确保在订阅之前建立 STOMP 套接字连接

Question

在我的 Angular 2 项目中，我使用ng2-stomp-service建立到服务器的套接字连接。

大多数时候它工作正常，但有时我在尝试订阅套接字时会收到此错误，表示我的连接尚未建立：

Error: Uncaught (in promise): Error: InvalidStateError: The connection has not been established yet Error: InvalidStateError: The connection has not been established yet
    at SockJS.send (main.js:158)
    at Client._transmit (stomp.js:159)
    at Client.subscribe (stomp.js:379)
    at StompService.subscribe (stomp.service.ts:132)
    at slide-manager.service.ts:129
    at ZoneDelegate.invoke (zone.js:391)
    at Object.onInvoke (core.es5.js:3933)
    at ZoneDelegate.invoke (zone.js:390)
    at Zone.run (zone.js:141)
    at zone.js:818
    at SockJS.send (main.js:158)
    at Client._transmit (stomp.js:159)
    at Client.subscribe (stomp.js:379)
    at StompService.subscribe (stomp.service.ts:132)
    at slide-manager.service.ts:129
    at ZoneDelegate.invoke (zone.js:391)
    at Object.onInvoke (core.es5.js:3933)
    at ZoneDelegate.invoke (zone.js:390)
    at Zone.run (zone.js:141)
    at zone.js:818
    at resolvePromise (zone.js:770)
    at zone.js:821
    at ZoneDelegate.invokeTask (zone.js:424)
    at Object.onInvokeTask (core.es5.js:3924)
    at ZoneDelegate.invokeTask (zone.js:423)
    at Zone.runTask (zone.js:191)
    at drainMicroTaskQueue (zone.js:584)
    at WebSocket.ZoneTask.invoke (zone.js:490)

这个堆栈跟踪中属于我的部分是slide-manager.service.ts:129。该代码位于该代码的 stomp.subscribe 行：

this.socketConfig = {
      host: 'http://' + getHost() + '/app-ws',
      debug: true,
      queue: {'init': false}
    };   

 this.stomp.configure(this.socketConfig);
 this.stomp.startConnect().then((frame) => {
 this.stomp.done('init');
 this.connected = true;
 this.spectraSubscription = this.stomp.subscribe('/topic/spectra', (spectra) => {
     if (spectra && (!this.theChart || hostElement.childElementCount === 0) && !this.refreshFlag) {
         this.makeChart(spectra.points, hostElement);
         this.refreshFlag = true;
     } else if (spectra && (this.theChart || hostElement.childElementCount > 0) || this.refreshFlag) {
         this.theChart.collectionView.sourceCollection = spectra.points;
         this.theChart.collectionView.refresh();
     }
  });

如何确保在尝试执行 .subscribe() 之前确实建立了连接？我认为将它放在 .then() 块中就可以解决问题，因为它只能在连接的承诺解决后执行，但显然即使不存在连接它仍然会被触发（根据错误）。

或者还有更多我错过的事情在起作用？

Answer 1

我能够通过检查 stomp 客户端的内置“状态”属性来解决此问题：

if (this.stomp.status === 'CONNECTED') {
    // do stuff that requires a connection, like establish subscriptions
}