[Flutter必备]-Flex布局完全解读

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。
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

2.轴向:direction:Axis

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。

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

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

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

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

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

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

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

enum TextDirection {
  ltr,//从左到右
  rtl,//从右到左
}

这个非常好理解，不多言了

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

enum VerticalDirection{
    up,
    down,
}

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

enum TextBaseline {
  alphabetic,
  ideographic,
}

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

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
复制代码

9.用Flex布局写个小例子

光说不练假把式,光练不说空把式,下面就用一个布局来实际看一下Flex的强大

• 首先简单的分析一下

1.由上中下三行，可以用Column
2.第一行由图标，文字和文字组成，其中两头分处左右，可以用Expanded处理  
3.中间比较复杂由一个Row中包含两部分，左边是一个两行Column的内容,右边是文字  
4.底部是一个Row,主轴对齐是start

• 布局代码

showItem() {
  var infoStyle = TextStyle(color: Color(0xff999999), fontSize: 13);
  var littleStyle = TextStyle(color: Colors.black, fontSize: 16);

  var top = Row(//顶部,通过Flex布局的Row进行横向排列,Expanded中间
    children: <Widget>[
      Image.asset("images/icon_head.png", width: 20, height: 20),
      Expanded(
        child: Padding(
          child: Text("张风捷特烈"),
          padding: EdgeInsets.only(left: 4),
        ),
      ),
      Text(
        "Flutter/Dart",
        style: infoStyle,
      )
    ],
  );

  var content = Column(//中间文字内容,交叉轴为start
    mainAxisSize: MainAxisSize.min,
    crossAxisAlignment: CrossAxisAlignment.start,
    children: <Widget>[
      Text(
        "[Flutter必备]-Flex布局完全解读",
        style: littleStyle,
        maxLines: 2,
        overflow: TextOverflow.ellipsis,
      ),
      Padding(
        child: Text("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",
            style: infoStyle, maxLines: 2, overflow: TextOverflow.ellipsis),
        padding: EdgeInsets.only(top: 5),
      ),
    ],
  );

  var center = Row(//中间的部分
    children: <Widget>[
      Expanded(
          child: Padding(
        child: content,
        padding: EdgeInsets.all(5),
      )),
      ClipRRect(
        borderRadius: BorderRadius.all(Radius.circular(5)),
        child: Image.asset("images/wy_300x200.jpg",
          width: 80, height: 80, fit: BoxFit.cover),)
    ],
  );

  var end = Row(//底部
    children: <Widget>[
      Icon(
        Icons.grade,
        color: Colors.green,
        size: 20,
      ),
      Text(
        "1000W",
        style: infoStyle,
      ),
      Padding(child:Icon(Icons.tag_faces, color: Colors.lightBlueAccent, size: 20),
          padding: EdgeInsets.symmetric(horizontal: 5),),
      Text("2000W", style: infoStyle),
    ],
  );

  var result = Card(//总体拼合
      child: Container(
          height: 160,
          color: Colors.white,
          padding: EdgeInsets.all(10),
          child: Column(children: <Widget>[top, Expanded(child: center), end])));
  return result;
}