What are the different ways to programmatically change CSS rules using JavaScript in a Firefox Add-on, leveraging XUL, SDK, or WebExtensions methods?

Question

What are the different ways to programmatically change CSS rules using JavaScript in a Firefox Add-on, leveraging XUL, SDK, or WebExtensions methods?

Looking to dynamically alter a CSS rule set through JavaScript within a Firefox Add-on using XUL, SDK or WebExtensions techniques. Specifically targeting support for Firefox versions 29.0b1 through 49.0a1.

The main issue I want to address is modifying the following elements:

file.xul: (contains this line at the beginning)

<?xml-stylesheet href="chrome://{GUID}/skin/text.css" type="text/css"?>

text.css: (relevant class in the file)

.myTextClass {
    max-width: 800px;
}

code:

// Make changes to the class selector rule set by adding another rule,
// such as font-family: DejaVu Sans Mono

// Later on, based on user input, update or remove this font from the class

This task may overlap with #20927075, which lacked satisfactory solutions. However, my focus also includes utilizing SDK or WebExtensions techniques to ensure the desired functionality can be implemented within an existing XUL-based add-on using newer APIs.

I aim to create a basic "font selector" UI feature that applies a new font to the class. The CSS rule set currently defines one class with one rule, to which I want to add another rule and reflect these changes in the add-on's interface.

Here are some options I've considered, but find them all somewhat cumbersome:

1) Utilize document.getElementById and manually set the style attribute. This works well for the demo area.

2) Similar to option 1, but use getElementByClassName. This method doesn't actually use a class, it finds nodes using a class and then updates each element's attribute, either style or class. This approach feels sluggish. While I could create a large CSS file with a class for every possible font and change the class attribute, this seems impractical compared to updating the style attribute directly.

3) Consider the style sheet service, although I am uncertain of its longevity given that it may be XPCOM-based and not part of Services.jsm. It appears to only load or unload entire style sheets, instead of allowing precise modifications to individual rules within a class definition.

4) jQuery is an option, but not preferable due to overhead concerns for a small add-on. Additionally, compatibility within XUL interface code is unknown.

5) Explore document.styleSheets which seems promising but falls short in terms of achieving the desired result (unless there is a misunderstanding on my part). Everything appears to be read-only, further complicating matters.

6) Resort to manual manipulation of entire style sheets via document.getElementsByTagName('head'), effectively overriding an entire CSS sheet rather than impacting a specific rule within a class.

If no better alternative exists, I might have to opt for option 2 above. Nevertheless, I was hoping for a solution that allows precise control over CSS from within a XUL document using JavaScript.

In attempting to understand how to accomplish this, I tested the following snippet:

for ( i = 0; i < document.styleSheets.length; i++ ) {
    styleSheet = document.styleSheets[ i ];

    console.log( 'style sheet: ' + i + ' ' + styleSheet.href );

    if ( styleSheet.href === 'chrome://{GUID}/skin/text.css' ) {
        for ( j = 0; j < styleSheet.cssRules.length; j++ ) {
            styleRule = styleSheet.cssRules[ j ];
            console.log( 'style sheet: ' + i + ' ' + 'styleRule: ' + styleRule.cssText );
        }
    }
}

Upon executing this code (placed in file.js referenced from a file.xul <script> tag), I observed log outputs similar to the following (in Firefox 29.0b1):

"style sheet: 0 chrome://git-addonbar/skin/addonbar.css"                              file.js:189
09:49:47.101 "style sheet: 1 chrome://browser/content/browser.css"                    file.js:189
09:49:47.101 "style sheet: 2 chrome://browser/content/places/places.css"              file.js:189
09:49:47.101 "style sheet: 3 chrome://browser/skin/devtools/common.css"               file.js:189
09:49:47.101 "style sheet: 4 chrome://browser/skin/customizableui/panelUIOverlay.css" file.js:189
09:49:47.101 "style sheet: 5 chrome://browser/skin/browser.css"                       file.js:189
09:49:47.101 "style sheet: 6 chrome://browser/skin/browser-lightweightTheme.css"      file.js:189
09:49:47.101 "style sheet: 7 chrome://global/skin/global.css"                         file.js:189
09:49:47.101 "style sheet: 8 chrome://{GUID}/skin/text.css"                           file.js:189
09:49:47.102 "style sheet: 8 styleRule: .myTextClass { max-width: 800px; }"           file.js:194
09:49:47.102 "style sheet: 9 null"                                                    file.js:189

javascript css dom firefox-addon xul

Answer 1

Answer №1

5) Is there a way to modify properties like document.styleSheets? It seems like everything is read-only.

