The use of navigation components is not the focus of this article. For specific use, please refer to Official documents , the navigation component framework is implemented through fragments. Its core classes can be mainly divided into three NavGraph, NavHostController and NavHostFragment. The functions of these three classes are:
NavGraph:
The objects obtained by parsing the navigation map xml mainly maintain a collection to store the destination. When navigating to the destination, an id will be passed in. This id may be the id of the fragment in the navigation map xml or the id of the action node under the fragment node. If it is the id of the action node, It will be internally converted into the id of the fragment (that is, the action node is OK without adding it), so that the corresponding fragment can be found.
NavHostController:
The core class of navigation control internally holds the object of parsing the navigation map xml. It also maintains the navigation fallback stack and manages the logical processing in navigation.
NavHostFragment:
The entry of navigation component is mainly to initialize some related classes. The most important thing is to hold NavHostController, which can control the whole navigation map.
Let's first look at the simple use in the layout file xml:
<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_constraintBottom_toTopOf="@id/nav_view"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:navGraph="@navigation/mobile_navigation" />The name attribute here specifies Android X navigation. fragment. NavHostFragment. Those familiar with fragments should know that NavHostFragment will be loaded here,
public class NavHostFragment extends Fragment implements NavHost {
private static final String KEY_GRAPH_ID = "android-support-nav:fragment:graphId";
private static final String KEY_START_DESTINATION_ARGS =
"android-support-nav:fragment:startDestinationArgs";
private static final String KEY_NAV_CONTROLLER_STATE =
"android-support-nav:fragment:navControllerState";
private static final String KEY_DEFAULT_NAV_HOST = "android-support-nav:fragment:defaultHost";
private NavHostController mNavController;
private Boolean mIsPrimaryBeforeOnCreate = null;
private View mViewParent;
// State that will be saved and restored
private int mGraphId;
private boolean mDefaultNavHost;
@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) {
// The xml navigation map will be parsed. The mGraphId is 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);
}
}
}
/**
* Create a navigation controller. In the navigation diagram, the destination of navigation can be fragment, activity, dialog and sub navigation diagram,
* Navigate to different destinations and use different controllers. dialog and fragment are provided here
*/
@SuppressWarnings({"WeakerAccess", "deprecation"})
@CallSuper
protected void onCreateNavController(@NonNull NavController navController) {
navController.getNavigatorProvider().addNavigator(
new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());
}
/**
* Create controller for fragment
*/
@SuppressWarnings("DeprecatedIsStillUsed")
@Deprecated
@NonNull
protected Navigator<? extends FragmentNavigator.Destination> createFragmentNavigator() {
return new FragmentNavigator(requireContext(), getChildFragmentManager(),
getContainerId());
}
@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;
}
... ...
}
}The NavHostFragment class has a small number of lines of code. Under the simplification, several methods in the initialization process are reserved. When the fragment tag is encountered in the layout, the view will be created first. When the NavHostFragment is executed, the onInflate() here will be executed first. You can see that the id of the navigation chart is obtained and assigned to the variable mgraphid. Then we call the fragment lifecycle method, which is the onCreate() method here, where we initialize the NavHostController object first, then we call the onCreateNavController() method. This method and the NavHostController constructor create the navigation controller and add the NavigatorProvider object. When you navigate to a specified page, you use the controller here, and then call mNavController.. setGraph(mGraphId):
public void setGraph(@NavigationRes int graphResId) {
setGraph(graphResId, null);
}
@CallSuper
public void setGraph(@NavigationRes int graphResId, @Nullable Bundle startDestinationArgs) {
setGraph(getNavInflater().inflate(graphResId), startDestinationArgs);
}
@CallSuper
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;
// The default display page of startDestination configured in the navigation diagram is processed in this method
onGraphCreated(startDestinationArgs);
}It can be seen that the navigation map xml is parsed here, and the final result is stored in NavGraph. The parsing of xml here is similar to the parsing of layout xml, so we won't go in here. Those who are interested can see for themselves. The startDestination attribute is usually configured under the root tab of the navigation map to specify the default fragment to start, The processing of this attribute is in the onGraphCreate() method:
private void onGraphCreated(@Nullable Bundle startDestinationArgs) {
... ...
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
navigate(mGraph, startDestinationArgs, null, null);
}
} else {
dispatchOnDestinationChanged();
}
}The navigation () method will be called here, passing the root object in the navigation diagram:
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
... ...
}Here, first get the navigation controller, and then navigate to the corresponding interface. About the addition of navigation controller, as mentioned earlier, let's take a look at the specific addition:
public NavController(@NonNull Context context) {
... ...
mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));
mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));
}The addNavigator() method of the navigator provider is called:
private final HashMap<String, Navigator<? extends NavDestination>> mNavigators = new HashMap<>();
public final Navigator<? extends NavDestination> addNavigator(
@NonNull Navigator<? extends NavDestination> navigator) {
String name = getNameForNavigator(navigator.getClass());
return addNavigator(name, navigator);
}
@CallSuper
@Nullable
public Navigator<? extends NavDestination> addNavigator(@NonNull String name,
@NonNull Navigator<? extends NavDestination> navigator) {
if (!validateName(name)) {
throw new IllegalArgumentException("navigator name cannot be an empty string");
}
return mNavigators.put(name, navigator);
}The name obtained here is the annotation on the navigation controller class, for example:
@Navigator.Name("navigation")
public class NavGraphNavigator extends Navigator<NavGraph> {
... ...
}The name obtained here is the navigation, and the name is used as the key to save the corresponding navigation controller. Here, go back to the navigation () method above:
private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
node.getNavigatorName());
Bundle finalArgs = node.addInDefaultArgs(args);
NavDestination newDest = navigator.navigate(node, finalArgs,
navOptions, navigatorExtras);
... ...
}The node passed in is the root object of the navigation map, node The value obtained by getnavigatorname() is navigation, so the navigation controller obtained here is NavGraphNavigator, and then call its navigate() method:
public NavDestination navigate(@NonNull NavGraph destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Extras navigatorExtras) {
int startId = destination.getStartDestination();
... ...
NavDestination startDestination = destination.findNode(startId, false);
... ...
Navigator<NavDestination> navigator = mNavigatorProvider.getNavigator(
startDestination.getNavigatorName());
return navigator.navigate(startDestination, startDestination.addInDefaultArgs(args),
navOptions, navigatorExtras);
}First obtain the default display view id configured in the navigation map, then find the corresponding navigation destination according to the id, and obtain the corresponding navigation controller according to the navigation destination. Take the following navigation map xml as an example:
<?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/mobile_navigation"
app:startDestination="@id/navigation_home">
<fragment
android:id="@+id/navigation_home"
android:name="com.tangedegushi.jetpack_navigation.ui.home.HomeFragment"
android:label="@string/title_home"
tools:layout="@layout/fragment_home" />
</navigation>startDestination.getNavigatorName() gets fragment, and the corresponding navigation controller is FragmentNavigator. Then call its navigate() method:
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<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;
}
}Here is the operation of fragment. After execution, the corresponding view will be displayed. The click navigation is similar, so I won't repeat it here.