Android Studio中点击按钮获取EditText文本未更新问题

开发登录界面时，点击Login按钮获取EditText控件文本内容，发现文本无法随用户输入更新，日志始终打印默认的“Name”和“Password”。

对应的布局XML代码：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#F8E00A"
    tools:context=".FirstFragment">

    <TextView
        android:id="@+id/textview_first"
        android:layout_width="186dp"
        android:layout_height="44dp"
        android:fontFamily="@font/alegreya_sc"
        android:text="Welcome"
        android:textAlignment="center"
        android:textColor="@android:color/holo_blue_bright"
        android:textSize="30sp"
        app:layout_constraintBottom_toTopOf="@id/login_button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.497"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/login_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="login"
        android:text="Login"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/textview_first" />

    <EditText
        android:id="@+id/username_box"
        android:layout_width="250dp"
        android:layout_height="60dp"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Name"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.496"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.421" />

    <EditText
        android:id="@+id/password_box"
        android:layout_width="250dp"
        android:layout_height="60dp"
        android:ems="10"
        android:inputType="textPassword"
        android:text="Password"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.496"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.535" />
</androidx.constraintlayout.widget.ConstraintLayout>

Java逻辑代码：

package com.example.myapplication;

import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.example.myapplication.databinding.FragmentFirstBinding;

public class FirstFragment extends Fragment {

    private FragmentFirstBinding binding;
    EditText usernameT;
    EditText passwordT;
    @Override
    public View onCreateView(
            @NonNull LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {
        View fragmentFirstLayout = inflater.inflate(R.layout.fragment_first, container, false);
        usernameT = (EditText) fragmentFirstLayout.findViewById(R.id.username_box);
        passwordT = (EditText) fragmentFirstLayout.findViewById(R.id.password_box);
        binding = FragmentFirstBinding.inflate(inflater, container, false);
        return binding.getRoot();

    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        view.findViewById(R.id.login_button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String username = usernameT.getText().toString();
                String password = passwordT.getText().toString();
                Log.d("debug", username);
                Log.d("debug", password);
                if (username.equals("Na") && password.equals("Pa")) {
                    NavHostFragment.findNavController(FirstFragment.this)
                            .navigate(R.id.action_FirstFragment_to_SecondFragment);
                } else {
                    Toast myToast = Toast.makeText(getActivity(), "Wrong Username or Password", Toast.LENGTH_SHORT);
                    myToast.show();
                }
            }
        });
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }
}

代码中存在核心错误：在onCreateView方法里，你先通过inflater.inflate创建了一个布局实例，并从中获取了EditText的引用，但随后又通过FragmentFirstBinding.inflate创建了另一个完全独立的布局实例，并返回了这个新实例的根View。这导致你操作的EditText是第一个未显示的布局里的控件，而用户实际输入的是第二个布局里的EditText，所以获取的永远是默认文本。

另外，布局XML中Button的android:onClick="login"属性，和代码中通过setOnClickListener设置的点击事件会产生冲突，建议删除该属性。

方案1：正确使用ViewBinding #

直接通过Binding对象获取控件引用，确保只inflate一次布局：

package com.example.myapplication;

import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

import com.example.myapplication.databinding.FragmentFirstBinding;

public class FirstFragment extends Fragment {

    private FragmentFirstBinding binding;

    @Override
    public View onCreateView(
            @NonNull LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {
        // 仅通过ViewBinding inflate一次布局
        binding = FragmentFirstBinding.inflate(inflater, container, false);
        return binding.getRoot();
    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        binding.loginButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // 通过Binding获取实时输入的文本
                String username = binding.usernameBox.getText().toString();
                String password = binding.passwordBox.getText().toString();
                Log.d("debug", username);
                Log.d("debug", password);
                if (username.equals("Na") && password.equals("Pa")) {
                    NavHostFragment.findNavController(FirstFragment.this)
                            .navigate(R.id.action_FirstFragment_to_SecondFragment);
                } else {
                    Toast.makeText(getActivity(), "Wrong Username or Password", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }

    @Override
    public void onDestroyView() {
        super.onDestroyView();
        binding = null;
    }
}

方案2：使用传统findViewById（放弃ViewBinding） #

确保只inflate一次布局，并用返回的根View查找所有控件：

package com.example.myapplication;

import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.fragment.app.Fragment;
import androidx.navigation.fragment.NavHostFragment;

public class FirstFragment extends Fragment {

    EditText usernameT;
    EditText passwordT;

    @Override
    public View onCreateView(
            @NonNull LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState
    ) {
        // 仅inflate一次布局
        View fragmentFirstLayout = inflater.inflate(R.layout.fragment_first, container, false);
        usernameT = fragmentFirstLayout.findViewById(R.id.username_box);
        passwordT = fragmentFirstLayout.findViewById(R.id.password_box);
        return fragmentFirstLayout;
    }

    public void onViewCreated(@NonNull View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        view.findViewById(R.id.login_button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String username = usernameT.getText().toString();
                String password = passwordT.getText().toString();
                Log.d("debug", username);
                Log.d("debug", password);
                if (username.equals("Na") && password.equals("Pa")) {
                    NavHostFragment.findNavController(FirstFragment.this)
                            .navigate(R.id.action_FirstFragment_to_SecondFragment);
                } else {
                    Toast.makeText(getActivity(), "Wrong Username or Password", Toast.LENGTH_SHORT).show();
                }
            }
        });
    }
}

内容的提问来源于stack exchange，提问作者millerstuff