Mailspring/docs-atom/advanced/scopes-and-scope-descriptors.md

Scoped Settings, Scopes and Scope Descriptors

Atom supports language-specific settings. You can soft wrap only Markdown files, or set the tab length to 4 in Python files.

Language-specific settings are a subset of something more general we call "scoped settings". Scoped settings allow targeting down to a specific syntax token type. For example, you could conceivably set a setting to target only Ruby comments, only code inside Markdown files, or even only JavaScript function names.

Scope names in syntax tokens

Each token in the editor has a collection of scope names. For example, the aformentioned JavaScript function name might have the scope names function and name. An open paren might have the scope names punctuation, parameters, begin.

Scope names work just like CSS classes. In fact, in the editor, scope names are attached to a token's DOM node as CSS classes.

Take this piece of JavaScript:

function functionName() {
  console.log('Log it out');
}

In the dev tools, the first line's markup looks like this.

All the class names on the spans are scope names. Any scope name can be used to target a setting's value.

Scope Selectors

Scope selectors allow you to target specific tokens just like a CSS selector targets specific nodes in the DOM. Some examples:

'.source.js' # selects all javascript tokens
'.source.js .function.name' # selects all javascript function names
'.function.name' # selects all function names in any language

Config::set accepts a scopeSelector. If you'd like to set a setting for JavaScript function names, you can give it the js function name scopeSelector:

atom.config.set('.source.js .function.name', 'my-package.my-setting', 'special value')

Scope Descriptors

A scope descriptor is an Object that wraps an Array of Strings. The Array describes a path from the root of the syntax tree to a token including all scope names for the entire path.

In our JavaScript example above, a scope descriptor for the function name token would be:

['source.js', 'meta.function.js', 'entity.name.function.js']

Config::get accepts a scopeDescriptor. You can get the value for your setting scoped to JavaScript function names via:

scopeDescriptor = ['source.js', 'meta.function.js', 'entity.name.function.js']
value = atom.config.get(scopeDescriptor, 'my-package.my-setting')

But, you do not need to generate scope descriptors by hand. There are a couple methods available to get the scope descriptor from the editor:

• Editor::getRootScopeDescriptor to get the language's descriptor. eg. [".source.js"]
• Editor::scopeDescriptorForBufferPosition to get the descriptor at a specific position in the buffer.
• Cursor::getScopeDescriptor to get a cursor's descriptor based on position. eg. if the cursor were in the name of the method in our example it would return ["source.js", "meta.function.js", "entity.name.function.js"]

Let's revisit our example using these methods:

editor = atom.workspace.getActiveTextEditor()
cursor = editor.getLastCursor()
valueAtCursor = atom.config.get(cursor.getScopeDescriptor(), 'my-package.my-setting')
valueForLanguage = atom.config.get(editor.getRootScopeDescriptor(), 'my-package.my-setting')