如何等待图像上传到Ionic 2中的firebase存储

我试图等到图像通过loader上传到firebase存储上,但效果并不好

我这样写我的代码,但我不知道我的错误在哪里,为什么警报显示在上传过程之前。

private uploadPhoto(): void {
    let loader = this.loadingCtrl.create({
        content: ""
    })
    loader.present().then(_=>{
        this.myPhotosRef.child(this.generateUUID()).child('myPhoto.JPEG')
        .putString(this.myPhoto, 'base64', { contentType: 'image/JPEG' })
        .then((savedPicture) => {
            this.myPhotoURL = savedPicture.downloadURL;
            this.afd.list('/Images/').push({
                nID: this.appService.id,
                url: this.myPhotoURL,
                date: new Date().toISOString()
            });
        });
    }).then(_=>{
        alert("image uploaded and stored in database");
        loader.dismiss();
    })
}