Flutter第4天--基础控件(下)+Flex布局详解

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字数统计: 1.2k | 阅读时长 ≈ 6
0.前言

Flex布局是Flutter的五虎上将之一,虎父无犬子，其子Row和Column也能力非凡
你有没有被mainAxisAlignment，crossAxisAlignment弄得晕头转向,本文将助你将他们纳入麾下。
先看一下父子三人在Flutter布局体系中的位置:多子组件布局

1.Flex的属性一览
属性名 类型 默认值 简介
direction Axis @required 轴向
mainAxisAlignment MainAxisAlignment start 主轴方向对齐方式
crossAxisAlignment CrossAxisAlignment center 交叉轴方向对齐方式
mainAxisSize MainAxisSize max 主轴尺寸
textDirection TextDirection null 文本方向
verticalDirection VerticalDirection down 竖直方向
textBaseline TextBaseline null 基线类型
children List [] 内部孩子
2.轴向:direction:Axis
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enum Axis {
  horizontal,//水平
  vertical,//竖直
}

也就是水平排放还是竖直排放，可以看出默认情况下都是主轴顶头,交叉轴居中
比如horizontal下主轴为水平轴，交叉轴则为竖直。也就是水平顶头，竖直居中
这里使用MultiShower快速展示,更好的对比出不同之处，MultiShower详见

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var direction =[Axis.horizontal,Axis.vertical];
var show = MultiShower(direction,(e){
  return Flex(
    direction: e,
    children: <Widget>[redBox,blueBox,yellowBox,greenBox],

  );
},color: Colors.black12,width: 300,height: 200);

var redBox= Container(
  color: Colors.red,
  height: 50,
  width: 50,
);

var blueBox= Container(
  color: Colors.blue,
  height: 30,
  width: 60,
);

var yellowBox= Container(
  color: Colors.yellow,
  height: 50,
  width: 100,
);

var greenBox= Container(
  color: Colors.green,
  height: 60,
  width: 60,
);
3.主轴方向:mainAxisAlignment:MainAxisAlignment

主轴方向的排布规则,这里以水平为例，主轴为水平方向。竖直类比即可

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enum MainAxisAlignment {
  start,//顶头
  end,//接尾
  center,//居中
  spaceBetween,//顶头接尾，其他均分
  spaceAround,//中间的孩子均分,两头的孩子空一半
  spaceEvenly,//均匀平分

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testMainAxisAlignment(){
  var redBox= Container(
    color: Colors.red,
    height: 50,
    width: 50,
  );

  var blueBox= Container(
    color: Colors.blue,
    height: 30,
    width: 60,
  );

  var yellowBox= Container(
    color: Colors.yellow,
    height: 10,
    width: 10,
  );

  var greenBox= Container(
    color: Colors.green,
    height: 50,
    width: 10,
  );

  var mainAxisAlignment =[
  MainAxisAlignment.start,MainAxisAlignment.center,
  MainAxisAlignment.end,MainAxisAlignment.spaceAround,
  MainAxisAlignment.spaceBetween,MainAxisAlignment.spaceEvenly];

  var show = MultiShower(mainAxisAlignment,(e){
    return Flex(
      direction: Axis.horizontal,
      mainAxisAlignment: e,
      children: <Widget>[redBox,blueBox,yellowBox,greenBox],

    );
  },color: Colors.black12,width: 200,height: 150);
  return show;
}
4.交叉轴方向:crossAxisAlignment:CrossAxisAlignment
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enum CrossAxisAlignment {
  start,//顶头
  end,//接尾
  center,//居中
  stretch,//伸展
  baseline,//基线
}

还是水平为例，交叉轴便是竖轴，这里可以看出他们的布局行为
其中需要注意的是CrossAxisAlignment.baseline使用时必须有textBaseline
其中textBaseline确定对齐的是那种基线,分为alphabetic和ideographic

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testCrossAxisAlignment(){
  var redBox= Container(
    color: Colors.red,
    height: 50,
    width: 50,
  );

  var blueBox= Container(
    color: Colors.blue,
    height: 30,
    width: 60,
  );

  var yellowBox= Container(
    color: Colors.yellow,
    height: 10,
    width: 10,
  );

  var greenBox= Container(
    color: Colors.green,
    height: 50,
    width: 10,
  );
  
  var crossAxisAlignment =[CrossAxisAlignment.start,CrossAxisAlignment.center,
    CrossAxisAlignment.end,CrossAxisAlignment.stretch,CrossAxisAlignment.baseline];

  var show = MultiShower(crossAxisAlignment,(e){
    return Flex(
      direction: Axis.horizontal,
      crossAxisAlignment: e,
      textBaseline: TextBaseline.alphabetic,//基线类型
      children: <Widget>[redBox,blueBox,yellowBox,greenBox],

