android-ui-dogfooding/jni/jni.odin

// Runtime support for generated JNI bindings: VM/env acquisition, exception
// handling, cached class lookup, and string marshalling.
//
package jni

import "core:strings"
import "shared:android"

Env :: ^android.JNIEnv

@(private)
vm: ^android.JavaVM

// Records the process JavaVM. Call once from android_main before any binding.
init :: proc(app: ^android.android_app) -> bool {
    if app == nil || app.activity == nil {
        return false
    }
    vm = app.activity.vm
    return vm != nil
}

// The Activity instance, usable anywhere a Context is expected.
activity :: proc(app: ^android.android_app) -> android.jobject {
    if app == nil || app.activity == nil {
        return nil
    }
    return app.activity.clazz
}

// The engine's loop runs on a glue-spawned thread, not the one holding
// activity.env, so the env has to come from the VM and the thread attached on
// first use. The thread lives as long as the app, so it is never detached.
@(thread_local)
tls_env: Env

env :: proc() -> Env {
    if tls_env != nil {
        return tls_env
    }
    if vm == nil {
        return nil
    }

    e: Env
    if vm^^.GetEnv(vm, cast(^rawptr)&e, android.JNI_VERSION_1_6) == android.JNI_EDETACHED {
        if vm^^.AttachCurrentThread(vm, &e, nil) != android.JNI_OK {
            return nil
        }
    }
    tls_env = e
    return e
}

//
// Exceptions
//

// Reports whether a Java exception is pending, printing and clearing it if so.
// Left pending, an exception makes every later JNI call on this thread fail.
failed :: proc(e: Env) -> bool {
    if e == nil {
        return true
    }
    if e^^.ExceptionCheck(e) == 0 {
        return false
    }
    e^^.ExceptionDescribe(e)
    e^^.ExceptionClear(e)
    return true
}

//
// Lookup
//

// Resolves a class and pins it with a global ref. Generated code caches the
// result in a static, so a class is only ever looked up once. A local ref would
// go stale the moment the native frame returns.
class_ref :: proc(e: Env, name: cstring) -> android.jclass {
    local := e^^.FindClass(e, name)
    if local == nil || failed(e) {
        return nil
    }
    global := e^^.NewGlobalRef(e, android.jobject(local))
    e^^.DeleteLocalRef(e, android.jobject(local))
    return android.jclass(global)
}

method_id :: proc(e: Env, cls: android.jclass, name, sig: cstring) -> android.jmethodID {
    id := e^^.GetMethodID(e, cls, name, sig)
    if failed(e) {
        return nil
    }
    return id
}

static_method_id :: proc(e: Env, cls: android.jclass, name, sig: cstring) -> android.jmethodID {
    id := e^^.GetStaticMethodID(e, cls, name, sig)
    if failed(e) {
        return nil
    }
    return id
}

field_id :: proc(e: Env, cls: android.jclass, name, sig: cstring) -> android.jfieldID {
    id := e^^.GetFieldID(e, cls, name, sig)
    if failed(e) {
        return nil
    }
    return id
}

static_field_id :: proc(e: Env, cls: android.jclass, name, sig: cstring) -> android.jfieldID {
    id := e^^.GetStaticFieldID(e, cls, name, sig)
    if failed(e) {
        return nil
    }
    return id
}

free_local :: proc(e: Env, obj: android.jobject) {
    if obj != nil {
        e^^.DeleteLocalRef(e, obj)
    }
}

//
// Strings
//

to_jstring :: proc(e: Env, s: string) -> android.jstring {
    c := strings.clone_to_cstring(s, context.temp_allocator)
    return e^^.NewStringUTF(e, c)
}

from_jstring :: proc(e: Env, s: android.jstring, allocator := context.allocator) -> string {
    if s == nil {
        return ""
    }
    utf := e^^.GetStringUTFChars(e, s, nil)
    if utf == nil {
        return ""
    }
    out := strings.clone(string(utf), allocator)
    e^^.ReleaseStringUTFChars(e, s, utf)
    return out
}

//
// Local reference frames
//
// Every object returned by a binding is a LOCAL ref, and locals are only
// reclaimed when control returns to Java -- which never happens on a native
// thread that owns its own loop. A sequence of object-returning calls made
// per-frame will therefore exhaust the local ref table (512 entries) and abort
// the process. Scope such a sequence in a frame:
//
//     if !jni.push_frame(env) { return false }
//     defer jni.pop_frame(env)
//
// Anything still needed after pop_frame must be a primitive, a copied string,
// or passed to pop_frame as `keep` to be promoted into the outer frame.
//

push_frame :: proc(e: Env, capacity: i32 = 16) -> bool {
    if e == nil {
        return false
    }
    return e^^.PushLocalFrame(e, android.jint(capacity)) == android.JNI_OK
}

pop_frame :: proc(e: Env, keep: android.jobject = nil) -> android.jobject {
    if e == nil {
        return nil
    }
    return e^^.PopLocalFrame(e, keep)
}