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# StyleSheet

The StyleSheet module normalizes the dynamic creation and modification of CSS stylesheets on a page. This makes it easy to manage the development, storage, and reapplication of personalized user stylesheets. Because it targets styles at the CSS level, it also allows you to modify styles applied to a CSS pseudo-element such as `p::first-letter`, or pseudo-class such as `a:hover`.

StyleSheet is capable of creating new stylesheets from scratch or modifying existing stylesheets held as properties of `<link>` or `<style>` elements. It should be noted that not all browsers support reading or modifying external stylesheets from other domains.

## Getting Started

To include the source files for StyleSheet and its dependencies, first load the YUI seed file if you haven't already loaded it.

    <script src="http://yui.yahooapis.com/3.10.3/build/yui/yui-min.js"></script>

Next, create a new YUI instance for your application and populate it with the modules you need by specifying them as arguments to the `YUI().use()` method. YUI will automatically load any dependencies required by the modules you specify.

    <script>
    // Create a new YUI instance and populate it with the required modules.
    YUI().use('stylesheet', function (Y) {
        // StyleSheet is available and ready for use. Add implementation
        // code here.
    });
    </script>

For more information on creating YUI instances and on the `use()` method, see the documentation for the YUI Global Object.

## Using the StyleSheet Utility

### Instantiating a `Y.StyleSheet`

The `Y.StyleSheet` constructor is written to support both function syntax and normal constructor syntax making the `new` prefix unnecessary (but harmless).

The constructor has no required parameters. Passing no arguments will create a new, empty StyleSheet instance.

    // These are equivalent; both create new empty StyleSheets
    var myStyleSheet = new Y.StyleSheet();

    var myOtherStyleSheet = Y.StyleSheet();

To seed a new StyleSheet with a number of CSS rules, you can pass the constructor any of the following:

1. a `<style>` or `<link>` node reference,
2. the id of a `<style>` or `<link>` node, or
3. a string of CSS

    <link id="local" type="text/css" rel="stylesheet" href="local_file.css">
    <style id="styleblock" type="text/css">
        .some select.or {
            margin-right: 2em;
        }
    </style>

    YUI().use('node','stylesheet', function (Y) {

        // Node or HTMLElement reference for a style or locally sourced link element
        var sheet = Y.StyleSheet(Y.one("#styleblock"));
        sheet = Y.StyleSheet(Y.DOM.byId('local'));

        // OR the id of a style element or locally sourced link element
        sheet = Y.StyleSheet('#local');

        // OR string of css text
        var css = ".moduleX .alert { background: #fcc; font-weight: bold; } " +
                  ".moduleX .warn  { background: #eec; } " +
                  ".hide_messages .moduleX .alert, " +
                  ".hide_messages .moduleX .warn { display: none; }";

        sheet = new Y.StyleSheet(css);

        //...
    });

Be aware that the Same Origin policy prevents access in some browsers to the style data of `<link>` elements with `href`s pointing to other domains. Attempts to seed a `Y.StyleSheet` instance with a cross-domain `<link>` may result in a security error.

    <link id="remote" type="text/css" rel="stylesheet" href="http://other.domain.com/remote_file.css">

    // ERROR - Same Origin Policy prevents access to remote stylesheets
    var styleSheet = Y.StyleSheet('remote');

### Getting a StyleSheet by registered name

`Y.StyleSheet` supports registering instances by name allowing them to be recalled by that same name elsewhere in your code. Internally, `Y.StyleSheet` maintains a registry of all created StyleSheet instances, using a unique generated id that the host node is tagged with. This allows future attempts to create a StyleSheet instance from the same node to return the previously created instance associated with that id.

Register a StyleSheet instance manually using the static `register` method or pass the desired name as a second parameter to the constructor.

    var sheetA = Y.StyleSheet('my_stylesheet');

    // Create a registry alias to sheetA.  We'll call it bob.
    Y.StyleSheet.register(sheetA, 'bob');

    // Create another StyleSheet passing the name as the second parameter
    var css = ".some .css { white-space: pre-wrap; color: pink; }";
    var sheetB = Y.StyleSheet(css, 'my sheet');

    // Meanwhile, elsewhere in your code

    // sheetA is the same instance as sheet1 and sheet2
    var sheet1 = Y.StyleSheet(Y.one('#my_stylesheet')),
        sheet2 = Y.StyleSheet('bob');

    // sheetB is the same instance as sheet3
    var sheet3 = Y.StyleSheet('my sheet');

If an unregistered name is passed as the first argument to the constructor, a new empty StyleSheet will be created and registered with that name. This allows you to use the following code pattern:

    // Whichever of these executes first, an empty StyleSheet will be created
    // and registered as 'MyApp'.

