text: Factor out kMonospaceTextStyle; handle iOS quirk about 'monospace'

To encapsulate handling the odd bug where Apple's font collection
doesn't respond to 'monospace'; see implementation comment.

Author: chrisbobbe

lib/widgets/content.dart

@@ -7,6 +7,7 @@ import '../model/content.dart';
 import '../model/store.dart';
 import 'store.dart';
 import 'lightbox.dart';
+import 'text.dart';
 
 /// The font size for message content in a plain unstyled paragraph.
 const double kBaseFontSize = 14;
@@ -371,18 +372,14 @@ InlineSpan inlineCode(InlineCodeNode node) {
   // ]);
 }
 
-const _kInlineCodeStyle = TextStyle(
+final _kInlineCodeStyle = kMonospaceTextStyle.merge(const TextStyle(
   backgroundColor: Color(0xffeeeeee),
-  fontSize: 0.825 * kBaseFontSize,
-  fontFamily: "Source Code Pro", // TODO supply font
-  fontFamilyFallback: ["monospace"]);
+  fontSize: 0.825 * kBaseFontSize));
 
-const _kCodeBlockStyle = TextStyle(
+final _kCodeBlockStyle = kMonospaceTextStyle.merge(const TextStyle(
   backgroundColor: Color.fromRGBO(255, 255, 255, 1),
   fontSize: 0.825 * kBaseFontSize,
-  fontFamily: "Source Code Pro", // TODO supply font
-  fontFamilyFallback: ["monospace"],
-);
+));
 
 // const _kInlineCodeLeftBracket = '⸤';
 // const _kInlineCodeRightBracket = '⸣';
@@ -639,8 +636,4 @@ InlineSpan _errorUnimplemented(UnimplementedNode node) {
 
 const errorStyle = TextStyle(fontWeight: FontWeight.bold, color: Colors.red);
 
-const errorCodeStyle = TextStyle(
-  color: Colors.red,
-  fontFamily: "Source Code Pro", // TODO supply font
-  fontFamilyFallback: ["monospace"],
-);
+final errorCodeStyle = kMonospaceTextStyle.merge(const TextStyle(color: Colors.red));

lib/widgets/text.dart

@@ -0,0 +1,19 @@
+import 'dart:io';
+import 'package:flutter/widgets.dart';
+
+/// A mergeable [TextStyle] with 'Source Code Pro' and platform-aware fallbacks.
+///
+/// Example:
+///
+/// ```dart
+/// kMonospaceTextStyle.merge(const TextStyle(color: Colors.red))
+/// ```
+final TextStyle kMonospaceTextStyle = TextStyle(
+  fontFamily: 'Source Code Pro', // TODO supply font
+
+  // Oddly, iOS doesn't handle 'monospace':
+  //   https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/flutter.3A.20monospace.20font.20fallback/near/1570622
+  fontFamilyFallback: Platform.isIOS ? ['Menlo', 'Courier'] : ['monospace'],
+
+  inherit: true,
+);