So far whenever I needed to use a conditional statement within a Widget I have done the following (Using Center and Containers as simplified dummy examples):
new Center(
child: condition == true ? new Container() : new Container()
)
Though when I tried using an if/else statement it would lead to an Dead code warning:
new Center(
child:
if(condition == true){
new Container();
}else{
new Container();
}
)
Interestingly enough I tried with a switch case statement and it gives me the same warning and thus I cannot run the code. Am I doing something wrong or is it so that one cannot use if/else or switch statements without flutter thinking there is dead code?
Lol after months of using ?: I just find out that I can use this:
Column(
children: [
if (true) Text('true') else Text('false'),
],
)
Here is the solution. I have fixed it. Here is the code
child: _status(data[index]["status"]),
Widget _status(status) {
if (status == "3") {
return Text('Process');
} else if(status == "1") {
return Text('Order');
} else {
return Text("Waiting");
}
}
With a button
bool _paused = false;
CupertinoButton(
child: _paused ? Text('Play') : Text('Pause'),
color: Colors.blue,
onPressed: () {
setState(() {
_paused = !_paused;
});
},
),
In such a case I would recommand using the ternary operator:
child: condition ? Container() : Center()
and try to avoid code of the form:
if (condition) return A else return B
which is needlessly more verbose than the ternary operator.
But if more logic is needed you may also:
The Builder widget is meant for allowing the use of a closure when a child widget is required:
A platonic widget that calls a closure to obtain its child widget.
It is convenient anytime you need logic to build a widget, it avoids the need to create a dedicated function.
You use the Builder widget as the child, you provide your logic in its builder method:
Center(
child: Builder(
builder: (context) {
// any logic needed...
final condition = _whateverLogicNeeded();
return condition
? Container();
: Center();
}
)
)
The Builder provides a convenient place to hold the creational logic. It is more straightforward than the immediate anonymous function proposed by atreeon.
Also I agree that the logic should be extracted from the UI code, but when it's really UI logic it is sometimes more legible to keep it there.
For the record, Dart 2.3 added the ability to use if/else statements in Collection literals. This is now done the following way:
return Column(children: <Widget>[
Text("hello"),
if (condition)
Text("should not render if false"),
Text("world")
],);
You can use ternary operator for conditional statements in dart, It's use is simple
(condition) ? statement1 : statement2
if the condition is true then the statement1 will be executed otherwise statement2.
Taking a practical example
Center(child: condition ? Widget1() : Widget2())
Remember if you are going to use null as Widget2 it is better to use SizedBox.shrink() because some parent widgets will throw an exception after getting a null child.
This is great article and conversation. I tried to use the ternary operator as described. But the code didn't work resulting in an error as mentioned.
Column(children: [ condition? Text("True"): null,],);
The ternary example above is miss leading. Dart will respond with an error that a null was returned instead of widget. You can't return null. The correct way will be to return a widget:
Column(children: [ condition? Text("True"): Text("false"),],);
In order for the ternary to work you need to return a Widget. If you don't want to return anything you can return a empty container.
Column(children: [ condition? Text("True"): Container(),],);
Good luck.
You can simply use a conditional statement a==b?c:d
For example :
Container(
color: Colors.white,
child: ('condition')
? Widget1(...)
: Widget2(...)
)
I hope you got the idea.
Suppose if there is no else condition you can use a SizedBox.shrink()
Container(
color: Colors.white,
child: ('condition')
? Widget1(...)
: SizedBox.shrink()
)
If it is a column no need to write ?: operator
Column(
children: <Widget>[
if('condition')
Widget1(...),
],
)
Another alternative: for 'switch's' like statements, with a lot of conditions, I like to use maps:
return Card(
elevation: 0,
margin: EdgeInsets.all(1),
child: conditions(widget.coupon)[widget.coupon.status] ??
(throw ArgumentError('invalid status')));
conditions(Coupon coupon) => {
Status.added_new: CheckableCouponTile(coupon.code),
Status.redeemed: SimpleCouponTile(coupon.code),
Status.invalid: SimpleCouponTile(coupon.code),
Status.valid_not_redeemed: SimpleCouponTile(coupon.code),
};
It's easier to add/remove elements to the condition list without touch the conditional statement.
Another example:
var condts = {
0: Container(),
1: Center(),
2: Row(),
3: Column(),
4: Stack(),
};
class WidgetByCondition extends StatelessWidget {
final int index;
WidgetByCondition(this.index);
@override
Widget build(BuildContext context) {
return condts[index];
}
}
Actually you can use if/else and switch and any other statement inline in dart / flutter.
class StatmentExample extends StatelessWidget {
Widget build(BuildContext context) {
return Text((() {
if(true){
return "tis true";}
return "anything but true";
})());
}
}
ie wrap your statements in a function
(() {
// your code here
}())
I would heavily recommend against putting too much logic directly with your UI 'markup' but I found that type inference in Dart needs a little bit of work so it can be sometimes useful in scenarios like that.
condition ? Text("True") : null,
children: [
...manyItems,
oneItem,
if(canIKickIt)
...kickTheCan
for (item in items)
Text(item)
child: getWidget()
Widget getWidget() {
if (x > 5) ...
