add a test for single- and multi-line paragraph layout (#22047)

* add a test for single- and multi-line paragraph layout

* use flutter_test

* use closeTo for double comparison

packages/flutter/test/engine/paragraph_test.dart

+// Copyright 2018 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:ui';
+
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  // Ahem font uses a constant ideographic/alphabetic baseline ratio.
+  const double kAhemBaselineRatio = 1.1662499904632568;
+
+  test('predictably lays out a single-line paragraph', () {
+    for (double fontSize in <double>[10.0, 20.0, 30.0, 40.0]) {
+      final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+        fontFamily: 'Ahem',
+        fontStyle: FontStyle.normal,
+        fontWeight: FontWeight.normal,
+        fontSize: fontSize,
+      ));
+      builder.addText('Test');
+      final Paragraph paragraph = builder.build();
+      paragraph.layout(ParagraphConstraints(width: 400.0));
+
+      expect(paragraph.height, closeTo(fontSize, 0.001));
+      expect(paragraph.width, closeTo(400.0, 0.001));
+      expect(paragraph.minIntrinsicWidth, closeTo(fontSize * 4.0, 0.001));
+      expect(paragraph.maxIntrinsicWidth, closeTo(fontSize * 4.0, 0.001));
+      expect(paragraph.alphabeticBaseline, closeTo(fontSize * .8, 0.001));
+      expect(
+        paragraph.ideographicBaseline,
+        closeTo(paragraph.alphabeticBaseline * kAhemBaselineRatio, 0.001),
+      );
+    }
+  });
+
+  test('predictably lays out a multi-line paragraph', () {
+    for (double fontSize in <double>[10.0, 20.0, 30.0, 40.0]) {
+      final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+        fontFamily: 'Ahem',
+        fontStyle: FontStyle.normal,
+        fontWeight: FontWeight.normal,
+        fontSize: fontSize,
+      ));
+      builder.addText('Test Ahem');
+      final Paragraph paragraph = builder.build();
+      paragraph.layout(ParagraphConstraints(width: fontSize * 5.0));
+
+      expect(paragraph.height, closeTo(fontSize * 2.0, 0.001)); // because it wraps
+      expect(paragraph.width, closeTo(fontSize * 5.0, 0.001));
+      expect(paragraph.minIntrinsicWidth, closeTo(fontSize * 4.0, 0.001));
+
+      // TODO(yjbanov): see https://github.com/flutter/flutter/issues/21965
+      expect(paragraph.maxIntrinsicWidth, closeTo(fontSize * 8.0, 0.001));
+      expect(paragraph.alphabeticBaseline, closeTo(fontSize * .8, 0.001));
+      expect(
+        paragraph.ideographicBaseline,
+        closeTo(paragraph.alphabeticBaseline * kAhemBaselineRatio, 0.001),
+      );
+    }
+  });
+}