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| # Android NDK Init | ||
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| This page is only needed to read if you want to use Android NDK in Rust code, | ||
| and see "android context was not initialized" error, or want to know more about`ndk_context` initialization. | ||
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| Related issues: | ||
| [#1323](https://github.com/fzyzcjy/flutter_rust_bridge/issues/1323), | ||
| [#1868](https://github.com/fzyzcjy/flutter_rust_bridge/issues/1868). | ||
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| ## Method 1 | ||
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| On android, when attempting to use crates that interact with the JavaVM through the JNI (like oboe-rs via cpal), you may | ||
| get panics that typically have this message: | ||
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| ``` | ||
| [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: FfiException(PANIC_ERROR, android context was not initialized, null) | ||
| ``` | ||
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| This is due to a interesting quirk of Rust NDK interaction, where | ||
| the [`ndk_context`](https://github.com/rust-mobile/ndk-context) crate does not have it's JVM and Android context | ||
| initialized. Typically, in a normal application, the Android JVM would `System.loadLibrary()` the library through the | ||
| Activity inside the JVM. It looks for symbols related to the JNI and executes them in accordance with the JNI standard. | ||
| This would initialize the `ndk_context` normally via `JNI_OnLoad`. However, using the DartVM this step is skipped while | ||
| loading the library, as the DartVM is not the JVM. So, the Android specific variables are not initialized, and therefore | ||
| you cannot interact with the system via the Java interface. | ||
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| ### MainActivity.kt | ||
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| Add these lines to your FlutterActivity subclass: | ||
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| ```kotlin | ||
| package com.example.frontend | ||
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| import io.flutter.embedding.android.FlutterActivity | ||
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| // https://github.com/dart-lang/sdk/issues/46027 | ||
| class MainActivity : FlutterActivity() { | ||
| // this `init` block, where "foo" is the name of your library | ||
| // ex: if it's libfoo.so, then use "foo" | ||
| init { | ||
| System.loadLibrary("foo") | ||
| } | ||
| } | ||
| ``` | ||
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| This handles loading the library before Dart does, and also executes the JNI related initialization. | ||
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| ### Rust | ||
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| #### Cargo.toml | ||
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| ``` | ||
| [target.'cfg(target_os = "android")'.dependencies] | ||
| jni = "0.21" | ||
| ndk-context = "0.1" | ||
| ``` | ||
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| #### lib.rs | ||
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| ```rust | ||
| #[cfg(target_os = "android")] | ||
| #[no_mangle] | ||
| pub extern "C" fn JNI_OnLoad(vm: jni::JavaVM, res: *mut std::os::raw::c_void) -> jni::sys::jint { | ||
| use std::ffi::c_void; | ||
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| let vm = vm.get_java_vm_pointer() as *mut c_void; | ||
| unsafe { | ||
| ndk_context::initialize_android_context(vm, res); | ||
| } | ||
| jni::JNIVersion::V6.into() | ||
| } | ||
| ``` | ||
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| This is the bit of JNI glue that allows for `ndk_context` to be initialized. | ||
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| ## Method 2 | ||
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| Alternative method, if you require direct access to the current Android context, without creating another one. | ||
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| ### MainActivity.kt | ||
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| Add these lines to your FlutterActivity subclass, as well as create a Plugin class: | ||
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| ```kotlin | ||
| package com.example.frontend | ||
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| import android.content.Context | ||
| import androidx.annotation.NonNull | ||
| import io.flutter.embedding.android.FlutterActivity | ||
| import io.flutter.embedding.engine.FlutterEngine | ||
| import io.flutter.embedding.engine.plugins.FlutterPlugin | ||
| import io.flutter.plugin.common.MethodCall | ||
| import io.flutter.plugin.common.MethodChannel.MethodCallHandler | ||
| import io.flutter.plugin.common.MethodChannel.Result | ||
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| class MainActivity : FlutterActivity() { | ||
| override fun configureFlutterEngine( | ||
| @NonNull flutterEngine: FlutterEngine, | ||
| ) { | ||
| super.configureFlutterEngine(flutterEngine) | ||
| flutterEngine.plugins.add(MyPlugin()) | ||
| } | ||
| } | ||
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| class MyPlugin : FlutterPlugin, MethodCallHandler { | ||
| companion object { | ||
| init { | ||
| System.loadLibrary("rust_lib_frontend") | ||
| } | ||
| } | ||
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| external fun init_android(ctx: Context) | ||
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| override fun onAttachedToEngine( | ||
| @NonNull flutterPluginBinding: FlutterPlugin.FlutterPluginBinding, | ||
| ) { | ||
| init_android(flutterPluginBinding.applicationContext) | ||
| } | ||
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| override fun onMethodCall( | ||
| @NonNull call: MethodCall, | ||
| @NonNull result: Result, | ||
| ) { | ||
| result.notImplemented() | ||
| } | ||
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| override fun onDetachedFromEngine( | ||
| @NonNull binding: FlutterPlugin.FlutterPluginBinding, | ||
| ) { | ||
| } | ||
| } | ||
| ``` | ||
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| ### Rust | ||
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| #### Cargo.toml | ||
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| ``` | ||
| [target.'cfg(target_os = "android")'.dependencies] | ||
| jni = "0.21" | ||
| ``` | ||
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| #### lib.rs | ||
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| ```rust | ||
| #[cfg(target_os = "android")] | ||
| use { | ||
| jni::{objects::JClass, objects::JObject, JNIEnv}, | ||
| mylib::setup_android, | ||
| }; | ||
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| #[cfg(target_os = "android")] | ||
| #[no_mangle] | ||
| pub extern "system" fn Java_com_example_spareshare_MyPlugin_init_1android( | ||
| env: JNIEnv, | ||
| _class: JClass, | ||
| ctx: JObject, | ||
| ) { | ||
| setup_android(env, ctx); | ||
| } | ||
| ``` |
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