在Android中使用firebase按ID查找对象的方法

好吧，我想创建这个方法，但是由于您无法从listener的方法返回，所以我无法找到在firebase找到它时为用户提供的方法：

public static User findByID(@NonNegative final int id) {
    getDatabase().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    dataSnapshot.getValue(User.class);    // this one must be returned
                }

                @Override public void onChildChanged(DataSnapshot dataSnapshot, String s) {}
                @Override public void onChildRemoved(DataSnapshot dataSnapshot) {}
                @Override public void onChildMoved(DataSnapshot dataSnapshot, String s) {}
                @Override public void onCancelled(DatabaseError databaseError) {}
            });
}

---

创建一个变量来接收值和返回也不起作用，因为firebase需要花费一些时间来查找用户。

数据检索在firebase中是异步的，因此您无法返回。我认为有两种方法可以克服你面临的问题

第一条路

如果应该在findById所在的同一类中的某个位置返回数据，则可以执行以下操作：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

第二条路

如果需要通知另一个类的实例，可以将侦听器作为参数传递给函数，然后调用该侦听器中的方法作为通知调用方的方式。示例调用方如下所示：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

鉴于您的静态函数可能位于一个类中，如下所示：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

---

数据检索在Firebase中是异步的，因此无法返回。我认为有两种方法可以克服你面临的问题

第一条路

如果应该在findById所在的同一类中的某个位置返回数据，则可以执行以下操作：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

第二条路

如果需要通知另一个类的实例，可以将侦听器作为参数传递给函数，然后调用该侦听器中的方法作为通知调用方的方式。示例调用方如下所示：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

鉴于您的静态函数可能位于一个类中，如下所示：

class A {

    private TextView nameTextView;

    ...

    public void findById(@NonNegative int id) {
        userRef().orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    onUserRetrieved(user);
                }
                ...
            });
    }

    private void onUserRetrived(User user) {
        // Now you can do whatever you want with the "user"
        // For instance, update name field in the layout...
        nameTextView.setText(user.getName);
        ...
    }
}

class Caller {

    ...

    private void someFunction() {
        ...
        String userId = 1; // Get user id somehow
        DatabaseUtil.findUserById(userId, new DatabaseUtil.Listener() {
            @Override
            public void onUserRetrieved(User user) {
                // Now you can do whatever you want with the "user"
            }
        });
        ...
    }
}

class DatabaseUtil {

    public static void findUserById(@NonNegative int id, Listener listener) {
        FirebaseDatabase.getInstance().getreference("user")
            .orderByChild("id")
            .equalTo(id)
            .limitToFirst(1)
            .addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    User user = dataSnapshot.getValue(User.class);
                    listener.onUserRetrieved(user);
                }
                ...
            });
    }

    interface Listener {
        void onUserRetrieved(User user);
    }
}

---

您无法返回异步对象，因此需要将EventListener传递给您创建的函数，当你找到用户时调用它。你所做的与我开始使用Firebase时所做的非常相似–基本上是尝试创建同步方法来检索数据，与使用传统数据库类似。不幸的是，这并不是Firebase的工作方式遗憾的是，Firebase的异步特性是其工作方式的固有特性，您需要构建应用程序的行为，有时会有点麻烦。您无法返回异步对象，因此您需要将EventListener传递给您创建的函数，当你找到用户时调用它。你所做的与我开始使用Firebase时所做的非常相似–基本上是尝试创建同步方法来检索数据，与使用传统数据库类似。不幸的是，这不是Firebase的工作方式可悲的是，Firebase的异步特性是其工作方式的固有特性，你需要围绕着它来构建你的应用程序的行为，这有时会有点麻烦。这对我来说有点抽象。你能举个例子吗？你打算在哪里使用你的FunByID方法？让我们考虑一下我的用户ID，想让他的名字显示在PrraveActudio上。哦，我明白你的意思了。第二种方法很好用。谢谢你的耐心！hi@mehmad有任何方法可以根据所选id查询数据。使用loopIt对我来说有点抽象。你能举个例子吗？你打算在哪里使用你的FunByID方法？让我们考虑一下我的用户ID，想让他的名字显示在PrraveActudio上。哦，我明白你的意思了。第二种方法很好用。谢谢你的耐心！hi@mehmad有任何方法可以根据所选id查询数据。使用循环