vue通过下拉框组件学习vue中的父子通讯

.select-box{
 position: relative;
 max-width: 250px;
 line-height: 35px;
 margin: 50px auto;
}
.select-title{
 position: relative;
 padding: 0 30px 0 10px;
 border: 1px solid #d8dce5;
 border-radius: 5px;
 transition-duration: 300ms;
 cursor: pointer;
}
.select-title:after{
 content: '';
 position: absolute;
 height: 0;
 width: 0;
 border-top: 6px solid #d8dce5;
 border-left: 6px solid transparent;
 border-right: 6px solid transparent;
 right: 9px;
 top: 0;
 bottom: 0;
 margin: auto;
 transition-duration: 300ms;
 transition-timing-function: ease-in-out;
}
.select-title-active{
 border-color: #409eff;
}
.select-title-active:after{
 transform: rotate(-180deg);
 border-top-color: #409eff;
}
.select-options{
 position: absolute;
 padding:10px 0;
 top: 45px;
 border:1px solid #d8dce5;
 width: 100%;
 border-radius: 5px;
}
.select-option-item{
 padding:0 10px;
 cursor: pointer;
 transition-duration: 300ms;
}
.select-option-item:hover,.select-option-active{
 background: #f1f1f1;
 color: #409eff;
}

.arrow-top{
 position: absolute;
 height: 0;
 width: 0;
 top: -7px;
 border-bottom: 7px solid #d8dce5;
 border-left: 7px solid transparent;
 border-right: 7px solid transparent;
 left: 0;
 right: 0;
 margin: auto;
 z-index: 99;
}
.arrow-top:after{
 content: '';
 position: absolute;
 display: block;
 height: 0;
 width: 0;
 border-bottom: 6px solid #fff;
 border-left: 6px solid transparent;
 border-right: 6px solid transparent;
 left: -6px;
 top: 1px;
 z-index: 99;
}

.slide-down-enter-active,.slide-down-leave{
 transition: all .3s ease-in-out;
 transform-origin:0 top;
 transform: scaleY(1);
}
.slide-down-enter{
 transform: scaleY(0);
}
.slide-down-leave-active{
 transition: all .3s ease-in-out;
 transform-origin:0 top;
 transform: scaleY(0);
}