Add ndk-context.

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 members = [
     "ndk",
+    "ndk-context",
     "ndk-macro",
     "ndk-build",
     "ndk-examples",

ndk-context/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "ndk-context"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

ndk-context/src/lib.rs

@@ -0,0 +1,86 @@
+//! Provides a stable api to rust crates for interfacing with the android platform. It is
+//! initialized by the runtime, usually [__ndk-glue__](https://crates.io/crates/ndk-glue),
+//! but could also be initialized by java or kotlin code when embedding in an existing android
+//! project.
+//!
+//! ```no_run
+//! let ctx = ndk_context::android_context();
+//! let vm = unsafe { jni::JavaVM::from_raw(ctx.vm() as *mut jni::sys::JavaVM) }?;
+//! let env = vm.attach_current_thread();
+//! let class_ctx = env.find_class("android/content/Context")?;
+//! let audio_service = env.get_static_field(class_ctx, "AUDIO_SERVICE", "Ljava/lang/String;")?;
+//! let audio_manager = env
+//!     .call_method(
+//!         ctx.context() as jni::sys::jobject,
+//!         "getSystemService",
+//!         "(Ljava/lang/String;)Ljava/lang/Object;",
+//!         &[audio_service],
+//!     )?
+//!     .l()?;
+//! ```
+use std::ffi::c_void;
+
+static mut ANDROID_CONTEXT: Option<AndroidContext> = None;
+
+/// [`AndroidContext`] provides the pointers required to interface with the jni on android
+/// platforms.
+#[derive(Clone, Copy, Debug)]
+pub struct AndroidContext {
+    java_vm: *mut c_void,
+    context_jobject: *mut c_void,
+}
+
+impl AndroidContext {
+    /// A handle to the `JavaVM` object.
+    ///
+    /// Usage with [__jni__](https://crates.io/crates/jni) crate:
+    /// ```no_run
+    /// let ctx = ndk_context::android_context();
+    /// let vm = unsafe { jni::JavaVM::from_raw(ctx.vm() as *mut jni::sys::JavaVM) }?;
+    /// let env = vm.attach_current_thread();
+    /// ```
+    pub fn vm(self) -> *mut c_void {
+        self.java_vm
+    }
+
+    /// A handle to an [android.content.Context](https://developer.android.com/reference/android/content/Context).
+    /// In most cases this will be a ptr to an `Activity`, but this isn't guaranteed.
+    ///
+    /// Usage with [__jni__](https://crates.io/crates/jni) crate:
+    /// ```no_run
+    /// let ctx = ndk_context::android_context();
+    /// let vm = unsafe { jni::JavaVM::from_raw(ctx.vm() as *mut jni::sys::JavaVM) }?;
+    /// let env = vm.attach_current_thread();
+    /// let class_ctx = env.find_class("android/content/Context")?;
+    /// let audio_service = env.get_static_field(class_ctx, "AUDIO_SERVICE", "Ljava/lang/String;")?;
+    /// let audio_manager = env
+    ///     .call_method(
+    ///         ctx.context() as jni::sys::jobject,
+    ///         "getSystemService",
+    ///         "(Ljava/lang/String;)Ljava/lang/Object;",
+    ///         &[audio_service],
+    ///     )?
+    ///     .l()?;
+    /// ```
+    pub fn context(self) -> *mut c_void {
+        self.context_jobject
+    }
+}
+
+/// Main entry point to this crate. Returns an [`AndroidContext`].
+pub fn android_context() -> AndroidContext {
+    unsafe { ANDROID_CONTEXT.expect("android context was initialized") }
+}
+
+/// Initializes the [`AndroidContext`]. [`AndroidContext`] is initialized by `ndk-glue` before
+/// main is called.
+///
+/// # Safety
+///
+/// The pointers need to be valid.
+pub unsafe fn initialize_android_context(java_vm: *mut c_void, context_jobject: *mut c_void) {
+    ANDROID_CONTEXT = Some(AndroidContext {
+        java_vm,
+        context_jobject,
+    });
+}

ndk-examples/Cargo.toml

@@ -10,6 +10,7 @@ jni = "0.18.0"
 libc = "0.2"
 log = "0.4.14"
 ndk = { path = "../ndk", features = ["trace"] }
+ndk-context = { path = "../ndk-context" }
 ndk-glue = { path = "../ndk-glue", features = ["logger"] }
 
 [[example]]

ndk-examples/examples/jni_audio.rs

@@ -7,9 +7,8 @@ const GET_DEVICES_OUTPUTS: jni::sys::jint = 2;
 
 fn enumerate_audio_devices() -> Result<(), Box<dyn std::error::Error>> {
     // Create a VM for executing Java calls
-    let native_activity = ndk_glue::native_activity();
-    let vm_ptr = native_activity.vm();
-    let vm = unsafe { jni::JavaVM::from_raw(vm_ptr) }?;
+    let ctx = ndk_context::android_context();
+    let vm = unsafe { jni::JavaVM::from_raw(ctx.vm() as *mut jni::sys::JavaVM) }?;
     let env = vm.attach_current_thread()?;
 
     // Query the global Audio Service
@@ -18,7 +17,7 @@ fn enumerate_audio_devices() -> Result<(), Box<dyn std::error::Error>> {
 
     let audio_manager = env
         .call_method(
-            native_activity.activity(),
+            ctx.context() as jni::sys::jobject,
             "getSystemService",
             // JNI type signature needs to be derived from the Java API
             // (ArgTys)ResultTy

ndk-glue/Cargo.toml

@@ -13,8 +13,9 @@ repository = "https://github.com/rust-windowing/android-ndk-rs"
 
 [dependencies]
 ndk = { path = "../ndk", version = "0.6.0" }
-ndk-sys = { path = "../ndk-sys", version = "0.3.0" }
+ndk-context = { path = "../ndk-context", version = "0.1.0" }
 ndk-macro = { path = "../ndk-macro", version = "0.3.0" }
+ndk-sys = { path = "../ndk-sys", version = "0.3.0" }
 lazy_static = "1.4.0"
 libc = "0.2.84"
 log = "0.4.14"

ndk-glue/src/lib.rs

@@ -165,6 +165,10 @@ pub unsafe fn init(
     callbacks.onLowMemory = Some(on_low_memory);
 
     let activity = NativeActivity::from_ptr(activity);
+    ndk_context::initialize_android_context(
+        activity.vm() as *mut std::ffi::c_void,
+        activity.activity() as *mut std::ffi::c_void,
+    );
     NATIVE_ACTIVITY = Some(activity);
 
     let mut logpipe: [RawFd; 2] = Default::default();