asset: move app.Open to its own package

This does not break the dependency on the app package's AndroidContext
for loading assets on android. A potential answer for gobind-based
apps: add a SetAndroidContext method to app.Context. But I'll explore
that separately after the long weekend.

Change-Id: I812f899740e288c379eee7900f42d9d53926d4ce
Reviewed-on: https://go-review.googlesource.com/11675
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

app/android.c

@@ -4,7 +4,6 @@
 
 // +build android
 
-#include <android/asset_manager_jni.h>
 #include <android/log.h>
 #include <dlfcn.h>
 #include <errno.h>

app/android.go

@@ -25,7 +25,6 @@ package app
 #cgo LDFLAGS: -llog -landroid
 
 #include <android/log.h>
-#include <android/asset_manager.h>
 #include <android/configuration.h>
 #include <android/native_activity.h>
 #include <time.h>
@@ -46,16 +45,9 @@ JavaVM* current_vm;
 jobject current_ctx;
 
 jclass app_find_class(JNIEnv* env, const char* name);
-
-// 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;
 */
 import "C"
 import (
-	"fmt"
-	"io"
 	"log"
 	"os"
 	"runtime"
@@ -92,8 +84,6 @@ func callMain(mainPC uintptr) {
 
 //export onCreate
 func onCreate(activity *C.ANativeActivity) {
-	C.asset_manager = activity.assetManager
-
 	config := C.AConfiguration_new()
 	C.AConfiguration_fromAssetManager(config, activity.assetManager)
 	density := C.AConfiguration_getDensity(config)
@@ -219,57 +209,6 @@ var (
 	windowRedrawNeeded = make(chan *C.ANativeWindow)
 )
 
-func openAsset(name string) (ReadSeekCloser, error) {
-	cname := C.CString(name)
-	defer C.free(unsafe.Pointer(cname))
-	a := &asset{
-		ptr:  C.AAssetManager_open(C.asset_manager, cname, C.AASSET_MODE_UNKNOWN),
-		name: name,
-	}
-	if a.ptr == nil {
-		return nil, a.errorf("open", "bad asset")
-	}
-	return a, nil
-}
-
-type asset struct {
-	ptr  *C.AAsset
-	name string
-}
-
-func (a *asset) errorf(op string, format string, v ...interface{}) error {
-	return &os.PathError{
-		Op:   op,
-		Path: a.name,
-		Err:  fmt.Errorf(format, v...),
-	}
-}
-
-func (a *asset) Read(p []byte) (n int, err error) {
-	n = int(C.AAsset_read(a.ptr, unsafe.Pointer(&p[0]), C.size_t(len(p))))
-	if n == 0 && len(p) > 0 {
-		return 0, io.EOF
-	}
-	if n < 0 {
-		return 0, a.errorf("read", "negative bytes: %d", n)
-	}
-	return n, nil
-}
-
-func (a *asset) Seek(offset int64, whence int) (int64, error) {
-	// TODO(crawshaw): use AAsset_seek64 if it is available.
-	off := C.AAsset_seek(a.ptr, C.off_t(offset), C.int(whence))
-	if off == -1 {
-		return 0, a.errorf("seek", "bad result for offset=%d, whence=%d", offset, whence)
-	}
-	return int64(off), nil
-}
-
-func (a *asset) Close() error {
-	C.AAsset_close(a.ptr)
-	return nil
-}
-
 func main(f func(App)) {
 	// Preserve this OS thread for the GL context created in windowDraw.
 	runtime.LockOSThread()

app/app.go

@@ -6,11 +6,7 @@
 
 package app
 
-import (
-	"io"
-
-	"golang.org/x/mobile/event"
-)
+import "golang.org/x/mobile/event"
 
