A direction in which text flows.
Some languages are written from the left to the right (for example, English, Tamil, or Chinese), while others are written from the right to the left (for example Aramaic, Hebrew, or Urdu). Some are also written in a mixture, for example Arabic is mostly written right-to-left, with numerals written left-to-right.
The text direction must be provided to APIs that render text or lay out boxes horizontally, so that they can determine which direction to start in: either right-to-left, TextDirection.rtl; or left-to-right, TextDirection.ltr.
Flutter is designed to address the needs of applications written in any of the world's currently-used languages, whether they use a right-to-left or left-to-right writing direction. Flutter does not support other writing modes, such as vertical text or boustrophedon text, as these are rarely used in computer programs.
It is common when developing user interface frameworks to pick a default text direction — typically left-to-right, the direction most familiar to the engineers working on the framework — because this simplifies the development of applications on the platform. Unfortunately, this frequently results in the platform having unexpected left-to-right biases or assumptions, as engineers will typically miss places where they need to support right-to-left text. This then results in bugs that only manifest in right-to-left environments.
In an effort to minimize the extent to which Flutter experiences this
category of issues, the lowest levels of the Flutter framework do not have a
default text reading direction. Any time a reading direction is necessary,
for example when text is to be displayed, or when a
writing-direction-dependent value is to be interpreted, the reading
direction must be explicitly specified. Where possible, such as in
statements, the right-to-left case is listed first, to avoid the impression
that it is an afterthought.
At the higher levels (specifically starting at the widgets library), an ambient Directionality is introduced, which provides a default. Thus, for instance, a widgets.Text widget in the scope of a MaterialApp widget does not need to be given an explicit writing direction. The Directionality.of static method can be used to obtain the ambient text direction for a particular BuildContext.
Known left-to-right biases in Flutter
Despite the design intent described above, certain left-to-right biases have nonetheless crept into Flutter's design. These include:
The Canvas origin is at the top left, and the x-axis increases in a left-to-right direction.
The default localization in the widgets and material libraries is American English, which is left-to-right.
Visual properties vs directional properties
Many classes in the Flutter framework are offered in two versions, a visually-oriented variant, and a text-direction-dependent variant. For example, EdgeInsets is described in terms of top, left, right, and bottom, while EdgeInsetsDirectional is described in terms of top, start, end, and bottom, where start and end correspond to right and left in right-to-left text and left and right in left-to-right text.
There are distinct use cases for each of these variants.
Text-direction-dependent variants are useful when developing user interfaces that should "flip" with the text direction. For example, a paragraph of text in English will typically be left-aligned and a quote will be indented from the left, while in Arabic it will be right-aligned and indented from the right. Both of these cases are described by the direction-dependent TextAlign.start and EdgeInsetsDirectional.start.
In contrast, the visual variants are useful when the text direction is known and not affected by the reading direction. For example, an application giving driving directions might show a "turn left" arrow on the left and a "turn right" arrow on the right — and would do so whether the application was localized to French (left-to-right) or Hebrew (right-to-left).
In practice, it is also expected that many developers will only be targeting one language, and in that case it may be simpler to think in visual terms.