<md-button ng-click="vm.task.isHideSubTasks=!vm.task.isHideSubTasks"
           class="md-icon-button collapse-sub-tasks-btn"
           aria-label="expand/collapse sub tasks"
           md-whiteframe="1">
  <ng-md-icon icon="add"
              aria-label="show sub tasks"
              ng-show="vm.task.isHideSubTasks"></ng-md-icon>
  <ng-md-icon icon="remove"
              aria-label="hide sub tasks"
              ng-show="!vm.task.isHideSubTasks"></ng-md-icon>
</md-button>
<task-list tasks="vm.task.subTasks"
           class="ani-expand-collapse"
           ng-show="!vm.task.isHideSubTasks"
           allow-task-selection="{{vm.allowTaskSelection}}"
           current-task-id="vm.currentTaskId"
           is-sub-tasks-disabled="true"
           parent-task="vm.task"></task-list>