Toast在Ionic2中加载不起作用？

Question

所以我打了一个http电话。在http调用加载出现之前，当响应到来（成功或错误）时，加载被取消并显示Toast。吐司应该持续3秒，但事实并非如此，它会显示0.5秒。每页加载只有一次。

如何在HTTP通话期间显示加载以及如何在加载后关闭后显示toast以便它可以工作？

这是HTTP调用

    this.presentLoading();
    this.http.post(this.mainUrl + '/api/v1/filters', obj, {headers: headers}).subscribe(data => {
      this.loader.dismiss();
      this.presentToast(data.json().success_message, 3000, "toast-success");
    }, err => {
      this.loader.dismiss();
      this.presentToast(err.json().error_message, 3000, "toast-error");
    });

presentToast功能

presentToast(msg, duration, cssClass) {
    let toast = this.toastCtrl.create({
      message: msg,
      duration: duration,
      position: 'bottom',
      dismissOnPageChange: true,
      cssClass: cssClass
    });

    toast.onDidDismiss(() => {
      // Do something
    });

    toast.present();
  }

presentLoading

presentLoading() {
    this.loader = this.loadingCtrl.create({
      content: 'Please wait...'
    });

    this.loader.dismiss().then(()=>{

    });

    this.loader.present();
  }

定义装载程序

 ...
 export class Filter {

  public loader = this.loadingCtrl.create({
    content: "Please wait.."
  });
  ...