x/mobile/app: Open function for basic asset management

LGTM=hyangah
R=hyangah
CC=golang-codereviews
https://golang.org/cl/187740044

app/android.go

@@ -12,11 +12,15 @@ 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 <pthread.h>
+
 #include <jni.h>
+#include <pthread.h>
+#include <stdlib.h>
 
 pthread_cond_t go_started_cond;
 pthread_mutex_t go_started_mu;
@@ -32,15 +36,22 @@ JavaVM* current_vm;
 */
 import "C"
 import (
+	"fmt"
+	"io"
 	"log"
+	"os"
 	"runtime"
 	"unsafe"
 
 	"golang.org/x/mobile/geom"
 )
 
+var assetManager *C.AAssetManager
+
 //export onCreate
 func onCreate(activity *C.ANativeActivity) {
+	assetManager = activity.assetManager
+
 	config := C.AConfiguration_new()
 	C.AConfiguration_fromAssetManager(config, activity.assetManager)
 	density := C.AConfiguration_getDensity(config)
@@ -150,6 +161,57 @@ var (
 	windowCreated   = 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(assetManager, 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 run(cb Callbacks) {
 	// We want to keep the event loop on a consistent OS thread.
 	runtime.LockOSThread()

app/app.go

@@ -4,7 +4,11 @@
 
 package app
 
-import "golang.org/x/mobile/event"
+import (
+	"io"
+
+	"golang.org/x/mobile/event"
+)
 
 // Run starts the app.
 //
@@ -26,10 +30,24 @@ type Callbacks struct {
 	Touch func(event.Touch)
 }
 
-/*
-TODO(crawshaw): Implement.
-var Start func()
-var Stop func()
-var Resume func()
-var Pause func()
-*/
+// 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
+}

app/desktop.go

@@ -0,0 +1,25 @@
+// Copyright 2014 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 !android
+
+package app
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// Logic shared by desktop debugging implementations.
+
+func openAsset(name string) (ReadSeekCloser, error) {
+	if !filepath.IsAbs(name) {
+		name = filepath.Join("assets", name)
+	}
+	f, err := os.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return f, nil
+}