Fix FontLoader does not remove the cache in web engine (flutter#14536)

lib/web_ui/lib/src/engine/text/font_collection.dart

@@ -200,6 +200,10 @@ class FontManager {
     final html.FontFace fontFace = html.FontFace(family, list);
     return fontFace.load().then((_) {
       html.document.fonts.add(fontFace);
+      // There might be paragraph measurements for this new font before it is
+      // loaded. They were measured using fallback font, so we should clear the
+      // cache.
+      TextMeasurementService.clearCache();
     }, onError: (dynamic exception) {
       // Failures here will throw an html.DomException which confusingly
       // does not implement Exception or Error. Rethrow an Exception so it can

lib/web_ui/lib/src/engine/text/measurement.dart

@@ -168,7 +168,8 @@ abstract class TextMeasurementService {
   /// Initializes the text measurement service with a specific
   /// [rulerCacheCapacity] that gets passed to the [RulerManager].
   static void initialize({@required int rulerCacheCapacity}) {
-    clearCache();
+    rulerManager?.dispose();
+    rulerManager = null;
     rulerManager = RulerManager(rulerCacheCapacity: rulerCacheCapacity);
   }
 
@@ -211,11 +212,10 @@ abstract class TextMeasurementService {
     return domInstance;
   }
 
-  /// Clears the cache of paragraph rulers.
-  @visibleForTesting
+  /// Clears the cache of paragraph rulers that are used for measuring paragraph
+  /// metrics.
   static void clearCache() {
-    rulerManager?.dispose();
-    rulerManager = null;
+    rulerManager?._evictAllRulers();
   }
 
   static bool _canUseCanvasMeasurement(EngineParagraph paragraph) {

lib/web_ui/test/text/font_loading_test.dart

@@ -7,6 +7,7 @@ import 'dart:typed_data';
 
 import 'package:test/test.dart';
 import 'package:ui/ui.dart' as ui;
+import 'package:ui/src/engine.dart';
 
 Future<void> main() async {
   await ui.webOnlyInitializeTestDomRenderer();
@@ -34,6 +35,29 @@ Future<void> main() async {
 
       expect(_containsFontFamily('Blehm'), true);
     });
+
+    test('loads font should clear measurement caches', () async {
+      final ui.ParagraphStyle style = ui.ParagraphStyle();
+      final ui.ParagraphBuilder builder = ui.ParagraphBuilder(style);
+      final ui.ParagraphConstraints constraints = ui.ParagraphConstraints(width: 30.0);
+      builder.addText('test');
+      final ui.Paragraph paragraph = builder.build();
+      // Triggers the measuring and verifies the result has been cached.
+      paragraph.layout(constraints);
+      expect(TextMeasurementService.rulerManager.rulers.length, 1);
+
+      // Now, loads a new font using loadFontFromList. This should clear the
+      // cache
+      final html.HttpRequest response = await html.HttpRequest.request(
+        _testFontUrl,
+        responseType: 'arraybuffer');
+      await ui.loadFontFromList(Uint8List.view(response.response),
+        fontFamily: 'Blehm');
+
+      // Verifies the font is loaded, and the cache is cleaned.
+      expect(_containsFontFamily('Blehm'), true);
+      expect(TextMeasurementService.rulerManager.rulers.length, 0);
+    });
   });
 }