On Android startup, obtain a Bluetooth adapter and check if it is

enabled.

ble_android.go

@@ -1,14 +1,22 @@
 package ble
 
 import (
+	"log"
 	"gioui.org/app"
 )
 
+/*
+#include <jni.h>
+*/
+import "C"
+
 // Types required for ble.go
 
 type bleState string
 
 type bleHandle struct {
+	adapter C.jobject
+	class C.jclass
 }
 
 type Peripheral struct {
@@ -22,12 +30,16 @@ type Characteristic string
 
 // Internal global variables
 
+var jvmContext uintptr
+
 // Functions required by API
 
 //Init needs to be called before the BLE library can be used. On Android we
 //need to set up the JVM by calling setJVM in jni_android.go.
 func Init() {
+	log.Print("ble.Init()")
 	h := app.PlatformHandle()
+	jvmContext = h.Context
 	setJVM(h.JVM, h.Context)
 }
 
@@ -98,6 +110,31 @@ func (p Peripheral) SetNotifyValue(c Characteristic) {
 //Bluetooth API.
 func NewBLE() *BLE {
 	ps := Peripherals{items: make([]PeripheralListItem, 0)}
-	return &BLE{events: make(chan interface{}), peripherals: ps}
+	h := bleHandle{}
+	RunInJVM(func(env *JNIEnv) {
+		h.adapter = GetBluetoothAdapter(env, jvmContext)
+		h.class = JniGetObjectClass(env, h.adapter)
+	})
+
+	ret := &BLE{
+		events: make(chan interface{}),
+		peripherals: ps,
+		handle: h,
+	}
+
+	//check if Bluetooth is enabled and if so, send an UpdateStateEvent
+	go func() {
+		RunInJVM(func(env *JNIEnv) {
+			mid := JniGetMethodID(env, h.class, "isEnabled", "()Z")
+			en := JniCallBooleanMethod(env, h.adapter, mid)
+			if en {
+				log.Print("It's enabled!")
+				ret.events <-UpdateStateEvent{"powered on"}
+			} else {
+				log.Print("It's not enabled")
+			}
+		})
+	}()
+	return ret
 }
 

jni_android.go

@@ -1,7 +1,7 @@
 package ble
 
 /*
-#cgo LDFLAGS: -landroid
+#cgo LDFLAGS: -landroid -llog
 
 #include <stdlib.h>
 #include <jni.h>
@@ -10,6 +10,13 @@ package ble
 static jobject gClassLoader;
 static jmethodID gFindClassMethod;
 
+jclass
+GetObjectClass(JNIEnv* env, jobject obj) {
+	jclass cls = (*env)->GetObjectClass(env, obj);
+	cls = (*env)->NewGlobalRef(env, cls);
+	return cls;
+}
+
 void
 SetLoader(JNIEnv* env, jobject context) {
 	jclass cclass = (*env)->GetObjectClass(env, context);
@@ -20,6 +27,68 @@ SetLoader(JNIEnv* env, jobject context) {
 	gFindClassMethod = (*env)->GetMethodID(env, lclass, "findClass", "(Ljava/lang/String;)Ljava/lang/Class;");
 }
 
