Question

我正在尝试在另一个画布上添加画布 - 如何让这个函数等到第一个画布创建之后才开始？

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;
    }

Answer 1

如果您可以访问创建画布的代码 - 只需在创建画布后立即调用该函数。

如果您无法访问该代码（例如，如果它是第三方代码，例如谷歌地图），那么您可以做的是测试间隔中是否存在：

var checkExist = setInterval(function() {
   if ($('#the-canvas').length) {
      console.log("Exists!");
      clearInterval(checkExist);
   }
}, 100); // check every 100ms

但请注意 - 很多时候第三方代码可以选择在加载完成后激活您的代码（通过回调或事件触发）。这可能是你可以放置你的功能的地方。区间解决方案实际上是一个糟糕的解决方案，只有在没有其他工作的情况下才能使用。

Answer 2

根据您需要支持的浏览器，可以选择MutationObserver。

根据这一点，应该可以解决这个问题：

// 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
});

N.B。我自己没有测试过这段代码，但这是一般的想法。

您可以轻松地将其扩展为仅搜索已更改的DOM部分。为此，使用mutations参数，它是MutationRecord个对象的数组。

Answer 3

这只适用于现代浏览器，但我发现使用then更容易，所以请先测试一下，但是：

<强>代码

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);
    }
}

或使用生成器功能

async function checkElement(selector) {
    while (document.querySelector(selector) === null) {
        await rafAsync()
    }
    return true;
}

<强>用法

checkElement('body') //use whichever selector you want
.then((element) => {
     console.info(element);
     //Do whatever you want now the element is there
});

Answer 4

等待元素的更现代的方法：

while(!document.querySelector(".my-selector")) {
  await new Promise(r => setTimeout(r, 500));
}
// now the element is loaded

请注意，此代码需要包含在async function。

中

Answer 5

如果您想要使用MutationObserver的通用解决方案，则可以使用此功能

// 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
示例如何使用它

elementReady('#someWidget').then((someWidget)=>{someWidget.remove();});

注意：MutationObserver具有强大的浏览器支持； https://caniuse.com/#feat=mutationobserver

等等！：）

Answer 6

最好在requestAnimationFrame中继电，而不是setTimeout。这是我在es6模块中的解决方案并使用Promises。

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
  }

plain js and promises：

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
  });

Answer 7

您可以通过设置超时来检查dom是否已经存在，直到它已经在dom中呈现。

var panelMainWrapper = document.getElementById('panelMainWrapper');
setTimeout(function waitPanelMainWrapper() {
    if (document.body.contains(panelMainWrapper)) {
        $("#panelMainWrapper").html(data).fadeIn("fast");
    } else {
        setTimeout(waitPanelMainWrapper, 10);
    }
}, 10);

Answer 8

与Jamie Hutber的回答相比，这里有一个小改进

const checkElement = async selector => {

while ( document.querySelector(selector) === null) {
    await new Promise( resolve =>  requestAnimationFrame(resolve) )
}

return document.querySelector(selector); };

Answer 9

这是使用可观察对象的解决方案。

waitForElementToAppear(elementId) {                                          

    return Observable.create(function(observer) {                            
            var el_ref;                                                      
            var f = () => {                                                  
                el_ref = document.getElementById(elementId);                 
                if (el_ref) {                                                
                    observer.next(el_ref);                                   
                    observer.complete();                                     
                    return;                                                  
                }                                                            
                window.requestAnimationFrame(f);                             
            };                                                               
            f();                                                             
        });                                                                  
}

现在您可以写

waitForElementToAppear(elementId).subscribe(el_ref => doSomethingWith(el_ref);

Answer 10

Iftah

的另一种形式

var counter = 10;
var checkExist = setInterval(function() {
  console.log(counter);
  counter--
  if ($('#the-canvas').length || counter === 0) {
    console.log("by bye!");
    clearInterval(checkExist);
  }
}, 200);

以防万一该元素从不显示，因此我们不会进行无限检查。

Answer 11

也许我有点晚了:)，但这里是 chrisjhoughton 的一个不错的简短解决方案，它允许在等待结束时执行回调函数。

https://gist.github.com/chrisjhoughton/7890303

var waitForEl = function(selector, callback) {
  if (jQuery(selector).length) {
    callback();
  } else {
    setTimeout(function() {
      waitForEl(selector, callback);
    }, 100);
  }
};

waitForEl(selector, function() {
  // work the magic
});

如果需要给回调函数传递参数，可以这样使用：

waitForEl("#" + elDomId, () => callbackFunction(param1, param2));

但是要小心！默认情况下，此解决方案可能会陷入无限循环的陷阱。

The GitHub thread 中还提供了对 topicstarter 建议的一些改进。

享受吧！

Answer 12

只需在递归中使用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，在限制之后不存在该元素时，将引发错误：

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();
}

使函数等到元素存在

12 个答案: