Trait TextMeasurer 

pub trait TextMeasurer: Send + Sync {
    // Required method
    fn measure(
        &self,
        text: &str,
        font_size: f32,
        available_width: Option<f32>,
    ) -> (f32, f32);

    // Provided methods
    fn hit_test(
        &self,
        _text: &str,
        _font_size: f32,
        _available_width: Option<f32>,
        _x: f32,
        _y: f32,
    ) -> usize { ... }
    fn get_line_metrics(
        &self,
        _text: &str,
        _font_size: f32,
        _available_width: Option<f32>,
    ) -> Vec<LineMetric> { ... }
    fn get_caret_position(
        &self,
        _text: &str,
        _font_size: f32,
        _available_width: Option<f32>,
        _caret_index: usize,
    ) -> (f32, f32) { ... }
    fn measure_rich_text(
        &self,
        _runs: &[TextRun],
        _available_width: Option<f32>,
    ) -> (f32, f32) { ... }
    fn layout_rich_text(
        &self,
        runs: &[TextRun],
        available_width: Option<f32>,
    ) -> RichTextLayoutInfo { ... }
    fn hit_test_rich(
        &self,
        runs: &[TextRun],
        _available_width: Option<f32>,
        x: f32,
        y: f32,
    ) -> usize { ... }
    fn resolve_rich_text_annotation_at_point(
        &self,
        _runs: &[TextRun],
        _available_width: Option<f32>,
        _x: f32,
        _y: f32,
        _paragraph_style: TextParagraphStyle,
        _annotations: &[RichTextAnnotation],
    ) -> Option<RichTextAnnotation> { ... }
}

A platform-provided text measurement backend.

The layout engine does not shape or measure text itself. Instead, platform backends implement TextMeasurer to wrap their native text engine (CoreText on macOS, DirectWrite on Windows, HarfBuzz + FreeType on Linux, etc.).

All methods have default implementations that return zero-sized results, so you only need to override the methods your backend supports.

Required

• measure – must be implemented to get correct text layout.

Optional

• hit_test – needed for click-to-cursor in text fields.
• get_line_metrics – needed for multi-line cursor navigation.
• get_caret_position – needed for drawing the text cursor.
• measure_rich_text – needed for mixed-style text.

Required Methods

fn measure( &self, text: &str, font_size: f32, available_width: Option<f32>, ) -> (f32, f32)

Measures single-style text and returns (width, height) in logical pixels.

If available_width is Some, the text should be wrapped at that width. If None, the text is measured as a single unwrapped line.

Provided Methods

fn hit_test( &self, _text: &str, _font_size: f32, _available_width: Option<f32>, _x: f32, _y: f32, ) -> usize

Returns the byte index of the character closest to the point (x, y), relative to the text’s origin. Used for click-to-cursor in text fields.

The default implementation returns 0.

fn get_line_metrics( &self, _text: &str, _font_size: f32, _available_width: Option<f32>, ) -> Vec<LineMetric>

Returns per-line metrics for the given text. Used for multi-line text fields and line-based cursor navigation.

The default implementation returns an empty vec.

fn get_caret_position( &self, _text: &str, _font_size: f32, _available_width: Option<f32>, _caret_index: usize, ) -> (f32, f32)

Returns the (x, y) position of the text cursor at caret_index (byte offset), relative to the text’s origin.

The default implementation returns (0.0, 0.0).

fn measure_rich_text( &self, _runs: &[TextRun], _available_width: Option<f32>, ) -> (f32, f32)

Measures multi-style (rich) text and returns (width, height) in logical pixels.

The default implementation returns (0.0, 0.0).

fn layout_rich_text( &self, runs: &[TextRun], available_width: Option<f32>, ) -> RichTextLayoutInfo

Measures rich text and returns positioned inline-widget boxes, if any.

Backends that understand inline rich-text widget markers should override this so layout can place the child widgets at the same coordinates used by text shaping.

fn hit_test_rich( &self, runs: &[TextRun], _available_width: Option<f32>, x: f32, y: f32, ) -> usize

Hit-test rich text (styled runs) at the given (x, y) position. Returns the byte offset into the concatenated text of all runs. Default falls back to plain hit_test using the first run’s font size.

fn resolve_rich_text_annotation_at_point( &self, _runs: &[TextRun], _available_width: Option<f32>, _x: f32, _y: f32, _paragraph_style: TextParagraphStyle, _annotations: &[RichTextAnnotation], ) -> Option<RichTextAnnotation>

Resolves the rich-text annotation at the given point, if any.

This is used for interactive rich-text spans that need hit testing against shaped rich text rather than box nodes.

Dyn Compatibility

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors