04_Flutter自定义Slider滑块

04_Flutter自定义Slider滑块

一.Slider控件基本用法

Column(mainAxisAlignment: MainAxisAlignment.start,children: <Widget>[Text("sliderValue: ${_sliderValue.toInt()}"),Slider(value: _sliderValue,min: 0,max: 100,divisions: 10,thumbColor: Colors.red,activeColor: Colors.red,onChanged: (value) {setState(() {_sliderValue = value;});})],
)

const Slider({super.key,required this.value,this.secondaryTrackValue,required this.onChanged,this.onChangeStart,this.onChangeEnd,this.min = 0.0,this.max = 1.0,this.divisions,this.label,this.activeColor,this.inactiveColor,this.secondaryActiveColor,this.thumbColor,this.overlayColor,this.mouseCursor,this.semanticFormatterCallback,this.focusNode,this.autofocus = false,})

几个比较重要的属性：

• value：slider控件显示的值
• min：slider控件滑动到最左边对应的值，即最小值
• max: slider控件滑动到最右边对应的值，即最大值
• divisions: 最小值到最大值之间被几等分
• activeColor: 滑块划过部分的颜色值，即选中的颜色值
• inactiveColor：滑块未划过部分的颜色值，即为选中的颜色值
• thumbColor：滑块的颜色值

二.如何修改滑块的大小以及滑块轨迹的高度

从上面的示例可以看到，通过Slider控件为我们提供的属性，只支持改变滑块的颜色，以及滑块轨迹的颜色，那么我们想要改变滑块的大小以及滑块轨迹的高度，是不是只能重新自定义呢?

NO! NO! NO!，细心的您在使用Flutter的AppBar时，可能会发现，为AppBar控件指定样式时，除了使用AppBar控件提供的属性外，也可以使用AppBarTheme来为AppBar设置某些特定的样式，既然如此，不妨查看下Flutter sdk的源码与Slider对应的是否有一个叫SliderTheme的控件呢? 嘿嘿，还真有。

final SliderThemeData data;
const SliderTheme({super.key,required this.data,required super.child,
});const SliderThemeData({this.trackHeight,this.thumbShape,...
});

仔细找SliderThemeData的trackHeight以及thumbShape的属性注释:

/// The height of the [Slider] track.
final double? trackHeight;/// The shape that will be used to draw the [Slider]'s thumb.
/// The default value is [RoundSliderThumbShape].
final SliderComponentShape? thumbShape;

此处省略…翻译软件的时间:

• trackHeight：[滑块]轨迹的高度
• thumbShape：默认值是一个RoundSliderThumbShape对象

看下RoundSliderThumbShape的源码怎么写的:

const RoundSliderThumbShape({this.enabledThumbRadius = 10.0,this.disabledThumbRadius,this.elevation = 1.0,this.pressedElevation = 6.0,
});

看到这里就不用做过多的解释了吧😂，因此要修改滑块的大小，可以重新指定thumbShape为RoundSliderThumbShape对象，并设置enabledThumbRadius的值。

Column(mainAxisAlignment: MainAxisAlignment.start,children: <Widget>[Text("sliderValue: ${_sliderValue.toInt()}"),SliderTheme(data: const SliderThemeData(trackHeight: 20,thumbShape: RoundSliderThumbShape(enabledThumbRadius: 20)),child: Slider(value: _sliderValue,min: 0,max: 100,divisions: 10,thumbColor: Colors.red,activeColor: Colors.red,onChanged: (value) {setState(() {_sliderValue = value;});}))],
)

三.使用本地资源图片作为自定义滑块

既然要自定义滑块，毫无疑问需要从SliderThemeData的thumbShape入手。

final SliderComponentShape? thumbShape;

thumbShape的类型为SliderComponentShape，继续查看SliderComponentShape源码:

abstract class SliderComponentShape {const SliderComponentShape();Size getPreferredSize(bool isEnabled, bool isDiscrete);void paint(PaintingContext context,Offset center, {required Animation<double> activationAnimation,required Animation<double> enableAnimation,required bool isDiscrete,required TextPainter labelPainter,required RenderBox parentBox,required SliderThemeData sliderTheme,required TextDirection textDirection,required double value,required double textScaleFactor,required Size sizeWithOverflow,});
}

因此我们可以定义一个类继承SliderComponentShape，并实现getPreferredSize和paint方法，getPreferredSize控制滑块大小，paint负责把滑块绘制到屏幕上。

