how to call a function when the page/DOM is ready for it

2018-06-03 15:38:49

This question already has an answer here:

$(document).ready equivalent without jQuery 31 answers

The simplest thing to do in the absence of a framework that does all the cross-browser compatibility for you is to just put a call to your code at the end of the body. This is faster to execute than an onload handler because this waits only for the DOM to be ready, not for all images to load. And, this works in every browser.

<html>
<head>
</head>
<body>
Your HTML here

<script>
// self executing function here
(function() {
   // your page initialization code here
   // the DOM will be available here

})();
</script>
</body>
</html>

If you really don't want to do it this way and you need cross browser compatibility and you don't want to wait for window.onload , then you probably should go look at how a framework like jQuery implements it's $(document).ready() method. It's fairly involved depending upon the capabilities of the browser.

To give you a little idea what jQuery does (which will work wherever the script tag is placed).

If supported, it tries the standard:

document.addEventListener('DOMContentLoaded', fn, false);

with a fallback to:

window.addEventListener('load', fn, false )

or for older versions of IE, it uses:

document.attachEvent("onreadystatechange", fn);

with a fallback to:

window.attachEvent("onload", fn);

And, there are some work-arounds in the IE code path that I don't quite follow, but it looks like it has something to do with frames.

Here is a full substitute for jQuery's .ready() written in plain javascript:

(function(funcName, baseObj) {
    // The public function name defaults to window.docReady
    // but you can pass in your own object and own function name and those will be used
    // if you want to put them in a different namespace
    funcName = funcName || "docReady";
    baseObj = baseObj || window;
    var readyList = [];
    var readyFired = false;
    var readyEventHandlersInstalled = false;

    // call this when the document is ready
    // this function protects itself against being called more than once
    function ready() {
        if (!readyFired) {
            // this must be set to true before we start calling callbacks
            readyFired = true;
            for (var i = 0; i < readyList.length; i++) {
                // if a callback here happens to add new ready handlers,
                // the docReady() function will see that it already fired
                // and will schedule the callback to run right after
                // this event loop finishes so all handlers will still execute
                // in order and no new ones will be added to the readyList
                // while we are processing the list
                readyList[i].fn.call(window, readyList[i].ctx);
            }
            // allow any closures held by these functions to free
            readyList = [];
        }
    }

    function readyStateChange() {
        if ( document.readyState === "complete" ) {
            ready();
        }
    }

    // This is the one public interface
    // docReady(fn, context);
    // the context argument is optional - if present, it will be passed
    // as an argument to the callback
    baseObj[funcName] = function(callback, context) {
        if (typeof callback !== "function") {
            throw new TypeError("callback for docReady(fn) must be a function");
        }
        // if ready has already fired, then just schedule the callback
        // to fire asynchronously, but right away
        if (readyFired) {
            setTimeout(function() {callback(context);}, 1);
            return;
        } else {
            // add the function and context to the list
            readyList.push({fn: callback, ctx: context});
        }
        // if document already ready to go, schedule the ready function to run
        if (document.readyState === "complete") {
            setTimeout(ready, 1);
        } else if (!readyEventHandlersInstalled) {
            // otherwise if we don't have event handlers installed, install them
            if (document.addEventListener) {
                // first choice is DOMContentLoaded event
                document.addEventListener("DOMContentLoaded", ready, false);
                // backup is window load event
                window.addEventListener("load", ready, false);
            } else {
                // must be IE
                document.attachEvent("onreadystatechange", readyStateChange);
                window.attachEvent("onload", ready);
            }
            readyEventHandlersInstalled = true;
        }
    }
})("docReady", window);

The latest version of the code is shared publicly on GitHub at https://github.com/jfriend00/docReady

Usage:

// pass a function reference
docReady(fn);

// use an anonymous function
docReady(function() {
    // code here
});

// pass a function reference and a context
// the context will be passed to the function as the first argument
docReady(fn, context);

// use an anonymous function with a context
docReady(function(context) {
    // code here that can use the context argument that was passed to docReady
}, ctx);

This has been tested in:

IE6 and up
Firefox 3.6 and up
Chrome 14 and up
Safari 5.1 and up
Opera 11.6 and up
Multiple iOS devices
Multiple Android devices

Working implementation and test bed: http://jsfiddle.net/jfriend00/YfD3C/

Here's a summary of how it works:

Create an IIFE (immediately invoked function expression) so we can have non-public state variables.