//more logic here and return a Widget
As another alternative to the ternary operator, you could create a function version of the switch statement such as in the following post https://stackoverflow.com/a/57390589/1058292.
child: case2(myInput,
{
1: Text("Its one"),
2: Text("Its two"),
}, Text("Default"));
child: Container(
child: isFile == true ?
Image.network(pathfile, width: 300, height: 200, fit: BoxFit.cover) :
Text(message.subject.toString(), style: TextStyle(color: Colors.white),
),
),
In my app I created a WidgetChooser widget so I can choose between widgets without conditional logic:
WidgetChooser(
condition: true,
trueChild: Text('This widget appears if the condition is true.'),
falseChild: Text('This widget appears if the condition is false.'),
);
This is the source for the WidgetChooser widget:
import 'package:flutter/widgets.dart';
class WidgetChooser extends StatelessWidget {
final bool condition;
final Widget trueChild;
final Widget falseChild;
WidgetChooser({@required this.condition, @required this.trueChild, @required this.falseChild});
@override
Widget build(BuildContext context) {
if (condition) {
return trueChild;
} else {
return falseChild;
}
}
}
Aside from the ternary operator, you can also use Builder widget if you have operation needs to be performed before the condition statement.
Container(
child: Builder(builder: (context) {
/// some operation here ...
if(someCondition) {
return Text('A');
}
else return Text('B');
})
)
I personally use if/else statement in children with this kind of block statement. It only supports on Dart version 2.3.0 above.
if / else
Column(
children: [
if (_selectedIndex == 0) ...[
DayScreen(),
] else ...[
StatsScreen(),
],
],
),
if / else if
Column(
children: [
if (_selectedIndex == 0) ...[
DayScreen(),
] else if(_selectedIndex == 1)...[
StatsScreen(),
],
],
),
I found out that an easy way to use conditional logic to build Flutter UI is to keep the logic outside of the UI. Here is a function to return two different colors:
Color getColor(int selector) {
if (selector % 2 == 0) {
return Colors.blue;
} else {
return Colors.blueGrey;
}
}
The function is used below to to set the background of the CircleAvatar.
new ListView.builder(
itemCount: users.length,
itemBuilder: (BuildContext context, int index) {
return new Column(
children: <Widget>[
new ListTile(
leading: new CircleAvatar(
backgroundColor: getColor(index),
child: new Text(users[index].name[0])
),
title: new Text(users[index].login),
subtitle: new Text(users[index].name),
),
new Divider(height: 2.0),
],
);
},
);
Very neat as you can reuse your color selector function in several widgets.
Only if vibrating widget
if(bool = true) Container(
child: ....
),
OR
if(bool = true) Container(
child: ....
) else new Container(child: lalala),
if you use a list of widgets you can use this:
class HomePage extends StatelessWidget {
bool notNull(Object o) => o != null;
@override
Widget build(BuildContext context) {
var condition = true;
return Scaffold(
appBar: AppBar(
title: Text("Provider Demo"),
),
body: Center(
child: Column(
children: <Widget>[
condition? Text("True"): null,
Container(
height: 300,
width: MediaQuery.of(context).size.width,
child: Text("Test")
)
].where(notNull).toList(),
)),
);
}
}
In Dart, if/else and switch are statements not expressions. They don't return a value so you can't pass them to constructor params. If you have a lot of conditional logic in your build method, then it is a good practice to try and simplify it. For example, you can move self-contained logic to methods, and use if/else statements to initialize local variables which you can later use.
Widget _buildChild() {
if (condition) {
return ...
}
return ...
}
Widget build(BuildContext context) {
return new Container(child: _buildChild());
}
if/elseWidget build(BuildContext context) {
Widget child;
if (condition) {
child = ...
} else {
child = ...
}
return new Container(child: child);
}
In flutter if you want to do conditional rendering, you may do this:
Column(
children: <Widget>[
if (isCondition == true)
Text('The condition is true'),
],
);
But what if you want to use a tertiary (if-else) condition? when the child widget is multi-layered.
You can use this for its solution flutter_conditional_rendering a flutter package which enhances conditional rendering, supports if-else and switch conditions.
If-Else condition:
Column(
children: <Widget>[
Conditional.single(
context: context,
conditionBuilder: (BuildContext context) => someCondition == true,
widgetBuilder: (BuildContext context) => Text('The condition is true!'),
fallbackBuilder: (BuildContext context) => Text('The condition is false!'),
),
],
);
Switch condition:
Column(
children: <Widget>[
ConditionalSwitch.single<String>(
context: context,
valueBuilder: (BuildContext context) => 'A',
caseBuilders: {
'A': (BuildContext context) => Text('The value is A!'),
'B': (BuildContext context) => Text('The value is B!'),
},
fallbackBuilder: (BuildContext context) => Text('None of the cases matched!'),
),
],
);
If you want to conditionally render a list of widgets (List<Widget>) instead of a single one. Use Conditional.list() and ConditionalSwitch.list()!
****You can also use conditions by using this method** **
int _moneyCounter = 0;
void _rainMoney(){
setState(() {
_moneyCounter += 100;
});
}
new Expanded(
child: new Center(
child: new Text('\$$_moneyCounter',
style:new TextStyle(
color: _moneyCounter > 1000 ? Colors.blue : Colors.amberAccent,
fontSize: 47,
fontWeight: FontWeight.w800
)
),
)
),
Source: Stackoverflow.com