feat: cache marker metrics

Author: hejsztynx

android/src/main/java/com/swmansion/enriched/common/spans/EnrichedOrderedListSpan.kt

@@ -27,12 +27,8 @@ open class EnrichedOrderedListSpan(
     paint: Paint,
     highestIndex: Int,
   ): Boolean {
-    val highestIndexText = "$highestIndex."
-
-    val originalTypeface = paint.typeface
-    paint.typeface = getTypeface(enrichedStyle.olMarkerFontWeight, originalTypeface)
-    val highestIndexWidth = ceil(paint.measureText(highestIndexText)).toInt()
-    paint.typeface = originalTypeface
+    val (digitWidth, dotWidth) = markerMetricsFor(paint, paint.typeface)
+    val highestIndexWidth = ceil(digitCountOf(highestIndex) * digitWidth + dotWidth).toInt()
 
     val newColumnMargin = max(enrichedStyle.olMarginLeft, highestIndexWidth)
     if (newColumnMargin == columnMargin) return false
@@ -99,4 +95,39 @@ open class EnrichedOrderedListSpan(
         Typeface.create(originalTypeface, Typeface.NORMAL)
       }
     }
+
+  private fun markerMetricsFor(
+    paint: Paint,
+    baseTypeface: Typeface,
+  ): Pair<Float, Float> {
+    val key = MarkerMetricsKey(baseTypeface, enrichedStyle.olMarkerFontWeight, paint.textSize)
+    return markerMetricsCache.getOrPut(key) {
+      val originalTypeface = paint.typeface
+      paint.typeface = getTypeface(enrichedStyle.olMarkerFontWeight, baseTypeface)
+      val digitWidth = paint.measureText("0")
+      val dotWidth = paint.measureText(".")
+      paint.typeface = originalTypeface
+      Pair(digitWidth, dotWidth)
+    }
+  }
+
+  private data class MarkerMetricsKey(
+    val baseTypeface: Typeface,
+    val fontWeight: Int?,
+    val textSize: Float,
+  )
+
+  companion object {
+    private val markerMetricsCache = mutableMapOf<MarkerMetricsKey, Pair<Float, Float>>()
+
+    private fun digitCountOf(n: Int): Int {
+      var count = 1
+      var value = max(n, 1)
+      while (value >= 10) {
+        value /= 10
+        count++
+      }
+      return count
+    }
+  }
 }

ios/styles/OrderedListStyle.mm

@@ -4,7 +4,13 @@
 #import "StyleUtils.h"
 #import "TextInsertionUtils.h"
 
-@implementation OrderedListStyle
+@implementation OrderedListStyle {
+  // we don't want to re-measure each marker's actual
+  // width. We estimate the width with cached metrics instead
+  UIFont *_cachedMarkerFont;
+  CGFloat _cachedDigitWidth;
+  CGFloat _cachedDotWidth;
+}
 
 + (StyleType)getType {
   return OrderedList;
@@ -87,17 +93,35 @@ - (void)recalculateListsAroundEditedRange:(NSRange)range {
   }
 }
 
+- (void)ensureMarkerMetricsForFont:(UIFont *)font {
+  if (_cachedMarkerFont != nil && [_cachedMarkerFont isEqual:font]) {
+    return;
+  }
+  _cachedMarkerFont = font;
+  NSDictionary *attrs = @{NSFontAttributeName : font};
+  _cachedDigitWidth = [@"0" sizeWithAttributes:attrs].width;
+  _cachedDotWidth = [@"." sizeWithAttributes:attrs].width;
+}
+
+- (NSInteger)digitCountOf:(NSInteger)n {
+  NSInteger count = 1;
+  NSInteger value = MAX(n, 1);
+  while (value >= 10) {
+    value /= 10;
+    count += 1;
+  }
+  return count;
+}
+
 // computes the shared marker-column indent for a list of the given item
 // count. The largest marker value equals the item count (numbering starts
 // at 1); if its width overflows the configured margin we expand to fit it
 - (CGFloat)headIndentForItemCount:(NSInteger)itemCount {
-  NSString *widestMarker =
-      [NSString stringWithFormat:@"%d.", (int)MAX(itemCount, 1)];
-  CGFloat widestMarkerWidth =
-      [widestMarker sizeWithAttributes:@{
-        NSFontAttributeName : [self.host.config orderedListMarkerFont]
-      }]
-          .width;
+  UIFont *markerFont = [self.host.config orderedListMarkerFont];
+  [self ensureMarkerMetricsForFont:markerFont];
+
+  NSInteger digitCount = [self digitCountOf:itemCount];
+  CGFloat widestMarkerWidth = digitCount * _cachedDigitWidth + _cachedDotWidth;
 
   CGFloat markerColumnWidth =
       MAX([self.host.config orderedListMarginLeft], widestMarkerWidth);