jQuery source code analysis notes (4) Ready function_jquery

This function is mentioned in the jQuery documentation in three equivalent forms:

Copy code The code is as follows:

// Defined in jQuery.fn.ready
$(document).ready(handler);
// It is the same as the previous one, not recommended
$().ready(handler) );
// Handle
$(handler) in the jQuery object alone;
// The definition of the above form:
if(jQuery.isFunction(selector) {
return rootjQuery. ready(selector);
}

So it actually boils down to one form: jQuery.fn.ready(fn) is defined as follows:

Copy code The code is as follows:

ready: function(fn) {
// Bind events to DOM
jQuery. bindReady();
// Trigger callback function
readyList.done(fn);
// Return jQuery object
return this;
}

Actual There is not only one reference to fn inside jQuery. The Deferred function is used here. On line 75, the readyList member is defined for the jQuery object, and this variable is initialized in the bindReady function:

Copy code The code is as follows:

if(readyList) {
return;
}
readyList = jQuery._Deferred();

In addition to initializing readyList, the bindReady function mainly handles the differences between browsers for binding events. IE uses attachEvent and other browsers use addEventHandler. After completion, the ready function uses readyList.resolveWith to trigger the callback function. In addition to this work, ready also handles holdReady. The function of this API is to delay the callback of the ready event. The main purpose is to set something before the ready event. Flag readyWait. When this flag is set, ready keeps calling setTimeout(jQuery.ready, 1) before calling readyList.resolveWith. That is, it calls itself recursively every fixed time (I don't know if the js engine will overflow if the hold time is long), so that when it is finally released by holdReady, setTimeout will come back along the call stack. In order not to trigger the ready callback function before this stack is completed. Each time setTimeout is called, the readyWait variable will be incremented. Used to indicate how many calls were delayed by the holdReady function.


###Several basic auxiliary functions
Starting on line 543, several noteworthy auxiliary functions are defined: parseJSON, parseXML and globalEval. parseJSON turns a string into a JSON object. We generally use eval. parseJSON encapsulates this operation, but eval is used as a last resort. Because the latest JavaScript standard adds JSON serialization and deserialization APIs. If the browser supports this standard, these two APIs are implemented in the JS engine using Native Code, and the efficiency is definitely much higher than eval. At present, both Chrome and Firefox4 support this API. parseJSON is used as follows:

Copy code The code is as follows:

// Native JSON API. Deserialization is JSON.stringify(object)
if(window.JSON && window.JSON.parse) {
return window.JSON.parse(data);
}
// .. . Roughly check the legality of the string
return (new Function("return " data))();

parseXML function is also mainly a package of standard API and IE. The standard API is the DOMParser object. IE uses the ActiveXObject object of Microsoft.XMLDOM. Definition:

Copy code The code is as follows:

if(window.DOMParser) {
tmp = new DOMParser();
xml = tmp.parseFromString(data, "text/xml");
} else {
xml = new ActiveXObject("Microsoft.XMLDOM");
xml .async = "false";
xml.loadXML(data);
}

The globalEval function loads a script into the global context. window.execScript can be used in IE. Other browsers require the use of eval. Because the entire jQuery code is an entire anonymous function, the current context is jQuery. Main code:

Copy code The code is as follows:

(window.execScript || function(data ) {
window["eval"].call(window, data); // Run in window context
})(data);