 // Main is called by the main.main function to run the mobile application.
 //
@@ -230,25 +226,3 @@ type Callbacks struct {
 	// Config is called by the app when configuration has changed.
 	Config func(new, old event.Config)
 }
-
-// Open opens a named asset.
-//
-// On Android, assets are accessed via android.content.res.AssetManager.
-// These files are stored in the assets/ directory of the app. Any raw asset
-// can be accessed by its direct relative name. For example assets/img.png
-// can be opened with Open("img.png").
-//
-// On iOS an asset is a resource stored in the application bundle.
-// Resources can be loaded using the same relative paths.
-//
-// For consistency when debugging on a desktop, assets are read from a
-// directoy named assets under the current working directory.
-func Open(name string) (ReadSeekCloser, error) {
-	return openAsset(name)
-}
-
-// ReadSeekCloser is an io.ReadSeeker and io.Closer.
-type ReadSeekCloser interface {
-	io.ReadSeeker
-	io.Closer
-}

asset/asset.go

@@ -0,0 +1,28 @@
+// Package asset provides access to application-bundled assets.
+//
+// On Android, assets are accessed via android.content.res.AssetManager.
+// These files are stored in the assets/ directory of the app. Any raw asset
+// can be accessed by its direct relative name. For example assets/img.png
+// can be opened with Open("img.png").
+//
+// On iOS an asset is a resource stored in the application bundle.
+// Resources can be loaded using the same relative paths.
+//
+// For consistency when debugging on a desktop, assets are read from a
+// directoy named assets under the current working directory.
+package asset
+
+import "io"
+
+// Open opens a named asset.
+//
+// Errors are of type *os.PathError.
+func Open(name string) (File, error) {
+	return openAsset(name)
+}
+
+// File is an open asset.
+type File interface {
+	io.ReadSeeker
+	io.Closer
+}

asset/asset_android.go

@@ -0,0 +1,120 @@
+package asset
+
+/*
+#cgo LDFLAGS: -llog -landroid
+#include <android/log.h>
+#include <android/asset_manager.h>
+#include <android/asset_manager_jni.h>
+#include <jni.h>
+#include <stdlib.h>
+
+#define LOG_FATAL(...) __android_log_print(ANDROID_LOG_FATAL, "Go/asset", __VA_ARGS__)
+
+// asset_manager is the asset manager of the app.
+AAssetManager* asset_manager;
+
+void asset_manager_init(void* java_vm, void* ctx) {
+	JavaVM* vm = (JavaVM*)(java_vm);
+	JNIEnv* env;
+	int err;
+	int attached = 0;
+
+	err = (*vm)->GetEnv(vm, (void**)&env, JNI_VERSION_1_6);
+	if (err != JNI_OK) {
+		if (err == JNI_EDETACHED) {
+			if ((*vm)->AttachCurrentThread(vm, &env, 0) != 0) {
+				LOG_FATAL("cannot attach JVM");
+			}
+			attached = 1;
+		} else {
+			LOG_FATAL("GetEnv unexpected error: %d", err);
+		}
+	}
+
+	// Equivalent to:
+	//	assetManager = ctx.getResources().getAssets();
+	jclass ctx_clazz = (*env)->FindClass(env, "android/content/Context");
+	jmethodID getres_id = (*env)->GetMethodID(env, ctx_clazz, "getResources", "()Landroid/content/res/Resources;");
+	jobject res = (*env)->CallObjectMethod(env, ctx, getres_id);
+	jclass res_clazz = (*env)->FindClass(env, "android/content/res/Resources");
+	jmethodID getam_id = (*env)->GetMethodID(env, res_clazz, "getAssets", "()Landroid/content/res/AssetManager;");
+	jobject am = (*env)->CallObjectMethod(env, res, getam_id);
+
+	// Pin the AssetManager and load an AAssetManager from it.
+	am = (*env)->NewGlobalRef(env, am);
+	asset_manager = AAssetManager_fromJava(env, am);
+
+	if (attached) {
+		(*vm)->DetachCurrentThread(vm);
+	}
+}
+*/
+import "C"
+import (
+	"fmt"
+	"io"
+	"os"
+	"sync"
+	"unsafe"
+
+	"golang.org/x/mobile/app"
+)
+
+var assetOnce sync.Once
+
+func assetInit() {
+	ctx := app.Context{}
+	C.asset_manager_init(ctx.JavaVM(), ctx.AndroidContext())
+}
+
+func openAsset(name string) (File, error) {
+	assetOnce.Do(assetInit)
+	cname := C.CString(name)
+	defer C.free(unsafe.Pointer(cname))
+	a := &asset{
+		ptr:  C.AAssetManager_open(C.asset_manager, cname, C.AASSET_MODE_UNKNOWN),
+		name: name,
+	}
+	if a.ptr == nil {
+		return nil, a.errorf("open", "bad asset")
+	}
+	return a, nil
+}
+
+type asset struct {
+	ptr  *C.AAsset
+	name string
+}
+
+func (a *asset) errorf(op string, format string, v ...interface{}) error {
+	return &os.PathError{
+		Op:   op,
+		Path: a.name,
+		Err:  fmt.Errorf(format, v...),
+	}
+}
+
+func (a *asset) Read(p []byte) (n int, err error) {
+	n = int(C.AAsset_read(a.ptr, unsafe.Pointer(&p[0]), C.size_t(len(p))))
+	if n == 0 && len(p) > 0 {
+		return 0, io.EOF
+	}
+	if n < 0 {
+		return 0, a.errorf("read", "negative bytes: %d", n)
+	}
+	return n, nil
+}
+
+func (a *asset) Seek(offset int64, whence int) (int64, error) {
+	// TODO(crawshaw): use AAsset_seek64 if it is available.
+	off := C.AAsset_seek(a.ptr, C.off_t(offset), C.int(whence))
+	if off == -1 {
+		return 0, a.errorf("seek", "bad result for offset=%d, whence=%d", offset, whence)
+	}
+	return int64(off), nil
+}
+
+func (a *asset) Close() error {
+	C.AAsset_close(a.ptr)
+	return nil
+}

