写在前面：Jetpack的更新速度非常快，可能你一个月前看WorkManager是这样用的，下个月这个使用方法就有可能被废弃了（我看源码的时候是遇到过的，而且源码也变了，但核心原理是不变的），所以我们这一系列文章偏重讲原理，使用就一带而过（因为讲了也没用啊，会变的。。。。。，读者使用最好看官方文档官方文档
，当然我这里讲的也是截止到目前的最新用法）。
Jetpack使用（一）Lifecycles核心原理
Jetpack使用（二）LiveData核心原理
Jetpack使用（三）DataBinding核心原理
Jetpack使用（四）ViewModel核心原理
Jetpack使用（五）Navigation核心原理
Jetpack使用（六） WorkManager的4种用法
Navigation是Jetpack里提供的用户导航的组件，比如我们在一个activity中实现3个fragment间的相互跳转，就可以用Navigation来实现
一、简单使用
- 1、build.gradle里导入
def nav_version = "2.3.0-alpha01"
implementation "androidx.navigation:navigation-fragment:$nav_version"
implementation "androidx.navigation:navigation-ui:$nav_version"
-
2、在res资源目录下新建navigation文件夹，并新建用于导航的xml文件
image.png
- 3、新建3个fragment里面放入导航跳转的按钮，例如
public class MainPage1Fragment extends Fragment {
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
// return super.onCreateView(inflater, container, savedInstanceState);
return inflater.inflate(R.layout.fragment_main_page1,container,false);
}
@Override
public void onViewCreated(@NonNull View view, @Nullable 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);
}
});
}
}
- 4、创建一个activity，并在activity的布局里，创建fragment标签，引入NavHostFragment
activity里本身没代码，最后运行代码，就能实现三个fragment之间的跳转了。
- 总结：我们在导航的xml图里看到了三个fragment的标签，并且每个fragment标签利用有action标签，action就是实现跳转的标签。app:startDestination是设置首个显示的fragment为=page1Fragment，然后我们再看main_activity.xml里就新建了一个NavHostFragment， app:defaultNavHost="true"设置为true是拦截系统返回键的，也就是在你fragment相互跳转后，按返回键，返回的顺序是跳转到顺序，而不是直接退出activity。
二、核心原理分析。
因为在MainActivity里新建了一个NavHostFragment，我们肯定要去看看NavHostFragment里面的源码的，
image.png
NavHostFragment代码太多，我就不全部贴出来了，
我们先看NavHostFragment的create方法，后面会具体分析每个生命周期代码，
第一步：解析导航图，并初始化导航相关类的数据
先看看NavHostFragment的create方法
- NavHostFragment#create
@NonNull
public static NavHostFragment create(@NavigationRes int graphResId,
@Nullable Bundle startDestinationArgs) {
Bundle b = null;
if (graphResId != 0) {
b = new Bundle();
b.putInt(KEY_GRAPH_ID, graphResId);
}
if (startDestinationArgs != null) {
if (b == null) {
b = new Bundle();
}
b.putBundle(KEY_START_DESTINATION_ARGS, startDestinationArgs);
}
final NavHostFragment result = new NavHostFragment();
if (b != null) {
result.setArguments(b);
}
return result;
}
（1）将nav_graph_main.xml文件的id，graphResId放进Bundle
（2）新建一个NavHostFragment，把bundle里的数据设置给NavHostFragment，最后返回一个新的NavHostFragment
也就相当于把activity_main.xml里的NavHostFragment和nav_graph_main.xml文件绑定了。
通过打断点，我们发现NavHostFragment里生命周期各个方法的执行顺序是onInflate、onCreate、onCreateView、onViewCreated。
- 1、onInflate
@CallSuper
@Override
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();
}
一开始就是解析activity_main.xml里的fragment标签包裹的一些xml属性值，
获取xml里的导航图的graphId，并将graphId赋值给NavHostFragment的成员变量mGraphId，最后设置defaultNavHost的值（defaultNavHost我们前面讲过了值为true就可以实现拦截系统back键）。
- 2、onCreate
@CallSuper
@Override
public void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
final Context context = requireContext();
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()
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);
}
}
}
@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}
*/
@SuppressLint("ResourceType")
@NonNull
public NavGraph inflate(@NavigationRes int graphResId) {
Resources res = mContext.getResources();
XmlResourceParser parser = res.getXml(graphResId);
final AttributeSet attrs = Xml.asAttributeSet(parser);
try {
int type;
while ((type = parser.next()) != XmlPullParser.START_TAG
&& type != XmlPullParser.END_DOCUMENT) {
// Empty loop
}
if (type != XmlPullParser.START_TAG) {
throw new XmlPullParserException("No start tag found");
}
String rootElement = parser.getName();
NavDestination destination = inflate(res, parser, attrs, graphResId);
if (!(destination instanceof NavGraph)) {
