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问题:

<select ng-model="machineSelected"
        ng-options="machine._id as machine.name for machine in machineList"
        ng-change="onChangeMachine(machineSelected)"
        ng-disabled="!mineSelected">
    <!--<option value=""></option>-->
    <option value="">Select</option>
</select>

When I add

$scope.machineSelected = "";

dynamically wherever in the controller, option in drop-down should set as "Select", but it is not updating.

回答1:

Use null or undefined, not an empty string, as the "not selected" value.

That's what the docs say:

Optionally, a single hard-coded <option> element, with the value set to an empty string, can be nested into the <select> element. This element will then represent the null or "not selected" option. See example below for demonstration.

It's not really clear that an empty string can't be used, but at least they mention null.

回答2:

Set default option in controller and dynamically change based on selected item

 <select ng-model="selectedItem"  ng-change="getSelectedText()">
    <option >All Items</option>
    <option >Active Subscriptions Only</option>
    <option >Expired Only</option>
 </select>

Controller

app.controller('SelectedTextController', function ($scope) {
    $scope.selectedItem = "All Items";
    $scope.getSelectedText = function () {
        var selectedItem=this.selectedItem;
    };       
});

回答3:

Use null for the unselected value:

̶$̶s̶c̶o̶p̶e̶.̶m̶a̶c̶h̶i̶n̶e̶S̶e̶l̶e̶c̶t̶e̶d̶ ̶=̶ ̶"̶"̶;̶
$scope.machineSelected = null;

The ng-options directive was re-factored with AngularJS V1.6. Before then the default could be selected with an empty string. Now the default is selected by assigning null to the model.

For more infomation, see

AngularJS API Reference - Using select with ngOptions and setting a default value
AngularJS ng-options Directive API Reference
AngularJS Developer Guide - Migrating to V1.6 - select directve
ng-options is hiding the blank value with AngularJS V1.5

The DEMO

angular.module("app",[])
.controller("ctrl",function($scope){
  $scope.machineList = [
    { _id:1, name: "Megatron" },
    { _id:2, name: "Starscream" },
    { _id:3, name: "Shockwave" },
  ];
  $scope.machineSelected = null;
})

<script src="//unpkg.com/angular/angular.js"></script> 
<body ng-app="app" ng-controller="ctrl">
    <select ng-model="machineSelected"
        ng-options="machine._id as machine.name for machine in machineList"
        ng-change="onChangeMachine(machineSelected)">
       <option value="">Select</option>
    </select>
    <br>
    machineSelected={{machineSelected}}
</body>