<attribute>

Sets an attribute of the object that contains it.

The <attribute> tag has two uses:

In a class definiiton, the <attribute> tag defines an attribute that can be set in instances of the class.
```
<canvas height="30">
  <class name="mybutton" extends="button">
    <attribute name="myattr" type="string" value="after"/>
  </class>
  <mybutton text="before" myattr="after"
            onclick="setAttribute('text', this.myattr)"/>
</canvas>
```
In an instance, the <attribute> tag attaches an attribute to the instance. The attribute can be accessed from script.
```
<canvas height="30">

  <class name="mybutton" extends="button">
      <attribute name="myattr" type="string" value="after"/>
  </class>
  <mybutton text="before" 
            onclick="setAttribute('text', this.myattr)">
    <attribute name="myattr" value="perhaps later"/>
  </mybutton>
</canvas>
```

See the Guide for a complete discussion.

Also see the documentation for <class> and the general description of LZX attributes. The Classes tutorial describes how to use attributes to define a class.

Attributes
Name	Usage	Type (Tag)	Type (JS)	Default	Category
name	Tag only	token			final
	The name of the attribute.

required	Tag only	boolean		false	final
	True if this is a required attribute.

setter	Tag only	script			final
	Code that is executed when setAttribute is used to set the value of this attribute. This code is executed within a context in which the variable with this attribute's name is bound to the value passed to setAttribute.

type	Tag only	boolean \| color \| expression \| number \| size \| string \| text \| html		expression	final
	The type of the attribute. This determines how the value is parsed. Attributes of type string are automatically quoted if when="immediate", for any other value of when, a literal string would have to be quoted to be used as a value.

value	Tag only	string			final
	The initial value of the attribute. The type of this value is specified by the value of the type attribute; if that attribute is not present, it's the type of this attribute in this element's tag. The attribute is given this value when the element is created.

when	Tag only	immediately \| once \| always		immediately	final
	When the initial value expression is to be evaluated. One of: `immediately` evaluate when the enclosing element is defined (must be a constant) `once` evaluate when the enclosing element is initialized `always` update whenever the value of the expression changes The default is `when="immediately"`. The setting for `when` can be overridden when assigning a value to an attribute by using the syntax `attribute name=$when value{expression}`. The default for `when value` is `always`, for example: `<view title="$once{computeTitle()}" />` sets the title of the view to a computed value and `<view title="${parent.subtitle}" \>` will cause the title of the view to track `parent.subtitle`.

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