throw new IllegalArgumentException("Root element <" + rootElement + ">"
+ " did not inflate into a NavGraph");
}
return (NavGraph) destination;
} catch (Exception e) {
throw new RuntimeException("Exception inflating "
+ res.getResourceName(graphResId) + " line "
+ parser.getLineNumber(), e);
} finally {
parser.close();
}
}
（1）New了一个控制器mNavController，把mNavController和lifecycle建立绑定关系（监听生命周期有关）；把mNavController和ViewModelStore建立关系（数据保存有关）。
（2）onCreateNavController(mNavController);
将mNavController传入，根据新建这个控制器对应的navigator，并把navigator和它对应的名字放进数组mNavigators里
（3）通过mNavController.setGraph(mGraphId)这句代码，根据导航图的mGraphId将导航图NavGraph和控制器mNavController关联起来，
NavGraph里又会通过inflate方法解析导航图xml文件，并最后通过addDestination将目的地信息添加到到NavDestination，
（控制器mNavController间接持有NavDestination数组： Deque
image.png
NavBackStackEntry类其实是包装了NavDestination类的
NavInflater主要就是解析导航图xml信息的
- 3、onCreateView
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
FragmentContainerView containerView = new FragmentContainerView(inflater.getContext());
// When added via XML, this has no effect (since this FragmentContainerView is given the ID
// automatically), but this ensures that the View exists as part of this Fragment's View
// hierarchy in cases where the NavHostFragment is added programmatically as is required
// for child fragment transactions
containerView.setId(getContainerId());
return containerView;
}
创建顶层容器FragmentContainerView，并且设置FragmentContainerView的id（FragmentContainerView是继承FrameLayout的，在1.0版本是直接创建FrameLayout的）
- 4、onViewCreated
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
if (!(view instanceof ViewGroup)) {
throw new IllegalStateException("created host view " + view + " is not a ViewGroup");
}
Navigation.setViewNavController(view, mNavController);
// When added programmatically, we need to set the NavController on the parent - i.e.,
// the View that has the ID matching this NavHostFragment.
if (view.getParent() != null) {
View rootView = (View) view.getParent();
if (rootView.getId() == getId()) {
Navigation.setViewNavController(rootView, mNavController);
}
}
}
将mNavController和view绑定起来。后面那段if判断的意思是：
（1）当通过XML添加时，父View是null，我们的view就是NavHostFragment的根FrameLayout。
（2）但是当以代码方式添加时，需要在父级上设置绑定NavController（我们也可以在MainActvity里直接创建NavHostFragment，并不一定在布局里创建）。
当你看完生命周期的每个方法和NavController的主要代码，差不多能画出我给出类图的右边那一部分了，也就是导航图的xml解析及各个相关导航类的赋值和引用关系的建立
Navigation核心类图
第二步：导航
我们再看看每个fragment里是到底怎么导航到相应的fragment里的，我们点进navigate方法
public void navigate(@IdRes int resId, @Nullable Bundle args, @Nullable NavOptions navOptions,
@Nullable Navigator.Extras navigatorExtras) {
NavDestination currentNode = mBackStack.isEmpty()
? mGraph
: mBackStack.getLast().getDestination();
if (currentNode == null) {
throw new IllegalStateException("no current navigation node");
}
@IdRes int destId = resId;
final NavAction navAction = currentNode.getAction(resId);
Bundle combinedArgs = null;
if (navAction != null) {
if (navOptions == null) {
navOptions = navAction.getNavOptions();
}
destId = navAction.getDestinationId();
Bundle navActionArgs = navAction.getDefaultArguments();
if (navActionArgs != null) {
combinedArgs = new Bundle();
combinedArgs.putAll(navActionArgs);
}
}
if (args != null) {
if (combinedArgs == null) {
combinedArgs = new Bundle();
}
combinedArgs.putAll(args);
}
if (destId == 0 && navOptions != null && navOptions.getPopUpTo() != -1) {
popBackStack(navOptions.getPopUpTo(), navOptions.isPopUpToInclusive());
return;
}
if (destId == 0) {
throw new IllegalArgumentException("Destination id == 0 can only be used"
+ " in conjunction with a valid navOptions.popUpTo");
}
NavDestination node = findDestination(destId);
if (node == null) {
