java - 从 Android 中的外部 XML 文件访问元素
问题描述
我正在开发一个 Android 应用程序。我为布局创建了另一个 XML 文件：-
聊天文件.XML
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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="@color/suitableblack"
android:orientation="vertical">
<Button
android:id="@+id/signout"
android:layout_width="422dp"
android:layout_height="64dp"
android:text="Button" />
<ListView
android:id="@+id/Msgdisplay"
android:layout_width="374dp"
android:layout_height="593dp"
android:layout_below="@id/signout"
android:layout_marginLeft="20dp"
android:forceDarkAllowed="false" />
<LinearLayout
android:layout_width="390dp"
android:layout_height="83dp"
android:layout_below="@id/Msgdisplay"
android:layout_marginLeft="20dp"
android:orientation="horizontal">
<EditText
android:id="@+id/Msginput"
android:layout_width="482dp"
android:layout_height="67dp"
android:layout_weight="1"
android:ems="10"
android:inputType="textPersonName"
android:text="Name" />
<Button
android:id="@+id/Sendbtn"
android:layout_width="match_parent"
android:layout_height="68dp"
android:layout_weight="1"
android:text="Button" />
</LinearLayout>
所以我想在我的 MainActivity.java 文件中使用 ID-Sendbtn 访问按钮，该部分的代码在这里：-
Button sendbtn=findViewById(R.id.Sendbtn);
sendbtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String msg=message.getText().toString();
if(msg.length()==0)
{
Toast.makeText(getApplicationContext(),"Blank message detected",Toast.LENGTH_SHORT);
}
else
{
FirebaseDatabase.getInstance().getReference().push().setValue(new chatcolumns(msg,FirebaseAuth.getInstance().getCurrentUser().getDisplayName()));
}
}
});
但是每当我运行此代码时，它都会给我以下错误：-
Process: com.example.anochatzone, PID: 18398
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.anochatzone/com.example.anochatzone.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.anochatzone.MainActivity.onCreate(MainActivity.java:87)
at android.app.Activity.performCreate(Activity.java:6679)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
我的主要问题是如何从 MainActivity.java 中的 chatfile.xml 访问按钮
更新 MainActivity.java
package com.example.anochatzone;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Layout;
import android.util.Log;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;
import android.widget.*;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.database.FirebaseDatabase;
import java.util.Objects;
public class MainActivity extends AppCompatActivity {
private FirebaseAuth authenticate;
private String email_str="";
private String password_str="";
private EditText email,password,message;
private Button loginbtn,resetbtn,sendbtn;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE); //will hide the title
Objects.requireNonNull(getSupportActionBar()).hide(); // hide the title bar
this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN); //enable full screen
setContentView(R.layout.activity_main);
email=findViewById(R.id.EmailInput);
password=findViewById(R.id.Passwordinput);
message=findViewById(R.id.Msginput);
loginbtn=findViewById(R.id.Loginbtn);
resetbtn= findViewById(R.id.ResetBtn);
sendbtn=findViewById(R.id.Sendbtn);
authenticate=FirebaseAuth.getInstance();
loginbtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(getApplicationContext(),"Please wait while we are processing your request........",Toast.LENGTH_SHORT).show();
email_str=email.getText().toString();
password_str=password.getText().toString();
if(email_str.length()==0||password_str.length()==0)
{
Toast.makeText(getBaseContext(),"Please check email and password",Toast.LENGTH_LONG).show();
}
else
{
authenticate.signInWithEmailAndPassword(email_str,password_str).addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
if(task.isSuccessful()){
Log.d("User display name" , FirebaseAuth . getInstance() . getCurrentUser().getDisplayName());
Toast.makeText(getApplicationContext(),"Welcome "+email_str+" you have sucessfully logged in",Toast.LENGTH_LONG).show();
setContentView(R.layout.chatfile);
}
else
{
Toast.makeText(getApplicationContext(),"Failed to sign in .Please check internet connection or try again later",Toast.LENGTH_SHORT).show();
}
}
});
}
}
});
sendbtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String msg=message.getText().toString();
if(msg.length()==0)
{
Toast.makeText(getApplicationContext(),"Blank message detected",Toast.LENGTH_SHORT);
}
else
{
FirebaseDatabase.getInstance().getReference().push().setValue(new chatcolumns(msg,FirebaseAuth.getInstance().getCurrentUser().getDisplayName())) ;
}
}
});
}
}
解决方案
根据您的目的，当您启动或启动应用程序时，会出现登录页面。如果登录成功，就会出现聊天页面。
因此，您需要将您的活动分为两个活动，例如LoginActivity启动器活动和ChatActivity(MainActivity)，如下所示。
对于登录页面，可能是一个activity_login.xml和LoginActivity.java
在activity_login.xml
<!-- You need one button for login action. -->
<Button
android:id="@+id/btnLogin"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:text="Login"
/>
在LoginActivity.java
@Override protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// This line is important to access your resource id
setContentView(R.layout.activity_login);
findViewById(R.id.btnLogin).setOnClickListener(new View.OnClickListener() {
@Override public void onClick(View v) {
// isLoginSuccess is an example condition for you
if (isLoginSuccess) {
// Navigate to mainpage
startActivity(new Intent(LoginActivity.this, MainActivity.class));
// Terminate the login activity for one time show.
finish();
} else {
// TODO: Try again implementations
}
}
});
}
对于主页，像上面一样，你需要一个activity_main.xml和一个MainActivity.java
在Chatfile.xml
// other codes....
<Button
android:id="@+id/Sendbtn"
android:layout_width="match_parent"
android:layout_height="68dp"
android:layout_weight="1"
android:text="Button" />
// other codes....
在MainActivity.java
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// This line is important for your 'chatfile.xml'
setContentView(R.layout.Chatfile);
// You can access id `SendBtn` right now.
findViewById(R.id.SendBtn).setOnClickListener(new View.OnClickListener() {
@Override public void onClick(View v) {
// TODO: implement something here.
}
});
}
在AndroidManifest.xml中，您需要将启动器活动更改为LoginActivity。
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".LoginActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".MainActivity"/>
</application>
根据我的解决方案，您不必考虑登录成功或失败时应该隐藏什么 ui 元素。
如果我的回答有任何问题，请随时告诉我。
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