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

如果你想让它在一段时间后停止查看(超时),那么下面的jQuery可以工作。10秒后会暂停。我需要使用这段代码而不是纯JS,因为我需要通过名称选择输入,并且在实现其他一些解决方案时遇到了麻烦。

 // Wait for element to exist.

    function imageLoaded(el, cb,time) {

        if ($(el).length) {
            // Element is now loaded.

            cb($(el));

            var imageInput =  $('input[name=product\\[image_location\\]]');
            console.log(imageInput);

        } else if(time < 10000) {
            // Repeat every 500ms.
            setTimeout(function() {
               time = time+500;

                imageLoaded(el, cb, time)
            }, 500);
        }
    };

    var time = 500;

    imageLoaded('input[name=product\\[image_location\\]]', function(el) {

     //do stuff here 

     },time);

其他回答

如果可以的话,我会尽量避开突变观察者,所以我想到了这个。它看起来类似于上面的一些其他答案。该函数将查找给定DOM调用中存在的第一个元素——className是预期的用法,但它也可以接受tagName或Id。如果您正在寻找具有给定类名或标记名的元素数量,则还可以为精确索引添加参数。

    async function waitUntilElementExits(domkey,domquery,maxtime){
        const delay = (ms) => new Promise(res => setTimeout(res, ms));
        for(let i=0; i<maxtime; i=i+200){
            await delay(200);
            let elm = document[domkey](domquery);
            if( (domkey == 'getElementById' && elm) || elm?.[0] ) break;
        }
    }
    // usage
    await waitUntilElementExits('getElementByClassName','some_class_name',10000)

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>

我采取@Yong Wong的解决方案,但它有一个可选的超时,你可以指定根节点从哪里你想等待元素。

完整的异步/等待。

const $ = (selector, opts) => {
  let timeout = undefined;
  let root = undefined;

  if (opts) {
    ({ root, timeout } = opts);
  }

  if (root === undefined) root = document.body;
  
  const nodeFound = root.querySelector(selector);
  if (nodeFound) return new Promise(resolve => resolve(nodeFound));

  return new Promise((resolve, reject) => {
    let callback = () => {
      observer.disconnect();
    };

    const _resolve = (node) => {
      callback();
      resolve(node);
    };

    const _reject = (err) => {
      callback();
      reject(err);
    };

    if (timeout && timeout > 0) {
      const handle = setTimeout(() => {
        _reject(new Error("Element not found: timeout exceeded."));
      }, timeout);
      callback = () => {
        observer.disconnect();
        clearTimeout(handle);
      };
    }

    const observer = new MutationObserver(mutations => {
      for (const mutation of mutations) {
        for (const addedNode of mutation.addedNodes) {
          if (addedNode.matches(selector)) {
            _resolve(addedNode);
            return;
          }
        }
      }
    });

    observer.observe(root, {
      childList: true,
      subtree: true,
    });
  });
}

示例调用:

// wait for 10 seconds for 'div.bla-bla-bla' to appear as a child of 'div.some-container'
await $("div.bla-bla-bla", {
  timeout: 10000,
  root: document.querySelector("div.some-container") 
});

一个返回承诺的解决方案,并允许使用超时(兼容IE 11+)。

对于单个元素(element类型):

"use strict";

function waitUntilElementLoaded(selector) {
    var timeout = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 0;

    var start = performance.now();
    var now = 0;

    return new Promise(function (resolve, reject) {
        var interval = setInterval(function () {
            var element = document.querySelector(selector);

            if (element instanceof Element) {
                clearInterval(interval);

                resolve();
            }

            now = performance.now();

            if (now - start >= timeout) {
                reject("Could not find the element " + selector + " within " + timeout + " ms");
            }
        }, 100);
    });
}

对于多个元素(类型为NodeList):

"use strict";

function waitUntilElementsLoaded(selector) {
    var timeout = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 0;

    var start = performance.now();
    var now = 0;

    return new Promise(function (resolve, reject) {
        var interval = setInterval(function () {
            var elements = document.querySelectorAll(selector);

            if (elements instanceof NodeList) {
                clearInterval(interval);

                resolve(elements);
            }

            now = performance.now();

            if (now - start >= timeout) {
                reject("Could not find elements " + selector + " within " + timeout + " ms");
            }
        }, 100);
    });
}

例子:

waitUntilElementLoaded('#message', 800).then(function(element) {
    // element found and available

    element.innerHTML = '...';
}).catch(function() {
    // element not found within 800 milliseconds
});

waitUntilElementsLoaded('.message', 10000).then(function(elements) {
    for(const element of elements) {
        // ....
    }
}).catch(function(error) {
    // elements not found withing 10 seconds
});

既适用于元素列表,也适用于单个元素。

这里有一个用Javascript编写的promise返回解决方案(没有混乱的回调)。默认情况下,它每200ms检查一次。

function waitFor(selector) {
    return new Promise(function (res, rej) {
        waitForElementToDisplay(selector, 200);
        function waitForElementToDisplay(selector, time) {
            if (document.querySelector(selector) != null) {
                res(document.querySelector(selector));
            }
            else {
                setTimeout(function () {
                    waitForElementToDisplay(selector, time);
                }, time);
            }
        }
    });
}