Android架构组件-Navigation的使用(一)
Android架构组件-Navigation的使用(二)
这篇主要介绍navigation和BottomNavigationView的使用，可以实现底部导航栏。
如果你的viewpager+tab是非滚动不左右滑动切换页面，可以更换了。

新建activity，选择Bottom Navigation Activity:

Bottom Navigation activity

如果新建activity没有该选项的话，请升级Android studio
代码自动产生activity，xml，menu，drawable。创建4个fragment，此处代码忽略，然后创建navigation，如何创建参考使用一
nav_bottom_navagation.xml代码如下：

<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"
    app:startDestination="@id/nav_home"
    android:id="@+id/nav_bottom_navagation">
    <fragment
        android:id="@+id/nav_home"
        android:name="com.ghp.demo.databindingdemoproject.navigation.bottomNavigationView.HomeFragment"
        android:label="@string/title_home"
        tools:layout="@layout/fragment_home">
    </fragment>
    <fragment
        android:id="@+id/nav_dashboard"
        android:name="com.ghp.demo.databindingdemoproject.navigation.bottomNavigationView.DashboardFragment"
        android:label="@string/title_dashboard"
        tools:layout="@layout/fragment_dashboard">
    </fragment>
    <fragment
        android:id="@+id/nav_notifications"
        android:name="com.ghp.demo.databindingdemoproject.navigation.bottomNavigationView.NotificationsFragment"
        android:label="@string/title_notifications"
        tools:layout="@layout/fragment_notifications">
    </fragment>
</navigation>

接着我们修改activity的XML，将自动产生的TextView替换为fragment

<layout 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.support.constraint.ConstraintLayout
        android:id="@+id/container"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".navigation.bottomNavigationView.BottomNavigationActivity">

        <fragment
            android:id="@+id/frag_nav_bottom_navigation"
            android:layout_width="0dp"
            android:layout_height="0dp"
            android:name="androidx.navigation.fragment.NavHostFragment"
            app:defaultNavHost="true"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintBottom_toTopOf="@id/navigation"
            app:navGraph="@navigation/nav_bottom_navagation"
            />

        <android.support.design.widget.BottomNavigationView
            android:id="@+id/navigation"
            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_navigation" />
    </android.support.constraint.ConstraintLayout>
</layout>

fragment的地方需要注意：

• 添加：android:name=”androidx.navigation.fragment.NavHostFragment”，如果自定义了NavHostFragment，就替换为自定义的路径+NavHostFragment
• 配置 app:defaultNavHost 和 app:navGraph属性：app:defaultNavHost=”true”
  app:navGraph=”@navigation/nav_bottom_navagation”

需要注意：navigation里fragment的id需要和menu里item的id对应

下面就是修改activity：

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_bottom_navigation)

        var navHostFragment: NavHostFragment = supportFragmentManager.findFragmentById(R.id.frag_nav_bottom_navigation) as NavHostFragment
        var navController: NavController = navHostFragment.navController
        NavigationUI.setupWithNavController(navigation, navController)
//        bottom_navigation.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener) 
    }
//需要重写onSupportNavigateUp方法
override fun onSupportNavigateUp(): Boolean {
        return Navigation.findNavController(this, R.id.frag_nav_bottom_navigation).navigateUp()
    }

需要将该自动产生的代码里的listener去掉，因为setupWithNavController里set了,可以看下面的源码

public static void setupWithNavController(
            @NonNull final BottomNavigationView bottomNavigationView,
            @NonNull final NavController navController) {
        bottomNavigationView.setOnNavigationItemSelectedListener(
                new BottomNavigationView.OnNavigationItemSelectedListener() {
                    @Override
                    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
                        return onNavDestinationSelected(item, navController, true);
                    }
                });
        navController.addOnNavigatedListener(new NavController.OnNavigatedListener() {
            @Override
            public void onNavigated(@NonNull NavController controller,
                    @NonNull NavDestination destination) {
                Menu menu = bottomNavigationView.getMenu();
                for (int h = 0, size = menu.size(); h < size; h++) {
                    MenuItem item = menu.getItem(h);
                    if (matchDestination(destination, item.getItemId())) {
                        item.setChecked(true);
                    }
                }
            }
        });
    }