• 首先第一步我们需要将本地图片为一个ImageInfo，例如传入一个"lib/assets/images/ic_slider_thumb.png"，最后得到一个ImageInfo，这里就直接奉上源码了，其实现也是参考了Image.asset的源码:

typedef AssertsWidgetBuilder = Widget Function(BuildContext context, ImageInfo? imageInfo);class AssertsImageBuilder extends StatefulWidget {final String assertsName;final AssertsWidgetBuilder builder;const AssertsImageBuilder(this.assertsName,{super.key,required this.builder,});@overrideState<StatefulWidget> createState() => _AssertsImageBuilderState();}class _AssertsImageBuilderState extends State<AssertsImageBuilder> {ImageInfo? _imageInfo;@overridevoid initState() {super.initState();_loadAssertsImage().then((value) {setState(() {_imageInfo = value;});});}@overridevoid didUpdateWidget(covariant AssertsImageBuilder oldWidget) {super.didUpdateWidget(oldWidget);if(oldWidget.assertsName != widget.assertsName) {_loadAssertsImage().then((value) {setState(() {_imageInfo = value;});});}}@overrideWidget build(BuildContext context) {return widget.builder!.call(context, _imageInfo);}Future<ImageInfo?> _loadAssertsImage() {final Completer<ImageInfo?> completer = Completer<ImageInfo?>();WidgetsBinding.instance.addPostFrameCallback((timeStamp) {final ImageProvider imageProvider = AssetImage(widget.assertsName);final ImageConfiguration config = createLocalImageConfiguration(context);final ImageStream stream = imageProvider.resolve(config);ImageStreamListener? listener;listener = ImageStreamListener((ImageInfo? image, bool sync) {if (!completer.isCompleted) {completer.complete(image);}SchedulerBinding.instance.addPostFrameCallback((Duration timeStamp) {stream.removeListener(listener!);});},onError: (Object exception, StackTrace? stackTrace) {stream.removeListener(listener!);completer.completeError(exception, stackTrace);},);stream.addListener(listener);});return completer.future;}}

• 自定义SliderComponentShape

import 'package:flutter/material.dart';
import 'dart:ui' as ui;class ImageSliderThumb extends SliderComponentShape {final Size size;final ui.Image? image;const ImageSliderThumb({required this.image,Size? size}): size = size ?? const Size(20, 20);@overrideSize getPreferredSize(bool isEnabled, bool isDiscrete) {return size;}@overridevoid paint(PaintingContext context, Offset center, {required Animation<double> activationAnimation, required Animation<double> enableAnimation, required bool isDiscrete, required TextPainter labelPainter, required RenderBox parentBox, required SliderThemeData sliderTheme, required TextDirection textDirection, required double value, required double textScaleFactor, required Size sizeWithOverflow}) {}}

• 绘制图片滑块

@override
void paint(PaintingContext context, Offset center, {required Animation<double> activationAnimation, required Animation<double> enableAnimation, required bool isDiscrete, required TextPainter labelPainter, required RenderBox parentBox, required SliderThemeData sliderTheme, required TextDirection textDirection, required double value, required double textScaleFactor, required Size sizeWithOverflow}) {//图片中心点double dx = size.width/2;double dy = size.height/2;if(image != null) {final Rect sourceRect = Rect.fromLTWH(0, 0, image!.width.toDouble(), image!.width.toDouble());//center会随着滑块的移动而改变，所以这里需要根据center计算图片绘制的位置double left = center.dx - dx;double top = center.dy - dy;double right = center.dx + dx;double bottom = center.dy + dy;Rect destinationRect = Rect.fromLTRB(left, top, right, bottom);final Canvas canvas = context.canvas;final Paint paint = new Paint();paint.isAntiAlias = true;//绘制滑块canvas.drawImageRect(image!, sourceRect, destinationRect, paint);}
}

四.怎么使用?

Column(mainAxisAlignment: MainAxisAlignment.start,children: <Widget>[Text("sliderValue: ${_sliderValue.toInt()}"),AssertsImageBuilder("lib/assets/images/ic_slider_thumb.png",builder: (context, imageInfo) {return SliderTheme(data: SliderThemeData(trackHeight: 10,thumbShape: ImageSliderThumb(image: imageInfo?.image,size: const Size(30, 30))),child: Slider(value: _sliderValue,min: 0,max: 100,divisions: 10,thumbColor: Colors.red,activeColor: Colors.red,onChanged: (value) {setState(() {_sliderValue = value;});}));}),],
)