Simplify Open to return io.ReadCloser.

This allows the browser implementation to return the underlying
io.ReaderCloser directly, without having to first cache the entire
thing. That enables streaming of the underlying source to be possible
(which is possible in browser after Fetch Transport, see
gopherjs/gopherjs#454.

If seeking is desired, it can be added by the user on top of the
returned io.ReadCloser. In many cases, seeking is not very critical and
easy to avoid, and definitely not worth the expensive overhead of
always supporting it.

This is a slightly breaking API change if you depend on seeking, but
easy to work around in client code.

Author: dmitshur

browser.go

@@ -3,17 +3,14 @@
 package glfw
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"log"
 	"net/http"
 	"runtime"
 
 	"github.com/gopherjs/gopherjs/js"
-	"golang.org/x/tools/godoc/vfs"
 	"honnef.co/go/js/dom"
 )
 
@@ -778,29 +775,18 @@ const (
 	ModSuper
 )
 
-// Open opens a named asset.
-func Open(name string) (vfs.ReadSeekCloser, error) {
+// Open opens a named asset. It's the caller's responsibility to close it when done.
+func Open(name string) (io.ReadCloser, error) {
 	resp, err := http.Get(name)
 	if err != nil {
 		return nil, err
 	}
-	defer resp.Body.Close()
 	if resp.StatusCode != 200 {
 		return nil, fmt.Errorf("non-200 status: %s", resp.Status)
 	}
-	b, err := ioutil.ReadAll(resp.Body)
-	if err != nil {
-		return nil, err
-	}
-	return nopCloser{bytes.NewReader(b)}, nil
+	return resp.Body, nil
 }
 
-type nopCloser struct {
-	io.ReadSeeker
-}
-
-func (nopCloser) Close() error { return nil }
-
 // ---
 
 func WaitEvents() {

desktop.go

@@ -3,11 +3,11 @@
 package glfw
 
 import (
+	"io"
 	"os"
 	"runtime"
 
 	"github.com/go-gl/glfw/v3.1/glfw"
-	"golang.org/x/tools/godoc/vfs"
 )
 
 func init() {
@@ -372,10 +372,10 @@ const (
 	ModSuper   = ModifierKey(glfw.ModSuper)
 )
 
-// Open opens a named asset.
+// Open opens a named asset. It's the caller's responsibility to close it when done.
 //
 // For now, assets are read directly from the current working directory.
-func Open(name string) (vfs.ReadSeekCloser, error) {
+func Open(name string) (io.ReadCloser, error) {
 	return os.Open(name)
 }