AppBar Widget

In this tutorial, you will learn more detail about the AppBar in the flutter.
Before start
In every tutorial, we need create a page to contain our code to show. So let's look at our code.
http://www.w3.org/2000/svg
import "package:flutter/material.dart";
import 'package:flutter_widgets/const/_const.dart';
​
class AppBarPage extends StatefulWidget {
  @override
  _AppBarState createState() => _AppBarState();
}
​
class _AppBarState extends State<AppBarPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text(PageName.APP_BAR),
        ),
        body: Text("body"));
  }
}
We have used the AppBar in our code above, just a simple one. It will show a app bar with a title. It is easy to use. It usually use with the appBar parameter of the  Scaffold . But in this tutorial, I will try to use it as a common widget to describe its parameter. Hope you do not misunderstand my style.
Constructor
We have use the AppBar above, So let's look at its constructor.
http://www.w3.org/2000/svg
AppBar({
    Key key,
    this.leading,
    this.automaticallyImplyLeading = true,
    this.title,
    this.actions,
    this.flexibleSpace,
    this.bottom,
    this.elevation = 4.0,
    this.backgroundColor,
    this.brightness,
    this.iconTheme,
    this.textTheme,
    this.primary = true,
    this.centerTitle,
    this.titleSpacing = NavigationToolbar.kMiddleSpacing,
    this.toolbarOpacity = 1.0,
    this.bottomOpacity = 1.0,
  }) 
So let's look at the its parameters as follows.
automaticallyImplyLeading
If you don't want the left-arrow on the app bar. you can use this parameter to hide it.
http://www.w3.org/2000/svg
  AppBar _autoLeadBar() => AppBar(
        title: Text(PageName.APP_BAR),
        automaticallyImplyLeading: false,
      );
But you should know, if you the leading is not null, then this parameter hasn't effect.
leading
This parameter controls the space as the left-arrow. So if you want to use some special function here. You can use this parameter, it is just a widget, but the space is strict. I often use the InkWell with a Icon as below.
http://www.w3.org/2000/svg
 AppBar _leadingBar() => AppBar(
        title: Text(PageName.APP_BAR),
        leading: InkWell(
          child: Icon(Icons.menu),
          onTap: () {
            print("click menu");
          },
        ),
      );
actions
If you want to add some widget in the right, the actions parameter you should know to use.
http://www.w3.org/2000/svg
 AppBar _actionBar() => AppBar(
        title: Text(PageName.APP_BAR),
        actions: <Widget>[
          InkWell(
            child: Icon(Icons.search),
            onTap: () {
              print("click search");
            },
          ),
          SizedBox(width: 10),
          InkWell(
            child: Icon(Icons.more_vert),
            onTap: () {
              print("click more");
            },
          ),
          SizedBox(width: 20)
        ],
      );
We have added the search and more icon at the right of the app bar.
bottom
When we look at the definition of the bottom parameter. we can see it is PreferredSizeWidget.
http://www.w3.org/2000/svg
  final PreferredSizeWidget bottom;
 It is an abstract class.So we need use its subclass. The PreferredSize is a good choice to use. The child is a widget, the preferredSize is the Size. 
http://www.w3.org/2000/svg
  Widget _bottomBar() => AppBar(
        title: Text(PageName.APP_BAR),
        bottom: PreferredSize(
          child: Container(
            alignment: Alignment.center,
            color: RED,
            constraints: BoxConstraints.expand(height: 50),
            child: Text(
              "bottom",
              style: TextStyle(fontSize: 30),
            ),
          ),
          preferredSize: Size(50, 50),
        ),
      );
It will show as follows.
elevation
This parameter controls the shadow of the AppBar.It is similar with RaiseButton I have tell before. 
http://www.w3.org/2000/svg
Widget _elevationBar() => AppBar(
        title: Text(PageName.APP_BAR),
        elevation: 10,
      );
This will show as right in the picture below.The left one is elevation = 4.
backgroundColor
If you want to change the color of the AppBar, you can use the backgroundColor as follows.
http://www.w3.org/2000/svg
 Widget _backgroundBar() => AppBar(
        title: Text(PageName.APP_BAR),
        backgroundColor: PURPLE,
      );
brightness
The parameter will change the text, icon color of the status bar.