运行试试看，一个简单的类似tabview+viewpager的效果。

点击BottomNavigationView的时候，发现当menu的数量>3的时候，出现了动画的效果，跟设计完全不符，查看源码会发现：

* support:design:27.1.1版本的：

BottomNavigationMenuView里有个Boolean变量mShiftingMode，他的值是根据mShiftingMode = mMenu.size() > 3;控制的，让人苦恼的他是private，且menu的布局在onMeasure时是根据他的值设置的，当他为false的时候，没有那个动画效果。那就想办法设置他为false吧

@SuppressLint("RestrictedApi")
fun BottomNavigationView.disableShiftMode() {
    val menuView = this.getChildAt(0) as BottomNavigationMenuView
    try {
        val shiftingMode = menuView.javaClass.getDeclaredField("mShiftingMode")
        shiftingMode.isAccessible = true
        shiftingMode.setBoolean(menuView, false)
        shiftingMode.isAccessible = false
        for (i in 0 until menuView.childCount) {
            val item = menuView.getChildAt(i) as BottomNavigationItemView

            item.setShiftingMode(false)
            // set once again checked value, so view will be updated

            item.setChecked(item.itemData.isChecked)
        }
    } catch (e: NoSuchFieldException) {
        Timber.e("Unable to get shift mode field: $e")
    } catch (e: IllegalAccessException) {
        Timber.e("Unable to change value of shift mode: $e")
    }
}

需要代码设置

bottomNavigationView.disableShiftMode()

运行看看，是不是4个menu均匀的分布了，这是27.1.1下的源码。

注意：27和28版本下navigation的源码是不一样的
如果你设置了4个menu均匀分布后，还会发现，他们带有选中变大的动画，这是因为，源码其实是2个view，他们有各自的大小值控制动画，附上源码：

public BottomNavigationItemView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        final Resources res = getResources();
        int inactiveLabelSize =
                res.getDimensionPixelSize(R.dimen.design_bottom_navigation_text_size);
        int activeLabelSize = res.getDimensionPixelSize(
                R.dimen.design_bottom_navigation_active_text_size);
        mDefaultMargin = res.getDimensionPixelSize(R.dimen.design_bottom_navigation_margin);
        mShiftAmount = inactiveLabelSize - activeLabelSize;
        mScaleUpFactor = 1f * activeLabelSize / inactiveLabelSize;
        mScaleDownFactor = 1f * inactiveLabelSize / activeLabelSize;

        LayoutInflater.from(context).inflate(R.layout.design_bottom_navigation_item, this, true);
        setBackgroundResource(R.drawable.design_bottom_navigation_item_background);
        mIcon = findViewById(R.id.icon);
        mSmallLabel = findViewById(R.id.smallLabel);
        mLargeLabel = findViewById(R.id.largeLabel);
    }

inactiveLabelSize和activeLabelSize，
如果不想要这样的动画的话，我们只需要将这2个值设为一样就可以了。在自己项目里添加：

<dimen name="design_bottom_navigation_active_text_size">14dp</dimen>
<dimen name="design_bottom_navigation_text_size">14dp</dimen>

* support:design:28版本的：

可能是意识到BottomNavigationView的问题，他新添加了方法setLabelVisibilityMode，可以设置不同的LabelVisibilityMode，看源码有4种

@Retention(RetentionPolicy.SOURCE)
public @interface LabelVisibilityMode {
    int LABEL_VISIBILITY_AUTO = -1;
    int LABEL_VISIBILITY_SELECTED = 0;
    int LABEL_VISIBILITY_LABELED = 1;
    int LABEL_VISIBILITY_UNLABELED = 2;
}