    // In one area of your app
    Y.StyleSheet('MyApp').set('.module .messages', { display: 'none' });

    //...

    // In another area of your app
    Y.StyleSheet('MyApp').unset('.module .messages','display');

### Summary of how the constructor handles the first argument

When nothing is passed as the first argument, a new StyleSheet instance is created.

When a `<style>` or `<link>` element is passed as the first argument, it is inspected for the id stamp that StyleSheet tags known host nodes with. If it finds one, it will return the associated StyleSheet from the registry. If not, it will stamp the node and seed the instance from the node's CSS content.

When a string is passed as the first argument, StyleSheet does the following things in order:

1. Check the registry for an instance associated to that name. If found, return the instance.
2. Check the DOM for a `<style>` or `<link>` node with that id. If found, check the registry for an instance associated to its tagged id if present. If found, return that instance. If not, use that node to seed a new StyleSheet instance.
3. Check the string for a curly brace { character. If found, create a new instance seeded with the string as initial `cssText`.
4. Create a new empty StyleSheet and register the instance by the provided string.

### Creating and modifying CSS style rules

The core method of StyleSheet instances is `set(selector, style_properties)`. It will create or alter a CSS rule using the property:value pairs in `style_properties` targeting the provided `selector`. In essence, it looks very much like natural CSS syntax, except style properties must be in JavaScript's camelCase.

    Y.StyleSheet('MyApp').set(
        "q.uoted select.or[str=ing]", {
            fontSize   : "150%",         // note the camel casing
            background : "#030 url(/images/bg_image.png) scroll repeat-y top left",
            cssFloat   : "left",
            opacity    : 0.5
        });

Rather than continually add new rules that will override one another, StyleSheet manages one rule per selector and modifies them in place. This may be relevant if you have two or more rules with selectors of the same specificity.

As with regular CSS syntax, comma-separated selectors are supported, but internally StyleSheet splits them up into individual rules because browser support for multiple selectors is not consistent. This means calling `set(..)` with such a selector string will incur multiple repaints or reflows, but limited to the number of atomic selectors.

    // This is valid, but will trigger 3 reflows
    Y.StyleSheet('MyApp').set(
        '.foo, .bar, .baz', {
            borderRight: "1em solid #f00"
        });

### Some style properties are normalized

Two style properties have differing implementation between browsers, namely `float` and `opacity`. StyleSheet instances will normalize these properties for you.

Because "float" is a reserved word in JavaScript, it is supported by the name `cssFloat` in W3C compliant browsers and `styleFloat` in IE. StyleSheet will accept any of these to set the `float` property.

    // Any of these will work
    Y.StyleSheet('MyApp').set('.foo', {
        "float"    : "left",   // "float" must be quoted
        cssFloat   : "right",
        styleFloat : "none"
    });

IE does not support the `opacity` style property, but has equivalent functionality offered by its proprietary `filter` property, though using a different value syntax. StyleSheet will translate `opacity` to `filter` for IE, but it will not translate `filter` to `opacity` for W3C-compliant browsers.

### Removing and resetting CSS style rules

When you want to remove a particular rule or style property from affecting the cascade, use `unset(selector,propert[y|ies])`.

`unset(..)` can be called in any of the following ways, with the noted result:

- `unset('.foo')` — removes the rule associated to the selector entirely.
- `unset('.foo','font')` — unsets the `font` property and any child properties (e.g. 'font-weight','font-variant','font-size','line-height', and 'font-family'). If there are no set properties left, the rule is removed.
- `unset('.foo',['font','border',...])` — same as above, but the rule is modified only once with the final applicable `cssText`.

It is important to note that there is a difference between setting a style property to its default value and unsetting it. The former can be achieved by calling `set(selector, { property: "auto" })` (or the respective default value for that property).

However, as the CSS is reapplied to the page, the "auto" value will override any value for that property that may have cascaded in from another rule. This is different than removing the property assignment entirely, as this allows cascading values through.

    Y.StyleSheet('MyApp').set('.foo', { background: 'auto' });

    // is NOT the same as

    Y.StyleSheet('MyApp').unset('.foo','background');

### A note on selector strings

Though the StyleSheet Utility takes selector strings as input to its API, it does not leverage the YUI selector engine. YUI's selector functionality supplements native CSS support for DOM access, but accomplishes this through efficient DOM traversal. Since the StyleSheet Utility uses the browser's built-in stylesheet and rule objects, it can not handle selectors that are not supported by the browser's native CSS parser.

