child of // the newly-added element, we need to use something like // node.querySelector() to find it. function(node) { return node.querySelector(".foo") !== null; }, // handlerFunc: Handler. function(node) { var elem = document.createElement("div"); elem.appendChild(document.createTextNode("Added node (" + node.innerText + ")")); document.getElementById("log").appendChild(elem); }, // observeSubtree true, // disconnectAfterMatch: If this is true the hanlerFunc will only run on // the first time that isMatchFunc returns true. If it's false, the handler // will continue to fire on matches. false); // Set up UI. Using JQuery here for convenience. $outerContent = $("#outerContent"); $innerContent = $("#innerContent"); $("#addOuter").on("click", function() { var newNode = $("<div><span class='foo'>Outer</span></div>"); $outerContent.append(newNode); }); $("#addInner").on("click", function() { var newNode = $("<div><span class='foo'>Inner</span></div>"); $innerContent.append(newNode); }); .content { padding: 1em; border: solid 1px black; overflow-y: auto; } #innerContent { height: 100px; } #outerContent { height: 200px; } #log { font-family: Courier; font-size: 10pt; } <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script> <h2>Create some mutations</h2> <div id="main"> <button id="addOuter">Add outer node</button> <button id="addInner">Add inner node</button> <div class="content" id="outerContent"> <div class="content" id="innerContent"></div> </div> </div> <h2>Log</h2> <div id="log"></div>
你可以这样做
$('#yourelement').ready(function() {
});
请注意,这只在从服务器请求元素时元素出现在DOM中时才有效。如果元素是通过JavaScript动态添加的,那么它将不起作用,您可能需要查看其他答案。
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.");
});