# Widgets 07 | AppBar ## Before start In every tutorial, we need create a page to contain our code to show. So let's look at our code. ```dart import "package:flutter/material.dart"; import 'package:flutter_widgets/const/_const.dart'; class AppBarPage extends StatefulWidget { @override _AppBarState createState() => _AppBarState(); } class _AppBarState extends State { @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. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWjOEI5RZb33AoexsCv%2F-LWjT_lah9yqWybbhqTO%2Fapp_bar_01.png?alt=media\&token=63355cde-2f8a-4e14-875b-0a44593d7a39) ## Constructor We have use the `AppBar` above, So let's look at its constructor. ```dart 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. ```dart AppBar _autoLeadBar() => AppBar( title: Text(PageName.APP_BAR), automaticallyImplyLeading: false, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWk1YT8c0oEQVJaiBw3%2Fapp_bar_02.png?alt=media\&token=96a6f3f5-83cc-4b11-93cd-c841ff54d6d6) 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. ```dart AppBar _leadingBar() => AppBar( title: Text(PageName.APP_BAR), leading: InkWell( child: Icon(Icons.menu), onTap: () { print("click menu"); }, ), ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWk2mOebzKCvCsWo9g0%2Fapp_bar_03.png?alt=media\&token=de084571-18a9-4543-bf39-84168a9daf5f) ## actions If you want to add some widget in the right, the actions parameter you should know to use. ```dart AppBar _actionBar() => AppBar( title: Text(PageName.APP_BAR), actions: [ 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. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWk4UzlrsnEzi_UFFPd%2Fapp_bar_04.png?alt=media\&token=6851ffda-bf53-4c70-9255-770a62f4880f) ## bottom When we look at the definition of the bottom parameter. we can see it is `PreferredSizeWidget`. ```dart 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`. ```dart 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. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWk7h7FnzqPrXOb6Pv0%2Fapp_bar_05.png?alt=media\&token=3c3acf47-a693-4c91-a01f-de13bd19045b) ## elevation This parameter controls the shadow of the `AppBar`.It is similar with `RaiseButton` I have tell before. ```dart 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`. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWkAEbqxJ4DlyUDgGJj%2Fapp_bar_06.png?alt=media\&token=a7e7a8a4-d666-4b38-bb73-69478b662450) ## backgroundColor If you want to change the color of the `AppBar`, you can use the `backgroundColor` as follows. ```dart Widget _backgroundBar() => AppBar( title: Text(PageName.APP_BAR), backgroundColor: PURPLE, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWkBOdp94PuzpW6AkMM%2Fapp_bar_07.png?alt=media\&token=c642f692-3c9e-4a89-927d-9a4c9b42d24b) ## brightness The parameter will change the text, icon color of the status bar. ```dart Widget _brightBar() => AppBar( title: Text(PageName.APP_BAR), brightness: Brightness.dark, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWk0NsEQcCb8Op_ZrKf%2F-LWkDEhDW_JVKvRbRze-%2Fapp_bar_08.png?alt=media\&token=f0da20dd-7a52-40c5-9366-bbf4ab7bf5dc) ## 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. ```dart IconThemeData({this.color, double opacity, this.size}) ``` Let's use it. ```dart 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. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkGJ2XaxNpWQOu9OLc%2Fapp_bar_09.png?alt=media\&token=86bd05e3-3865-4980-9ca9-bab0f02d3f3a) But you should know, if you change the size, it doesn't have an effect. ## textTheme This will control the title of the Text. ```dart 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. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkIAdfiE4JJIGNUPO0%2Fapp_bar_10.png?alt=media\&token=24909dde-c943-47a8-9aec-19ec948543ae) ## primary The default value of this parameter is true. When we set to false, the icon and title will move up. ```dart Widget _primaryBar() => AppBar( title: Text(PageName.APP_BAR), primary: false, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkJtNY19LbNP6UA7nR%2Fapp_bar_11.png?alt=media\&token=3cb1f79d-ec0e-4034-ab2d-c00612ef590f) ## centerTitle This parameter is bool, the default value is false.It controls the title whether or not show in the center of the AppBar. ```dart Widget _centerBar() => AppBar( title: Text(PageName.APP_BAR), centerTitle: true, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkKaXZyHO4b-qdUmvW%2Fapp_bar_12.png?alt=media\&token=285d245d-088a-401a-8ebf-bb55dfac7273) ## titleSpacing This parameter will change the distance of the title from left. So the direction is horizontal and the default value is zero. ```dart Widget _spaceBar() => AppBar( title: Text(PageName.APP_BAR), titleSpacing: 20, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkMRH3qHnPD0yor0Av%2Fapp_bar_13.png?alt=media\&token=8d98d8bf-2a52-46ae-b0d2-c73b8889ceef) ## toolbarOpacity It will change the transparency of the elements of the `AppBar`. ```dart Widget _toolBar() => AppBar( title: Text(PageName.APP_BAR), toolbarOpacity: 0.5, ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkO6ZVzDihStx4T07e%2Fapp_bar_14.png?alt=media\&token=f61b385c-6ce4-494f-9520-d0659080aaa5) ## bottomOpacity We have used the `bottom` above, so the `bottomOpacity` will control the transparency of the part of the `bottom`. ```dart 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), ), ); ``` ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkPGnIgBc8FmPm1_5j%2Fapp_bar_15.png?alt=media\&token=d15d2386-cc32-48dd-9cca-36d5b5d176d2) ## 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. ```dart Scaffold( body: CustomScrollView( slivers: [ 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](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkSheNMBWqC_LPmfcb%2Fapp_bar_16.png?alt=media\&token=8776cd16-54a9-4ec8-885b-e6c007dccdbc) When you scroll to top, It will show as follows. ![](https://3262132263-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LW5X4rCYpodwj4P1c8s%2F-LWkDW59JzwYMpEYfvMP%2F-LWkStLZbcBjqncCElAv%2Fapp_bar_17.png?alt=media\&token=02711a93-4f6d-4a81-97b5-f129dcd99d95) 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! 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