Flutter AlertDialog Tutorial
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AlertDialog is a dialog box that is used to notify the user of some situations which confirmation is requested. It includes an optional title, an optional content and optional action buttons under the content.
If the content is too large for the screen vertically, you can wrap it in a SingleChildScrollView to avoid its overflowing. Nevertheless, it is notable that AlertDialog tries to size itself based on the intrinsic size of its children; therefore, child widgets such as ListView, GridView and CustomScrollView should be considered using in content creation.
The lazy data display mode does not work in AlertDialog. If this is problematic, you can use the Dialog class directly in order to get more creative space.
- TODO Link?
If you want a simple dialog containing only one title and one list of options, take the SimpleDialog into consideration.
AlertDialog Constructor:
AlertDialog Constructor
const AlertDialog(
{Key key,
Widget title,
Widget content,
List<Widget> actions,
ShapeBorder shape,
double elevation,
EdgeInsetsGeometry titlePadding,
TextStyle titleTextStyle,
EdgeInsetsGeometry contentPadding: const EdgeInsets.fromLTRB(24.0, 20.0, 24.0, 24.0),
TextStyle contentTextStyle,
EdgeInsetsGeometry actionsPadding: EdgeInsets.zero,
VerticalDirection actionsOverflowDirection,
double actionsOverflowButtonSpacing,
EdgeInsetsGeometry buttonPadding,
Color backgroundColor,
String semanticLabel,
EdgeInsets insetPadding: _defaultInsetPadding,
Clip clipBehavior: Clip.none,
bool scrollable: false}
)
First of all, take a look at the simple but complete example of the AlertDialog. It will help you answer the following basic questions:
- How to create an AlertDialog.
- How to return a value from AlertDialog.
- How to handle the returned value.
main.dart (ex1)
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'o7planning.org',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key}) : super(key: key);
@override
State<StatefulWidget> createState() {
return MyHomePageState();
}
}
class MyHomePageState extends State<MyHomePage> {
String answer = "?";
@override
Widget build(BuildContext context) {
return Scaffold (
appBar: AppBar(
title: Text("Flutter AlertDialog Example")
),
body: Center (
child: Row (
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.center,
children: [
ElevatedButton (
child: Text("Question"),
onPressed: () {
showMyAlertDialog(context);
},
),
SizedBox(width:5, height:5),
Text("Answer: " + this.answer.toString())
]
)
)
);
}
showMyAlertDialog(BuildContext context) {
// Create AlertDialog
AlertDialog dialog = AlertDialog(
title: Text("Election 2020"),
content: Text("Will you vote for Trump?"),
actions: [
ElevatedButton(
child: Text("Yes"),
onPressed: (){
Navigator.of(context).pop("Yes, Of course!"); // Return value
}
),
ElevatedButton(
child: Text("No"),
onPressed: (){
Navigator.of(context).pop("No, I will vote for Biden"); // Return value
}
),
],
);
// Call showDialog function to show dialog.
Future<String> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
Stream<String> stream = futureValue.asStream();
stream.listen((String data) {
String answerValue = data;
this.setState(() {
this.answer = answerValue;
});
}, onDone : () {
print("Done!");
} , onError: (error) {
print("Error! " + error.toString());
});
}
}
shape property is used to define the border shape of the AlertDialog. The default value of the shape is a RoundedRectangleBorder with the radius of 4.0 pixels at the four corners.
ShapeBorder shape
- TODO Link?
shape (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
content: Text("Are you sure to remove this item?"),
shape: RoundedRectangleBorder(
side: BorderSide(color: Colors.green,width: 3),
borderRadius: BorderRadius.all(Radius.circular(15))
),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
elevation property defines the Z-axis of the AlertDialog. Its default value is 24.0 pixels.
double elevation
title property is an option to set the title for AlertDialog, which in most use cases, it is a Text object.
Widget title
Here is a simple title example of Text object:
title (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Are you sure to remove this item?"),
content: Text("It will delete all items permanently."),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
This is another example of a more complicated title:
title (ex2)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Row (
children: [
Icon(Icons.delete, color: Colors.red),
SizedBox(width: 5, height: 5),
Text("Are you sure to remove this item?"),
]
),
content: Text("It will delete all items permanently."),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
titlePadding property is used to add a padding around the AlertDialog title. If title is null, titlePadding will not be used.
By default, titlePadding offers 24 pixels at the top, the left and the right of the title. It also offers 20 pixels at the bottom of the title if the content is null.
