photo_url_on_app_bar_leading.dart

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Screen title'),
        leading: _buildProfileIconButton(),
      ),
      body:  /* Main contents */
    )
  }

  Widget _buildProfileIconButton() {
    const iconSize = 32.0;
    return IconButton(
      icon: photoUrl == null
          ? Icon(
              Icons.account_circle,
              size: iconSize,
            )
          : CircleAvatar(
              backgroundImage: NetworkImage(photoUrl),
              backgroundColor: Colors.transparent,
              radius: iconSize / 2,
            ),
      onPressed: /* Action */,
    );
  }