Indeed, the styleSheets property is read-only, but you can still make modifications to the stylesheet object you retrieve.

If you're targeting a specific browser, this task becomes much simpler compared to trying to achieve it cross-browser as seen in Changing a CSS rule-set from Javascript. Here's a basic example:

function updateCSS() {
  let myRule = document.styleSheets[0].cssRules[0];
  if (myRule.style.backgroundColor == "white")
    myRule.style.backgroundColor = "black";
  else
    myRule.style.backgroundColor = "white";
}

.my-class {
  background-color: white;
}

<div class="my-class">Sample Div</div>
<button onclick="updateCSS()">Change Background!</button>

You'll need to tailor this code to locate the stylesheet and rule you want to modify - reference this post with an example of how to do that.

Answer 2

5) Is there a way to modify properties like document.styleSheets? It seems like everything is read-only.

Indeed, the styleSheets property is read-only, but you can still make modifications to the stylesheet object you retrieve.

If you're targeting a specific browser, this task becomes much simpler compared to trying to achieve it cross-browser as seen in Changing a CSS rule-set from Javascript. Here's a basic example:

function updateCSS() {
  let myRule = document.styleSheets[0].cssRules[0];
  if (myRule.style.backgroundColor == "white")
    myRule.style.backgroundColor = "black";
  else
    myRule.style.backgroundColor = "white";
}

.my-class {
  background-color: white;
}

<div class="my-class">Sample Div</div>
<button onclick="updateCSS()">Change Background!</button>

You'll need to tailor this code to locate the stylesheet and rule you want to modify - reference this post with an example of how to do that.

Answer 3

Answer №2

After some experimentation, I was able to come up with a solution that effectively addressed my issue. Here are the key points of interest:

document.styleSheets[ i ].href
document.styleSheets[ i ].cssRules[ j ].selectorText
document.styleSheets[ i ].cssRules[ j ].style.fontFamily

The process hinges on the document.styleSheets array-like object, which is then easily manipulated using DOM methods. One could potentially create a general function for handling this task. When dealing with a known stylesheet location and a unique selector, one can streamline the search process by skipping unnecessary checks on other loaded sheets in Firefox. This represents the simplest scenario.

// Load font and apply style sheet class

// Am using the .href to find the sheet faster because location is known and rule is unique
var myCSShref = 'chrome://{GUID}/skin/text.css';
var mySelector = '.myTextClass';

// fonts stored as a string in a user pref, later split to array inline
// fonts could be stored as array during runtime, remove splits accordingly
// fonts get appended to a XUL menulist element
// list index 0 is for default, 1+ for fonts below
var fonts = 'monospace|Anonymous Pro|Apple Menlo|Bitstream Vera Sans Mono|Consolas|DejaVu Sans Mono|Droid Sans Mono|Everson Mono|Liberation Mono|Lucida Console|Source Code Pro';
var font_index = 2; // Anonymous Pro, the 2 is pulled from a user pref

// for loop vars
var i, styleSheet, j, cssRule, oldFontFamily, newFontFamily;

// console.logs for demonstration purposes only

// The following code only needed to be run once for me,
// but it can easily be changed to a reusable function.

if ( document.styleSheets ) {
    for ( i = 0; i < document.styleSheets.length; i++ ) {
        styleSheet = document.styleSheets[ i ];

        console.log( 'style sheet: ' + i + ' ' + styleSheet.href );

        if ( styleSheet.href === myCSShref ) {
            for ( j = 0; j < styleSheet.cssRules.length; j++ ) {
                cssRule = styleSheet.cssRules[ j ];
                console.log( 'style sheet: ' + i + ' cssRule.selectorText: ' + cssRule.selectorText );

                if ( cssRule && cssRule.selectorText === mySelector ) {
                    oldFontFamily = ( cssRule.style.fontFamily ) ? cssRule.style.fontFamily : '';
                    // font_index 0 is the menu default option, so we have 1 extra
                    newFontFamily = ( font_index === 0 ) ? 'inherit' : fonts.split( '|' )[ font_index - 1 ];
                    console.log( 'style sheet: ' + i + ' cssRule.style.fontFamily: ' + oldFontFamily + ' (before)' );
                    console.log( 'style sheet: ' + i + ' cssRule.style.fontFamily: ' + newFontFamily + ' (after)' );
                    cssRule.style.fontFamily = newFontFamily;
                }
            }
        }
    }
}

Answer 4

After some experimentation, I was able to come up with a solution that effectively addressed my issue. Here are the key points of interest:

document.styleSheets[ i ].href
document.styleSheets[ i ].cssRules[ j ].selectorText
document.styleSheets[ i ].cssRules[ j ].style.fontFamily

The process hinges on the document.styleSheets array-like object, which is then easily manipulated using DOM methods. One could potentially create a general function for handling this task. When dealing with a known stylesheet location and a unique selector, one can streamline the search process by skipping unnecessary checks on other loaded sheets in Firefox. This represents the simplest scenario.

