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<title>Example: Rebuilding Class Instances from JSON Data</title>
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<h1>Example: Rebuilding Class Instances from JSON Data</h1>
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<p>This example illustrates one method of serializing and recreating class instances by using the <code>replacer</code> and <code>reviver</code> parameters to <code>JSON.stringify</code> and <code>JSON.parse</code> respectively.</p>
</div>
<div class="example yui3-skin-sam">
<style scoped>
#demo pre {
background: #fff;
border: 1px solid #ccc;
margin: 1em;
padding: 1em;
white-space: pre-wrap; /* css-3 */
word-wrap: break-word; /* Internet Explorer 5.5+ */
}
</style>
<div id="demo">
<input type="button" id="demo_freeze" value="Freeze">
<input type="button" id="demo_thaw" disabled="disabled" value="Thaw">
<pre id="demo_frozen">(stringify results here)</pre>
<div id="demo_thawed"></div>
</div>
<script type="text/javascript">
YUI().use("node", "json", function(Y) {
var example = {
data : null,
jsonString : null,
dateRE : /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d+)?Z$/,
cryo : function (k,o) {
return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
},
revive : function (k,v) {
// Turn anything that looks like a UTC date string into a Date instance
var match = Y.Lang.isString(v) ? v.match(example.dateRE) : null,
d;
if (match) {
d = new Date();
d.setUTCFullYear(match[1], (match[2] - 1), match[3]);
d.setUTCHours(match[4], match[5], match[6]);
return d;
}
// Check for cavemen by the _class key
if (v instanceof Object && v._class == 'CaveMan') {
return CaveMan.thaw(v);
}
// default to returning the value unaltered
return v;
}
};
function CaveMan(name,discovered) {
this.name = name;
this.discovered = discovered;
};
CaveMan.prototype.getName = function () {
return this.name + ", the cave man";
}
// Static methods to convert to and from a basic object structure
CaveMan.thaw = function (o) {
return new CaveMan(o.n, o.d);
};
// Convert to a basic object structure including a class identifier
CaveMan.freeze = function (cm) {
return {
_class : 'CaveMan',
n : cm.name,
d : cm.discovered // remains a Date for standard JSON serialization
};
};
example.data = {
count : 1,
type : 'Hominid',
specimen : [
new CaveMan('Ed',new Date(1946,6,6))
]
};
Y.one('#demo_freeze').on('click',function (e) {
// Format the string with 4 space indentation
example.jsonString = Y.JSON.stringify(example.data, example.cryo, 4);
Y.one('#demo_frozen').set('text', example.jsonString);
Y.one('#demo_thaw').set('disabled',false);
});
Y.one('#demo_thaw').on('click',function (e) {
var parsedData = Y.JSON.parse(example.jsonString, example.revive);
cm = parsedData.specimen[0];
Y.one('#demo_thawed').set('innerHTML',
"<p>Specimen count: " + parsedData.count + "</p>"+
"<p>Specimen type: " + parsedData.type + "</p>"+
"<p>Instanceof CaveMan: " + (cm instanceof CaveMan) + "</p>"+
"<p>Name: " + cm.getName() + "</p>"+
"<p>Discovered: " + cm.discovered + "</p>");
});
// Expose the example objects for inspection
example.CaveMan = CaveMan;
YUI.example = example;
});
</script>
</div>
<h3>The CaveMan class</h3>
<p>For this example, we'll use a class CaveMan, with a property <code>discovered</code> that holds a <code>Date</code> instance, and a method <code>getName</code>.</p>
<pre class="code prettyprint">YUI().use("node", "json", function(Y) {
function CaveMan(name,discovered) {
this.name = name;
this.discovered = discovered;
};
CaveMan.prototype.getName = function () {
return this.name + ", the cave man";
}
...</pre>
<h3>Add <code>freeze</code> and <code>thaw</code> static methods</h3>
<p>We'll add the methods responsible for serializing and reconstituting instances to the CaveMan class as static methods.</p>
<pre class="code prettyprint">// Static method to convert to a basic structure with a class identifier
CaveMan.freeze = function (cm) {
return {
_class : 'CaveMan',
n : cm.name,
d : cm.discovered // remains a Date for standard JSON serialization
};
};
// Static method to reconstitute a CaveMan from the basic structure
