Add Style::number_formatter as the default used by DragValue (#4740)

This allows users to customize how numbers are converted into strings in
a `DragValue`, as a global default.

This can be used (for instance) to show thousands separators, or use `,`
as a decimal separator.

crates/egui/src/style.rs

@@ -2,7 +2,7 @@
 
 #![allow(clippy::if_same_then_else)]
 
-use std::collections::BTreeMap;
+use std::{collections::BTreeMap, ops::RangeInclusive, sync::Arc};
 
 use epaint::{Rounding, Shadow, Stroke};
 
@@ -11,6 +11,51 @@ use crate::{
     RichText, WidgetText,
 };
 
+/// How to format numbers in e.g. a [`crate::DragValue`].
+#[derive(Clone)]
+pub struct NumberFormatter(
+    Arc<dyn 'static + Sync + Send + Fn(f64, RangeInclusive<usize>) -> String>,
+);
+
+impl NumberFormatter {
+    /// The first argument is the number to be formatted.
+    /// The second argument is the range of the number of decimals to show.
+    ///
+    /// See [`Self::format`] for the meaning of the `decimals` argument.
+    #[inline]
+    pub fn new(
+        formatter: impl 'static + Sync + Send + Fn(f64, RangeInclusive<usize>) -> String,
+    ) -> Self {
+        Self(Arc::new(formatter))
+    }
+
+    /// Format the given number with the given number of decimals.
+    ///
+    /// Decimals are counted after the decimal point.
+    ///
+    /// The minimum number of decimals is usually automatically calculated
+    /// from the sensitivity of the [`crate::DragValue`] and will usually be respected (e.g. include trailing zeroes),
+    /// but if the given value requires more decimals to represent accurately,
+    /// more decimals will be shown, up to the given max.
+    #[inline]
+    pub fn format(&self, value: f64, decimals: RangeInclusive<usize>) -> String {
+        (self.0)(value, decimals)
+    }
+}
+
+impl std::fmt::Debug for NumberFormatter {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str("NumberFormatter")
+    }
+}
+
+impl PartialEq for NumberFormatter {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.0, &other.0)
+    }
+}
+
 // ----------------------------------------------------------------------------
 
 /// Alias for a [`FontId`] (font of a certain size).
@@ -182,6 +227,12 @@ pub struct Style {
     /// The style to use for [`DragValue`] text.
     pub drag_value_text_style: TextStyle,
 
+    /// How to format numbers as strings, e.g. in a [`crate::DragValue`].
+    ///
+    /// You can override this to e.g. add thousands separators.
+    #[cfg_attr(feature = "serde", serde(skip))]
+    pub number_formatter: NumberFormatter,
+
     /// If set, labels, buttons, etc. will use this to determine whether to wrap the text at the
     /// right edge of the [`Ui`] they are in. By default, this is `None`.
     ///
@@ -231,6 +282,12 @@ pub struct Style {
     pub always_scroll_the_only_direction: bool,
 }
 
+#[test]
+fn style_impl_send_sync() {
+    fn assert_send_sync<T: Send + Sync>() {}
+    assert_send_sync::<Style>();
+}
+
 impl Style {
     // TODO(emilk): rename style.interact() to maybe… `style.interactive` ?
     /// Use this style for interactive things.
@@ -1060,6 +1117,7 @@ impl Default for Style {
             override_text_style: None,
             text_styles: default_text_styles(),
             drag_value_text_style: TextStyle::Button,
+            number_formatter: NumberFormatter(Arc::new(emath::format_with_decimals_in_range)),
             wrap: None,
             wrap_mode: None,
             spacing: Spacing::default(),
@@ -1355,6 +1413,7 @@ impl Style {
             override_text_style,
             text_styles,
             drag_value_text_style,
+            number_formatter: _, // can't change callbacks in the UI
             wrap: _,
             wrap_mode: _,
             spacing,

crates/egui/src/widgets/drag_value.rs

@@ -176,6 +176,8 @@ impl<'a> DragValue<'a> {
     /// A custom formatter takes a `f64` for the numeric value and a `RangeInclusive<usize>` representing
     /// the decimal range i.e. minimum and maximum number of decimal places shown.
     ///
+    /// The default formatter is [`Style::number_formatter`].
+    ///
     /// See also: [`DragValue::custom_parser`]
     ///
     /// ```
@@ -481,7 +483,10 @@ impl<'a> Widget for DragValue<'a> {
 
         let value_text = match custom_formatter {
             Some(custom_formatter) => custom_formatter(value, auto_decimals..=max_decimals),
-            None => emath::format_with_decimals_in_range(value, auto_decimals..=max_decimals),
+            None => ui
+                .style()
+                .number_formatter
+                .format(value, auto_decimals..=max_decimals),
         };
 
         let text_style = ui.style().drag_value_text_style.clone();
@@ -676,16 +681,19 @@ fn parse(custom_parser: &Option<NumParser<'_>>, value_text: &str) -> Option<f64>
     }
 }
 
-fn default_parser(value_text: &str) -> Option<f64> {
-    let value_text: String = value_text
+/// The default egui parser of numbers.
+///
+/// It ignored whitespaces anywhere in the input, and treats the special minus character (U+2212) as a normal minus.
+fn default_parser(text: &str) -> Option<f64> {
+    let text: String = text
         .chars()
         // Ignore whitespace (trailing, leading, and thousands separators):
         .filter(|c| !c.is_whitespace())
         // Replace special minus character with normal minus (hyphen):
         .map(|c| if c == '−' { '-' } else { c })
         .collect();
 
-    value_text.parse().ok()
+    text.parse().ok()
 }
 
 fn clamp_value_to_range(x: f64, range: RangeInclusive<f64>) -> f64 {

crates/egui/src/widgets/slider.rs

@@ -318,7 +318,9 @@ impl<'a> Slider<'a> {
     /// A custom formatter takes a `f64` for the numeric value and a `RangeInclusive<usize>` representing
     /// the decimal range i.e. minimum and maximum number of decimal places shown.
     ///
-    /// See also: [`DragValue::custom_parser`]
+    /// The default formatter is [`Style::number_formatter`].
+    ///
+    /// See also: [`Slider::custom_parser`]
     ///
     /// ```
     /// # egui::__run_test_ui(|ui| {
@@ -361,7 +363,7 @@ impl<'a> Slider<'a> {
     /// A custom parser takes an `&str` to parse into a number and returns `Some` if it was successfully parsed
     /// or `None` otherwise.
     ///
-    /// See also: [`DragValue::custom_formatter`]
+    /// See also: [`Slider::custom_formatter`]
     ///
     /// ```
     /// # egui::__run_test_ui(|ui| {