Attribute: Getters, Setters and Validators
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+ The Basic Configuration example shows how we can add attributes to a host class, and set up default values for them using the attribute configuration object. This example explores how you can configure setter, getter and validator functions for individual attributes, which can be used to modify or normalize attribute values during get and set invocations, and prevent invalid values from being stored.
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- Try entering valid and invalid values for x, y; or values which attempt to position the box outside it's parent (parent box co-ordinates are displayed next to the text box). +
- Try entering rgb, hex or keyword color values [
rgb(255,0,0),#ff0000,red].
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+Getter, Setter And Validator Functions
+ +Attribute lets you configure getter and setter functions for each attribute. These functions are invoked when the user calls Attribute's get and set methods, and provide a way to modify the value returned or the value stored respectively.
You can also define a validator function for each attribute, which is used to validate the final value before it gets stored.
All these functions receive the value and name of the attribute being set or retrieved, as shown in the example code below. The name is not used in this example, but is provided to support use cases where you may wish to share the same function between different attributes.
+ +Creating The Box Class - The X, Y And XY Attributes
+ +In this example, we'll set up a custom Box class representing a positionable element, with x, y and xy attributes.
Only the xy attribute will actually store the page co-ordinate position of the box. The x and y attributes provide the user a convenient way to set only one of the co-ordinates.
+However we don't want to store the actual values in the x and y attributes, to avoid having to constantly synchronize all three.
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+The getter and setter functions provide us with an easy way to achieve this. We'll define getter and setter functions for both the x and y attributes, which simply pass through to the xy attribute to store and retrieve values:
The validator function for xy ensures that only valid values finally end up being stored.
The xy attribute also has a setter function configured, which makes sure that the box is always constrained to it's parent element. The constrain method which it delegates to, takes the xy value the user is trying to set and returns a constrained value if the x or y values fall outside the parent box. The value which is returned by the setter is the value which is ultimately stored for the xy attribute:
The setter, getter and validator functions are invoked with the host object as the context, so that they can refer to the host object using "this", as we see in the setter function for xy.
The Color Attribute - Normalizing Stored Values Through Get
+ +The Box class also has a color attribute which also has a getter and validator functions defined:
The role of the getter handler in this case is to normalize the actual stored value of the color attribute, so that users always receive the hex value, regardless of the actual value stored, which maybe a color keyword (e.g. "red"), an rgb value (e.g.rbg(255,0,0)), or a hex value (#ff0000). The validator ensures the the stored value is one of these three formats.
Syncing Changes Using Attribute Change Events
+ +Another interesting aspect of this example, is it's use of attribute change events to listen for changes to the attribute values. Box's _bind method configures a set of attribute change event listeners which monitor changes to the xy, color and parent attributes and update the rendered DOM for the Box in response:
Since only xy stores the final co-ordinates, we don't need to monitor the x and y attributes individually for changes.
DOM Event Listeners And Delegation
+ +Although not an integral part of the example, it's worth highlighting the code which is used to setup the DOM event listeners for the form elements used by the example:
+ + + +Rather than attach individual listeners to each button, the above code uses YUI 3's delegate support, to listen for clicks from buttons with an action class which bubble up to the attrs element.
The delegate listener uses the Event Facade which normalizes cross-browser access to DOM event properties, such as currentTarget, to route to the appropriate button handler. Note the use of selector syntax when we specify the elements for the listener (e.g. #attrs, button.actions) and the use of the Node facade when dealing with references to HTML elements (e.g. xTxt, yTxt, colorTxt).