Understanding TextPainter Size Differences: Android vs. Unit Tests in Flutter

Discover the key parameters to adjust in Flutter's `TextPainter` to align text sizes across Android and Unit Tests. Learn about line height discrepancies and how they affect layout.
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Understanding TextPainter Size Differences: Android vs. Unit Tests in Flutter

When developing a Flutter app that requires layouting, you might face unexpected differences in text size when running unit tests compared to rendering on actual devices like Android or web browsers. This article will dive into a specific issue surrounding the TextPainter and the differing Size outputs across platforms. We'll explore the factors contributing to these discrepancies and how to resolve them effectively.

The Problem: Discrepancies in Text Sizes

In Flutter, TextPainter is a foundational class that allows rendering and layout of text in a custom way. The original concern arises from the following observations:

On running unit tests, the TextPainter produces different size outputs compared to the Android device.

Even when explicitly setting the fontFamily to "Roboto-Black" (known as the default font), the sizes reported differ substantially.

Size Output Overview

Here's a look at some example outputs for the word "test" using both TextPainter and Paragraph across different platforms:

TextPainter Outputs:

With fontFamily left empty:

Unit Test: Size(400, 100)

Android: Size(171, 117)

With font set to Roboto-Black:

Unit Test: Size(175, 120)

Android: Size(175, 117)

Paragraph Outputs:

With fontFamily left empty:

Unit Test: (200, 200)

Android: (170.1, 117)

With font set to Roboto-Black:

Unit Test: (174.31640625, 120)

Android: (174.3, 117)

This suggests that the unit tests expect a certain precision in size measurements that Android does not replicate.

The Solution: Adjusting Line Height

Upon investigation, it was found that the discrepancy in size was primarily due to differences in the line height used in the rendering process across platforms:

Flutter unit tests default to a line height of 1.2.

Flutter on Android has a line height of 1.17.

This difference explains why the text rendered with a font size of 100 appears differently sized across these environments; notably, Android renders it with a height of 117 while the unit tests render it with a height of 120.

Standardizing Line Height

To achieve consistent text size across both unit tests and Android, you can standardize the line height. By setting the height to 1.0, you can expect a more unified output:

[[See Video to Reveal this Text or Code Snippet]]

Default Styles on Android

At present, the default style for Roboto on Android devices may have additional parameters defined, which can impact text rendering and ultimately the reported sizes.

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Understanding the nuances of text rendering in Flutter can significantly improve the way your application behaves across different platforms. Adjusting the line height to a consistent value of 1.0 will help align the output sizes from unit tests and Android, reducing discrepancies that can introduce unnecessary complexity in your development process.

By following these adjustments, you ensure a smoother development experience and, more importantly, a consistent appearance of text in your Flutter applications across different environments.