CaveMan.thaw = function (o) {
return new CaveMan(o.n, o.d);
};</pre>
<h3>Reference the methods in replacer and reviver functions</h3>
<p>We'll create an <code>example</code> namespace to hold our moving parts. In it, we'll add a method to pass to <code>JSON.stringify</code> that calls our custom serializer, and another method to pass to <code>JSON.parse</code> that detects the serialized structure and calls our thawing method.</p>
<pre class="code prettyprint">var example = {
cryo : function (k,o) {
return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
},
revive : function (k,v) {
// Check for cavemen by the _class key
if (v instanceof Object && v._class == 'CaveMan') {
return CaveMan.thaw(v);
}
// default to returning the value unaltered
return v;
}
};</pre>
<h3>The data to be serialized</h3>
<p>We'll create a CaveMan instance and nest it in another object structure to illustrate how the thawing process still operates normally for all other data.</p>
<pre class="code prettyprint">example.data = {
count : 1,
type : 'Hominid',
specimen : [
new CaveMan('Ed',new Date(1946,6,6))
]
};</pre>
<h3>Thawing from the inside out and the <code>Date</code> instance</h3>
<p>The reviver function passed to <code>JSON.parse</code> is applied to all key:value pairs in the raw parsed object from the deepest keys to the highest level. In our case, this means that the <code>name</code> and <code>discovered</code> properties will be passed through the reviver, and <em>then</em> the object containing those keys will be passed through.</p>
<p>We'll take advantage of this by watching for UTC formatted date strings (the default JSON serialization for Dates) and reviving them into proper <code>Date</code> instances before the containing object gets its turn in the reviver.</p>
<pre class="code prettyprint">var example = {
dateRE : /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d+)?Z$/,
cryo : function (k,o) {
return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
},
revive : function (k,v) {
// Turn anything that looks like a UTC date string into a Date instance
var match = Y.Lang.isString(v) ? v.match(example.dateRE) : null,
d;
if (match) {
d = new Date();
d.setUTCFullYear(match[1], (match[2] - 1), match[3]);
d.setUTCHours(match[4], match[5], match[6]);
return d;
}
// Check for cavemen by the _class key
if (v instanceof Object && v._class == 'CaveMan') {
return CaveMan.thaw(v);
}
// default to returning the value unaltered
return v;
}
};</pre>
<p>Now when the reviver function is evaluating the object it determines to be a CaveMan, the <code>discovered</code> property is correctly containing a <code>Date</code> instance.</p>
<h3>Choose your serialization</h3>
<p>You'll note there are two freeze and thaw operations going on in this example. One for our CaveMan class and one for <code>Date</code> instances. Their respective serialization and recreation techniques are very different. You are free to decide the serialized format of your objects. Choose whatever makes sense for your application.</p>
<p><em>Note</em>: There is no explicit <code>Date</code> serialization method listed inline because <code>JSON</code> natively supports <code>Date</code> serialization. However, it is outside the scope of the parser's duty to create Date instances, so it's up to you to recreate them in the <code>parse</code> phase. Feel free to use the method included here.</p>
<h3>Show and Tell</h3>
<p>Now we add the event handlers to the example buttons to call <code>JSON.stringify</code> and <code>parse</code> with our <code>example.cryo</code> and <code>example.revive</code> methods, respectively.</p>
<pre class="code prettyprint">Y.one('#demo_freeze').on('click',function (e) {
example.jsonString = Y.JSON.stringify(example.data, example.cryo);
Y.one('#demo_frozen').set('innerHTML', example.jsonString);
Y.one('#demo_thaw').set('disabled',false);
});
Y.one('#demo_thaw').on('click',function (e) {
var x = Y.JSON.parse(example.jsonString, example.revive);
cm = x.specimen[0];
Y.one('#demo_thawed').set('innerHTML',
"<p>Specimen count: " + x.count + "</p>"+
"<p>Specimen type: " + x.type + "</p>"+
