Android 活动中的默认片段

我已经看过很多关于这个问题的帖子，但我的代码无法理解。我创建了一个选项卡式活动，并在底部设置了三个按钮。按钮在不同片段之间切换。MainActivity中的按钮打开此选项卡式活动，但问题是片段布局为空（请参见下图）

按钮工作正常，将我发送到正确的片段，但当它打开时，没有加载片段

我根据YouTube教程构建的代码如下所示：

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="TabbedActivity">

    <FrameLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintTop_toBottomOf="@+id/appbar">

    </FrameLayout>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="0dp"
        android:layout_marginStart="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_tabbed_activity);

    BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

    if(savedInstanceState == null) {
       getSupportFragmentManager().
       beginTransaction().replace(R.id.content,new Fragment_B1()).commit();
    }
 }

TabbedActivity.class

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.view.MenuItem;

public class TabbedActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_tabbed_activity);

        BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
        navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);
    }

    private BottomNavigationView.OnNavigationItemSelectedListener mOnNavigationItemSelectedListener
            = new BottomNavigationView.OnNavigationItemSelectedListener() {


        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            android.support.v4.app.FragmentManager fragmentManager = getSupportFragmentManager();
            android.support.v4.app.FragmentTransaction transaction = fragmentManager.beginTransaction();
            int intentFragment = getIntent().getExtras().getInt("frgToLoad");
            switch (item.getItemId()) {
                case R.id.navigation_home:
                    transaction.replace(R.id.content,new Fragment_B1()).commit();
/*                    mTextMessage.setText(R.string.title_home);*/
                    return true;
                case R.id.navigation_dashboard:
                    transaction.replace(R.id.content,new Fragment_B2()).commit();
                    /*mTextMessage.setText(R.string.title_dashboard);*/
                    return true;
                case R.id.navigation_notifications:
                    transaction.replace(R.id.content,new Fragment_B3()).commit();
                    /*mTextMessage.setText(R.string.title_notifications);*/
                    return true;

            }
            return false;
        }
    };

}

片段内部有相同的代码模式，这是基本的，在这里添加它没有意义。TabbedActivity的xml如下所示：

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="TabbedActivity">

    <FrameLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintTop_toBottomOf="@+id/appbar">

    </FrameLayout>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="0dp"
        android:layout_marginStart="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_tabbed_activity);

    BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

    if(savedInstanceState == null) {
       getSupportFragmentManager().
       beginTransaction().replace(R.id.content,new Fragment_B1()).commit();
    }
 }

，但仍然不起作用
 在最后一次添加中，
navigation.setSelectedItemId（navigation\u home）
您可以直接在onCreate上添加原始片段，如下所示：

<?xml version="1.0" encoding="utf-8"?>

<android.support.constraint.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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="TabbedActivity">

    <FrameLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_constraintTop_toBottomOf="@+id/appbar">

    </FrameLayout>

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/navigation"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginEnd="0dp"
        android:layout_marginStart="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/navigation" />

</android.support.constraint.ConstraintLayout>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_tabbed_activity);

    BottomNavigationView navigation = (BottomNavigationView) findViewById(R.id.navigation);
    navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener);

    if(savedInstanceState == null) {
       getSupportFragmentManager().
       beginTransaction().replace(R.id.content,new Fragment_B1()).commit();
    }
 }

这与js有什么关系？打勾接受它作为答案吧@医生不能马上做。显然，至少有10分钟的等待时间。