[−][src]Trait piet::TextLayout

pub trait TextLayout: Clone {
    fn width(&self) -> f64;

    fn update_width(&mut self, new_width: f64) -> Result<(), Error>;

    fn line_text(&self, line_number: usize) -> Option<&str>;

    fn line_metric(&self, line_number: usize) -> Option<LineMetric>;

    fn line_count(&self) -> usize;

    fn hit_test_point(&self, point: Point) -> HitTestPoint;

    fn hit_test_text_position(
        &self, 
        text_position: usize
    ) -> Option<HitTestTextPosition>;
}

Text Layout

Line Breaks

A text layout may be broken into multiple lines in order to fit within a given width. Line breaking is generally done between words (whitespace-separated).

When resizing the width of the text layout, calling update_width on the text layout will recalculate line breaks and modify in-place.

A line's text and LineMetrics can be accessed line-by-line by 0-indexed line number.

Fields on ['LineMetric`] include:

  • line start offset from text layout beginning (in UTF-8 code units)
  • line end offset from text layout beginning (in UTF-8 code units)
  • line trailing whitespace (in UTF-8 code units)
  • line's baseline, distance of the baseline from the top of the line
  • line height
  • cumulative line height (includes previous line heights)

The trailing whitespace distinction is important. Lines are broken at the grapheme boundary after whitespace, but that whitepace is not necessarily rendered since it's just the trailing whitepace at the end of a line. Keeping the trailing whitespace data available allows API consumers to determine their own trailing whitespace strategy.

Text Position

A text position is the offset in the underlying string, defined in utf-8 code units, as is standard for Rust strings.

However, text position is also related to valid cursor positions. Therefore:

  • The beginning of a line has text position 0.
  • The end of a line is a valid text position. e.g. text.len() is a valid text position.
  • If the text position is not at a code point or grapheme boundary, undesirable behavior may occur.

Required methods

fn width(&self) -> f64

Measure the advance width of the text.

fn update_width(&mut self, new_width: f64) -> Result<(), Error>

Used for changing the width of a text layout. Given a width, returns a TextLayout struct with recalculated lines and line metrics.

fn line_text(&self, line_number: usize) -> Option<&str>

Given a line number, return a reference to that line's underlying string.

fn line_metric(&self, line_number: usize) -> Option<LineMetric>

Given a line number, return a reference to that line's metrics.

fn line_count(&self) -> usize

Returns total number of lines in the text layout.

fn hit_test_point(&self, point: Point) -> HitTestPoint

Given a Point, determine the corresponding text position.

Return value:

Returns a HitTestPoint describing the results of the test.

HitTestPoint field is_inside is true if the tested point falls within the bounds of the text, false otherwise.

HitTestPoint field metrics is a HitTestMetrics struct. HitTestMetrics field text_position is the text position closest to the tested point.

Notes:

Some text position will always be returned; if the tested point is inside, it returns the appropriate text position; if it's outside, it will return the nearest text position (either 0 or text.len()).

For more on text positions, see docs for the TextLayout trait.

fn hit_test_text_position(
    &self,
    text_position: usize
) -> Option<HitTestTextPosition>

Given a text position, determine the corresponding pixel location. (currently consider the text layout just one line)

Return value:

Returns a HitTestTextPosition describing the results of the test.

HitTestTextPosition field point is the point offset of the boundary of the grapheme cluster that the text position is a part of.

HitTestTextPosition field metrics is a HitTestMetrics struct. HitTestMetrics field text_position is the original text position (unless out of bounds).

Notes:

In directwrite, if a text position is not at code point boundary, this method will panic. Cairo and web are more lenient and may not panic.

For text position that is greater than text.len(), web/cairo will return the HitTestTextPosition as if text_position == text.len(). In directwrite, the method will panic, as the text position is out of bounds.

For more on text positions, see docs for the TextLayout trait.

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Implementors

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