Android Jetpack架构组件（六）Navigation简单使用

参考：
https://developer.android.google.cn/guide/navigation/
https://developer.android.google.cn/guide/navigation/navigation-getting-started
https://blog.csdn.net/hongxue8888/article/details/103672351
https://www.jianshu.com/p/0bb65607bdf8

阐述Navigation如何管理MainActivity中的NavigationPage1Fragment、NavigationPage2Fragment、NavigationPage3Fragment三个fragment。

Gradle依赖

def nav_version = "2.3.0-alpha06"
// Java language implementation
implementation "androidx.navigation:navigation-fragment:$nav_version"
implementation "androidx.navigation:navigation-ui:$nav_version"

navigation导航文件

在app的res目录下，创建一个navigation文件夹，在此文件夹下新增一个navigation的xml文件，此处新增一个nav_graph_main.xml，内容如下：

<?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_main.xml"
    app:startDestination="@id/page1Fragment">

    <fragment
        android:id="@+id/page1Fragment"
        android:name="cn.appblog.jetpack.navigation.fragment.NavigationPage1Fragment"
        android:label="fragment_page1"
        tools:layout="@layout/fragment_navigation_page1">
        <!--
            action:程序中使用id跳到destination对应的类
        -->
        <action
            android:id="@+id/action_page2"
            app:destination="@id/page2Fragment" />
    </fragment>

    <fragment
        android:id="@+id/page2Fragment"
        android:name="cn.appblog.jetpack.navigation.fragment.NavigationPage2Fragment"
        android:label="fragment_page2"
        tools:layout="@layout/fragment_navigation_page2">
        <action
            android:id="@+id/action_page1"
            app:destination="@id/page1Fragment" />
        <action
            android:id="@+id/action_page3"
            app:destination="@id/page3Fragment" />
    </fragment>

    <!--<navigation-->
    <!--    android:id="@+id/nav_graph_page3"-->
    <!--    app:startDestination="@id/page3Fragment">-->
    <fragment
        android:id="@+id/page3Fragment"
        android:name="cn.appblog.jetpack.navigation.fragment.NavigationPage3Fragment"
        android:label="fragment_page3"
        tools:layout="@layout/fragment_navigation_page3">
        <action
            android:id="@+id/action_page2"
            app:destination="@id/page2Fragment" />
    </fragment>

    <!--</navigation>-->

</navigation>

在Design中可以看到非常直观的fragment切换流程，如下：

主页面

<?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"
    >

    <!--
    app:defaultNavHost="true"
    拦截系统back键，在fragment中就可以用back回退上一个fragment
    app:navGraph="@navigation/nav_graph_main"
    连接使用nav_graph_main.xml，获取fragment之间的导航结构
    -->
    <fragment
        android:id="@+id/my_nav_host_fragment"
        android:name="androidx.navigation.fragment.NavHostFragment"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:defaultNavHost="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:navGraph="@navigation/nav_graph_main" />

</androidx.constraintlayout.widget.ConstraintLayout>

• app:defaultNavHost="true"

拦截系统back键，在fragment中就可以用back回退上一个fragment

• app:navGraph="@navigation/nav_graph_main"

连接使用nav_graph_main.xml，获取fragment之间的导航结构

• android:name="androidx.navigation.fragment.NavHostFragment"

name 必须指定，表示这是一个可以切换fragment的控件

三个Fragment

Navigation.findNavController(view).navigate(R.id.action_page2);

实现点击按钮后Fragment页面的跳转

public class NavigationPage1Fragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_navigation_page1, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Button btn = view.findViewById(R.id.btn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Navigation.findNavController(view).navigate(R.id.action_page2);
            }
        });
    }
}

public class NavigationPage2Fragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_navigation_page2, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Button btn = view.findViewById(R.id.btn);
        //返回MainPage1Fragment
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Navigation.findNavController(view).navigate(R.id.action_page1);
            }
        });

        Button btn2 = view.findViewById(R.id.btn2);
        //前往MainPage3Fragment
        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Navigation.findNavController(view).navigate(R.id.action_page3);
            }
        });
    }
}

public class NavigationPage3Fragment extends Fragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_navigation_page3, container, false);
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Button btn = view.findViewById(R.id.btn);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Navigation.findNavController(view).navigate(R.id.action_page2);
                //回退上一步
//                Navigation.findNavController(view).navigateUp();
            }
        });
    }
}

遗留问题

尽管通过Navigation方式管理Fragment的代码简洁而且直观，但是有一个非常致命的问题，就是同一个Fragment会被重复创建，这样会导致每次切换Fragment的时候都会重新去请求数据，非常浪费系统资源，而且降低了APP的用户体验。