谷歌推出Navigation主要是为了统一应用内页面跳转行为。
'androidx.navigation:navigation-fragment:2.1.0'
'androidx.navigation:navigation-ui:2.1.0'
'androidx.navigation:navigation-fragment-ktx:2.1.0'
'androidx.navigation:navigation-ui-ktx:2.1.0'
添加依赖
Navigation的使用很简单,在创建新项目的时候可以直接选择 Bottom Navigation Activity 项目,这样默认就已经帮我们实现了相关页面逻辑。
Navigation的源码也很简单,但是却涉及到很多的类,主要有以下几个:
- Navigation 提供查找NavController方法
- NavHostFragment 用于承载导航的内容的容器
- NavController 通过navigate实现页面的跳转
- Navigator 是一个abstract,有是个主要实现类
- NavDestination 导航节点
- NavGraph 导航节点页面集合
navigation三要素:
- navigation graph(xml resource): 可视化的navigation 资源文件
- NavHostFragment:当前Fragment容器
- NavController: //导航控制者
Navigation简单使用
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val navView: BottomNavigationView = findViewById(R.id.nav_view) //1.底部导航View
val navController = findNavController(R.id.nav_host_fragment) //2.导航控制者
navView.setupWithNavController(navController)
}
}
我们首先从NavHostFragment入手查看,因为他是直接定义在我们的XML文件中的,我们直接查看器生命周期方法 onCreate :
@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context context = requireContext();
mNavController = new NavHostController(context); //1
mNavController.setLifecycleOwner(this);
mNavController.setOnBackPressedDispatcher(requireActivity().getOnBackPressedDispatcher());
// Set the default state - this will be updated whenever
// onPrimaryNavigationFragmentChanged() is called
mNavController.enableOnBackPressed(
mIsPrimaryBeforeOnCreate != null && mIsPrimaryBeforeOnCreate);
mIsPrimaryBeforeOnCreate = null;
mNavController.setViewModelStore(getViewModelStore());
onCreateNavController(mNavController); //2
Bundle navState = null;
if (savedInstanceState != null) {
navState = savedInstanceState.getBundle(KEY_NAV_CONTROLLER_STATE);
if (savedInstanceState.getBoolean(KEY_DEFAULT_NAV_HOST, false)) {
mDefaultNavHost = true;
getParentFragmentManager().beginTransaction()
.setPrimaryNavigationFragment(this)
.commit();
}
mGraphId = savedInstanceState.getInt(KEY_GRAPH_ID);
}
if (navState != null) {
// Navigation controller state overrides arguments
mNavController.restoreState(navState);
}
if (mGraphId != 0) {
// Set from onInflate()
mNavController.setGraph(mGraphId);
} else {
// See if it was set by NavHostFragment.create()
final Bundle args = getArguments();
final int graphId = args != null ? args.getInt(KEY_GRAPH_ID) : 0;
final Bundle startDestinationArgs = args != null
? args.getBundle(KEY_START_DESTINATION_ARGS)
: null;
if (graphId != 0) {
mNavController.setGraph(graphId, startDestinationArgs);
}
}
}
步骤1直接创建了NavHostController 并通过 findNavController 方法暴露给外部调用者。NavHostController是继承自NavController的。步骤2处代码如下:
@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
navController.getNavigatorProvider().addNavigator(
new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}
通过navController获取NavigatorProvider并向其中添加了两个Navigator,分别为DialogFragmentNavigator和FragmentNavigator。另外在NavController的构造方法中还添加了另外两个Navigator,如下:
public NavController(@NonNull Context context) {
....
mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));
mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));
}
他们都是Navigator的实现类。分别对应于DialogFragment、Fragment和Activity的页面跳转。大家可能对于NavGraphNavigator一些好奇,它是用在什么地方的呢? 其实我们在XML中配置的navGraph对应的navigation跟节点文件中的 startDestination 就是通过NavGraphNavigator来实现跳转的。这也是它目前唯一的用途。
各个Navigator通过复写 navigate 方法来实现各自的跳转逻辑。这里重点强调下 FragmentNavigator 的实现逻辑:
@Nullable
@Override
public NavDestination navigate(@NonNull Destination destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
if (mFragmentManager.isStateSaved()) {
Log.i(TAG, "Ignoring navigate() call: FragmentManager has already"
+ " saved its state");
return null;
}
String className = destination.getClassName();
if (className.charAt(0) == '.') {
className = mContext.getPackageName() + className;
}
final Fragment frag = instantiateFragment(mContext, mFragmentManager,
className, args);
frag.setArguments(args);
final FragmentTransaction ft = mFragmentManager.beginTransaction();
int enterAnim = navOptions != null ? navOptions.getEnterAnim() : -1;
int exitAnim = navOptions != null ? navOptions.getExitAnim() : -1;
int popEnterAnim = navOptions != null ? navOptions.getPopEnterAnim() : -1;
int popExitAnim = navOptions != null ? navOptions.getPopExitAnim() : -1;
if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
enterAnim = enterAnim != -1 ? enterAnim : 0;
exitAnim = exitAnim != -1 ? exitAnim : 0;
popEnterAnim = popEnterAnim != -1 ? popEnterAnim : 0;
popExitAnim = popExitAnim != -1 ? popExitAnim : 0;
ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim);
}
ft.replace(mContainerId, frag);//关键代码:使用replase替换
ft.setPrimaryNavigationFragment(frag);
final @IdRes int destId = destination.getId();
final boolean initialNavigation = mBackStack.isEmpty();
// TODO Build first class singleTop behavior for fragments
final boolean isSingleTopReplacement = navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast() == destId;
boolean isAdded;
if (initialNavigation) {
isAdded = true;
} else if (isSingleTopReplacement) {
// Single Top means we only want one instance on the back stack
if (mBackStack.size() > 1) {
// If the Fragment to be replaced is on the FragmentManager's
// back stack, a simple replace() isn't enough so we
// remove it from the back stack and put our replacement
// on the back stack in its place
mFragmentManager.popBackStack(
generateBackStackName(mBackStack.size(), mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE);
ft.addToBackStack(generateBackStackName(mBackStack.size(), destId));
}
isAdded = false;
} else {
ft.addToBackStack(generateBackStackName(mBackStack.size() + 1, destId));
isAdded = true;
}
if (navigatorExtras instanceof Extras) {
Extras extras = (Extras) navigatorExtras;
for (Map.Entry<View, String> sharedElement : extras.getSharedElements().entrySet()) {
ft.addSharedElement(sharedElement.getKey(), sharedElement.getValue());
}
}
ft.setReorderingAllowed(true);
ft.commit();
// The commit succeeded, update our view of the world
if (isAdded) {
mBackStack.add(destId);
return destination;
} else {
return null;
}
}
最关键的一行代码就是注释处。他是通过 replace 来加载 Fragment 的 ,这就会导致导航切换时重走Fragment生命周期,这不符合我们实际的开发逻辑。
回到上文中的 navController 获取的 NavigatorProvider 其内部是维护了一个HashMap来存储相关的Navigator信息。通过获取到Navigator的注解 Name 为key 和 Navigator 的 getClass为 value 进行存储。
我们在回到上文中的 onCreate 方法:
@CallSuper @Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); final Context context = requireContext();
....
if (mGraphId != 0) {
mNavController.setGraph(mGraphId);
} else {
....
if (graphId != 0) {
mNavController.setGraph(graphId, startDestinationArgs);
}
}
} 这里通过 mNavController 调用了 setGraph 。这里主要是为了解析我们的 XML 中配置的mobile_navigation节点信息文件。会根据不同的节点来各自解析。
@NonNull private NavDestination inflate(@NonNull Resources res, @NonNull XmlResourceParser parser, @NonNull AttributeSet attrs, int graphResId) throws XmlPullParserException, IOException {
Navigator navigator = mNavigatorProvider.getNavigator(parser.getName());
final NavDestination dest = navigator.createDestination();
dest.onInflate(mContext, attrs);
....
