How to Scroll to a Specific Position in Flutter

Scrolling to a Specific Widget Position in SingleChildScrollView in Flutter

When developing user interfaces in Flutter, a common requirement is to scroll to a specific widget in UIScrollView. This can be achieved using GlobalKey and the Scrollable.ensureVisible() method. Let's delve into how you can implement this:

Scrollable

Using GlobalKey in Flutter

First, you need to assign a GlobalKey to your target widget. A GlobalKey is a unique identifier that provides information about a widget's state and position. Here's how you can create a GlobalKey:

GlobalKey _globalKey = GlobalKey();

Attaching GlobalKey to the Target Widget

Next, assign the GlobalKey you created to your target widget. In this example, we are using a Container widget:

Container(
  key: _globalKey,
  ...
)

Implementing Scrollable.ensureVisible() Method

When you want to scroll to a specific widget, call the Scrollable.ensureVisible() method. This method takes the GlobalKey's currentContext as an argument and scrolls to make the corresponding widget visible on the screen.

void scrollToWidget() {
  Scrollable.ensureVisible(
    _globalKey.currentContext,
  );
}

Now, let's look at the complete code:

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  GlobalKey _globalKey = GlobalKey();

  void scrollToWidget() {
    Scrollable.ensureVisible(
      _globalKey.currentContext,
    );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Scroll to Widget Example'),
        ),
        body: SingleChildScrollView(
          child: Column(
            children: [
              Container(
                height: 200,
                color: Colors.yellow,
              ),
              ElevatedButton(
                onPressed: scrollToWidget,
                child: Text('Scroll to Target Widget'),
              ),
              Container(
                key: _globalKey,
                height: 200,
                color: Colors.red,
              ),
              Container(
                height: 200,
                color: Colors.blue,
              ),
            ],
          ),
        ),
      ),
   );
  }
}

After running this code, the screen will scroll to the red Container when the button is pressed.

How to Use Scrollable.ensureVisible() Method: Examples

The Scrollable.ensureVisible() method is a powerful tool to adjust the scroll position in order to make a specific widget visible on the screen. Here's how you can use this feature:

Creating a GlobalKey in Flutter

First, create a key to identify the widget that you will reference for scrolling later.

GlobalKey _globalKey = GlobalKey();

Assigning GlobalKey to the Scroll Target Widget

Next, assign the created GlobalKey to your widget to enable scrolling to its position.

Container(
  key: _globalKey,
  ...
)

Calling Scrollable.ensureVisible() Method

When you need to adjust the scroll position, call the Scrollable.ensureVisible() method. This method uses GlobalKey's currentContext as an argument and moves the scroll to the corresponding widget's position.

void scrollToTarget() {
  Scrollable.ensureVisible(
    _globalKey.currentContext,
  );
}

Example Code for Scrollable.ensureVisible()

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  GlobalKey _globalKey = GlobalKey();

  void scrollToTarget() {
    Scrollable.ensureVisible(
      _globalKey.currentContext,
    );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Scroll to Widget Example'),
        ),
        body: SingleChildScrollView(
          child: Column(
            children: [
              Container(
                height: 200,
                color: Colors.yellow,
              ),
              ElevatedButton(
                onPressed: scrollToTarget,
                child: Text('Scroll to Target Widget'),
              ),
              Container(
                key: _globalKey,
                height: 200,
                color: Colors.red,
              ),
              Container(
                height: 200,
                color: Colors.blue,
              ),
            ],
          ),
        ),
      ),
    );
  }
}

In this example, yellow, red, and blue Containers are sequentially arranged on the screen. When the user clicks on the ElevatedButton, it automatically scrolls to the red Container.

Why ListView.builder() and ListView are not suitable for Scrollable.ensureVisible()

When using Scrollable.ensureVisible() to scroll to a specific position, you need a reference to that position with GlobalKey. However, ListView.builder() and ListView have some limitations that make them less suitable for this purpose.

Situations where GlobalKey cannot be used

ListView.builder() and ListView use item reuse and deactivation mechanisms to ensure high efficiency when handling many items. However, this can cause problems when assigning GlobalKey to each item.

Specifically, when using ListView.builder() with a large number of items, assigning GlobalKey to each item may cause a 'multiple widgets used the same GlobalKey' error. This error arises when multiple widgets using the same key are active simultaneously, causing a conflict.

Obtaining scroll position is difficult with ListView.builder() and ListView

The item reuse and deactivation mechanism of ListView.builder() and ListView can make it difficult to calculate the scroll position when there is no fixed height between items. It may also be impossible to accurately determine the position of a specific item in advance.

Alternative to ListView.builder() and ListView: SingleChildScrollView

To overcome these limitations, you can use SingleChildScrollView. SingleChildScrollView renders all child widgets of SingleChildScrollView and manipulates a viewport that shows only the content within the screen. This makes moving to a specific widget position using GlobalKey easier and more reliable.

However, be aware that the overall rendering performance may degrade, or memory issues may occur when using a large number of items or complex widget structures in the page. Therefore, you should choose the most appropriate widget for your application based on these characteristics.

SingleChildScrollView

Conclusion

In conclusion, the Scrollable.ensureVisible() method is a valuable tool for navigating to specific widgets in a ScrollView. However, it is important to consider the limitations and potential issues of the widgets you are using, such as ListView.builder() and ListView, and select the most suitable alternative for your specific needs.