可以在代码里设置：

navigation.labelVisibilityMode = LabelVisibilityMode.LABEL_VISIBILITY_SELECTED 

也可以在xml里设置：

app:labelVisibilityMode="labeled"

每种效果不一样，具体的你可以每个运行看看。

有的同学的menu图片使用了PNG，也是可以的，但需要图片四周是透明的，不然会是整块都是颜色值。
如果不想使用他的特性，就想使用自己彩色的图片，可以代码设置：

bottomNavigationView.itemIconTintList = null

运行的过程中发现，每次点击切换menu的时候，fragment都会重新oncreate。

如何避免fragment被回收重新创建呢？
还有印象我前面提到的自定义NavHostFragment吗？

查看源码发现NavHostFragment在create的时候，会在NavigatorProvider里添加创建的FragmentNavigator，而在FragmentNavigator里的navigate方法里对fragment做了处理

@Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        final Context context = requireContext();

        mNavController = new NavController(context);
        mNavController.getNavigatorProvider().addNavigator(createFragmentNavigator());

       ...
    }

我们自定义个FragmentNavigator，重写NavHostFragment的createFragmentNavigator

@Navigator.Name("tab_fragment")  // Use as custom tag at navigation.xml
class TabNavigator (
        private val context: Context,
        private val manager: FragmentManager,
        private val containerId: Int
) : FragmentNavigator(context, manager, containerId) {

    override fun navigate(destination: Destination, args: Bundle?, navOptions: NavOptions?) {
        val tag = destination.id.toString()
        val transaction = manager.beginTransaction()

        val currentFragment = manager.primaryNavigationFragment
        if (currentFragment != null) {
            transaction.hide(currentFragment)
        }

        var fragment = manager.findFragmentByTag(tag)
        if (fragment == null) {
            fragment = destination.createFragment(args)
            transaction.add(containerId, fragment, tag)
        } else {
            transaction.show(fragment)
        }

        transaction.setPrimaryNavigationFragment(fragment)
        transaction.setReorderingAllowed(true)
        transaction.commit()

        dispatchOnNavigatorNavigated(destination.id, BACK_STACK_DESTINATION_ADDED)
    }
}

class TabNavHostFragment: NavHostFragment() {
    override fun createFragmentNavigator(): Navigator<out FragmentNavigator.Destination> {
        return TabNavigator(requireContext(), childFragmentManager, id)
    }
}

TabNavigator我们重命名了@Navigator.Name(“tab_fragment”)，在
navigation里我们需要更改fragment为tab_fragment

<?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"
    app:startDestination="@id/nav_home"
    android:id="@+id/nav_bottom_navagation">
    <tab_fragment
        android:id="@+id/nav_home"
        android:name="com.ghp.demo.databindingdemoproject.navigation.bottomNavigationView.HomeFragment"
        android:label="@string/title_home"
        tools:layout="@layout/fragment_home">
    </tab_fragment>
    ...
</navigation>

同时我们需要将xml里的fragment修改：

<fragment
   ...
    android:name="com.ghp.demo.databindingdemoproject.navigation.bottomNavigationView.TabNavHostFragment"
    ...
    />

activity里在获取NavHostFragment也需要修改：

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_bottom_navigation)
        //使用自定义的TabNavHostFragment，优化fragment每次重新create问题
        var navHostFragment: TabNavHostFragment = supportFragmentManager.findFragmentById(R.id.frag_nav_bottom_navigation) as TabNavHostFragment
        var navController: NavController = navHostFragment.navController
        NavigationUI.setupWithNavController(navigation, navController)

在每个fragment的oncreate方法添加log，运行查看是不是在点击menu的时候，不会每次都执行oncreate了，而且只有在点击menu的时候才会创建fragment，实现了类似懒加载的方式。