final String name = parser.getName();
if (TAG_ARGUMENT.equals(name)) { // argument 节点
inflateArgumentForDestination(res, dest, attrs, graphResId);
} else if (TAG_DEEP_LINK.equals(name)) { // deeplink 节点
inflateDeepLink(res, dest, attrs);
} else if (TAG_ACTION.equals(name)) { // action 节点
inflateAction(res, dest, attrs, parser, graphResId);
} else if (TAG_INCLUDE.equals(name) && dest instanceof NavGraph) { // include 节点
final TypedArray a = res.obtainAttributes(attrs, R.styleable.NavInclude);
final int id = a.getResourceId(R.styleable.NavInclude_graph, 0);
((NavGraph) dest).addDestination(inflate(id));
a.recycle();
} else if (dest instanceof NavGraph) { // NavGraph 节点
((NavGraph) dest).addDestination(inflate(res, parser, attrs, graphResId));
}
}
return dest;
} 通过获取 NavInflater 来对其进行解析。解析后返回 NavGraph ,NavGraph是继承自 NavDestination的。里面主要是保存了所有解析出来的节点信息。
最后简单的总结下就是通过 NavHostFragment 获取到NavContorl并存储了相关的Navigator信息。通过各自的navigate方法进行页面的跳转。通过setGraph来解析配置的页面节点信息,并封装为NavGraph对象。里面通过SparseArray来存储 Destination 信息。
回到我们上面说的问题,如何解决页面切换导致的Fragment生命周期重走的问题,自定义FragmentNavigator。
自定义FragmentNavigator
上文中我们说了需要自定义自己的 Navigator 用于承载 Fragment 。主要的实现思路就是继承现有的 FragmentNavigator 并复写其 navigate 方法,将其中的 replace 方法 替换为 show 和 hide 方法 来完成 Fragment 的切换。
那么我们自定义的 Navigator 如何才能让系统识别呢? 这也简单,只要给我们的 类加上注解 @Navigator.Name(value) 那么他就是一个 Navigator 了。最后通过上文中分析的思路 在将其加入到NavigatorProvider 中 即可。
@Navigator.Name("fragment") //此处名字必须和FragmentNavigator中的名字一致,不然不起作用
class FixFragmentNavigator(context: Context, manager: FragmentManager, conditionId: Int) : FragmentNavigator(context, manager, conditionId) {
private val mContext = context
private val mManager = manager
private val mContainerId = conditionId
private val TAG = "FixFragmentNavigator"
override fun navigate(destination: Destination,
args: Bundle?,
navOptions: NavOptions?,
navigatorExtras: Navigator.Extras?):
NavDestination? {
Log.d(TAG,"124454546456")
if (mManager.isStateSaved) {
Log.i(TAG, "Ignoring navigate() call: FragmentManager has already" + " saved its state")
return null
}
var className = destination.className
if (className[0] == '.') {
className = mContext.packageName + className
}
val ft = mManager.beginTransaction()
var enterAnim = navOptions?.enterAnim ?: -1
var exitAnim = navOptions?.exitAnim ?: -1
var popEnterAnim = navOptions?.popEnterAnim ?: -1
var popExitAnim = navOptions?.popExitAnim ?: -1
if (enterAnim != -1 || exitAnim != -1 || popEnterAnim != -1 || popExitAnim != -1) {
enterAnim = if (enterAnim != -1) enterAnim else 0
exitAnim = if (exitAnim != -1) exitAnim else 0
popEnterAnim = if (popEnterAnim != -1) popEnterAnim else 0
popExitAnim = if (popExitAnim != -1) popExitAnim else 0
ft.setCustomAnimations(enterAnim, exitAnim, popEnterAnim, popExitAnim)
}
// ft.replace(mContainerId, frag)
/**
* 1、先查询当前显示的fragment 不为空则将其hide
* 2、根据tag查询当前添加的fragment是否不为null,不为null则将其直接show
* 3、为null则通过instantiateFragment方法创建fragment实例
* 4、将创建的实例添加在事务中
*/
val fragment = mManager.primaryNavigationFragment //当前显示的fragment
if (fragment != null) {
ft.hide(fragment)
}
var frag: Fragment?