final String dest = NavDestination.getDisplayName(mContext, destId);
throw new IllegalArgumentException("navigation destination " + dest
+ (navAction != null
? " referenced from action " + NavDestination.getDisplayName(mContext, resId)
: "")
+ " is unknown to this NavController");
}
navigate(node, combinedArgs, navOptions, navigatorExtras);
}
一开始会查询到NavDestination，然后根据不同的Navigator实现页面导航。
navigate 方法
（1）如果回退栈为null返回NavGraph，不为null返回回退栈中的最后一项。
（2）根据id，获取对应的NavAction。然后在通过NavAction获取目的地id。
（4）利用目的地ID属性，通过findDestination方法，找到准备导航的目的地。
（5）根据导航目的地的名字，调用getNavigator方法，获取Navigator对象。这里对应的是FragmentNavigator。
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);跳下一步
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
if (navOptions != null) {
if (navOptions.getPopUpTo() != -1) {
popped = popBackStackInternal(navOptions.getPopUpTo(),
navOptions.isPopUpToInclusive());
}
}
Navigator navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
if (newDest != null) {
if (!(newDest instanceof FloatingWindow)) {
// We've successfully navigating to the new destination, which means
// we should pop any FloatingWindow destination off the back stack
// before updating the back stack with our new destination
//noinspection StatementWithEmptyBody
while (!mBackStack.isEmpty()
&& mBackStack.peekLast().getDestination() instanceof FloatingWindow
&& popBackStackInternal(
mBackStack.peekLast().getDestination().getId(), true)) {
// Keep popping
}
}
// The mGraph should always be on the back stack after you navigate()
if (mBackStack.isEmpty()) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, mGraph, finalArgs,
mLifecycleOwner, mViewModel);
mBackStack.add(entry);
}
// Now ensure all intermediate NavGraphs are put on the back stack
// to ensure that global actions work.
ArrayDeque hierarchy = new ArrayDeque<>();
NavDestination destination = newDest;
while (destination != null && findDestination(destination.getId()) == null) {
NavGraph parent = destination.getParent();
if (parent != null) {
NavBackStackEntry entry = new NavBackStackEntry(mContext, parent, finalArgs,
mLifecycleOwner, mViewModel);
hierarchy.addFirst(entry);
}
destination = parent;
}
mBackStack.addAll(hierarchy);
// And finally, add the new destination with its default args
NavBackStackEntry newBackStackEntry = new NavBackStackEntry(mContext, newDest,
newDest.addInDefaultArgs(finalArgs), mLifecycleOwner, mViewModel);
mBackStack.add(newBackStackEntry);
}
updateOnBackPressedCallbackEnabled();
if (popped || newDest != null) {
dispatchOnDestinationChanged();
}
}
（1）从mNavigatorProvider拿出对应的navigator，
然后调用Navigator的navigate，将目的地，动画参数，跳转参数传入实现跳转
，而真正实现这个抽象方法的是在FragmentNavigator和ActivityNavigator的跳转方法里。我们看到FragmentNavigator里（ActivityNavigator里的实现更简单，我们这里不做阐述）
抽象方法navigate的具体实现
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);
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 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;
}
}
一开始根据导航目的地destination拿到className，然后初始化fragment的一些操作，还有出入场的动画，（其实出入场动画都在NavOptions类里，这里不再深入介绍），最后通过mFragmentManager把fragment出栈，入栈最后通过事务的提交fragment。
至此我们所有Navigation的核心源码都分析完整了。我们再回顾下每个类的作用
- NavHosFragment
就是activty要绑定的Fragment，它和我们的导航xml绑定在一起，可以理解为实现导航的主要的Fragment - NavHostController
导航控制器，也是整个Navigation源码里的核心类，是在NavHosFragment的onCreate方法里初始化和做一些关联操作的。用于中转控制xml解析，navigate导航等一系列主要操作。 - NavInflater
主要用于导航xml图的解析工作 - NavGraph
保存NavInflater解析后的目的地信息 - NavDestination
目的地实体类 - NavigatorProvider
导航Navigator类的提供者 - Navigator
导航类，提供导航 - NavAction
导航动作信息类，保存导航入场动画等信息
看完每个类的总结，再回顾我画的类图，对于Navigation的核心原理应该都能了然于胸了，如果对于一些类的细节还想继续深入了解的，也可以结合我这张图，再深入分析，但注意一点，我这张图画的时候，Navigation是2.3.0-alpha01，按照谷歌的尿性，现在在大力推广Jetpack，更新可能会很快，但核心原来不变。
Navigation核心类图