onselected event in md-select in Angular 4

For Material version >= 6

Use the (selectionChange) event on mat-select.

<mat-form-field>
    <mat-select placeholder="State" (selectionChange)="someMethod($event.value)">
        <mat-option *ngFor="let state of states" [value]="state.value">
            {{ state.viewValue }}
        </mat-option>
    </mat-select>
</mat-form-field>

In the event method, $event.value contains the currently selected [value]. Reference to the official documentation.

@Output() selectionChange: EventEmitter< MatSelectChange >

Event emitted when the selected value has been changed by the user.

Here is a link to Stackblitz Demo.

For Material version < 6

Use the (change) output event.

<md-select placeholder="State" (change)="someMethod()">
  <md-option *ngFor="let state of states" [value]="state.value">
    {{ state.viewValue }}
  </md-option>
</md-select>

Another way is to use (onSelectionChange) event on <md-option>:

<md-select placeholder="State">
  <md-option *ngFor="let state of states" [value]="state.code" 
             (onSelectionChange)="anotherMethod($event, state)">
    {{ state.name }}
  </md-option>
</md-select>

Link to Stackblitz Demo.