val tag = destination.id.toString()
frag = mManager.findFragmentByTag(tag)
if (frag != null) {
ft.show(frag)
} else {
frag = instantiateFragment(mContext, mManager, className, args)
frag.arguments = args
ft.add(mContainerId, frag, tag)
}
ft.setPrimaryNavigationFragment(frag)
@IdRes val destId = destination.id
/**
* 通过反射的方式获取 mBackStack
*/
val mBackStack: ArrayDeque<Int>
val field = androidx.navigation.fragment.FragmentNavigator::class.java.getDeclaredField("mBackStack")
field.isAccessible = true
mBackStack = field.get(this) as ArrayDeque<Int>
val initialNavigation = mBackStack.isEmpty()
// TODO Build first class singleTop behavior for fragments
val isSingleTopReplacement = (navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast() == destId)
val isAdded: Boolean
if (initialNavigation) {
isAdded = true
} else if (isSingleTopReplacement) {
// Single Top means we only want one instance on the back stack
if (mBackStack.size > 1) {
// If the Fragment to be replaced is on the FragmentManager's
// back stack, a simple replace() isn't enough so we
// remove it from the back stack and put our replacement
// on the back stack in its place
mManager.popBackStack(
generateBackStackName(mBackStack.size, mBackStack.peekLast()),
FragmentManager.POP_BACK_STACK_INCLUSIVE
)
ft.addToBackStack(generateBackStackName(mBackStack.size, destId))
}
isAdded = false
} else {
ft.addToBackStack(generateBackStackName(mBackStack.size + 1, destId))
isAdded = true
}
if (navigatorExtras is Extras) {
val extras = navigatorExtras as Extras?
for ((key, value) in extras!!.sharedElements) {
ft.addSharedElement(key, value)
}
}
ft.setReorderingAllowed(true)
ft.commit()
// The commit succeeded, update our view of the world
if (isAdded) {
mBackStack.add(destId)
return destination
} else {
return null
}
}
/**
* 在父类是 private的 直接定义一个方法即可
*/
private fun generateBackStackName(backIndex: Int, destid: Int): String {
return "$backIndex - $destid"
}
}
此处需要注意的点:@Navigator.Name("fragment") 此处名字必须和FragmentNavigator中的名字一致,不然不起作用,上面提到存储Nacitator的是Map集合,只有名字相同才能覆盖初始添加的FragmentNavigator。
自定义完成好,需要删除布局文件中NavHostFragment的节点引用
<fragment
android:id="@+id/nav_host_fragment"
android:name="androidx.navigation.fragment.NavHostFragment"
android:layout_width="match_parent"
android:layout_height="match_parent"
app:defaultNavHost="true"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/release_navigation" /> //把此节点删除
在代码中手动将 FixFragmentNavigator 和 NavControl 进行关联。
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val navView: BottomNavigationView = findViewById(R.id.nav_view) //1.底部导航View
val navController = findNavController(R.id.nav_host_fragment) //2.导航控制者
//自定义Navigator
//1. 创建NavHostFragment
val fragment = supportFragmentManager.findFragmentById(R.id.nav_host_fragment) as NavHostFragment
//2. 创建自定义navigator
val navigator = FixFragmentNavigator(this,supportFragmentManager,fragment!!.id)
//3. 添加navigator
navController.navigatorProvider.addNavigator(navigator)
//4. 设置xml导航
navController.setGraph(R.navigation.mobile_navigation)
navView.setupWithNavController(navController)
}
}
这样就完成了自定义 Navigator 实现切换 Tab 的时候 Fragment 生命周期不会重新执行了。
谢谢作者的帮助。