How to test the intrinsic size of a render object in Flutter

Question

I have previously asked about testing the size of a widget in Flutter.

However, now I am trying to test the intrinsic size of the underlying render object.

I tried to do this

testWidgets('MongolRichText has correct min instrinsic width',
    (WidgetTester tester) async {
  const String myString = 'A string';
  await tester.pumpWidget(
    Center(child: MongolText(myString)),
  );

  MongolRenderParagraph text = tester.firstRenderObject(find.byType(MongolRenderParagraph));
  expect(text, isNotNull);
  expect(text.getMinIntrinsicHeight(double.infinity), 100);
});

where MongolText creates a MongolRenderParagraph (similarly to how Text ends up creating a Paragraph). However, I get the following error:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════
The following StateError was thrown running a test:
Bad state: No element

How do I get the underlying render object to run tests on it?

I found the answer so I am adding this as a self answer Q&A. My answer is below.

Solution

You can create the render object directly, so you don't need to pump a widget.

A simple example is here:

testWidgets('MongolRichText has correct min instrinsic width', (WidgetTester tester) async {
  MongolRenderParagraph paragraph = MongolRenderParagraph(TextSpan(text: 'A string'));
  final double textWidth = paragraph.getMaxIntrinsicWidth(double.infinity);
  expect(textWidth, greaterThan(0));
});

I found the solution by looking at the Flutter source code for paragraph_intrinsics_test.dart:

// Copyright 2015 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 'package:flutter/rendering.dart';
import '../flutter_test_alternative.dart';

void main() {
  test('list body and paragraph intrinsics', () {
    final RenderParagraph paragraph = RenderParagraph(
      const TextSpan(
        style: TextStyle(height: 1.0),
        text: 'Hello World',
      ),
      textDirection: TextDirection.ltr,
    );
    final RenderListBody testBlock = RenderListBody(
      children: <RenderBox>[
        paragraph,
      ],
    );

    final double textWidth = paragraph.getMaxIntrinsicWidth(double.infinity);
    final double oneLineTextHeight = paragraph.getMinIntrinsicHeight(double.infinity);
    final double constrainedWidth = textWidth * 0.9;
    final double wrappedTextWidth = paragraph.getMinIntrinsicWidth(double.infinity);
    final double twoLinesTextHeight = paragraph.getMinIntrinsicHeight(constrainedWidth);
    final double manyLinesTextHeight = paragraph.getMinIntrinsicHeight(0.0);

    // paragraph
    expect(wrappedTextWidth, greaterThan(0.0));
    expect(wrappedTextWidth, lessThan(textWidth));
    expect(oneLineTextHeight, lessThan(twoLinesTextHeight));
    expect(twoLinesTextHeight, lessThan(oneLineTextHeight * 3.0));
    expect(manyLinesTextHeight, greaterThan(twoLinesTextHeight));
    expect(paragraph.getMaxIntrinsicHeight(double.infinity), equals(oneLineTextHeight));
    expect(paragraph.getMaxIntrinsicHeight(constrainedWidth), equals(twoLinesTextHeight));
    expect(paragraph.getMaxIntrinsicHeight(0.0), equals(manyLinesTextHeight));

    // vertical block (same expectations)
    expect(testBlock.getMinIntrinsicWidth(double.infinity), equals(wrappedTextWidth));
    expect(testBlock.getMaxIntrinsicWidth(double.infinity), equals(textWidth));
    expect(testBlock.getMinIntrinsicHeight(double.infinity), equals(oneLineTextHeight));
    expect(testBlock.getMinIntrinsicHeight(constrainedWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(double.infinity), equals(oneLineTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(constrainedWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMinIntrinsicWidth(0.0), equals(wrappedTextWidth));
    expect(testBlock.getMaxIntrinsicWidth(0.0), equals(textWidth));
    expect(testBlock.getMinIntrinsicHeight(wrappedTextWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(wrappedTextWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMinIntrinsicHeight(0.0), equals(manyLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(0.0), equals(manyLinesTextHeight));

    // horizontal block (same expectations again)
    testBlock.axisDirection = AxisDirection.right;
    expect(testBlock.getMinIntrinsicWidth(double.infinity), equals(wrappedTextWidth));
    expect(testBlock.getMaxIntrinsicWidth(double.infinity), equals(textWidth));
    expect(testBlock.getMinIntrinsicHeight(double.infinity), equals(oneLineTextHeight));
    expect(testBlock.getMinIntrinsicHeight(constrainedWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(double.infinity), equals(oneLineTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(constrainedWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMinIntrinsicWidth(0.0), equals(wrappedTextWidth));
    expect(testBlock.getMaxIntrinsicWidth(0.0), equals(textWidth));
    expect(testBlock.getMinIntrinsicHeight(wrappedTextWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(wrappedTextWidth), equals(twoLinesTextHeight));
    expect(testBlock.getMinIntrinsicHeight(0.0), equals(manyLinesTextHeight));
    expect(testBlock.getMaxIntrinsicHeight(0.0), equals(manyLinesTextHeight));
  });
}

Comments

[Suragch]

I'd still like to know why tester.firstRenderObject() didn't work, though.