Angular Toast无法在Ionic2中加载？

所以我打了一个http电话。在显示http调用加载之前，当出现响应（成功或错误）时，加载将被取消，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。并且每页只加载1次

如何在HTTP调用期间显示加载，以及如何在加载解除后显示toast以使其正常工作？？

这是HTTP呼叫

    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函数

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();
  }

当前加载

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

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

    });

    this.loader.present();
  }

定义加载器

 ...
 export class Filter {

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