0.前言

Flex布局是Flutter的五虎上將之一,虎父無犬子穆壕，其子Row和Column也能力非凡
你有沒有被mainAxisAlignment，crossAxisAlignment弄得暈頭轉(zhuǎn)向,本文將助你將他們納入麾下署照。
先看一下父子三人在Flutter布局體系中的位置:多子組件布局

image

1.Flex的屬性一覽

2.軸向:direction:Axis

enum Axis {
  horizontal,//水平
  vertical,//豎直
}

也就是水平排放還是豎直排放臭脓，可以看出默認(rèn)情況下都是主軸頂頭,交叉軸居中
比如horizontal下主軸為水平軸酗钞，交叉軸則為豎直。也就是水平頂頭来累，豎直居中
這里使用MultiShower快速展示,更好的對比出不同之處砚作，MultiShower詳見

image

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.主軸方向:mainAxisAlignment:MainAxisAlignment

主軸方向的排布規(guī)則,這里以水平為例，主軸為水平方向嘹锁。豎直類比即可

enum MainAxisAlignment {
  start,//頂頭
  end,//接尾
  center,//居中
  spaceBetween,//頂頭接尾葫录，其他均分
  spaceAround,//中間的孩子均分,兩頭的孩子空一半
  spaceEvenly,//均勻平分

image

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.交叉軸方向:crossAxisAlignment:CrossAxisAlignment

enum CrossAxisAlignment {
  start,//頂頭
  end,//接尾
  center,//居中
  stretch,//伸展
  baseline,//基線
}

還是水平為例，交叉軸便是豎軸领猾，這里可以看出他們的布局行為
其中需要注意的是CrossAxisAlignment.baseline使用時(shí)必須有textBaseline
其中textBaseline確定對齊的是那種基線,分為alphabetic和ideographic

image

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,//基線類型
      children: <Widget>[redBox,blueBox,yellowBox,greenBox],

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

  return show;
}

5.主軸尺寸:mainAxisSize

enum MainAxisSize {
  min,
  max,
}

當(dāng)父容器的寬未約束,Flex默認(rèn)會(huì)將自身盡可能延伸米同，這便是MainAxisSize.max

image

此時(shí)改為MainAxisSize.min時(shí),它不會(huì)延伸自己的區(qū)域骇扇，自會(huì)包裹內(nèi)容

image

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,//從左到右
  rtl,//從右到左
}

這個(gè)非常好理解，不多言了

image

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.豎直方向排序:verticalDirection:VerticalDirection

enum VerticalDirection{
    up,
    down,
}

image

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.基線對齊方式:textBaseline:TextBaseline

enum TextBaseline {
  alphabetic,
  ideographic,
}

image

testTextBaseline(){
  var redBox= Text(
    "張風(fēng)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;
}

至此面粮，F(xiàn)lutter中的Flex布局就已經(jīng)完全解讀完了少孝。
其中Flutter的孩子Row是direction: Axis.horizontal,Column是direction: Axis.vertical,
其他的屬性都是類似的，相當(dāng)于Flex的簡化版熬苍，所以Flex在手韭山，天下我有。

Expanded與Flex的搭配--- [更新于2019.1.22]

還要一點(diǎn)是關(guān)于Expanded冷溃，也比較保用钱磅，它能與Flex布局結(jié)合，變更孩子尺寸

c1:綠色  c2:紅色  c3:黃色
1).Expanded--c2：c1和c3將不變似枕，c2延伸自己占滿剩余部分
2).同時(shí)Expanded--c2和c3盖淡，最終c2和c3的長度是一樣的
3).同時(shí)Expanded--c1，c2和c3凿歼，最終c1,c2褪迟，c3長度都是一樣的

expaned.png

9.用Flex布局寫個(gè)小例子

光說不練假把式,光練不說空把式,下面就用一個(gè)布局來實(shí)際看一下Flex的強(qiáng)大

image

• 首先簡單的分析一下

image

1.由上中下三行，可以用Column
2.第一行由圖標(biāo)答憔，文字和文字組成味赃，其中兩頭分處左右，可以用Expanded處理  
3.中間比較復(fù)雜由一個(gè)Row中包含兩部分虐拓，左邊是一個(gè)兩行Column的內(nèi)容,右邊是文字  
4.底部是一個(gè)Row,主軸對齊是start

• 布局代碼

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

  var top = Row(//頂部,通過Flex布局的Row進(jìn)行橫向排列,Expanded中間
    children: <Widget>[
      Image.asset("images/icon_head.png", width: 20, height: 20),
      Expanded(
        child: Padding(
          child: Text("張風(fēng)捷特烈"),
          padding: EdgeInsets.only(left: 4),
        ),
      ),
      Text(
        "Flutter/Dart",
        style: infoStyle,
      )
    ],
  );

  var content = Column(//中間文字內(nèi)容,交叉軸為start
    mainAxisSize: MainAxisSize.min,
    crossAxisAlignment: CrossAxisAlignment.start,
    children: <Widget>[
      Text(
        "[Flutter必備]-Flex布局完全解讀",
        style: littleStyle,
        maxLines: 2,
        overflow: TextOverflow.ellipsis,
      ),
      Padding(
        child: Text("也就是水平排放還是豎直排放心俗，可以看出默認(rèn)情況下都是主軸頂頭,"
            "交叉軸居中比如horizontal下主軸為水平軸，交叉軸則為豎直蓉驹。也就是水平頂頭城榛，豎直居中"
            "這里使用MultiShower快速展示,更好的對比出不同之處",
            style: infoStyle, maxLines: 2, overflow: TextOverflow.ellipsis),
        padding: EdgeInsets.only(top: 5),
      ),
    ],
  );

  var center = Row(//中間的部分
    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(//總體拼合
      child: Container(
          height: 160,
          color: Colors.white,
          padding: EdgeInsets.all(10),
          child: Column(children: <Widget>[top, Expanded(child: center), end])));
  return result;
}

結(jié)語

本文到此接近尾聲了，如果想快速嘗鮮Flutter态兴，《Flutter七日》會(huì)是你的必備佳品;如果想細(xì)細(xì)探究它,那就跟隨我的腳步狠持，完成一次Flutter之旅。
另外本人有一個(gè)Flutter微信交流群瞻润，歡迎小伙伴加入喘垂，共同探討Flutter的問題，本人微信號:zdl1994328绍撞，期待與你的交流與切磋正勒。