flutter--Stack & Positioned & Align 层叠布局与定位

Stack在flutter–模拟登录中出现过，这里来详细了解一下这个Widget，类似于css中的position: relative，但是会强制子Widget层叠显示，Positioned类似于css中的position: absolute，可以设置坐标。

Stack

Stack({
    Key key,
    AlignmentGeometry alignment: AlignmentDirectional.topStart,
    TextDirection textDirection,
    StackFit fit: StackFit.loose,
    Overflow overflow: Overflow.clip,
    List<Widget> children: const[]
})

先看一个只有children的demo

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Stack(
          children: <Widget>[
            Text('1'),
            Text('2'),
            Text('3')
          ],
        )
      )
    );
  }
}

1、2、3叠在了一起，哪一个在上哪一个在下，把Text换成Container再看一下，

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Stack(
          children: <Widget>[
            Container(
              width: 100,
              height: 100,
              color: Colors.red
            ),
            Container(
              width: 50,
              height: 50,
              color: Colors.green
            )
          ],
        )
      )
    );
  }
}

可以看见和css里一样，后写的在上。

alignment

子Widget的对齐方式
AlignmentDirectional.topStart (默认值)、AlignmentDirectional.topCenter、AlignmentDirectional.topEnd、AlignmentDirectional.centerStart、AlignmentDirectional.center、AlignmentDirectional.centerEnd、AlignmentDirectional.bottomStart、AlignmentDirectional.bottomCenter、AlignmentDirectional.bottomEnd。
简单易懂，看个demo。

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Stack(
          alignment: AlignmentDirectional.bottomCenter,
          children: <Widget>[
            Container(
              width: 100,
              height: 100,
              color: Colors.red
            ),
            Container(
              width: 50,
              height: 50,
              color: Colors.green
            ),
            Container(
              width: 30,
              height: 30,
              color: Colors.blue
            )
          ],
        )
      )
    );
  }
}

textDirection

子Widget的排列方式，默认值是TextDirection.ltr，从左往右，当设置为TextDirection.rtl时效果如下：

fit

子Widget中未定位元素的大小，只有两个值：
StackFit.loose：不对其大小进行约束(默认值)
StackFit.expand：最大
StackFit.passthrough：父级的约束直接传递给子Widgert

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Container(
          width: 200,
          height: 200,
          child: Stack(
            fit: StackFit.passthrough,
            children: <Widget>[
              Container(
                width: 50,
                height: 50,
                color: Colors.red
              )
            ],
          )
        )
      )
    );
  }
}

这里需要注意的是fit值如果不是默认值StackFit.loose，子Widget设置的尺寸将失去作用。

overflow

和css里效果一样，只有两个值
Overflow.clip：溢出将被剪切
Overflow.visible：不对溢出的部分做处理

Positioned

定位元素，用于Stack的子Widget

Positioned({
    Key key,
    double left,
    double top,
    double right,
    double bottom,
    double width,
    double height,
    @required Widget child
})

都是基础属性，left、top、right、bottom是相对于父级的坐标。

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Stack(
          children: <Widget>[
            Positioned(
              width: 100,
              height: 100,
              left: 100,
              bottom: 100,
              child: Container(
                color: Colors.red
              )
            )
          ],
        )
      )
    );
  }
}

相对于左下角的距离是100、100.

Align

这个更简单一点，只是设定子Widget相对于Align的位置。

Align({
    Key key,
    AlignmentGeometry alignment: Alignment.center,
    double widthFactor,
    double heightFactor,
    Widget child
})

alignment

子Widget相对于Align的位置。

widthFactor & heightFactor

这两个相当于是系数，乘以子Widget的宽高，就是Align的尺寸。

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Align(
          widthFactor: 2,
          heightFactor: 2,
          alignment: Alignment.bottomRight,
          child: Container(
            width: 50,
            height: 50,
            color: Colors.blue
          )
        )
      )
    );
  }
}

这里需要注意一点，就是如果Align的父级设有宽高widthFactor和heightFactor将市区作用，大小为最大。

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: Scaffold(
        body: Container(
          width: 300,
          height: 300,
          color: Colors.red,
          child: Align(
            widthFactor: 2,
            heightFactor: 2,
            alignment: Alignment.bottomRight,
            child: Container(
              width: 50,
              height: 50,
              color: Colors.blue
            )
          )
        )
      )
    );
  }
}