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increase-coverage

Testing & Quality
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Systematic guidelines for executing code coverage tools, analyzing gaps in testing, and implementing targeted tests to maximize code coverage across the reactive framework.

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Code Coverage Execution and Maximization Guide

This developer skill provides step-by-step instructions and best practices for executing code coverage tools, identifying untested paths (gaps), and writing high-fidelity unit tests to drive coverage to 100% across the monorepo.

1. Executing and Aggregating Monorepo Coverage

Our monorepo coordinates testing across multiple packages using Melos. Follow this command workflow to generate and merge coverage data:

A. Run Package Tests with Coverage

Execute the monorepo test runner. This runs flutter test --coverage (or dart test --coverage) across all packages:

melos run test

B. Combine Package Coverage Files

Run the combination script to merge individual package-level coverage/lcov.info files into a unified root lcov.info file:

melos run coverage

[!NOTE] The aggregation script (scripts/combine_coverage.sh) dynamically rewrites path roots to be absolute and deletes temporary package coverage directories to maintain workspace hygiene.

C. Generate the HTML Coverage Report

Translate the unified lcov.info file into an interactive HTML visualization report using genhtml (part of the lcov toolkit):

genhtml lcov.info -o coverage/html

2. Pinpointing Uncovered Lines (Gap Analysis)

To quickly locate untested lines of code without browsing the entire HTML report, run the following grep and awk pipeline on the root lcov.info file:

grep -E "SF:|DA:" lcov.info | awk -F: '/SF/ {file=$2} /DA/ {split($2,a,","); if(a[2]==0) print file, a[1]}'

This output displays the absolute file path followed by the exact line number of every uncovered statement, for example:

/workspace/packages/preact_signals/lib/src/model.dart 21
/workspace/packages/preact_signals/lib/src/model.dart 25

3. Strategies for Reaching 100% Coverage

Once you identify the uncovered lines, apply these targeted testing strategies to write meaningful tests:

A. Testing Model and Option Basics

Ensure value models, options, and parameters are fully exercised:

  • copyWith Methods: Test with both null (to assert fallback to current value) and explicit values.
  • Equality (==) and hashCode:
    • Test comparing identical references (identical(this, other)).
    • Test comparing distinct instances with identical properties (expect equal).
    • Test comparing distinct instances with differing properties (expect unequal).
    • Test comparing against null or instances of completely different classes.

B. Exercising Complex Branch/Condition Logic

Examine every nested if, else, and ternary (? :) statement:

  • Construct test inputs that force the path to evaluate to true.
  • Construct test inputs that force the path to evaluate to false.
  • Ensure bounds checks, empty collections, and null-safety fallbacks are explicitly triggered.

C. Simulating and Asserting Exception/Catch Paths

To cover recovery logic or diagnostic assertions:

  • catch Blocks: Pass incompatible types or construct scenarios that cause runtime TypeError or AssertionError to verify fallback branches.
  • Dispose and Lifecycle Errors: Call .dispose() on a signal/primitive, then attempt a read or write operation to assert it throws SignalsReadAfterDisposeError or SignalsWriteAfterDisposeError.

D. Deep Recursive Collections

Verify that custom deep collection assertions (such as lists, maps, sets, or nested objects) evaluate both matching and mismatching configurations at every depth.

4. Housekeeping and Version Safety

Before finalizing the coverage maximization run, verify repository hygiene:

  1. Format test files: 
    dart format .
  2. Run Lints & Static Analysis: 
    melos run analyze
  3. Clean Git State: Never check coverage files (lcov.info or coverage/) into Git history. If they are accidentally staged, remove them: 
    git rm -r --cached coverage/
git rm lcov.info