http://www.w3.org/2000/svg
Widget _brightBar() => AppBar(
        title: Text(PageName.APP_BAR),
        brightness: Brightness.dark,
      );
iconTheme
When we look at the parameter carefully, we can see it is IconThemeData, its constructor is below. We have used the color, size several times, so I do not explain it. The opacity controls the  transparency of the Icon. The value is from 0 to 1.
http://www.w3.org/2000/svg
IconThemeData({this.color, double opacity, this.size})
Let's use it.
http://www.w3.org/2000/svg
     Widget _iconThemeBar() => AppBar(
        title: Text(PageName.APP_BAR),
        iconTheme: IconThemeData(color: Colors.white, size: 20, opacity: 0.9),
      );
The left-arrow will change to white.
But you should know, if you change the size, it doesn't have an effect.
textTheme
This will control the title of the Text.
http://www.w3.org/2000/svg
 Widget _textThemeBar() => AppBar(
        title: Text(PageName.APP_BAR),
        textTheme: TextTheme(title: TextStyle(color: Colors.white,fontSize: 30)),
      );
It will change the color of the title to white and the fontSize to 30.
primary 
The default value of this parameter is true. When we set to false, the icon and title will move up.
http://www.w3.org/2000/svg
  Widget _primaryBar() => AppBar(
        title: Text(PageName.APP_BAR),
        primary: false,
      );
centerTitle
This parameter is bool, the default value is false.It controls the title whether or not show in the center of the AppBar. 
http://www.w3.org/2000/svg
 Widget _centerBar() => AppBar(
        title: Text(PageName.APP_BAR),
        centerTitle: true,
      );
titleSpacing
This parameter will change the distance of the title from left. So the direction is horizontal and the default value is zero.
http://www.w3.org/2000/svg
  Widget _spaceBar() => AppBar(
        title: Text(PageName.APP_BAR),
        titleSpacing: 20,
      );
toolbarOpacity
It will change the transparency of the elements of the AppBar.
http://www.w3.org/2000/svg
 Widget _toolBar() =>
      AppBar(
        title: Text(PageName.APP_BAR),
        toolbarOpacity: 0.5,
      );
bottomOpacity
We have used the bottom above, so the bottomOpacity will control the transparency of the part of the bottom.
http://www.w3.org/2000/svg
 Widget _bottomOpBar() => AppBar(
        title: Text(PageName.APP_BAR),
        bottomOpacity: 0.5,
        bottom: PreferredSize(
          child: Container(
            alignment: Alignment.center,
            color: RED,
            constraints: BoxConstraints.expand(height: 50),
            child: Text(
              "bottom with  bottomOpacity = 0.5 ",
              style: TextStyle(fontSize: 20),
            ),
          ),
          preferredSize: Size(50, 50),
        ),
      );
flexibleSpace
I put this parameter in the last, because it is a bit difficult to use. But if you use it in the AppBar, it is not so useful. So I will use a SliverAppBar to replace the AppBar to describe this parameter. 
http://www.w3.org/2000/svg
Scaffold(
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
//              title: Text("hello"),
            expandedHeight: 100,
            pinned: true,
            flexibleSpace: FlexibleSpaceBar(
              title: Text(PageName.APP_BAR),
              centerTitle: false,
              collapseMode: CollapseMode.parallax,
              background: Container(
                color: RED,
                constraints: BoxConstraints.expand(height: 100),
                child: Image.asset(
                  PageImage.FLUTTER_OPEN,
                  fit: BoxFit.cover,
                ),
              ),
            ),
          ),
          SliverList(
            delegate: SliverChildListDelegate([
              SizedBox(
                height: 1000,
              ),
            ]),
          )
        ],
      ),
    );
We should replace all code  of the Scaffold with these code. It will show as below.
default
When you scroll to top, It will show as follows.
That's nice!
Conclusion
We have learned how to use the AppBar in the flutter. It is so common and important to us. So just keep going, we can learn very well about the widgets in the flutter.
Thanks for your reading!
The end.
Whole code in GitHub,star to support.