device/file: implement IsRunning method for file

device/file/file.go

@@ -36,8 +36,9 @@ import (
 // AVFile is an implementation of the AVDevice interface for a file containg
 // audio or video data.
 type AVFile struct {
-	f   io.ReadCloser
-	cfg config.Config
+	f         io.ReadCloser
+	cfg       config.Config
+	isRunning bool
 }
 
 // NewAVFile returns a new AVFile.
@@ -62,11 +63,19 @@ func (m *AVFile) Start() error {
 	if err != nil {
 		return fmt.Errorf("could not open media file: %w", err)
 	}
+	m.isRunning = true
 	return nil
 }
 
 // Stop will close the file such that any further reads will fail.
-func (m *AVFile) Stop() error { return m.f.Close() }
+func (m *AVFile) Stop() error {
+	err := m.f.Close()
+	if err == nil {
+		m.isRunning = false
+		return nil
+	}
+	return err
+}
 
 // Read implements io.Reader. If start has not been called, or Start has been
 // called and Stop has since been called, an error is returned.
@@ -79,6 +88,5 @@ func (m *AVFile) Read(p []byte) (int, error) {
 
 // IsRunning is used to determine if the AVFile device is running.
 func (m *AVFile) IsRunning() bool {
-	panic("not implemented")
-	return false
+	return m.f != nil && m.isRunning
 }