Declare a public function docReady(fn, context)

When docReady(fn, context) is called, check if the ready handler has already fired. If so, just schedule the newly added callback to fire right after this thread of JS finishes with setTimeout(fn, 1) .

If the ready handler has not already fired, then add this new callback to the list of callbacks to be called later.

Check if the document is already ready. If so, execute all ready handlers.

If we haven't installed event listeners yet to know when the document becomes ready, then install them now.

If document.addEventListener exists, then install event handlers using .addEventListener() for both "DOMContentLoaded" and "load" events. The "load" is a backup event for safety and should not be needed.

If document.addEventListener doesn't exist, then install event handlers using .attachEvent() for "onreadystatechange" and "onload" events.

In the onreadystatechange event, check to see if the document.readyState === "complete" and if so, call a function to fire all the ready handlers.

In all the other event handlers, call a function to fire all the ready handlers.

In the function to call all the ready handlers, check a state variable to see if we've already fired. If we have, do nothing. If we haven't yet been called, then loop through the array of ready functions and call each one in the order they were added. Set a flag to indicate these have all been called so they are never executed more than once.

Clear the function array so any closures they might be using can be freed.

Handlers registered with docReady() are guaranteed to be fired in the order they were registered.

If you call docReady(fn) after the document is already ready, the callback will be scheduled to execute as soon as the current thread of execution completes using setTimeout(fn, 1) . This allows the calling code to always assume they are async callbacks that will be called later, even if later is as soon as the current thread of JS finishes and it preserves calling order.

I would like to mention some of the possible ways here together with a pure javascript trick which works across all browsers :

// with jQuery 
$(document).ready(function(){ /* ... */ });

// shorter jQuery version 
$(function(){ /* ... */ });

// without jQuery (doesn't work in older IEs)
document.addEventListener('DOMContentLoaded', function(){ 
    // your code goes here
}, false);

// and here's the trick (works everywhere)
function r(f){/in/.test(document.readyState)?setTimeout('r('+f+')',9):f()}
// use like
r(function(){
    alert('DOM Ready!');
});

The trick here, as explained by the original author, is that we are checking the document.readyState property. If it contains the string in (as in uninitialized and loading , the first two DOM ready states out of 5) we set a timeout and check again. Otherwise, we execute the passed function.

And here's the jsFiddle for the trick which works across all browsers.

Thanks to Tutorialzine for including this in their book.

Tested in IE9, and latest Firefox and Chrome and also supported in IE8.

document.onreadystatechange = function () {
  var state = document.readyState;
  if (state == 'interactive') {
      init();
  } else if (state == 'complete') {
      initOnCompleteLoad();
  }
};

Example: http://jsfiddle.net/electricvisions/Jacck/

UPDATE - reusable version

I have just developed the following. It's a rather simplistic equivalent to jQuery or Dom ready without backwards compatibility. It probably needs further refinement. Tested in latest versions of Chrome, Firefox and IE (10/11) and should work in older browsers as commented on. I'll update if I find any issues.

window.readyHandlers = [];
window.ready = function ready(handler) {
  window.readyHandlers.push(handler);
  handleState();
};

window.handleState = function handleState () {
  if (['interactive', 'complete'].indexOf(document.readyState) > -1) {
    while(window.readyHandlers.length > 0) {
      (window.readyHandlers.shift())();
    }
  }
};

document.onreadystatechange = window.handleState;

Usage:

ready(function () {
  // your code here
});

It's written to handle async loading of JS but you might want to sync load this script first unless you're minifying. I've found it useful in development.

Modern browsers also support async loading of scripts which further enhances the experience. Support for async means multiple scripts can be downloaded simultaneously all while still rendering the page. Just watch out when depending on other scripts loaded asynchronously or use a minifier or something like browserify to handle dependencies.

链接地址: http://www.djcxy.com/p/12298.html