Jetpack学习-Navigation
Navigation是什么
Navigation翻译过来就是导航。
导航是指支持用户导航、进入和退出应用中不同内容片段的交互。Android Jetpack 的导航组件可帮助您实现导航，无论是简单的按钮点击，还是应用栏和抽屉式导航栏等更为复杂的模式，该组件均可应对。导航组件还通过遵循一套既定原则来确保一致且可预测的用户体验。
导航组件由以下三个关键部分组成：
- 导航图：在一个集中位置包含所有导航相关信息的 XML 资源。这包括应用内所有单个内容区域（称为目标）以及用户可以通过应用获取的可能路径。
- NavHost：显示导航图中目标的空白容器。导航组件包含一个默认 NavHost 实现 (NavHostFragment)，可显示 Fragment 目标。
- NavController：在 NavHost 中管理应用导航的对象。当用户在整个应用中移动时，NavController 会安排 NavHost 中目标内容的交换。
在应用中导航时，您告诉 NavController，您想沿导航图中的特定路径导航至特定目标，或直接导航至特定目标。NavController 便会在 NavHost 中显示相应目标。
导航组件提供各种其他优势，包括以下内容：
- 处理 Fragment 事务。
- 默认情况下，正确处理往返操作。
- 为动画和转换提供标准化资源。
- 实现和处理深层链接。
- 包括导航界面模式（例如抽屉式导航栏和底部导航），用户只需完成极少的额外工作。
- Safe Args - 可在目标之间导航和传递数据时提供类型安全的 Gradle 插件。
- ViewModel 支持 - 您可以将 ViewModel 的范围限定为导航图，以在图表的目标之间共享与界面相关的数据。
- 此外，您还可以使用 Android Studio 的 Navigation Editor 来查看和编辑导航图。
以上内容来自官方文档(我只是一个搬运工\(^o^)/)
简单使用
引入Navigation
在需要使用Navigation的模块的build.gradle中引入
def nav_version = "2.3.0-alpha01"
implementation "androidx.navigation:navigation-fragment:$nav_version"
implementation "androidx.navigation:navigation-ui:$nav_version"建立导航图
在res目录右键-New-Android Resource File
在弹出的界面中，File name可随意输入，Resource type选择Navigation,点击确定
点击确定后，会在res目录下创建navigation目录，以及刚才定义的导航文件
双击打开刚才创建的导航文件，在Design界面可以看到目前还没有内容，可以点击上方的+号图标添加fragment，也可以自己手动在xml中添加
我们需要为这个文件指定startDestination，即起始的界面
startDestination指定为mainFragment，mainFragment对应的布局为fragment_main
Navigation首先会加载一个默认的Fragment，这个需要在Activity中指定
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:layout_width="match_parent"
android:layout_height="match_parent">
<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:navGraph="@navigation/nav_graph" />
</LinearLayout>配置defaultNavHost为true,即指定这个fragment为默认的NavHost，每个导航图只能指定一个默认的NavHost。这里的name配置为androidx.navigation.fragment.NavHostFragment,navGraph配置为nav_graph，即指定nav_graph为导航图。这样当Activity启动时，会首先通过activity布局里的fragment去加载导航图中的startDestination配置的fragment。
导航
通过一个Fragment导航到另一个Fragment，可以通过
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_main, container, false);
loginBtn = view.findViewById(R.id.fragment_main_login);
loginBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Bundle bundle = new Bundle();
bundle.putString("name", "zs");
Navigation.findNavController(v).navigate(R.id.action_mainFragment_to_loginFragment, bundle);
}
});
return view;
}这里通过点击一个按钮进行跳转，通过Navigation.findNavController(v).navigate()方法导航。这里还可以通过Bundle进行传值。
在目的Fragment,可以通过getArguments()来获取到传递过来的数据
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
String name = getArguments().getString("name", "null");
Toast.makeText(getContext(), name, Toast.LENGTH_SHORT).show();
View view = inflater.inflate(R.layout.fragment_login, container, false);
backBtn = view.findViewById(R.id.fragment_login_back);
backBtn.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Navigation.findNavController(v).popBackStack();
}
});
return view;
}在目的Fragment，还可以通过一个按钮返回上一个Fragment：Navigation.findNavController(v).popBackStack()
原理
Navigation的简单使用流程就介绍到这，可以在官方文档上看更多相关的使用方法。下面来分析下Navigation的流程
显示起始Fragment
在Activity启动时，会先实例化NavHostFragment,这个是我们前面在布局中指定的。
首先会执行onInflate方法
public void onInflate(@NonNull Context context, @NonNull AttributeSet attrs,
@Nullable Bundle savedInstanceState) {
super.onInflate(context, attrs, savedInstanceState);
final TypedArray navHost = context.obtainStyledAttributes(attrs,
androidx.navigation.R.styleable.NavHost);
final int graphId = navHost.getResourceId(
androidx.navigation.R.styleable.NavHost_navGraph, 0);
if (graphId != 0) {
mGraphId = graphId;
}
navHost.recycle();
final TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.NavHostFragment);
final boolean defaultHost = a.getBoolean(R.styleable.NavHostFragment_defaultNavHost, false);
if (defaultHost) {
mDefaultNavHost = true;
}
a.recycle();
}这个方法，其实就是解析出来我们要使用哪个导航图，获取到了graphId。还获取了是否为默认的Host：defaultHost
然后会执行onAttach方法
public void onAttach(@NonNull Context context) {
super.onAttach(context);
// TODO This feature should probably be a first-class feature of the Fragment system,
// but it can stay here until we can add the necessary attr resources to
// the fragment lib.
