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Hot Reload Runtime

The hot-reload-runtime library is the core component that enables true hot-reloading on Android devices by loading dynamic DEX patches without restarting the application.

Overview​

When you make a change to a Kotlin file, the JetStart dev server compiles it into a DEX patch and sends it to your device. The hot-reload-runtime library handles:

  1. WebSocket Communication - Receiving DEX payloads from the dev server.
  2. DEX Loading - Using a custom ClassLoader to load the received bytecode into the running process.
  3. State Management - Managing the reloadVersion state that triggers Jetpack Compose recomposition.
  4. Method Swapping - Redirecting method calls from existing classes to the newly loaded $Override classes.

Installation​

The library is included as a dependency in your app-level build.gradle:

dependencies {
    // ...
    implementation 'com.jetstart:hot-reload-runtime:1.0.0'
}

Key Components​

HotReload Manager​

The HotReload object is the primary API used in your MainActivity.kt:

  • HotReload.connect(context, url, sessionId) - Establishes a connection to the dev server.
  • HotReload.reloadVersion - A StateFlow that increments whenever a new hot-reload patch is applied.
  • HotReload.disconnect() - Safely closes the connection.

HotReloadClassLoader​

A specialized ClassLoader that implements a child-first loading strategy for hot-reload patches. This allows it to prioritize newly received DEX bytecode over the original classes bundled in the APK.

IncrementalChange​

The interface used for method-level instrumentation, allowing the runtime to swap implementations at the method level (based on the InstantRun pattern).

Usage in App​

The runtime is designed to be nearly invisible. Its main usage is in the app's entry point:

// Observe the reload version
val reloadVersion by HotReload.reloadVersion.collectAsState()

// Use reloadVersion as a key to force recomposition
key(reloadVersion) {
    AppContent()
}

Performance​

The runtime is optimized for speed, typically applying updates and triggering recomposition in under 20ms once the DEX payload is received.