期望结果
使用Navigation UI的BottomNavigationView
和FragmentContainerView
创建底部应用栏导航,类似于示例应用程序NavigationAdavancedSample。
请注意,NavigationAndvancedSample应用程序似乎并没有使用文档中描述的相同实现,因为它使用了自定义扩展方法setupWithNavController
来实现。
观察结果
在MainActivity.kt中使用FragmentContainerView
创建BottomNavigationView
时,出现空的NavHostFragment
。
可重现的错误和完整代码示例可在BottomNavigationViewSample存储库中找到。
错误信息
2020-05-15 12:39:19.117 18747-18747/com.example.bottomnavigationviewsample E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.bottomnavigationviewsample, PID: 18747 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bottomnavigationviewsample/com.example.bottomnavigationviewsample.MainActivity}: kotlin.TypeCastException: null不能转换为非空类型androidx.navigation.fragment.NavHostFragment at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) at android.os.Handler.dispatchMessage(Handler.java:107) at android.os.Looper.loop(Looper.java:214) at android.app.ActivityThread.main(ActivityThread.java:7356) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) Caused by: kotlin.TypeCastException: null不能转换为非空类型androidx.navigation.fragment.NavHostFragment at com.example.bottomnavigationviewsample.MainActivity.onCreate(MainActivity.kt:16) at android.app.Activity.performCreate(Activity.java:7802) at android.app.Activity.performCreate(Activity.java:7791) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) at android.os.Handler.dispatchMessage(Handler.java:107) at android.os.Looper.loop(Looper.java:214) at android.app.ActivityThread.main(ActivityThread.java:7356) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 2020-05-15 12:39:19.132 18747-18747/com.example.bottomnavigationviewsample I/Process: Sending signal. PID: 18747 SIG: 9为第一个视图home.xml创建一个导航图,在app > res > navigation下显示在
BottomNavigationView
中。a. 当提示Add Project Dependency时,选择OK。
b. 在build.gradle (:someAppModule)中添加
kotlinOptions { jvmTarget = '1.8' }
以启用AppBarConfiguration
的内联字节码。c. 向home.xml添加一个片段以在
BottomNavigationView
中显示。
home.xml
<?xml version="1.0" encoding="utf-8"?>
<navigation 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/home"
app:startDestination="@id/homeFragment">
<fragment
android:id="@+id/homeFragment"
android:name="com.example.bottomnavigationviewsample.HomeFragment"
android:label="fragment_home"
tools:layout="@layout/fragment_home" />
</navigation>
build.gradle
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'
android {
compileSdkVersion 29
buildToolsVersion "29.0.3"
kotlinOptions { jvmTarget = '1.8' }
defaultConfig {
applicationId "com.example.bottomnavigationviewsample"
minSdkVersion 21
targetSdkVersion 29
versionCode 1
versionName "1.0"
testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
}
}
}
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
implementation 'androidx.appcompat:appcompat:1.1.0'
implementation 'androidx.core:core-ktx:1.2.0'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'androidx.test.ext:junit:1.1.1'
androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
implementation 'androidx.navigation:navigation-fragment-ktx:2.2.2'
implementation 'androidx.navigation:navigation-ui-ktx:2.2.2'
}
在app > res > menu下创建
BottomNavigationView
菜单bottom_nav.xml。a. 为菜单
id
和title
添加字符串值。b. 为菜单
icon
添加矢量可绘制图形。
<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android">
<item
android:id="@+id/home"
android:icon="@drawable/ic_home_black_24dp"
android:contentDescription="@string/cd_home"
android:title="@string/title_home" />
</menu>
- 在MainActivity.kt的activity_main.xml布局中添加
FragmentContainerView
和BottomNavigationView
。
activity_main.xml
<LinearLayout 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:orientation="vertical"
tools:context=".MainActivity">
<androidx.fragment.app.FragmentContainerView
android:id="@+id/nav_host_container"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" />
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/bottom_nav"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:menu="@menu/bottom_nav"/>
</LinearLayout>
在MainActivity.kt中启用
BottomNavigationView
。a. 创建
AppBarConfiguration
。 参见:使用NaviationUI更新UI组件 > AppBarConfigurationb. 创建
NavHostFragment
和NavController
。 参见:StackOverflow解决方案,使用findNavController的FragmentContainerViewc. 设置操作栏导航。
d. 设置
BottomNavigationView
导航。 参见:使用NavigationUI更新UI组件 > 底部导航
package com.example.bottomnavigationviewsample
import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.navigation.fragment.NavHostFragment
import androidx.navigation.ui.AppBarConfiguration
import androidx.navigation.ui.setupActionBarWithNavController
import androidx.navigation.ui.setupWithNavController
import kotlinx.android.synthetic.main.activity_main.*
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val appBarConfiguration = AppBarConfiguration(setOf(R.id.home))
val navHostFragment = supportFragmentManager.findFragmentById(R.id.nav_host_container) as NavHostFragment
val navController = navHostFragment.navController
setupActionBarWithNavController(navController, appBarConfiguration)
bottom_nav.setupWithNavController(navController)
}
}
尝试的解决方案
按照文档中的说明,使用
findNavController(R.id.nav_host_container)
创建NavController
。在 activity_main.xml 中实现一个
fragment
视图,而不是FragmentContainerView
。
activity_main.xml
<LinearLayout 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:orientation="vertical"
tools:context=".MainActivity">
<fragment
android:id="@+id/nav_host_container"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" />
<com.google.android.material.bottomnavigation.BottomNavigationView
android:id="@+id/bottom_nav"
android:layout_width="match_parent"
android:layout_height="wrap_content"
app:menu="@menu/bottom_nav"/>
</LinearLayout>