asset/asset_darwin_armx.go

@@ -0,0 +1,25 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build darwin
+// +build arm arm64
+
+package app
+
+import (
+	"os"
+	"path/filepath"
+)
+
+func openAsset(name string) (File, error) {
+	if !filepath.IsAbs(name) {
+		name = filepath.Join("assets", name)
+	}
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
+
+}

asset/asset_desktop.go

@@ -2,18 +2,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux,!android darwin
+// +build linux,!android darwin,!arm,!arm64
 
-package app
+package asset
 
 import (
 	"os"
 	"path/filepath"
 )
 
-// Logic shared by desktop debugging implementations.
-
-func openAsset(name string) (ReadSeekCloser, error) {
+func openAsset(name string) (File, error) {
 	if !filepath.IsAbs(name) {
 		name = filepath.Join("assets", name)
 	}

example/audio/main.go

@@ -39,6 +39,7 @@ import (
 
 	"golang.org/x/mobile/app"
 	"golang.org/x/mobile/app/debug"
+	"golang.org/x/mobile/asset"
 	"golang.org/x/mobile/event"
 	"golang.org/x/mobile/exp/audio"
 	"golang.org/x/mobile/exp/sprite"
@@ -71,7 +72,7 @@ func main() {
 }
 
 func start() {
-	rc, err := app.Open("boing.wav")
+	rc, err := asset.Open("boing.wav")
 	if err != nil {
 		log.Fatal(err)
 	}
@@ -152,7 +153,7 @@ func boing() {
 }
 
 func loadGopher() sprite.SubTex {
-	a, err := app.Open("gopher.jpeg")
+	a, err := asset.Open("gopher.jpeg")
 	if err != nil {
 		log.Fatal(err)
 	}

example/sprite/main.go

@@ -36,6 +36,7 @@ import (
 
 	"golang.org/x/mobile/app"
 	"golang.org/x/mobile/app/debug"
+	"golang.org/x/mobile/asset"
 	"golang.org/x/mobile/event"
 	"golang.org/x/mobile/exp/sprite"
 	"golang.org/x/mobile/exp/sprite/clock"
@@ -131,7 +132,7 @@ const (
 )
 
 func loadTextures() []sprite.SubTex {
-	a, err := app.Open("waza-gophers.jpeg")
+	a, err := asset.Open("waza-gophers.jpeg")
 	if err != nil {
 		log.Fatal(err)
 	}