o7planning

Flutter LinearProgressIndicator Tutorial

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1- LinearProgressIndicator

LinearProgressIndicator is a subclass of ProgressIndicator. It creates a horizontal progress bar indicator, but you can also create a vertical progress bar indicator if you place it in a RotatedBox rotating 90 degrees.
  • TODO Link!
LinearProgressIndicator is divided into two types:

Determinate

Determinate: a progress indicator that shows the user the percentage of work completed based on the value of the value property (in the range of 0 and 1).

Indeterminate

Indeterminate: a progress indicator that neither identifies the percentage of work completed nor indicates the end time.
LinearProgressIndicator Constructor:
LinearProgressIndicator constructor
const LinearProgressIndicator(
    {Key key,
    double value,
    Color backgroundColor,
    Animation<Color> valueColor,
    double minHeight,
    String semanticsLabel,
    String semanticsValue}  
)

2- Example: Indeterminate

Let's begin with the simplest example, a LinearProgressIndicator simulates an active process but does not know the percentage of work that has been completed and does not know when to finish.
main.dart (ex 1)
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,
      home: HomePage(),
    );
  }
}

class HomePage extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter LinearProgressIndicator Example'),
      ),
      body: Center(
        child:  LinearProgressIndicator(
          backgroundColor: Colors.cyan[100],
          valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
        ),
      ),
    );
  }
}
By default, LinearProgressIndicator has a fairly small size, but if you want to customize its size, let's put it in a​​​​​​​ SizedBox.
SizedBox(
  width: 250,
  height: 20,
  child: LinearProgressIndicator(
    backgroundColor: Colors.cyan[100],
    valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
  ),
)
The valueColor parameter is used to specify a color animation to the progress of LinearProgressIndicator, for example:
valueColor: new AlwaysStoppedAnimation<Color>(Colors.red)
AlwaysStoppedAnimation<Color> will always stop the animation of LinearProgressIndicator if the value parameter is a specific value and not a null value.
SizedBox(
  width: 250,
  height: 20,
  child: LinearProgressIndicator(
    value: 0.3,
    backgroundColor: Colors.cyan[100],
    valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
  ),
)
Based on the aforementioned characteristics of the value and valueColor parameters you can control the behavior of LinearProgressIndicator, let's see an example:
main.dart (ex 1d)
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,
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {

  @override
  State<StatefulWidget> createState() {
    return _HomePageState();
  }
}

class _HomePageState extends State<HomePage> {

  bool _working = false;

  void startWorking() async {
    this.setState(() {
      this._working = true;
    });
  }

  void finishWorking() {
    this.setState(() {
      this._working = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter LinearProgressIndicator Example'),
      ),
      body: Center(
          child:  Column (
              mainAxisAlignment: MainAxisAlignment.center,
              children:  [
                SizedBox(
                  width: 250,
                  height: 20,
                  child: LinearProgressIndicator(
                    value: this._working? null: 1,
                    backgroundColor: Colors.cyan[100],
                    valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
                  ),
                ),
                SizedBox(width:10, height:10),
                ElevatedButton(
                    child: Text("Start"),
                    onPressed: this._working? null: () {
                      this.startWorking();
                    }
                ),
                ElevatedButton(
                    child: Text("Finish"),
                    onPressed: !this._working? null: () {
                      this.finishWorking();
                    }
                )
              ]
          )
      ),
    );
  }
}

3- Example: Determinate

The following example uses LinearProgressIndicator to present a progress with the percentage of work completed. The value parameter has a value from 0 to 1.
main.dart (ex 2)
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,
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {

  @override
  State<StatefulWidget> createState() {
    return _HomePageState();
  }
}

class _HomePageState extends State<HomePage> {

  double _value = 0;
  bool _working = false;

  void startWorking()  async {
    this.setState(() {
      this._working = true;
      this._value = 0;
    });
    for(int i = 0; i< 10; i++) {
      if(!this._working)  {
        break;
      }
      await Future.delayed(Duration(seconds: 1));
      this.setState(() {
        this._value += 0.1;
      });
    }
    this.setState(() {
      this._working = false;
    });
  }

  void stopWorking() {
    this.setState(() {
      this._working = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter LinearProgressIndicator Example'),
      ),
      body: Center(
          child:  Column (
              mainAxisAlignment: MainAxisAlignment.center,
              children:  [
                SizedBox(
                  width: 250,
                  height: 20,
                  child: LinearProgressIndicator(
                    value: this._value,
                    backgroundColor: Colors.cyan[100],
                    valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
                  ),
                ),
                SizedBox(width:10, height: 10),
                Text(
                  "Percent: " + (this._value * 100).round().toString()+ "%",
                  style: TextStyle(fontSize: 20),
                ),
                ElevatedButton(
                    child: Text("Start"),
                    onPressed: this._working? null: () {
                      this.startWorking();
                    }
                ),
                ElevatedButton(
                    child: Text("Stop"),
                    onPressed: !this._working? null: () {
                      this.stopWorking();
                    }
                )
              ]
          )
      ),
    );
  }
}

4- Example: Vertical LinearProgressIndicator

LinearProgressIndicator basically does not support the vertical direction. However, if you want a vertical progress bar indicator, you can place it in a RotatedBox with a 90 degree rotation.
For example:
RotatedBox(
    quarterTurns: -1,
    child: SizedBox(
        width: 250,
        height: 25,
        child : LinearProgressIndicator(
          backgroundColor: Colors.cyan[100],
          valueColor: new AlwaysStoppedAnimation<Color>(Colors.green),
        )
    )
)
  • The quarterTurns = 1 parameter means 90 degrees clockwise rotation.
  • Otherwise, quarterTurns = -1 means 90 degrees counterclockwise rotation.

5- backgroundColor

backgroundColor is used to set the background color of LinearProgressIndicator.
Color backgroundColor

6- value

double value

7- valueColor

valueColor is used to specify a color animation to the progress.
Animation<Color> valueColor
For example:
LinearProgressIndicator(
  backgroundColor: Colors.cyanAccent,
  valueColor: new AlwaysStoppedAnimation<Color>(Colors.red),
)
  • TODO Link!

8- minHeight

minHeight is used to set the minimum height of LinearProgressIndicator.
double minHeight

9- semanticsLabel

semanticsLabel is used to describe the intended use of LinearProgressIndicator, which is completely hidden on the interface and is purposeful for Screen readers, such as screen readers for the blind.
String semanticsLabel

10- semanticsValue

semanticsValue is completely hidden on the screen. Its purpose is to provide information about the current progress for screen readers.
By default, the value of the semanticsValue is the percentage of the work completed, for example, the value of the value property is 0.3 meaning that the semanticsValue is "30%".
String semanticsValue
Example:
LinearProgressIndicator(
  value: this._value,
  backgroundColor: Colors.cyanAccent,
  valueColor: new AlwaysStoppedAnimation<Color>(Colors.red),
  semanticsLabel: "Downloading file abc.mp3",
  semanticsValue: "Percent " + (this._value * 100).toString() + "%",
)

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