if (mDefaultNavHost) {
getParentFragmentManager().beginTransaction()
.setPrimaryNavigationFragment(this)
.commit();
}
}由于在onInflate已经获取到mDefaultNavHost为true,因此这里会将当前Fragment通过commit加入到FragmentManager()中
然后执行onCreate方法
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context context = requireContext();
//设置NavHostController,NavHostController里初始化了NavigatorProvider
mNavController = new NavHostController(context);
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);
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()
//前面执行onInflate后，已经获取到mGraphId，因此会执行下面的setGraph代码
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);
}
}
}在这个方法里，设置了NavHostController及NavigatorProvider，然后执行NavController.setGraph方法
public void setGraph(@NavigationRes int graphResId) {
setGraph(graphResId, null);
}继续调用setGraph
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}继续调用setGraph
public void setGraph(@NonNull NavGraph graph, @Nullable Bundle startDestinationArgs) {
if (mGraph != null) {
// Pop everything from the old graph off the back stack
popBackStackInternal(mGraph.getId(), true);
}
mGraph = graph;
onGraphCreated(startDestinationArgs);
}调用onGraphCreated
private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
if (mNavigatorStateToRestore != null) {
ArrayList<String> navigatorNames = mNavigatorStateToRestore.getStringArrayList(
KEY_NAVIGATOR_STATE_NAMES);
if (navigatorNames != null) {
for (String name : navigatorNames) {
Navigator<?> navigator = mNavigatorProvider.getNavigator(name);
Bundle bundle = mNavigatorStateToRestore.getBundle(name);
if (bundle != null) {
navigator.onRestoreState(bundle);
}
}
}
}
if (mBackStackToRestore != null) {
for (Parcelable parcelable : mBackStackToRestore) {
NavBackStackEntryState state = (NavBackStackEntryState) parcelable;
NavDestination node = findDestination(state.getDestinationId());
if (node == null) {
throw new IllegalStateException("unknown destination during restore: "
+ mContext.getResources().getResourceName(state.getDestinationId()));
}
Bundle args = state.getArgs();
if (args != null) {
args.setClassLoader(mContext.getClassLoader());
}
NavBackStackEntry entry = new NavBackStackEntry(mContext, node, args,
mLifecycleOwner, mViewModel,
state.getUUID(), state.getSavedState());
mBackStack.add(entry);
}
updateOnBackPressedCallbackEnabled();
mBackStackToRestore = null;
}
if (mGraph != null && mBackStack.isEmpty()) {
boolean deepLinked = !mDeepLinkHandled && mActivity != null
&& handleDeepLink(mActivity.getIntent());
if (!deepLinked) {
// Navigate to the first destination in the graph
// if we haven't deep linked to a destination
//首次进入时，会执行这个代码
navigate(mGraph, startDestinationArgs, null, null);
}
}
}首次进入Activity，会最终执行navigate(mGraph, startDestinationArgs, null, null)方法来导航到起始的目的Fragment
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
//...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
//...
}这个方法，会调用navigator.navigate(node, finalArgs,navOptions, navigatorExtras)方法，这个方法的实现在NavGraphNavigator
public NavDestination navigate(@NonNull NavGraph destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Extras navigatorExtras) {
int startId = destination.getStartDestination();
if (startId == 0) {
throw new IllegalStateException("no start destination defined via"
+ " app:startDestination for "
+ destination.getDisplayName());
}
NavDestination startDestination = destination.findNode(startId, false);
if (startDestination == null) {
final String dest = destination.getStartDestDisplayName();
throw new IllegalArgumentException("navigation destination " + dest
+ " is not a direct child of this NavGraph");
}
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
startDestination.getNavigatorName());
return navigator.navigate(startDestination, startDestination.addInDefaultArgs(args),
navOptions, navigatorExtras);
}在这个方法里调用navigator.navigate方法，这个方法实现在FragmentNavigator
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;
}
//通过反射实例化Fragment
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);
}
//替换fragment
ft.replace(mContainerId, frag);
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);
//提交
ft.commit();
// The commit succeeded, update our view of the world
if (isAdded) {
mBackStack.add(destId);
return destination;
} else {
return null;
}
}在这个方法中，通过反射实例化目的Fragment，然后replace原来的Fragment,并commit，这样目的Fragment就显示出来了。
导航到其它Fragment
通过Navigation.findNavController(v).navigate(resId)可以导航到指定的Fragment
public static NavController findNavController(@NonNull View view) {
NavController navController = findViewNavController(view);
if (navController == null) {
throw new IllegalStateException("View " + view + " does not have a NavController set");
}
return navController;
}然后调用findViewNavController
private static NavController findViewNavController(@NonNull View view) {
while (view != null) {
NavController controller = getViewNavController(view);
if (controller != null) {
return controller;
}
ViewParent parent = view.getParent();
view = parent instanceof View ? (View) parent : null;
}
return null;
}这里通过子View往父View不停查找NavController，这个NavController在前面onCreate的时候已经附加到了view上。
找到NavController后，调用navigate。这个过程和前面第一次导航到起始Fragment是一样的流程，这里不再分析。
其实这里只是比较粗的一个梳理，涉及很多细节并没有具体去看，暂且先有一个流程的印象吧。
附上一张时序图
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