Function html_canvas_to_2d_context

web/src/canvas.rs:42–77

Returns the 2D rendering context for a given `canvas` element.

(canvas: HtmlCanvasElement)

/// Returns the 2D rendering context for a given `canvas` element.
fn html_canvas_to_2d_context(canvas: HtmlCanvasElement) -> io::Result<CanvasRenderingContext2d> {
    let attrs = ContextAttributes2d::new();

    // We don't use transparency for anything, so disable the alpha channel for performance reasons.
    attrs.set_alpha(false);

    // Chrome recommends setting this to true because we read from the canvas to move the cursor
    // and to scroll the console, but these operations needn't be fast.  It seems better to keep
    // this disabled to optimize for the rendering path of graphical applications.
    attrs.set_will_read_frequently(false);

    let context = match canvas
        .get_context_with_context_options("2d", &attrs)
        .map_err(js_value_to_io_error)?
    {
        Some(context) => context,
        None => {
            return Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                "Failed to get 2D context from canvas",
            ));
        }
    };

    let context = match context.dyn_into::<CanvasRenderingContext2d>() {
        Ok(context) => context,
        Err(_) => {
            return Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                "Returned 2D context for canvas does not have the correct type",
            ));
        }
    };

    Ok(context)
}

/// Implementation of a console that renders on an HTML canvas.
pub(crate) struct CanvasRasterOps {