    // This will not cause a style change in IE 6, for example
    Y.StyleSheet('MyApp').set('input[type=checkbox]:checked', {
        verticalAlign : 'super'
    });

### Disabling and enabling a StyleSheet

Disabling a StyleSheet effectively turns it off; no CSS from that stylesheet is applied to the page. Disabling a StyleSheet does not remove the host node from the page, and style can be reapplied by enabling the StyleSheet again.

When StyleSheets are disabled, it is still possible to change their style rules via `set` and `unset`.

    var sheet = Y.StyleSheet(styleNode);

    sheet.disable();
    sheet.set('.foo', { backgroundColor: '#900', color: '#fff' });
    sheet.set('.bar', { borderBottomColor: '#369' });
    sheet.unset('.baz');
    sheet.enable();

### Static methods

`Y.StyleSheet` exposes a few static methods.

Method | Use for
--- | ---
    Y.StyleSheet.register(instance, name)
| Use to assign a named registry entry for a StyleSheet instance.
    Y.StyleSheet.toCssText(property_obj, starting_cssText)
| Use to translate an object of style property:value pairs to a single `cssText` string. The optional second argument is a `cssText` string of a style's "before" state.

`Y.StyleSheet.toCssText` is used internally to assemble the `cssText` strings for updating the stylesheet rules. However, it may also be helpful for avoiding reflow overhead when substantially modifying a single element's style.

    var el           = Y.one('some_element'),
        changes      = { color : '#684', fontWeight: 'bold', padding: '2em' },
        currentStyle = el.getStyle('cssText');

    el.setStyle('cssText',
        Y.StyleSheet.toCssText(changes, currentStyle));

## How `Y.StyleSheet` works

Browsers grant access via the DOM API to stylesheets included in markup as `<link>` or `<style>` elements. Despite differing implementations across the browser spectrum, they all support adding, removing, and modifying CSS rules.

CSS rules are comprised of a selector and collection of style property:value pairs enclosed in curly braces.

    /* |            This is a CSS rule          |
       |    selectorText    |  style properties | */
       div.this-is a .rule  { font-color: #f00; }

In JavaScript, each rule object has a `selectorText` property and a `style` property that operates in the same way as the `style` property on regular DOM elements, such as `<p>` or `<strong>` elements.

Arguably the most valuable property of the style collection is `cssText` which corresponds to the serialized summary of property:value pairs applied by this collection (e.g. "font-size: 100%; color: #FF0000;"). The reason this property is important is that modifications to the string value will cause changes to repopulate the individual style properties while only triggering a single repaint or reflow by the browser.

    var el = document.getElementById('some_element');

    el.style.borderBottom = '3px solid #eee'; // reflow
    el.style.borderTop    = '3px solid #ccc'; // another reflow
    el.style.fontWeight   = 'bold';           // another reflow

    // Vs. three changes in one reflow
    el.style.cssText += '; border-bottom: 3px solid #eee; border-top: 3px solid #ccc; font-weight: bold';

`Y.StyleSheet` leverages this mechanism in addition to applying modifications at the CSS rule level rather than modifying each targeted DOM node directly. This means changing multiple style properties on multiple elements (that can be identified by a single selector) will only ever incur one repaint or reflow.

It must be noted that all reflows are not the same. The scope of a reflow is greatly affected by what element triggered it. For example, changing a style of an absolutely positioned element will trigger a very limited reflow because the browser understands that not much could change as a result. Stylesheet modifications on the other hand are not tied to an element, but the page as a whole. The CSS cascade must be recalculated and applied, resulting in a full page reflow. This means it may be more efficient to individually update many elements than to change the stylesheet.

## Known Issues

Unable to set style values with `!important`.

CSS syntax for declaring that a style value has `important` priority is to include the `!important` flag after the value.

    .some-class {
        color: #000 !important;
    }

However, the DOM API for modifying stylesheets does not parse out the `!important` flag from the assigned value string, and thus considers the entire string to be an invalid value.

    el.style.color = "#000 !important"; // Error

StyleSheet will support `!important` in the value string in a future release, but for now the only way to assign an `!important` style is by creating a new StyleSheet, passing a CSS text string to the constructor.

    var sheet = new Y.StyleSheet();
    sheet.set(".foo", { color: "#000 !important" }); // FAIL

    new Y.StyleSheet(".foo { color: #000 !important; }"); // WORKS