EdgeInsetsGeometry titlePadding
For example:
titlePadding (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
titlePadding: EdgeInsets.fromLTRB(24, 24, 24, 50),
content: Text("Are you sure to remove this item?"),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
titleTextStyle property is used to define the text style of the title area.
TextStyle titleTextStyle
titleTextStyle (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
titleTextStyle: TextStyle(color: Colors.red, fontSize: 24),
content: Text("Are you sure to remove this item?"),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
content property is an option to define a content to be displayed in the center of the AlertDialog. It should be a SingleChildScrollView object if the content is long.
Widget content
content (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
titleTextStyle: TextStyle(color: Colors.red, fontSize: 24),
content: Row (
children: [
Icon(Icons.dangerous, size: 30, color: Colors.red),
SizedBox(width:5, height:5),
Text("Are you sure to remove this item?")
]
),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: () {
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
- TODO Link?
contentPadding property is used to add a padding around the content of AlertDialog. If content is null, contentPadding will not be used.
By default, contentPadding offers 20 pixels on the top of the content and 24 pixels on the left, the right and the bottom of the content.
EdgeInsetsGeometry contentPadding: const EdgeInsets.fromLTRB(24.0, 20.0, 24.0, 24.0)
contentPadding (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
content: Text("Are you sure to remove this item?"),
contentPadding: EdgeInsets.fromLTRB(24, 24, 24, 50),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
contentTextStyle property is used to define the text style of the content area.
TextStyle contentTextStyle
contentTextStyle (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
content: Text("Are you sure to remove this item?"),
contentTextStyle: TextStyle(color: Colors.red, fontStyle: FontStyle.italic),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
),
],
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
actions property is an option to define action buttons located at the bottom of the AlertDialog. Specifically, it is a list of TextButton(s) or ElevatedButton(s) wrapped in a ButtonBar.
List<Widget> actions
- TODO Link?
actionsPadding property is used to add a padding around the ButtonBar of the AlertDialog. If actions are null, actionsPadding will not be used. The default value of actionsPadding is EdgeInsets.zero (0,0,0,0).
Instead of using the actionsPadding property, you can use the buttonPadding property in order to create paddings around each button.
EdgeInsetsGeometry actionsPadding: EdgeInsets.zero
actionsPadding (ex1)
void openMyAlertDialog(BuildContext context) {
// Create a AlertDialog.
AlertDialog dialog = AlertDialog(
title: Text("Confirm"),
content: Text("Are you sure to remove this item?"),
actions: [
ElevatedButton(
child: Text("Yes Delete"),
onPressed: (){
Navigator.of(context).pop(true); // Return true
}
),
ElevatedButton(
child: Text("Cancel"),
onPressed: (){
Navigator.of(context).pop(false); // Return false
}
)
],
actionsPadding: EdgeInsets.fromLTRB(24, 24, 24, 50),
);
// Call showDialog function.
Future<bool> futureValue = showDialog(
context: context,
builder: (BuildContext context) {
return dialog;
}
);
futureValue.then( (value) {
print("Return value: " + value.toString()); // true/false
});
}
actions are usually placed on a row. However, if the horizontal space is not enough, they will be placed on a column. The actionsOverflowDirection property will specify how to order them on a column, top to bottom (Default) or bottom to top.
VerticalDirection actionsOverflowDirection
// enum VerticalDirection
VerticalDirection.up
VerticalDirection.down (Default)
- actionsOverflowDirection: VerticalDirection.up
- actionsOverflowDirection: VerticalDirection.down (Default)
actions are usually placed on a row. However, if the horizontal space is not enough, they will be placed on a column. The actionsOverflowButtonSpacing property specifies the space among the actions in the vertical direction.
double actionsOverflowButtonSpacing
EdgeInsetsGeometry buttonPadding
backgroundColor property is used to specify the background color of the AlertDialog.
Color backgroundColor
EdgeInsets insetPadding: _defaultInsetPadding
Clip clipBehavior: Clip.none
- TODO Link?
semanticLabel is a descriptive text of the AlertDialog which is not visible on the user's interface. When the user opens or closes the AlertDialog, the system will read this description to the user if the accessibility mode is enabled.
String semanticLabel
scrollable property is outdated and no longer in use.
@Deprecated( 'Set scrollable to `true`. This parameter will be removed and ' 'was introduced to migrate AlertDialog to be scrollable by ' 'default. For more information, see ' 'https://flutter.dev/docs/release/breaking-changes/scrollable_alert_dialog. ' 'This feature was deprecated after v1.13.2.' ) bool scrollable: false
