Angular4中的checkbox 全选按钮启用禁用

<div class="modal-header">
          <p class="modal-title">Logout Warnning</p>
        </div>
        <div class="modal-body">
                <p>Your have working order(s). if you logout the order(s) will be returned to the Pick Queue and all unconfirmed handling units will remove. Do you want to continue logout"table">
                <thead>
                  <tr>
                    <th>
                        <input type="checkbox" name="checkedAll" [checked]="isAllChecked()" (change)="checkAll($event)">
                    </th>
                    <th>Phase Code</th>
                    <th>Delivery</th>
                    <th>Product</th>
                  </tr>
                </thead>
                <tbody>
                  <tr *ngFor="let task of this.releaseTasks.releaseTaskDetails index as i;"(click)="onModalSelectedRows(task)"
>
                    <td>
                        <input type="checkbox" [(ngModel)]="task.isChecked" name="checkedTask{{i}}" #checkedTask="ngModel"/>
                    </td>
                    <td>
                        {{task.phaseCode}}
                    </td>
                    <td>
                        {{task.saP_DeliveryOrder_ID}}
                    </td>
                    <td>
                        {{task.saP_ProductOrder_ID|removeLeadingZeros}}
                    </td>
                  </tr>
                </tbody>
              </table>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-secondary" (click)="c('Yes')" [disabled]="!canRelease">Release and logout</button>
          <button type="button" class="btn btn-primary" (click)="c('No')">Logout only</button>
        </div>

​​checkAll(ev: any) {​​
 ​​this.releaseTasks.releaseTaskDetails.forEach((x:any) => x.isChecked = ev.target.checked)​​
 ​​ }​​
 ​​isAllChecked() {​​
 ​​if(this.releaseTasks.releaseTaskDetails.length > 0​​
 ​​return this.canRelease = this.releaseTasks.releaseTaskDetails.every((_:any) => _.isChecked);​​
 ​​ }​​
 ​​return false;​​
 ​​ }​​
 ​​​​​​
    onModalSelectedRows(task
     : 
     any) {
    
    task.
     isChecked 
     = 
     !
     task.
     isChecked;
    
    let 
     len 
     = 
     0;
    
    this.
     releaseTasks.
     releaseTaskDetails.
     forEach((item
     : 
     any) 
     =>
    if(
     item.
     isChecked) {
    
    len 
     ++;
    
    
      }
    
    
      });
    
    if(
     len 
     === 
     0) {
    
    this.
     canRelease 
     = 
     false;
    
    else{
    
    this.
     canRelease 
     = 
     true;
    
    
      }
    
    
      }
    ​​​​后台的viewmodel代码​​

var activityQuery = from op in _context.OperatorActivities
                                where op.Operator_ID == userName && !op.IsComplete && !op.IsReleased && !op.IsException 
                                select op;
            
            ReleaseTask relesaseTask = new ReleaseTask();

            if(activityQuery.Any()){
                foreach (var activity in activityQuery)
                {
                    ReleaseTaskViewModel taskDetail = new ReleaseTaskViewModel();
                    taskDetail.SAP_DeliveryOrder_ID = getOrderById(activity.DeliveryOrder_ID);
                    taskDetail.SAP_ProductOrder_ID = getProductOrderById(activity.ProductionOrder_ID);
                    taskDetail.PhaseCode = activity.ActivityCode;
                    taskDetail.isChecked = true;
                    taskDetail.OperatorActivityId = activity.OperatorActivity_ID;
                    taskDetail.DeliveryOrder_ID = activity.DeliveryOrder_ID;
                    taskDetail.ProductionOrder_ID = activity.ProductionOrder_ID;
                    taskDetail.Operator_ID = activity.Operator_ID;
                    taskDetail.OrderId = activity.ActivityCode == "MAKE" ? activity.ProductionOrder_ID : activity.DeliveryOrder_ID;
                    taskDetail.isPersistent = isPersistent(activity.ProductionOrder_ID);
                    if(!taskDetail.isPersistent) {
                        relesaseTask.ReleaseTaskDetails.Add(taskDetail);
                    }
                }
            }
            return(Ok(new { success = true, data = relesaseTask}));