Flutter组件--Align和AnimatedAlign

示意图:

Align介绍

Align 一般是用来确定子控件在父布局中的位置，比如居中、左上等多个对齐方式。

什么情况下使用Align?

当子组件需要设置位于父组件的某个位置时，需要用到Align.

Align构造函数

const Align({
    Key? key,
    this.alignment = Alignment.center,  // 子组件在父组件中的对齐方式
    this.widthFactor,  // 如果设置该值，Align的宽度始终是child宽度的两倍
    this.heightFactor, // 如果设置该值，Align的高度始终是child高度的两倍
    Widget? child,	// 子widget
  }) : assert(alignment != null),
       assert(widthFactor == null || widthFactor >= 0.0),
       assert(heightFactor == null || heightFactor >= 0.0),
       super(key: key, child: child);

import 'package:flutter/material.dart';
 
class AlignExample extends StatefulWidget {
  @override
  _AlignExampleState createState() => _AlignExampleState();
}
 
class _AlignExampleState extends State<AlignExample> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("AlignExample"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Container(
              width: 200,
              height: 200,
              color: Colors.blueAccent,
              child: Align(
                alignment: Alignment.topRight,
                widthFactor: 100.0,
                heightFactor: 2.0,
                child: Text("Align"),
              ),
            )
          ],
        ),
      ),
    );
  }
}

AlignmentGeometry介绍

AlignmentGeometry 是一个如何对齐child 的一个组件，一般我们都是使用它的子类 Alignment 来进行对齐设置。

Alignment详解

Alignment继承自AlignmentGeometry，表示矩形内的一个点，他有两个属性x、y，分别表示在水平和垂直方向的偏移。

const Alignment(this.x, this.y)
  : assert(x != null),
		assert(y != null);

 Alignment属性

/// 顶部左侧对齐
static const Alignment topLeft = Alignment(-1.0, -1.0);
 
/// 顶部中间对齐
static const Alignment topCenter = Alignment(0.0, -1.0);
 
/// 顶部右侧对齐
static const Alignment topRight = Alignment(1.0, -1.0);
 
/// 中间左侧对齐
static const Alignment centerLeft = Alignment(-1.0, 0.0);
 
/// 垂直居中对齐
static const Alignment center = Alignment(0.0, 0.0);
 
/// 中间右侧对齐
static const Alignment centerRight = Alignment(1.0, 0.0);
 
/// 底部左侧对齐
static const Alignment bottomLeft = Alignment(-1.0, 1.0);
 
/// 底部中间对齐
static const Alignment bottomCenter = Alignment(0.0, 1.0);
 
/// 底部右侧对齐
static const Alignment bottomRight = Alignment(1.0, 1.0);

AnimatedAlign构造函数

const AnimatedAlign({
  Key? key,
  required this.alignment,  //必传， 子组件在父组件中的对齐方式 
  this.child,		// 子组件
  this.heightFactor,  // 如果设置该值，Align的高度始终是child高度的两倍
  this.widthFactor,  // 如果设置该值，Align的宽度始终是child宽度的两倍 
  Curve curve = Curves.linear,  // 动画的运动速率
  required Duration duration,   // 必传，动画的持续时间
  VoidCallback? onEnd,  // 动画结束时的回调
}) : assert(alignment != null),
assert(widthFactor == null || widthFactor >= 0.0),
assert(heightFactor == null || heightFactor >= 0.0),
super(key: key, curve: curve, duration: duration, onEnd: onEnd);

AnimatedAlign完整示例代码

import 'package:flutter/material.dart';
 
class AnimatedAlignExample extends StatefulWidget {
  @override
  _AnimatedAlignExampleState createState() => _AnimatedAlignExampleState();
}
 
class _AnimatedAlignExampleState extends State<AnimatedAlignExample> {
  bool selected = false;
 
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("AnimatedAlignExample"),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Container(
              width: 250.0,
              height: 250.0,
              color: Colors.green,
              child: AnimatedAlign(
                alignment: selected ? Alignment.topRight : Alignment.bottomLeft,
                duration: Duration(milliseconds: 1000),
                curve: Curves.fastOutSlowIn,
                child: Icon(Icons.ac_unit, size: 40,),
                widthFactor: 2.0,
                heightFactor: 2.0,
                onEnd: () {
                  print("动画结束时进入");
                },
              ),
            ),
            ElevatedButton(
              child: Text('改变alignment参数'),
              onPressed: () {
                setState(() {
                  selected = !selected;
                });
              }),
          ],
        ),
      ),
    );
  }
}

总结

当子组件需要设置位于父组件的某个位置时，需要用到Align组件，而AnimatedAlign 是Align 组件的动画版本，只要对齐方式发生改变，它就会在给定的持续时间内自动转换子组件的位置.