// Load font and apply style sheet class

// Am using the .href to find the sheet faster because location is known and rule is unique
var myCSShref = 'chrome://{GUID}/skin/text.css';
var mySelector = '.myTextClass';

// fonts stored as a string in a user pref, later split to array inline
// fonts could be stored as array during runtime, remove splits accordingly
// fonts get appended to a XUL menulist element
// list index 0 is for default, 1+ for fonts below
var fonts = 'monospace|Anonymous Pro|Apple Menlo|Bitstream Vera Sans Mono|Consolas|DejaVu Sans Mono|Droid Sans Mono|Everson Mono|Liberation Mono|Lucida Console|Source Code Pro';
var font_index = 2; // Anonymous Pro, the 2 is pulled from a user pref

// for loop vars
var i, styleSheet, j, cssRule, oldFontFamily, newFontFamily;

// console.logs for demonstration purposes only

// The following code only needed to be run once for me,
// but it can easily be changed to a reusable function.

if ( document.styleSheets ) {
    for ( i = 0; i < document.styleSheets.length; i++ ) {
        styleSheet = document.styleSheets[ i ];

        console.log( 'style sheet: ' + i + ' ' + styleSheet.href );

        if ( styleSheet.href === myCSShref ) {
            for ( j = 0; j < styleSheet.cssRules.length; j++ ) {
                cssRule = styleSheet.cssRules[ j ];
                console.log( 'style sheet: ' + i + ' cssRule.selectorText: ' + cssRule.selectorText );

                if ( cssRule && cssRule.selectorText === mySelector ) {
                    oldFontFamily = ( cssRule.style.fontFamily ) ? cssRule.style.fontFamily : '';
                    // font_index 0 is the menu default option, so we have 1 extra
                    newFontFamily = ( font_index === 0 ) ? 'inherit' : fonts.split( '|' )[ font_index - 1 ];
                    console.log( 'style sheet: ' + i + ' cssRule.style.fontFamily: ' + oldFontFamily + ' (before)' );
                    console.log( 'style sheet: ' + i + ' cssRule.style.fontFamily: ' + newFontFamily + ' (after)' );
                    cssRule.style.fontFamily = newFontFamily;
                }
            }
        }
    }
}

Answer 5

Answer №3

3) The style sheet service, though still uncertain of its current status or potential obsoletion (based on XPCOM?), remains a rather rudimentary tool that can only load or unload entire style sheets. Unfortunately, this limitation hinders my ability to simply modify a single rule within a class definition.

At present, other addon APIs act as abstractions for the sheet service or utilize windowutils.loadSheet. While Mozilla may eventually phase out this service, these abstractions are likely to be upheld through alternative methods. However, any such changes appear to be years away at this point.

The addon-sdk offers an alternative method for loading styles using its page-mod module.

For Webextensions, support is available either imperatively via tabs.insertCSS or declaratively within the manifest.json file under content_scripts.

That said, these options primarily target content frames. To make modifications to the browser chrome itself, one must resort to xpcom APIs since there are no specific addon APIs designed for this purpose.

Answer 6

3) The style sheet service, though still uncertain of its current status or potential obsoletion (based on XPCOM?), remains a rather rudimentary tool that can only load or unload entire style sheets. Unfortunately, this limitation hinders my ability to simply modify a single rule within a class definition.

At present, other addon APIs act as abstractions for the sheet service or utilize windowutils.loadSheet. While Mozilla may eventually phase out this service, these abstractions are likely to be upheld through alternative methods. However, any such changes appear to be years away at this point.

The addon-sdk offers an alternative method for loading styles using its page-mod module.

For Webextensions, support is available either imperatively via tabs.insertCSS or declaratively within the manifest.json file under content_scripts.

That said, these options primarily target content frames. To make modifications to the browser chrome itself, one must resort to xpcom APIs since there are no specific addon APIs designed for this purpose.

What are the different ways to programmatically change CSS rules using JavaScript in a Firefox Add-on, leveraging XUL, SDK, or WebExtensions methods?

Answer №1

Answer №2

Answer №3

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