    );
  },color: Colors.black12,width: 200,height: 140);

  return show;
}
5.主轴尺寸:mainAxisSize
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enum MainAxisSize {
  min,
  max,
}

当父容器的宽未约束,Flex默认会将自身尽可能延伸，这便是MainAxisSize.max

此时改为MainAxisSize.min时,它不会延伸自己的区域，自会包裹内容

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testMainAxisSize(){
  var redBox= Container(
    color: Colors.red,
    height: 50,
    width: 50,
  );

  var blueBox= Container(
    color: Colors.blue,
    height: 30,
    width: 60,
  );

  var yellowBox= Container(
    color: Colors.yellow,
    height: 10,
    width: 10,
  );

  var greenBox= Container(
    color: Colors.green,
    height: 50,
    width: 10,
  );

  return Center(child: Flex(
    direction: Axis.horizontal,
    mainAxisSize: MainAxisSize.max,
    children: <Widget>[redBox,blueBox,yellowBox,greenBox],

  ),);
}
6.文字方向:textDirection:TextDirection
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enum TextDirection {
  ltr,//从左到右
  rtl,//从右到左
}

这个非常好理解，不多言了

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testTextDirection(){
  var redBox= Container(
    color: Colors.red,
    height: 50,
    width: 50,
  );

  var blueBox= Container(
    color: Colors.blue,
    height: 30,
    width: 60,
  );

  var yellowBox= Container(
    color: Colors.yellow,
    height: 10,
    width: 10,
  );

  var greenBox= Container(
    color: Colors.green,
    height: 50,
    width: 10,
  );

  var textDirection =[TextDirection.ltr,TextDirection.rtl];
  var show = MultiShower(textDirection,(e){
    return Flex(
      direction: Axis.horizontal,
      textDirection: e,
      children: <Widget>[redBox,blueBox,yellowBox,greenBox],

    );
  },color: Colors.black12,width: 200,height: 140);
  return show;
}
7.竖直方向排序:verticalDirection:VerticalDirection
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enum VerticalDirection{
    up,
    down,
}

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testVerticalDirection(){
  var redBox= Container(
    color: Colors.red,
    height: 50,
    width: 50,
  );

  var blueBox= Container(
    color: Colors.blue,
    height: 30,
    width: 60,
  );

  var yellowBox= Container(
    color: Colors.yellow,
    height: 10,
    width: 10,
  );

  var greenBox= Container(
    color: Colors.green,
    height: 50,
    width: 10,
  );

  var verticalDirection =[VerticalDirection.up,VerticalDirection.down];

  var show = MultiShower(verticalDirection,(e){
    return Flex(
      direction: Axis.vertical,
      verticalDirection: e
      children: <Widget>[redBox,blueBox,yellowBox,greenBox],

    );
  },color: Colors.black12,width: 200,height: 140);

  return show;
}
8.基线对齐方式:textBaseline:TextBaseline
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enum TextBaseline {
  alphabetic,
  ideographic,
}

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testTextBaseline(){
  var redBox= Text(
    "张风捷特烈",style: TextStyle(fontSize: 20,backgroundColor: Colors.red),
  );

  var blueBox= Text(
    "toly",style: TextStyle(fontSize: 50,backgroundColor: Colors.blue),
  );

  var yellowBox=  Text(
    "1994",style: TextStyle(fontSize: 30,backgroundColor: Colors.green),
  );

  var textBaseline =[TextBaseline.alphabetic,TextBaseline.ideographic];

  var show = MultiShower(textBaseline,(e){
    return Flex(
      direction: Axis.horizontal,
      crossAxisAlignment: CrossAxisAlignment.baseline,
      textBaseline: e,
      children: <Widget>[redBox,blueBox,yellowBox],
    );
  },color: Colors.black12,width: 300,height: 140);

  return show;
}

至此，Flutter中的Flex布局就已经完全解读完了。
其中Flutter的孩子Row是direction: Axis.horizontal,Column是direction: Axis.vertical,
其他的属性都是类似的，相当于Flex的简化版，所以Flex在手，天下我有。

Expanded与Flex的搭配— [更新于2019.1.22]

还要一点是关于Expanded，也比较保用，它能与Flex布局结合，变更孩子尺寸 

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c1:绿色  c2:红色  c3:黄色
1).Expanded--c2：c1和c3将不变，c2延伸自己占满剩余部分
2).同时Expanded--c2和c3，最终c2和c3的长度是一样的
3).同时Expanded--c1，c2和c3，最终c1,c2，c3长度都是一样的

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