AgentSkillsCN

Condition-Based Waiting

用条件轮询取代任意超时设置，打造更可靠的异步测试

SKILL.md
--- frontmatter
name: Condition-Based Waiting
description: Replace arbitrary timeouts with condition polling for reliable async tests
when_to_use: when tests have race conditions, timing dependencies, or inconsistent pass/fail behavior
version: 1.1.0
languages: all
progressive_disclosure:
  entry_point:
    summary: "Replace arbitrary timeouts with condition polling for reliable async tests"
    when_to_use: "Tests with setTimeout/sleep, flaky tests, or timing-dependent async operations"
    quick_start: |
      1. Identify arbitrary delays in tests (setTimeout, sleep, time.sleep())
      2. Replace with condition-based waiting (waitFor pattern)
      3. Use domain-specific helpers for common scenarios
      See @example.ts for complete working implementation
    core_pattern: |
      // ❌ Guessing at timing
      await new Promise(r => setTimeout(r, 50));

      // ✅ Waiting for condition
      await waitFor(() => getResult() !== undefined);
  references:
    - path: references/patterns-and-implementation.md
      purpose: Detailed waiting patterns, implementation guide, and common mistakes
      when_to_read: When implementing waitFor or debugging timing issues

Condition-Based Waiting

Overview

Flaky tests often guess at timing with arbitrary delays. This creates race conditions where tests pass on fast machines but fail under load or in CI.

Core principle: Wait for the actual condition you care about, not a guess about how long it takes.

When to Use

dot
digraph when_to_use {
    "Test uses setTimeout/sleep?" [shape=diamond];
    "Testing timing behavior?" [shape=diamond];
    "Document WHY timeout needed" [shape=box];
    "Use condition-based waiting" [shape=box];

    "Test uses setTimeout/sleep?" -> "Testing timing behavior?" [label="yes"];
    "Testing timing behavior?" -> "Document WHY timeout needed" [label="yes"];
    "Testing timing behavior?" -> "Use condition-based waiting" [label="no"];
}

Use when:

  • •Tests have arbitrary delays (setTimeout, sleep, time.sleep())
  • •Tests are flaky (pass sometimes, fail under load)
  • •Tests timeout when run in parallel
  • •Waiting for async operations to complete

Don't use when:

  • •Testing actual timing behavior (debounce, throttle intervals)
  • •Always document WHY if using arbitrary timeout

Core Pattern

typescript
// ❌ BEFORE: Guessing at timing
await new Promise(r => setTimeout(r, 50));
const result = getResult();
expect(result).toBeDefined();

// ✅ AFTER: Waiting for condition
await waitFor(() => getResult() !== undefined);
const result = getResult();
expect(result).toBeDefined();

Quick Patterns

ScenarioPattern
Wait for eventwaitFor(() => events.find(e => e.type === 'DONE'))
Wait for statewaitFor(() => machine.state === 'ready')
Wait for countwaitFor(() => items.length >= 5)
Wait for filewaitFor(() => fs.existsSync(path))
Complex conditionwaitFor(() => obj.ready && obj.value > 10)

Implementation

Generic polling function:

typescript
async function waitFor<T>(
  condition: () => T | undefined | null | false,
  description: string,
  timeoutMs = 5000
): Promise<T> {
  const startTime = Date.now();

  while (true) {
    const result = condition();
    if (result) return result;

    if (Date.now() - startTime > timeoutMs) {
      throw new Error(`Timeout waiting for ${description} after ${timeoutMs}ms`);
    }

    await new Promise(r => setTimeout(r, 10)); // Poll every 10ms
  }
}

See @example.ts for complete implementation with domain-specific helpers (waitForEvent, waitForEventCount, waitForEventMatch).

For detailed patterns, implementation guide, and common mistakes, see @references/patterns-and-implementation.md

Real-World Impact

From debugging session (2025-10-03):

  • •Fixed 15 flaky tests across 3 files
  • •Pass rate: 60% → 100%
  • •Execution time: 40% faster
  • •No more race conditions