"<p>Instanceof CaveMan: " + (cm instanceof CaveMan) + "</p>"+
"<p>Name: " + cm.getName() + "</p>"+
"<p>Discovered: " + cm.discovered + "</p>");
});
}); // end of YUI(..).use(.., function (Y) {</pre>
<h3>Full Code Listing</h3>
<pre class="code prettyprint"><style scoped>
#demo pre {
background: #fff;
border: 1px solid #ccc;
margin: 1em;
padding: 1em;
white-space: pre-wrap; /* css-3 */
word-wrap: break-word; /* Internet Explorer 5.5+ */
}
</style>
<div id="demo">
<input type="button" id="demo_freeze" value="Freeze">
<input type="button" id="demo_thaw" disabled="disabled" value="Thaw">
<pre id="demo_frozen">(stringify results here)</pre>
<div id="demo_thawed"></div>
</div>
<script type="text/javascript">
YUI().use("node", "json", function(Y) {
var example = {
data : null,
jsonString : null,
dateRE : /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})(?:\.\d+)?Z$/,
cryo : function (k,o) {
return (o instanceof CaveMan) ? CaveMan.freeze(o) : o;
},
revive : function (k,v) {
// Turn anything that looks like a UTC date string into a Date instance
var match = Y.Lang.isString(v) ? v.match(example.dateRE) : null,
d;
if (match) {
d = new Date();
d.setUTCFullYear(match[1], (match[2] - 1), match[3]);
d.setUTCHours(match[4], match[5], match[6]);
return d;
}
// Check for cavemen by the _class key
if (v instanceof Object && v._class == 'CaveMan') {
return CaveMan.thaw(v);
}
// default to returning the value unaltered
return v;
}
};
function CaveMan(name,discovered) {
this.name = name;
this.discovered = discovered;
};
CaveMan.prototype.getName = function () {
return this.name + ", the cave man";
}
// Static methods to convert to and from a basic object structure
CaveMan.thaw = function (o) {
return new CaveMan(o.n, o.d);
};
// Convert to a basic object structure including a class identifier
CaveMan.freeze = function (cm) {
return {
_class : 'CaveMan',
n : cm.name,
d : cm.discovered // remains a Date for standard JSON serialization
};
};
example.data = {
count : 1,
type : 'Hominid',
specimen : [
new CaveMan('Ed',new Date(1946,6,6))
]
};
Y.one('#demo_freeze').on('click',function (e) {
// Format the string with 4 space indentation
example.jsonString = Y.JSON.stringify(example.data, example.cryo, 4);
Y.one('#demo_frozen').set('text', example.jsonString);
Y.one('#demo_thaw').set('disabled',false);
});
Y.one('#demo_thaw').on('click',function (e) {
var parsedData = Y.JSON.parse(example.jsonString, example.revive);
cm = parsedData.specimen[0];
Y.one('#demo_thawed').set('innerHTML',
"<p>Specimen count: " + parsedData.count + "</p>"+
"<p>Specimen type: " + parsedData.type + "</p>"+
"<p>Instanceof CaveMan: " + (cm instanceof CaveMan) + "</p>"+
"<p>Name: " + cm.getName() + "</p>"+
"<p>Discovered: " + cm.discovered + "</p>");
});
// Expose the example objects for inspection
example.CaveMan = CaveMan;
YUI.example = example;
});
</script></pre>
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<h2 class="no-toc">Examples</h2>
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<li data-description="Use JSON to parse data received via XMLHttpRequest via Y.io calls — a simple JSON use case.">
<a href="json-connect.html">JSON with Y.io</a>
</li>
<li data-description="Use the replacer and reviver parameters to reconstitute object instances that have been serialized to JSON.">
<a href="json-freeze-thaw.html">Rebuilding Class Instances from JSON Data</a>
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<li data-description="Use a currency conversion calculation to add a new price member to a JSON response, demonstrating how JSON data, once retrieved, can be transformed during parsing.">
<a href="json-convert-values.html">Adding New Object Members During Parsing</a>
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<a href="../app/app-todo.html">Todo List</a>
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<li data-description="Portal style example using Drag & Drop Event Bubbling and Animation.">
<a href="../dd/portal-drag.html">Portal Style Example</a>
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};
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