javascript - ionic + JavaScript : How to create promise with Firebase

Question

我有 doRegisterUser() 的简单函数，它基本上使用提供程序中定义的函数来检查用户名是否已注册。在下面的代码中，我打印到控制台，调用提供程序的函数，然后打印到控制台。简而言之，我这样做只是为了观察执行顺序。我想要的是让 checkUsernameReserved() 执行，然后进行任何控制台打印。

如何实现这一点？

doRegisterUser() {
        var self = this;
        /*Step A - Check username provided by user is taken or not. If no username taken, create random username*/
        console.log("Before checkUsernameReserved() execution");



        self.firebaseProvider.checkUsernameReserved(self.username);



        console.log("After checkUsernameReserved() execution");
    }

这是使用 firebase 的提供程序函数:

checkUsernameReserved(username:string): any{
    /*Check usernamesTaken table for provided username*/
    firebase.database().ref('/usernames_taken/' + username).once('value').then(function(snapshot) {

      /*Check if username is taken.*/
      if(snapshot.val() != null && snapshot.val() != "")
      {
        console.log("Username Taken");
      }else{
        console.log("Username Available");
      }
    })
  }

我在控制台中得到的当前输出是:

• 在 checkUsernameReserved() 执行之前
• 执行 checkUsernameReserved() 后
• 用户名已被占用

Answer 1

有两件事:

• 返回 checkUsernameReserved 的 promise
• 将必须在检查后运行的代码放入该 Promise 的 .then 中。

所以:

doRegisterUser() {
    var self = this;
    /*Step A - Check username provided by user is taken or not. If no username taken, create random username*/
    console.log("Before checkUsernameReserved() execution");
    self.firebaseProvider.checkUsernameReserved(self.username).then(() => {
        // Put code that must run after the check in here...
        console.log("After checkUsernameReserved() execution");
    }); 
}

checkUsernameReserved(username:string): any{
    /*Check usernamesTaken table for provided username*/
    // !! note the return !!
    return firebase.database().ref('/usernames_taken/' + username).once('value').then(function(snapshot) {

        /*Check if username is taken.*/
        if(snapshot.val() != null && snapshot.val() != "") {
            console.log("Username Taken");
        } else {
            console.log("Username Available");
        }
    });
}