我似乎无法让我的片段加载 webview 而不会崩溃

Question

所以我正在制作一个应用程序，底部导航可让您点击不同的网页，但我遇到了一个问题。我似乎无法获得 webview 在片段类中正确运行的位置。

activity_main

<?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:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/fragment_container"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:layout_marginBottom="56dp">
    </FrameLayout>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/nav_view"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="0dp"
        android:layout_marginEnd="0dp"
        android:background="?android:attr/windowBackground"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:menu="@menu/bottom_nav_menu" />

</androidx.constraintlayout.widget.ConstraintLayout>

fragment_clever.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent">

    <WebView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/clever_web"></WebView>

</RelativeLayout>

cleverfragment.java（我目前正在使用的那个）

package com.port.schoool;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.webkit.WebView;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;

public class CleverFragment extends Fragment {
    WebView webview;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

        webview.findViewById(R.id.clever_web);
        webview.getSettings().setJavaScriptEnabled(true);
        webview.loadUrl("google.com");

        return inflater.inflate(R.layout.fragment_clever, null);
    }
}

mainactivity.java

package com.port.schoool;

import android.os.Bundle;
import android.view.MenuItem;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;

import com.google.android.material.bottomnavigation.BottomNavigationView;

public class MainActivity extends AppCompatActivity implements BottomNavigationView.OnNavigationItemSelectedListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        BottomNavigationView navView = findViewById(R.id.nav_view);
        navView.setOnNavigationItemSelectedListener(this);

        loadFragment(new ReadWorksFragment());
    }

    private boolean loadFragment(Fragment fragment) {
        if (fragment != null) {
            getSupportFragmentManager()
                    .beginTransaction()
                    .replace(R.id.fragment_container, fragment)
                    .commit();
            return true;
        }
        return false;
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
        Fragment fragment = null;
        switch (menuItem.getItemId()) {
            case R.id.navigation_home:
                fragment = new ReadWorksFragment();
                break;

            case R.id.navigation_dashboard:
                fragment = new CleverFragment();
                break;
            case R.id.navigation_notifications:
                fragment = new PortalFragment();
                break;
        }

        return loadFragment(fragment);
    }
}

这是我的 logcat 崩溃日志：

2019-08-04 18:05:25.164 7425-7425/com.port.schoool E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.port.schoool, PID: 7425
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.webkit.WebView.findViewById(int)' on a null object reference
        at com.port.schoool.CleverFragment.onCreateView(CleverFragment.java:20)
        at androidx.fragment.app.Fragment.performCreateView(Fragment.java:2439)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1460)
        at androidx.fragment.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1784)
        at androidx.fragment.app.FragmentManagerImpl.moveToState(FragmentManager.java:1852)
        at androidx.fragment.app.BackStackRecord.executeOps(BackStackRecord.java:802)
        at androidx.fragment.app.FragmentManagerImpl.executeOps(FragmentManager.java:2625)
        at androidx.fragment.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2411)
        at androidx.fragment.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2366)
        at androidx.fragment.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2273)
        at androidx.fragment.app.FragmentManagerImpl$1.run(FragmentManager.java:733)
        at android.os.Handler.handleCallback(Handler.java:790)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6960)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:441)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1408)

Answer 1

将您的片段替换为：

public class CleverFragment extends Fragment {
    WebView webview;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_clever, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        webview = view.findViewById(R.id.clever_web);
        webview.getSettings().setJavaScriptEnabled(true);
        webview.loadUrl("https://google.com");
    }
}

注意：

• 您的原始代码无法编译，因为Fragment 上没有findViewById()

• inflate() 在onCreateView() 中使用的正确版本是将container 和false 作为第二和第三个参数的版本

• 您在onViewCreated()中配置片段的视图

• 您需要在膨胀布局上调用findViewById()（onViewCreated() 中的view）并将其分配给webview

• 您需要使用有效的 URL 和 loadUrl()

这类内容应该包含在您的 Android 应用开发一书中。