Angular 4材料芯片占位符不能正常工作

在下面添加了GIF

我的代码是

<mat-form-field>
  <mat-chip-list #chipList>
    <mat-chip *ngFor="let keyword of keywords" [selectable]="selectable"
             [removable]="removable" (remove)="remove(keyword)">
      {{keyword}}
      <mat-icon matChipRemove *ngIf="removable">cancel</mat-icon>
    </mat-chip>
    <input placeholder="Keywords"
           [matChipInputFor]="chipList"
           [matChipInputSeparatorKeyCodes]="separatorKeysCodes"
           [matChipInputAddOnBlur]="addOnBlur"
           (matChipInputTokenEnd)="add($event)" />
  </mat-chip-list>
</mat-form-field>

和ts是

visible: boolean = true;
  selectable: boolean = true;
  removable: boolean = true;
  addOnBlur: boolean = true;
  separatorKeysCodes = [ENTER,COMMA];
  keywords= [];   // At time load i need this to be empty


public add(event: MatChipInputEvent): void {
    let input = event.input;
    let value = event.value;
    if ((value || '').trim()) {
        this.keywords.push(value.trim());
    }
    if (input) {
        input.value = '';
    }
}

public remove(keyword: any): void {
    let index = this.keywords.indexOf(keyword);
    if (index >= 0) {
        this.keywords.splice(index,1);
    }
}

我使用了相同的代码,这些代码在材料文档上给出,但只有改变我完成的是代替加载数组值我在时间加载时传递空数组.请有人帮我解决这个问题