flex布局，占满余下宽度或余下高度

.father{
    display: flex;
    flex-flow: row nowrap;
    .son1{
        width: 20px;
    }
    .son2{
        flex-grow: 1;
    }
}

占满宽度，flex-direction: row
占满高度，flex-direction: column
最后一个元素flex-frow: 1;