類型錯誤：無法讀取null的屬性“長度”

Question

我想運行這段簡單的代碼，基本上它會根據狀態（字符串）更改標簽和圖標

  <ng-container *ngIf="container.length > 0">
                <span *ngIf="!container.status.includes('Paused') && container.status.includes('Up')"
                      class="label label-success label-lg-status"><span class="fa fa-check"></span> Completed - {{ container.status }}</span>
                <span *ngIf="container.status.includes('Paused')" class="label label-warning label-lg-status"><span
                  class="fa fa-pause"></span> Completed - {{ container.status }}</span>
                <span *ngIf="container.status.includes('Exited')" class="label label-danger label-lg-status"><span
                  class="fa fa-stop"></span> Completed - {{ container.status }}</span>
   </ng-container>

但是我收到一個錯誤消息，說不能使用null的包含，所以我放了*ngIf="container.length > 0" ，現在我得到ReportComponent.html:22 ERROR TypeError: Cannot read property 'length' of null

如何使html等待get請求的執行？

export class ReportComponent implements OnInit {

  private containerName:string;
  private processes = [];
  private container = [];
  private settings = [];
  private environment = [];
  private id:Number;

  constructor(private _route:ActivatedRoute, private dockerService:DockerService, private environmentService:EnvironmentService, private settingsService:SettingsService) { }

  ngOnInit() {
      // Get Environment id
      this._route.params.subscribe(params => {
        this.id = params['id'];
        // Get Environment
        this.environmentService.getEnvironment(this.id).subscribe( environment => {
          this.environment = environment;
          console.log(environment);
          //this.containerName = this.environment.name;
        });
        // Get Docker processes
        this.dockerService.getProcesses(this.containerName).subscribe(processes => {
          this.processes = processes;
        });
        // Get Docker Container Object
        this.dockerService.getContainer(this.containerName).subscribe( container => {
          this.container = container;
        });
        // Get Settings
        this.settingsService.getSettings().subscribe( settings => {
          this.settings = settings;
        });
    });
  }
}

Answer 1

像這樣使用它：

*ngIf="container && (container.length > 0)"

錯誤將被消除。

除非container數組可用，否則它不會檢查length 。

Answer 2

是的，可以像這樣使用它，它將更像Angular

  <ng-container *ngIf="container?.length > 0">

它也會工作。

Answer 3

您的確切問題是服務調用是異步的。 因此，在將值綁定到container對象之前就開始DOM渲染。 因為DOM渲染不等待異步服務響應完成

解決方案是，您應該在nginit中初始化數組。

嘗試這個

 ngOnInit() {
       this.container = []//add this
      // Get Environment id
      this._route.params.subscribe(params => {
        this.id = params['id'];
        // Get Environment
        this.environmentService.getEnvironment(this.id).subscribe( environment => {
          this.environment = environment;
          console.log(environment);
          //this.containerName = this.environment.name;
        });
        // Get Docker processes
        this.dockerService.getProcesses(this.containerName).subscribe(processes => {
          this.processes = processes;
        });
        // Get Docker Container Object
        this.dockerService.getContainer(this.containerName).subscribe( container => {
          this.container = container;
        });
        // Get Settings
        this.settingsService.getSettings().subscribe( settings => {
          this.settings = settings;
        });
    });
  }
}