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React Native Timing

packages/react-native/ReactCommon/react/timing/__docs__/README.md

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React Native Timing

🏠 Home

This directory contains the shared C++ implementation of high-resolution timing primitives for React Native. These primitives provide precise time measurement capabilities that align with Web standards while being optimized for the React Native environment.

🚀 Usage

The timing primitives in this module are primarily used by internal React Native systems that require precise time measurements. The monotonic clock is used for these primitives: they should be used for measuring time intervals. These primitives should not be used for wall times. They are not expected to be used directly by application developers but serve as foundational components for various React Native features.

Key primitives include:

  • HighResTimeStamp: A class representing a specific point in time with high precision.
  • HighResDuration: A class representing a duration of time with high precision.

These primitives support various operations:

cpp
// Getting the current high-resolution timestamp
auto start = HighResTimeStamp::now();

// Creating durations
auto duration = HighResDuration::fromNanoseconds(100);
auto durationMs = HighResDuration::fromMilliseconds(100);

// Arithmetic operations
auto later = start + duration;
auto elapsed = later - start;

// Converting to absolute time units of highest precision
auto end = HighResTimeStamp::now();
int64_t nanoseconds = (end - start).toNanoseconds();

// Converting to DOMHighResTimeStamp (for JavaScript interoperability)
double jsTimeValue = now.toDOMHighResTimeStamp();

📐 Design

The timing primitives are designed to align with Web standards while leveraging C++'s type system and the performance characteristics of native code. The implementation uses std::chrono internally but provides a more specialized interface tailored to React Native's needs.

HighResTimeStamp

This class represents a specific point in time with high precision. It encapsulates a std::chrono::steady_clock::time_point and provides methods to:

  • Convert to DOMHighResTimeStamp for JavaScript interoperability.
  • Perform arithmetic operations with durations.
  • Compare with other timestamps.

HighResDuration

This class represents a duration of time with high precision. It encapsulates a std::chrono::duration and provides methods to:

  • Convert to DOMHighResTimeStamp for JavaScript interoperability.
  • Convert to an absolute number of nanoseconds.
  • Perform arithmetic operations.
  • Compare with other durations.

🔗 Relationship with other systems

Used by

  • Event Loop: Uses timing primitives for measuring task execution times and scheduling.
  • Web Performance API: Timing primitives are used to implement performance measurement APIs like PerformanceObserver entries (e.g., longtask and event).
  • React Native DevTools: The timing primitives integrate with the React Native DevTools tracing infrastructure to report the timing of tasks and events.
  • Web timing APIs: The timing primitives are designed to be compatible with Web timing concepts, making it easier to implement Web-compatible APIs in React Native.