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# YUI Library Examples: Attribute: Read-Only and Write-Once Attributes

## Attribute: Read-Only and Write-Once Attributes

Attributes can be configured to be `readOnly`, stopping them from being modified by the end user, or `writeOnce` allowing them to be set by the end user, but only once. This example demonstrates how to setup attributes for your class as `readOnly` or `writeOnce` attributes, and shows how their behavior differs when the end user attempts to set their values.

View example in new window.

Construct o1, setting initial values for both foo and bar in the constructor:

foo: bar:

Try setting values again, after they've been set once:

foo: bar:

Call a MyClass method, which sets foo internally (using _set):

foo:

### ReadOnly And WriteOnce

Attribute supports the ability to configure attributes to be `readOnly` or `writeOnce`. `readOnly` attributes cannot be set by the end user, either through initial values passed to the constructor, or by invoking the `set` method. `writeOnce` attributes on the other hand, can be set by the user, but only once, either during initialization or through a call to `set`. Once a value is established for a `writeOnce` attribute, it cannot be reset to another value by the user.

### Configuring ReadOnly And WriteOnce Attributes

This example sets up a custom class, `MyClass`, with two attributes, `foo` and `bar`. `foo` is configured to be a `readOnly` attribute, and `bar` is configured to be a `writeOnce` attribute:

    // Setup a custom class with attribute supportfunction MyClass(cfg) {     // Attribute configuration    var attrs = {        "foo" : {            value: "Default Foo",            readOnly: true        },         "bar" : {            value: "Default Bar",            writeOnce: true        }    }     this.addAttrs(attrs, cfg);}

    // Setup a custom class with attribute support
    function MyClass(cfg) {

        // Attribute configuration
        var attrs = {
            "foo" : {
                value: "Default Foo",
                readOnly: true
            },

            "bar" : {
                value: "Default Bar",
                writeOnce: true
            }
        }

        this.addAttrs(attrs, cfg);
    }

// Setup a custom class with attribute support function MyClass(cfg) { // Attribute configuration var attrs = { "foo" : { value: "Default Foo", readOnly: true }, "bar" : { value: "Default Bar", writeOnce: true } } this.addAttrs(attrs, cfg); }

### Attempting To Set Values

We first attempt to set values for both attributes in the constructor (used to intialize the attributes) and see that only `bar`, the `writeOnce` attribute, gets set to the user provided value:

    var fooVal = Y.one("#writeInitial .fooVal").get("value");var barVal = Y.one("#writeInitial .barVal").get("value"); o1 = new MyClass({    foo: fooVal,    bar: barVal}); displayValues(o1, "Attempt to set initial values in constructor",                 "#writeInitial .example-out");

    var fooVal = Y.one("#writeInitial .fooVal").get("value");
    var barVal = Y.one("#writeInitial .barVal").get("value");

    o1 = new MyClass({
        foo: fooVal,
        bar: barVal
    });

    displayValues(o1, "Attempt to set initial values in constructor",
                    "#writeInitial .example-out");

var fooVal = Y.one("#writeInitial .fooVal").get("value"); var barVal = Y.one("#writeInitial .barVal").get("value"); o1 = new MyClass({ foo: fooVal, bar: barVal }); displayValues(o1, "Attempt to set initial values in constructor", "#writeInitial .example-out");

We then attempt to set values for both attributes again, using `set`, and see that niether of the values are modified:

    var fooVal = Y.one("#writeAgain .fooVal").get("value");var barVal = Y.one("#writeAgain .barVal").get("value"); // Attempt to reset values:o1.set("foo", fooVal);o1.set("bar", barVal); displayValues(o1, "Attempt to set values again, using set",                 "#writeAgain .example-out");

    var fooVal = Y.one("#writeAgain .fooVal").get("value");
    var barVal = Y.one("#writeAgain .barVal").get("value");

    // Attempt to reset values:
    o1.set("foo", fooVal);
    o1.set("bar", barVal);

    displayValues(o1, "Attempt to set values again, using set",
                    "#writeAgain .example-out");

var fooVal = Y.one("#writeAgain .fooVal").get("value"); var barVal = Y.one("#writeAgain .barVal").get("value"); // Attempt to reset values: o1.set("foo", fooVal); o1.set("bar", barVal); displayValues(o1, "Attempt to set values again, using set", "#writeAgain .example-out");

### Setting The State Of ReadOnly Values Internally

Although the user cannot update the value of `readOnly` 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 `_set` property on the host:

    MyClass.prototype.doSomething = function(val) {    // ... 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", val);}; ... var val = Y.one("#writeInternally .fooVal").get("value"); // Call method, which sets foo internally...o1.doSomething(val); displayValues(o1, "Set value of foo (readOnly) interally",                 "#writeInternally .example-out");

    MyClass.prototype.doSomething = function(val) {
        // ... 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", val);
    };

    ...

    var val = Y.one("#writeInternally .fooVal").get("value");

    // Call method, which sets foo internally...
    o1.doSomething(val);

    displayValues(o1, "Set value of foo (readOnly) interally",
                    "#writeInternally .example-out");

MyClass.prototype.doSomething = function(val) { // ... 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", val); }; ... var val = Y.one("#writeInternally .fooVal").get("value"); // Call method, which sets foo internally... o1.doSomething(val); displayValues(o1, "Set value of foo (readOnly) interally", "#writeInternally .example-out");

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