React Native FlexBox布局

这段时间熟悉了一下ReactNative，里面的布局感觉很有意思，跟我以前接触的布局思想有很大不同，所以就自己测试了一些FlexBox的属性，一下是效果图：

RN的布局主要依赖于FlexBox系统，它有如下几个主要属性：
flex 视图的比重，这里注意了：In React Native flex does not work the same way that it does in CSS
flexDirection 子视图在容器中排布的方向,这是用来确定主轴的 flexDirection controls which directions children of a container go
justifyContent 子视图在主轴的排列方式 justifyContent aligns children in the main direction
alignItems 子视图在主轴的交叉轴（次轴）排列方式 alignItems aligns children in the cross direction
alignSelf 子视图在次轴的排列方式，主要是用来重写父视图的alignItems
alignSelf controls how a child aligns in the cross direction, overriding the alignItems of the parent.
position 布局方式，默认是相对布局relative
flexWrap 是否包裹 ，默认值是nowrap
接下来看代码实现：
首先我们也仿照官方的列子写个Circle类来描述作为子视图的球

class Circle extends Component {
    render() {
        var size = this.props.size || 20
        var backgroundColor = this.props.backgroundColor || ‘#dcdcdc‘
        return(
            <View
                style={{
                    borderRadius:size/2,
                    backgroundColor:backgroundColor,
                    width:size,
                    height:size,
                    margin:2,
                    shadowOffset:{width:4,height:4},
                    shadowColor:‘#a9a9a9‘,
                    shadowOpacity:0.6
                }}
            />
        )}
}

其次我们再写个CircleBlock类作为容器，其实就是小球的父视图：

class Circle extends Component {
    render() {
        var size = this.props.size || 20
        var backgroundColor = this.props.backgroundColor || ‘#dcdcdc‘
        return(
            <View
                style={{
                    borderRadius:size/2,
                    backgroundColor:backgroundColor,
                    width:size,
                    height:size,
                    margin:2,
                    shadowOffset:{width:4,height:4},
                    shadowColor:‘#a9a9a9‘,
                    shadowOpacity:0.6
                }}
            />
        )}
}

先看看flexDirection这个属性有哪些值：

  flexDirection: ReactPropTypes.oneOf([
    ‘row‘, //我的理解就是横排
    ‘row-reverse‘, //横排颠倒，其实从效果上来看就是把row的一切都反过来
    ‘column‘, //竖排
    ‘column-reverse‘ //竖排颠倒
  ]),

以下几个属性的实际效果：

                <Text>flexDirection:row</Text>
                <CircleBlock style={{flexDirection:‘row‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>flexDirection:column</Text>
                <CircleBlock style={{flexDirection:‘column‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>flexDirection:row-reverse</Text>
                <CircleBlock style={{flexDirection:‘row-reverse‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>flexDirection:column-reverse</Text>
                <CircleBlock style={{flexDirection:‘column-reverse‘}}>
                    {minCircles}
                </CircleBlock>

justifyContent作为子视图在主轴的排列方式有这些值：

justifyContent: ReactPropTypes.oneOf([
    ‘flex-start‘, //从开始位置排列，值从左到右或从上到下
    ‘flex-end‘, //从结尾位置排列
    ‘center‘, //居中
    ‘space-between‘, //空间在子视图之间，从效果来看就是子视图等间距
    ‘space-around‘ //空间环绕子视图，我的理解是每个子视图占据相同的空白
  ]),

看代码：

                <Text>justifyContent:flex-start</Text>
                <CircleBlock style={{justifyContent:‘flex-start‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>justifyContent:center</Text>
                <CircleBlock style={{justifyContent:‘center‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>justifyContent:flex-end</Text>
                <CircleBlock style={{justifyContent:‘flex-end‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>justifyContent:space-between</Text>
                <CircleBlock style={{justifyContent:‘space-between‘}}>
                    {minCircles}
                </CircleBlock>
                <Text>justifyContent:space-around</Text>
                <CircleBlock style={{justifyContent:‘space-around‘}}>
                    {minCircles}
                </CircleBlock>

至于效果，是这样的

alignItems是子视图在主轴交叉轴的排列方式，有这些值

alignItems: ReactPropTypes.oneOf([
    ‘flex-start‘, //从开始位置排列
    ‘flex-end‘, //从结尾处排列
    ‘center‘, //居中
    ‘stretch‘ //拉伸
  ]),

alignSelf做为对alignItems的重写，值与alignItems一样

            <Text>alignItems: flex-start</Text>
                <CircleBlock style={{alignItems:‘flex-start‘,height:100}}>
                    {minCircles}
                </CircleBlock>
                <Text> alignItems:center</Text>
                <CircleBlock style={{alignItems:‘center‘,height:100}}>
                    {minCircles}
                </CircleBlock>
                <Text>alignItems:flex-end</Text>
                <CircleBlock style={{alignItems:‘flex-end‘,height:100}}>
                    {minCircles}
                </CircleBlock>

flexWrap包裹

 flexWrap: ReactPropTypes.oneOf([
    ‘wrap‘, //子视图超过一行的长度后将会换行
    ‘nowrap‘ //子视图超过一行的长度后不换行
  ]),

下面是代码:

                <Text>flexWrap:wrap</Text>
                <CircleBlock style={{flexWrap:‘wrap‘,height:150}}>
                    {[minCircles,minCircles,minCircles,minCircles,minCircles,minCircles]}
                </CircleBlock>
                <Text>flexWrap:nowrap</Text>
                <CircleBlock style={{flexWrap:‘nowrap‘,height:150}}>
                    {minCircles}
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,marginVertical:40}}>
                        {minCircles}
                    </CircleBlock>
                </CircleBlock>
                <CircleBlock style={{flexWrap:‘wrap‘,height:150}}>
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,position:‘absolute‘,marginVertical:40}}>
                        {minCircles}
                    </CircleBlock>
                    {minCircles}
                </CircleBlock>
                <CircleBlock style={{flexWrap:‘nowrap‘,height:150}}>
                    {minCircles}
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,alignSelf:‘center‘}}>
                        {minCircles}
                    </CircleBlock>
                </CircleBlock>

position当子视图需要在父视图里面精准定位是需要将子视图的position设置为absoulute

 position: ReactPropTypes.oneOf([
    ‘absolute‘, //相对布局
    ‘relative‘ //绝对布局
  ]),

而且是这里面的绝对布局可以理解是对容器剩余空间的布局，要注意区分这段代码的实现和效果：
实现

<CircleBlock style={{height:150}}>
                    {minCircles}
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,marginVertical:40}}>
                        {minCircles}
                    </CircleBlock>
                </CircleBlock>
                <CircleBlock style={{height:150}}>
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,position:‘absolute‘,marginVertical:40}}>
                        {minCircles}
                    </CircleBlock>
                    {minCircles}
                </CircleBlock>
                <CircleBlock style={{height:150}}>
                    {minCircles}
                    <CircleBlock style={{width:100,flexWrap:‘wrap‘,alignSelf:‘center‘}}>
                        {minCircles}
                    </CircleBlock>
                </CircleBlock>

效果：

源码已上传到git上面了，下载源码