BidiFormatter Class
Bidi stands for Bi-directional text. According to Wikipedia: Bi-directional text is text containing text in both text directionalities, both right-to-left (RTL) and left-to-right (LTR). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text directionality in each row.
Utility class for formatting display text in a potentially opposite-directionality context without garbling layout issues. Mostly a very "slimmed-down" and dart-ified port of the Closure Birectional formatting libary. If there is a utility in the Closure library (or ICU, or elsewhere) that you would like this formatter to make available, please contact the Dart team.
Provides the following functionality:
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BiDi Wrapping When text in one language is mixed into a document in another, opposite- directionality language, e.g. when an English business name is embedded in a Hebrew web page, both the inserted string and the text following it may be displayed incorrectly unless the inserted string is explicitly separated from the surrounding text in a "wrapper" that declares its directionality at the start and then resets it back at the end. This wrapping can be done in HTML mark-up (e.g. a 'span dir=rtl' tag) or - only in contexts where mark-up can not be used - in Unicode BiDi formatting codes (LRE|RLE and PDF). Providing such wrapping services is the basic purpose of the BiDi formatter.
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Directionality estimation How does one know whether a string about to be inserted into surrounding text has the same directionality? Well, in many cases, one knows that this must be the case when writing the code doing the insertion, e.g. when a localized message is inserted into a localized page. In such cases there is no need to involve the BiDi formatter at all. In the remaining cases, e.g. when the string is user-entered or comes from a database, the language of the string (and thus its directionality) is not known a priori, and must be estimated at run-time. The BiDi formatter does this automatically.
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Escaping When wrapping plain text - i.e. text that is not already HTML or HTML- escaped - in HTML mark-up, the text must first be HTML-escaped to prevent XSS attacks and other nasty business. This of course is always true, but the escaping cannot be done after the string has already been wrapped in mark-up, so the BiDi formatter also serves as a last chance and includes escaping services.
Thus, in a single call, the formatter will escape the input string as specified, determine its directionality, and wrap it as necessary. It is then up to the caller to insert the return value in the output.
Constructors
Code new BidiFormatter.LTR([alwaysSpan = false]) #
Create a formatting object with a direction. If
alwaysSpan is true we
should always use a span tag, even when the input directionality is
neutral or matches the context, so that the DOM structure of the output
does not depend on the combination of directionalities.
BidiFormatter.LTR([alwaysSpan=false]) : contextDirection = TextDirection.LTR,
_alwaysSpan = alwaysSpan;
Code new BidiFormatter.RTL([alwaysSpan = false]) #
BidiFormatter.RTL([alwaysSpan=false]) : contextDirection = TextDirection.RTL,
_alwaysSpan = alwaysSpan;
Code new BidiFormatter.UNKNOWN([alwaysSpan = false]) #
BidiFormatter.UNKNOWN([alwaysSpan=false]) :
contextDirection = TextDirection.UNKNOWN, _alwaysSpan = alwaysSpan;
Methods
Code TextDirection estimateDirection(String text, [bool isHtml = false]) #
Estimates the directionality of text using the best known general-purpose method (using relative word counts). A TextDirection.UNKNOWN return value indicates completely neutral input. isHtml is true if text HTML or HTML-escaped.
TextDirection estimateDirection(String text, [bool isHtml=false]) {
return estimateDirectionOfText(text, isHtml); //TODO~!!!
}
Code bool get isRTL() #
Is true if the known context direction for this formatter is RTL.
bool get isRTL() => contextDirection == TextDirection.RTL;
Code String wrapWithSpan(String text, [bool isHtml = false, bool resetDir = true, TextDirection direction]) #
Formats a string of a given (or estimated, if not provided)
direction for use in HTML output of the context directionality, so
an opposite-directionality string is neither garbled nor garbles what
follows it.
If the input string's directionality doesn't match the context
directionality, we wrap it with a span tag and add a dir attribute
(either "dir=rtl" or "dir=ltr").
If alwaysSpan was true when constructing the formatter, the input is always
wrapped with span tag, skipping the dir attribute when it's not needed.
If resetDir is true and the overall directionality or the exit directionality of text is opposite to the context directionality, a trailing unicode BiDi mark matching the context directionality is appended (LRM or RLM). If isHtml is false, we HTML-escape the text.
String wrapWithSpan(String text, [bool isHtml=false, bool resetDir=true,
TextDirection direction]) {
if (direction == null) direction = estimateDirection(text, isHtml);
var result;
if (!isHtml) text = htmlEscape(text);
var directionChange = contextDirection.isDirectionChange(direction);
if (_alwaysSpan || directionChange) {
var spanDirection = '';
if (directionChange) {
spanDirection = ' dir=${direction.spanText}';
}
result= '<span$spanDirection>$text</span>';
} else {
result = text;
}
return result.concat(resetDir? _resetDir(text, direction, isHtml) : '');
}
Code String wrapWithUnicode(String text, [bool isHtml = false, bool resetDir = true, TextDirection direction]) #
Format text of a known (if specified) or estimated direction for use in plain-text output of the context directionality, so an opposite-directionality text is neither garbled nor garbles what follows it. Unlike wrapWithSpan, this makes use of unicode BiDi formatting characters instead of spans for wrapping. The returned string would be RLE+text+PDF for RTL text, or LRE+text+PDF for LTR text.
If resetDir is true, and if the overall directionality or the exit directionality of text are opposite to the context directionality, a trailing unicode BiDi mark matching the context directionality is appended (LRM or RLM).
In HTML, the only valid use of this function is inside of elements that do not allow markup, e.g. an 'option' tag. This function does not do HTML-escaping regardless of the value of isHtml. isHtml is used to designate if the text contains HTML (escaped or unescaped).
String wrapWithUnicode(String text, [bool isHtml=false, bool resetDir=true,
TextDirection direction]) {
if (direction == null) direction = estimateDirection(text, isHtml);
var result = text;
if (contextDirection.isDirectionChange(direction)) {
result = '''${direction == TextDirection.RTL ? RLE : LRE}$text$PDF''';
}
return result.concat(resetDir? _resetDir(text, direction, isHtml) : '');
}
Fields
Code TextDirection contextDirection #
The direction of the surrounding text (the context).
TextDirection contextDirection;