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/**
* Copyright (c) 2013-present, Facebook, Inc.
*
* This source code is licensed under the MIT license found in the
* LICENSE file in the root directory of this source tree.
*
* @providesModule UnicodeBidiDirection
* @typechecks
* @flow
*/
/**
* Constants to represent text directionality
*
* Also defines a *global* direciton, to be used in bidi algorithms as a
* default fallback direciton, when no better direction is found or provided.
*
* NOTE: Use `setGlobalDir()`, or update `initGlobalDir()`, to set the initial
* global direction value based on the application.
*
* Part of the implementation of Unicode Bidirectional Algorithm (UBA)
* Unicode Standard Annex #9 (UAX9)
* http://www.unicode.org/reports/tr9/
*/
'use strict';
const invariant = require('./invariant');
export type BidiDirection = 'LTR' | 'RTL' | 'NEUTRAL';
export type HTMLDir = 'ltr' | 'rtl';
const NEUTRAL = 'NEUTRAL'; // No strong direction
const LTR = 'LTR'; // Left-to-Right direction
const RTL = 'RTL'; // Right-to-Left direction
let globalDir: ?BidiDirection = null;
// == Helpers ==
/**
* Check if a directionality value is a Strong one
*/
function isStrong(dir: BidiDirection): boolean {
return dir === LTR || dir === RTL;
}
/**
* Get string value to be used for `dir` HTML attribute or `direction` CSS
* property.
*/
function getHTMLDir(dir: BidiDirection): HTMLDir {
invariant(isStrong(dir), '`dir` must be a strong direction to be converted to HTML Direction');
return dir === LTR ? 'ltr' : 'rtl';
}
/**
* Get string value to be used for `dir` HTML attribute or `direction` CSS
* property, but returns null if `dir` has same value as `otherDir`.
* `null`.
*/
function getHTMLDirIfDifferent(dir: BidiDirection, otherDir: BidiDirection): ?HTMLDir {
invariant(isStrong(dir), '`dir` must be a strong direction to be converted to HTML Direction');
invariant(isStrong(otherDir), '`otherDir` must be a strong direction to be converted to HTML Direction');
return dir === otherDir ? null : getHTMLDir(dir);
}
// == Global Direction ==
/**
* Set the global direction.
*/
function setGlobalDir(dir: BidiDirection): void {
globalDir = dir;
}
/**
* Initialize the global direction
*/
function initGlobalDir(): void {
setGlobalDir(LTR);
}
/**
* Get the global direction
*/
function getGlobalDir(): BidiDirection {
if (!globalDir) {
this.initGlobalDir();
}
invariant(globalDir, 'Global direction not set.');
return globalDir;
}
const UnicodeBidiDirection = {
// Values
NEUTRAL,
LTR,
RTL,
// Helpers
isStrong,
getHTMLDir,
getHTMLDirIfDifferent,
// Global Direction
setGlobalDir,
initGlobalDir,
getGlobalDir
};
module.exports = UnicodeBidiDirection;
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