后栈在Jetpack导航中无法正常工作

Question

我创建了一个简单的演示项目来测试Android Jetpack导航组件的默认后台功能<我有一个主要活动和两个片段。应用程序运行时会显示主片段。主片段有一个按钮。点击导航到另一个片段<但是每当我按下“系统后退”按钮时，我的整个应用程序就完成了，而不是通常的后堆栈行为，我应该看到主片段<感谢您的帮助。提前谢谢
代码如下：
main活动。千吨

package com.callsamik.jetpacknavigationcomponentdemo

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

活动_main。xml（主要活动的布局）

<?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"
    tools:context=".MainActivity">

    <androidx.fragment.app.FragmentContainerView
        android:id="@+id/nave_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:navGraph="@navigation/nav_graph"
        />

</androidx.constraintlayout.widget.ConstraintLayout>

HomeFragment。千吨

package com.callsamik.jetpacknavigationcomponentdemo

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import androidx.navigation.fragment.findNavController
import com.callsamik.jetpacknavigationcomponentdemo.databinding.FragmentHomeBinding

class HomeFragment: Fragment() {
    var binding: FragmentHomeBinding? = null
    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        binding = FragmentHomeBinding.inflate(layoutInflater)
        return binding?.root
    }
    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        binding?.loginButton?.setOnClickListener{
            val action = HomeFragmentDirections.actionHomeFragmentToLoginFragment()
            findNavController().navigate(action)
        }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        binding = null
    }
}

fragment_主页。xml（HomeFragment的布局）

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    tools:context= ".HomeFragment">
    <androidx.appcompat.widget.AppCompatTextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginBottom="16dp"
        android:text="Home Screen"
        android:textAppearance="@style/TextAppearance.AppCompat.Large"/>

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/loginButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Login"/>
</LinearLayout>

LoginFragment.kt（第二个片段）

package com.callsamik.jetpacknavigationcomponentdemo

import androidx.fragment.app.Fragment

class LoginFragment: Fragment(R.layout.fragment_login) {
}

fragment\u登录。xml（第二个片段布局）

<?xml version="1.0" encoding="utf-8"?>
<androidx.appcompat.widget.LinearLayoutCompat xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:padding="16dp"
    android:orientation="vertical"
    tools:context=".LoginFragment">

    <androidx.appcompat.widget.AppCompatEditText
        android:id="@+id/usernameEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Username"/>

    <androidx.appcompat.widget.AppCompatEditText
        android:id="@+id/passwordEditText"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Password"/>

    <androidx.appcompat.widget.AppCompatButton
        android:id="@+id/confirmButton"
        android:text="Confirm"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</androidx.appcompat.widget.LinearLayoutCompat>

nav_graph.xml（导航图）

<?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/nav_graph"
    app:startDestination="@id/homeFragment">

    <fragment
        android:id="@+id/homeFragment"
        android:name="com.callsamik.jetpacknavigationcomponentdemo.HomeFragment"
        android:label="fragment_home"
        tools:layout="@layout/fragment_home" >
        <action
            android:id="@+id/action_homeFragment_to_loginFragment"
            app:destination="@id/loginFragment"
            app:enterAnim="@anim/slide_in_right"
            app:exitAnim="@anim/slide_out_left"
            app:popEnterAnim="@anim/slide_in_left"
            app:popExitAnim="@anim/slide_out_right" />
    </fragment>
    <fragment
        android:id="@+id/loginFragment"
        android:name="com.callsamik.jetpacknavigationcomponentdemo.LoginFragment"
        android:label="fragment_login"
        tools:layout="@layout/fragment_login" />
</navigation>

顶级构建。格拉德尔

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = "1.5.31"
    ext.nav_version = "2.3.5"
    repositories {
        google()
        mavenCentral()
    }
    dependencies {
        classpath "com.android.tools.build:gradle:7.0.3"
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "androidx.navigation:navigation-safe-args-gradle-plugin:$nav_version"

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

应用程序模块构建。格拉德尔

plugins {
    id 'com.android.application'
    id 'kotlin-android'
    id 'androidx.navigation.safeargs.kotlin'
}

android {
    compileSdk 31

    defaultConfig {
        applicationId "com.callsamik.jetpacknavigationcomponentdemo"
        minSdk 22
        targetSdk 31
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
    }

    buildFeatures {
        viewBinding true
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = '1.8'
    }
}

dependencies {

    implementation 'androidx.core:core-ktx:1.7.0'
    implementation 'androidx.appcompat:appcompat:1.3.1'
    implementation 'com.google.android.material:material:1.4.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.1.1'
    implementation "androidx.navigation:navigation-fragment-ktx:$nav_version"
    implementation "androidx.navigation:navigation-ui-ktx:$nav_version"

    testImplementation 'junit:junit:4.+'
    androidTestImplementation 'androidx.test.ext:junit:1.1.3'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'
}

Answer 1

根据文件：

• app:defaultNavHost=“true”属性确保您的NavHostFragment拦截系统后退按钮。请注意，只有一个导航主机可以作为默认值。如果在同一布局中有多个主机（例如，两个窗格布局），请确保仅指定一个默认主机

您缺少该属性，因此导航不会处理系统后退按钮。

<androidx.fragment.app.FragmentContainerView
    android:id="@+id/nave_host_fragment"
    android:name="androidx.navigation.fragment.NavHostFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:navGraph="@navigation/nav_graph"
    app:defaultNavHost="true"
    />