+jobject
+GetBluetoothAdapter(JNIEnv* env, jobject ctx) {
+	jclass cls = (*env)->GetObjectClass(env, ctx);
+	if (cls == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get context class");
+		return NULL;
+	}
+	jfieldID id = (*env)->GetStaticFieldID(env, cls, "BLUETOOTH_SERVICE", "Ljava/lang/String;");
+	if (id == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get field ID");
+		return NULL;
+	}
+	jstring btsrv = (jstring)(*env)->GetStaticObjectField(env, cls, id);
+	if (btsrv == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get static object field");
+		return NULL;
+	}
+	jmethodID mid = (*env)->GetMethodID(env, cls, "getSystemService", "(Ljava/lang/String;)Ljava/lang/Object;");
+	if (mid == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get method ID");
+		return NULL;
+	}
+	jobject manager = (*env)->CallObjectMethod(env, ctx, mid, btsrv);
+	if (manager == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get bluetooth manager");
+		return NULL;
+	}
+	cls = (*env)->GetObjectClass(env, manager);
+	if (cls == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get class of bluetooth manager");
+		return NULL;
+	}
+	mid = (*env)->GetMethodID(env, cls, "getAdapter", "()Landroid/bluetooth/BluetoothAdapter;");
+	if (mid == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get methodID for getAdapter()");
+		return NULL;
+	}
+	jobject ad = (*env)->CallObjectMethod(env, manager, mid);
+	if (ad == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Can't get adapter");
+	} else {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetBluetoothAdapter(): Found");
+	}
+	ad = (*env)->NewGlobalRef(env, ad);
+	return ad;
+}
+
+jstring
+GetObjectStaticStringField(JNIEnv* env, jobject obj, char* fld) {
+	jclass cls = (*env)->GetObjectClass(env, obj);
+	if (cls == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetObjectStaticStringField(): Can't get class");
+		return NULL;
+	}
+	jfieldID id = (*env)->GetStaticFieldID(env, cls, fld, "Ljava/lang/String;");
+	if (id == NULL) {
+		__android_log_write(ANDROID_LOG_DEBUG, "gio", "GetObjectStaticStringField(): Can't find FieldID");
+		return NULL;
+	}
+	return (jstring)(*env)->GetStaticObjectField(env, cls, id);
+}
+
 jclass
 FindClass(JNIEnv* env, char* name) {
 	jstring strClassName = (*env)->NewStringUTF(env, name);
@@ -36,6 +105,10 @@ void CallVoidMethod(JNIEnv *env, jobject obj, jmethodID methodID) {
 	(*env)->CallVoidMethod(env, obj, methodID);
 }
 
+jboolean CallBooleanMethod(JNIEnv *env, jobject obj, jmethodID mid) {
+	return (*env)->CallBooleanMethod(env, obj, mid);
+}
+
 jint CallIntMethod(JNIEnv *env, jobject obj, jmethodID methodID) {
 	return (*env)->CallIntMethod(env, obj, methodID);
 }
@@ -74,7 +147,7 @@ type JNIEnv = C.JNIEnv
 func setJVM(jvm, context uintptr) {
 	log.Print("set theJVM")
 	theJVM = (*C.JavaVM)(unsafe.Pointer(jvm))
-	log.Printf("theJVM = %d", uintptr(unsafe.Pointer(theJVM)))
+	//log.Printf("theJVM = %d", uintptr(unsafe.Pointer(theJVM)))
 	RunInJVM(func(env *C.JNIEnv) {
 		C.SetLoader(env, (C.jobject)(context))
 	})
@@ -86,10 +159,28 @@ func FindClass(env *C.JNIEnv, name string) C.jclass {
 	return C.FindClass((*C.JNIEnv)(env), cname)
 }
 
+func JniGetObjectClass(env *C.JNIEnv, obj C.jobject) C.jclass {
+	return C.GetObjectClass(env, obj)
+}
+
+func JniGetObjectStaticStringField(env *C.JNIEnv, obj C.jobject, fld string) C.jstring {
+	cstring := C.CString(fld)
+	defer C.free(unsafe.Pointer(cstring))
+	return C.GetObjectStaticStringField(env, obj, cstring)
+}
+
+func GetBluetoothAdapter(env *C.JNIEnv, ctx uintptr) C.jobject {
+	return C.GetBluetoothAdapter(env, (C.jobject)(ctx))
+}
+
 func JniCallVoidMethod(env *C.JNIEnv, obj C.jobject, methodID C.jmethodID) {
 	C.CallVoidMethod(env, obj, methodID)
 }
 
+func JniCallBooleanMethod(env *C.JNIEnv, obj C.jobject, methodID C.jmethodID) bool {
+	return C.CallBooleanMethod(env, obj, methodID) == 0
+}
+
 func JniCallIntMethod(env *C.JNIEnv, obj C.jobject, methodID C.jmethodID) int {
 	return (int)(C.CallIntMethod(env, obj, methodID))
 }
@@ -111,7 +202,7 @@ func RunInJVM(f func(env *C.JNIEnv)) {
 	defer runtime.UnlockOSThread()
 	var env *C.JNIEnv
 	var detach bool
-	log.Printf("RunInJVM(): theJVM = %d", uintptr(unsafe.Pointer(theJVM)))
+	//log.Printf("RunInJVM(): theJVM = %d", uintptr(unsafe.Pointer(theJVM)))
 	if res := C.GetEnv(theJVM, &env, C.JNI_VERSION_1_6); res != C.JNI_OK {
 		if res != C.JNI_EDETACHED {
 			panic(fmt.Errorf("JNI GetEnv failed with error %d", res))