Angularjs单选改为多选的开发过程及问题解析

<md-input-container flex="35" class="md-input-has-value"> 
                    <label>产品类型</label> 
                    <md-select ng-model="params.productType" md-on-close="clearSearchTerm()" ng-change="change(params.productType)" data-md-container-class="selectHeader" multiple> 
                      <div> 
                        <button ng-click="pTCheckNone()" class="md-button md-ink-ripple"><i class="zmdi zmdi-undo ng-scope"></i>重置</button> 
                      </div> 
                      <md-select-header class="select-header"> 
                        <input ng-model="searchTerm" type="search" placeholder="Search for a product.." class="header-searchbox md-text" > 
                      </md-select-header> 
                      <md-optgroup label="productTypes"> 
                        <md-option value="{{item.key}}" ng-repeat="item in productTypes | filter:searchTerm">{{item.value}}</md-option> 
                      </md-optgroup> 
                    </md-select> 
                  </md-input-container> 

$scope.productTypes = [ 
   {"key":"SecureSiteProEV1", "value":"product1"}, 
   {"key":"SecureSiteProEV2", "value":"product2"}, 
   {"key":"SecureSiteProEV3", "value":"product3"}, 
   {"key":"SecureSiteProEV4", "value":"product4"}, 
   {"key":"SecureSiteProEV5", "value":"product5"}]; 

md-input-container label:not(.md-no-float):not(.md-container-ignore), 
md-input-container .md-placeholder { 
  white-space: normal; 
}