feat: add basic rust JNI demo

.gitignore

@@ -8,3 +8,4 @@
 .externalNativeBuild
 .cxx
 local.properties
+jniLibs

app/src/main/java/net/mbess/popequer/Native.kt

@@ -0,0 +1,8 @@
+package net.mbess.popequer
+
+object Native {
+    init {
+        System.loadLibrary("rustsandbox")
+    }
+    external fun add(a: Int, b: Int): Int
+}

app/src/main/java/net/mbess/popequer/ui/MainActivity.kt

@@ -8,10 +8,14 @@ import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.material3.*
 import androidx.compose.ui.platform.LocalContext
 import net.mbess.popequer.AppContext
+import net.mbess.popequer.Native
 
 
 class MainActivity : ComponentActivity() {
     override fun onCreate(savedInstanceState: Bundle?) {
+        Native.add(143, 54).let {
+            println("From foreign rust result: $it")
+        }
         super.onCreate(savedInstanceState)
 
         val appContext = AppContext(this)

rust-sandbox/.gitignore

@@ -0,0 +1,20 @@
+# Created by https://www.toptal.com/developers/gitignore/api/rust
+# Edit at https://www.toptal.com/developers/gitignore?templates=rust
+
+### Rust ###
+# Generated by Cargo
+# will have compiled files and executables
+debug/
+target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk
+
+# MSVC Windows builds of rustc generate these, which store debugging information
+*.pdb
+
+# End of https://www.toptal.com/developers/gitignore/api/rust

rust-sandbox/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "rust-sandbox"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+name = "rustsandbox"
+crate-type = ["cdylib"]
+
+[dependencies]
+jni = "0.21.1"
+
+[target.'cfg(target_os = "android")'.dependencies]

rust-sandbox/src/lib.rs

@@ -0,0 +1,15 @@
+use jni::{
+    objects::{JClass, JObject},
+    JNIEnv,
+};
+
+#[no_mangle]
+pub extern "system" fn Java_net_mbess_popequer_Native_add(
+    _env: JNIEnv,
+    _class: JClass,
+    a: i32,
+    b: i32
+) -> i32 {
+    a + b
+}
+