Flutter: Displaying Dynamic Contents using ListView.builder
Daksh Gupta
Jul 29, 2018·5 min read
Flutter is a mobile App SDK by Google which helps in creating modern mobile apps for iOS and Android using a single[almost] code base. Its a new entrant in the cross platform mobile application development and unlike other frameworks like React Native, it doesnt use JavaScript but DART as a Programming Language.
If youre new to Flutter, Its highly recommended to read my first post Flutter: From Zero To Comfortable to get a good sense of how Flutter widgets work. You may also read my other stories on Flutter to get an overall view of different type of flutter widgets
What is meant by Dynamic Contents?
Unless an app is very basic and created for learning purposes, it will always use dynamic data i.e. Contents of the screen will change based on user actions and/or with change in network data.
Most of the time, the dynamic behaviour is achieved by changing the contents that is being displayed in the body. Flutter provides ListView.builder which can be used to generate dynamic contents from external sources.
Using ListView.builder[]
ListView.builder is a way of constructing the list where childrens [Widgets] are built on demand. However, instead of returning a static widget, it calls a function which can be called multiple times [based on itemCount ] and its possible to return different widget at each call.
For example, lets create a simple application containing basic MaterialApp with Scaffold contains AppBar and Body
void main[] => runApp[new MyApp[]];class MyApp extends StatelessWidget {
@override
Widget build[BuildContext ctxt] {
return new MaterialApp[
home: new ListDisplay[],
];
}
}class ListDisplay extends StatelessWidget {
@override
Widget build [BuildContext ctxt] {
return new Scaffold[
appBar: new AppBar[title: new Text["Dynamic Demo"],],
body: // ListView.builder[] shall be used here.
],
];
}
}
To use ListView.builder, we have to create an instance of it by calling its constructor using new , it takes multiple parameters, however, two of them are of special importance. They are
- itemCount
- itemBuilder
The itemCount parameter decides how many times the callback function in itemBuilder will be called.
To demonstrate the same, lets assume that we have a global list like this in our application
List litems = ["1","2","Third","4"];Lets use the ListView.builder[...] to display the list item in the body. The code for the same shall look like this
body: new ListView.builder[
itemCount: litems.length,
itemBuilder: [BuildContext ctxt, int index] {
return new Text[litems[index]];
}
]
In here, the in-place callback function shall be called 4 times and each call will generate a Text Widget which shall be displayed in the body.
We can also write a separate function which can be called from itemBuilder as
Widget buildBody[BuildContext ctxt, int index] {
return new Text[litems[index]];
}body: new ListView.builder
[
itemCount: litems.length,
itemBuilder: [BuildContext ctxt, int index] => buildBody[ctxt, index]
],
In both the cases the output shall look like
We can use ListView.builder[..] in Stateless Widgets if we want to display dynamic [different] contents every time the widget is loaded within an application. With Stateful widgets, it can change the contents of the screen dynamically.
ListView.builder[] with Stateful Widgets
Lets first convert the existing widget as a Stateful widget by changing the code as
class ListDisplay extends StatefulWidget {@override
State createState[] => new DyanmicList[];
}class DyanmicList extends State {
@override
Widget build [BuildContext ctxt] {
return new Scaffold[...]
}
To properly understand the usage of ListView.builder[..] in a stateful widget, lets create a very simple ToDo app with Stateful Widget. Well create a Text input area [i.e TextField which allows one to add user defined items in the ToDo list. As soon as the item is added in the ToDo list, it will be automatically displayed onto the screen.
Since well be creating more than one widget in Body: , we have to use a widget which is capable of holding multiple widgets like Column: , However, ListView.builder[...] will not work under Column unless its wrapped inside the Expanded widget. The Expanded widget allows Column elements to expand and fill the available space in the main axis, which is needed by ListView.builder[...]
So the overall hierarchy shall look like
To add text items into the list, we need an empty list and TextEditingController
final TextEditingController eCtrl = new TextEditingController[];
Here is how the TextField widget shall be created.
new TextField[controller: eCtrl,
onSubmitted: [text] {
litems.add[text]; // Append Text to the list
eCtrl.clear[]; // Clear the Text area
setState[[] {}]; // Redraw the Stateful Widget
},
],
Nothing shall be changed in the implementation of ListView.builder[...] but it will be wrapped inside Expanded widget
new Expanded[child: new ListView.builder
[
itemCount: lItems.length,
itemBuilder: [BuildContext ctxt, int Index] {
return new Text[lItems[Index]];
}
]
]
And were ready to dynamically change the contents of the screen. Here is how the action shall look like
Here is the complete code for class DynamicList
class DyanmicList extends State {List litems = [];
final TextEditingController eCtrl = new TextEditingController[];
@override
Widget build [BuildContext ctxt] {
return new Scaffold[
appBar: new AppBar[title: new Text["Dynamic Demo"],],
body: new Column[
children: [
new TextField[
controller: eCtrl,
onSubmitted: [text] {
litems.add[text];
eCtrl.clear[];
setState[[] {}];
},
],
new Expanded[
child: new ListView.builder
[
itemCount: litems.length,
itemBuilder: [BuildContext ctxt, int Index] {
return new Text[litems[Index]];
}
]
]
],
]
];
}
}
Thats all for this particular post. I hope I was able to explain how we can use ListView.builder[...] for generating dynamic contents.
Thanks for reading.!!!
Daksh