app: initialize asset manager for gobind android app.

Tested manually by modifying example/libhello to read an asset file,
and checking crawshaw's balloon app continues to work.

Fixes golang/go#9422.

Change-Id: I17b25d9456a4023c68be1de82ee071ea01299e68
Reviewed-on: https://go-review.googlesource.com/5680
Reviewed-by: David Crawshaw <crawshaw@golang.org>

app/android.c

@@ -4,6 +4,7 @@
 
 // +build android
 
+#include <android/asset_manager_jni.h>
 #include <android/log.h>
 #include <dlfcn.h>
 #include <errno.h>
@@ -210,6 +211,18 @@ void ANativeActivity_onCreate(ANativeActivity *activity, void* savedState, size_
 JNIEXPORT void JNICALL
 Java_go_Go_run(JNIEnv* env, jclass clazz, jobject ctx) {
 	current_ctx = (*env)->NewGlobalRef(env, ctx);
+
+	if (current_ctx != NULL) {
+		// Init asset_manager.
+		jclass context_clazz = find_class(env, "android/content/Context");
+		jmethodID getassets = find_method(
+			env, context_clazz, "getAssets", "()Landroid/content/res/AssetManager;");
+		// Prevent the java AssetManager from being GC'd
+		jobject asset_manager_ref = (*env)->NewGlobalRef(
+			env, (*env)->CallObjectMethod(env, current_ctx, getassets));
+		asset_manager = AAssetManager_fromJava(env, asset_manager_ref);
+	}
+
 	init_go_runtime(NULL);
 }
 

app/android.go

@@ -40,6 +40,11 @@ jobject current_ctx;
 // current_native_activity is the Android ANativeActivity. May be NULL.
 ANativeActivity* current_native_activity;
 
+// asset_manager is the asset manager of the app.
+// For all-Go app, this is initialized in onCreate.
+// For go library app, this is set from the context passed to Go.run.
+AAssetManager* asset_manager;
+
 // build_auxv builds an ELF auxiliary vector for initializing the Go
 // runtime. While there does not appear to be any spec for this
 // format, there are some notes in
@@ -67,11 +72,9 @@ import (
 	"golang.org/x/mobile/geom"
 )
 
-var assetManager *C.AAssetManager
-
 //export onCreate
 func onCreate(activity *C.ANativeActivity) {
-	assetManager = activity.assetManager
+	C.asset_manager = activity.assetManager
 
 	config := C.AConfiguration_new()
 	C.AConfiguration_fromAssetManager(config, activity.assetManager)
@@ -192,7 +195,7 @@ func openAsset(name string) (ReadSeekCloser, error) {
 	cname := C.CString(name)
 	defer C.free(unsafe.Pointer(cname))
 	a := &asset{
-		ptr:  C.AAssetManager_open(assetManager, cname, C.AASSET_MODE_UNKNOWN),
+		ptr:  C.AAssetManager_open(C.asset_manager, cname, C.AASSET_MODE_UNKNOWN),
 		name: name,
 	}
 	if a.ptr == nil {