如果你想动态的手动添加menu和fragment，不妨往下继续看。

在xml里将fragment去除app:navGraph的配置，
BottomNavigationView去除app:menu的配置

<layout 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">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        tools:context=".navigation.bottomNavigationView.BtNavShiftingActivity">

        <fragment
            android:id="@+id/fragment"
            android:layout_width="match_parent"
            android:layout_height="0dp"
            android:layout_weight="1"
            android:name="androidx.navigation.fragment.NavHostFragment"
            app:defaultNavHost="true"
            />

        <android.support.design.widget.BottomNavigationView
            android:id="@+id/bottomNavigationView"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_marginStart="0dp"
            android:layout_marginEnd="0dp"
            android:background="?android:attr/windowBackground"
            app:labelVisibilityMode="labeled"
            />

    </LinearLayout>
</layout>

代码手动的添加：

class BtNavTransactionActivity : AppCompatActivity() {

    var homeFragment: HomeFragment? = null
    var notificationsFragment: NotificationsFragment? = null
    var dashboardFragment: DashboardFragment? = null
    var testFragment: DashboardFragment? = null
    var currentFragment: Fragment? = null

    private val mOnNavigationItemSelectedListener = BottomNavigationView.OnNavigationItemSelectedListener { item ->
        when (item.itemId) {
            0 -> {
                if (homeFragment == null) {
                    homeFragment = HomeFragment()
                }
                settingFragment(homeFragment!!)
            }
            1 -> {
                if (notificationsFragment == null) {
                    notificationsFragment = NotificationsFragment()
                }
                settingFragment(notificationsFragment!!)
            }
            2 -> {
                if (dashboardFragment == null) {
                    dashboardFragment = DashboardFragment()
                }
                settingFragment(dashboardFragment!!)
            }
            3 -> {
                if (testFragment == null) {
                    testFragment = DashboardFragment()
                }
                settingFragment(testFragment!!)
            }
        }
        true
    }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_bt_nav_transaction)
        bottomNavigationView.menu.add(0, 0, 0, resources.getString(R.string.title_home)).setIcon(R.drawable.ic_home_black_24dp)
        bottomNavigationView.menu.add(0, 1, 1, resources.getString(R.string.title_dashboard)).setIcon(R.drawable.ic_dashboard_black_24dp)
        bottomNavigationView.menu.add(0, 2, 2, resources.getString(R.string.title_notifications)).setIcon(R.drawable.ic_notifications_black_24dp)
        bottomNavigationView.menu.add(0, 3, 3, resources.getString(R.string.title_test)).setIcon(R.drawable.ic_notifications_black_24dp)

        if (homeFragment == null) {
            homeFragment = HomeFragment()
        }
        fragmentAdd(R.id.fragment, homeFragment!!)
        currentFragment = homeFragment
//        bottomNavigationView.itemIconTintList = null //如果使用的是自己的彩色png图片，需要设置
        bottomNavigationView.setOnNavigationItemSelectedListener(mOnNavigationItemSelectedListener)
    }

    /**
      *用FragmentTransaction控制fragment的show和hide
      *避免每次的oncreate
      **/
    private fun settingFragment(fragment: Fragment) {
        if (currentFragment != fragment) {
            val transaction = supportFragmentManager.beginTransaction()
            if(!fragment.isAdded){
                transaction.hide(currentFragment!!).add(R.id.fragment, fragment).commit()
            } else {
                transaction.hide(currentFragment!!).show(fragment).commit()
            }
            currentFragment = fragment
        }
    }

    fun fragmentAdd(viewId: Int, fragment: Fragment) {
        supportFragmentManager
                .beginTransaction()
                .add(viewId, fragment)
                .commitAllowingStateLoss()
    }
}

OK，算是这几天navigation的使用总结吧。

补充注意:
按照上面步骤写代码的话，会出现fragment重叠的bug，可以重写onSaveInstanceState处理，这里就不介绍相关fragment重叠问题了。