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<div id="pagetitle"><h1>YUI Library Examples: Attribute: readOnly and writeOnce Configuration</h1></div>
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<h2>Attribute: readOnly and writeOnce Configuration</h2>
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<p>
Attributes can be configured to be <code>readOnly</code>, stopping them from being modified by the end user, or <code>writeOnce</code> allowing them to only be set by the end user, but only once. This example demonstrates how to setup attributes for your class as <code>readOnly</code> or <code>writeOnce</code> attributes, and shows how their behavior differs when the end user attempts to set their values.
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<a href="attribute-rw_clean.html" target="_blank">View example in new window.</a>
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<!--BEGIN SOURCE CODE FOR EXAMPLE =============================== -->
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<script type="text/javascript">
YUI({base:"../../build/", timeout: 10000}).use("node", "attribute", function(Y) {
// Shortcut to print (unrelated to example)
var print = YUI.example.print;
// Setup a custom class with attribute support
function MyClass(cfg) {
this.addAttrs(Y.merge(MyClass.ATTRIBUTES), cfg);
}
// Setup attribute configuration
MyClass.ATTRIBUTES = {
"foo" : {
value: "Default Foo",
readOnly: true
},
"bar" : {
value: "Default Bar",
writeOnce: true
}
};
MyClass.prototype.doSomething = function() {
// ... Do something which requires
// MyClass to change the value
// of foo ...
// Host code can reset value of
// readOnly attributes interally,
// by working with the private _set
// property
this._set("foo", "New Default Foo");
};
Y.augment(MyClass, Y.Attribute);
var o1 = new MyClass({
foo: "User Foo",
bar: "User Bar"
});
print("Initial Set", "header");
print("foo value (readOnly):", "subheader");
print(o1.get("foo"));
print("bar value (writeOnce):", "subheader");
print(o1.get("bar"));
// Attempt to reset values:
print("Try Setting Again", "header");
o1.set("foo", "User Reset Foo");
o1.set("bar", "User Reset Bar");
print("Final foo value (readOnly)", "subheader");
print(o1.get("foo"));
print("Final bar value (writeOnce)", "subheader");
print(o1.get("bar"));
print("Set Value Of Foo Interally", "header");
print("foo value (readOnly):", "subheader");
o1.doSomething();
print(o1.get("foo"));
});
</script>
<!--END SOURCE CODE FOR EXAMPLE =============================== -->
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<h3>ReadOnly And WriteOnce</h3>
<p>Attribute supports the ability to configure attributes to be <code>readOnly</code> or <code>writeOnce</code>. <code>readOnly</code> attributes cannot be set by the end user, either through initial values passed to the constructor, or by invoking the <code>set</code> method. <code>writeOnce</code> attributes on the other hand, can be set by the user, but only once, either during initialization or through a call to <code>set</code>. Once a value is established for a <code>writeOnce</code> attribute, it cannot be reset to another value by the user.</p>
<h3>Configuring ReadOnly And WriteOnce Attributes</h3>
<p>This example sets up a custom class, <code>MyClass</code>, with two attributes, <code>foo</code> and <code>bar</code>. <code>foo</code> is configured to be a <code>readOnly</code> attribute, and <code>bar</code> is configured to be a <code>writeOnce</code> attribute:</p>
<textarea name="code" class="JScript" rows="1" cols="60">
// Setup a custom class with attribute support
function MyClass(cfg) {
this.addAttrs(Y.merge(MyClass.ATTRIBUTES), cfg);
}
// Setup attribute configuration
MyClass.ATTRIBUTES = {
"foo" : {
value: "Default Foo",
readOnly: true
},
"bar" : {
value: "Default Bar",
writeOnce: true
}
};
</textarea>
<h3>Attempting To Set Values</h3>
<p>We first attempt to set values for both attributes in the constructor (used to intialize the attributes) and see that only <code>bar</code>, the <code>writeOnce</code> attribute, gets set to the user provided value:</p>
<textarea name="code" class="JScript" rows="1" cols="60">
var o1 = new MyClass({
foo: "User Foo",
bar: "User Bar"
});
print("Initial Set", "header");
print("foo value (readOnly):", "subheader");
print(o1.get("foo"));
print("bar value (writeOnce):", "subheader");
print(o1.get("bar"));
</textarea>
<p>We then attempt to set values for both attributes again, using <code>set</code>, and see that niether of the values are modified:</p>
<textarea name="code" class="JScript" rows="1" cols="60">
print("Try Setting Again", "header");
o1.set("foo", "User Reset Foo");
o1.set("bar", "User Reset Bar");
print("Final foo value (readOnly)", "subheader");
print(o1.get("foo"));
print("Final bar value (writeOnce)", "subheader");
print(o1.get("bar"));
</textarea>
<h3>Setting The State Of ReadOnly Values Internally</h3>
<p>Although the user cannot update the value of <code>readOnly</code> attributes, it maybe neccessary for the host object to update it's value internally. The example shows how this can be done, using the private <code>_set</code> property on the host:</p>
<textarea name="code" class="JScript" rows="1" cols="60">
MyClass.prototype.doSomething = function() {
// ... Do something which requires
// MyClass to change the value
// of foo ...
// Host code can reset value of
// readOnly attributes interally,
// by working with the private _set property
this._set("foo", "New Default Foo");
};
</textarea>
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<h4>
Attribute Examples:</h4>
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<li><a href='../attribute/attribute-basic.html'>Basic Configuration</a></li><li><a href='../attribute/attribute-event.html'>Change Events</a></li><li><a href='../attribute/attribute-getset.html'>Getters, Setters and Validators</a></li><li class='selected'><a href='../attribute/attribute-rw.html'>readOnly and writeOnce Configuration</a></li> </ul>
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<h4>More Attribute Resources:</h4>
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