Jetpack之Navigation(2)

Jetpack之Navigation(1)

2.原理

初始化过程 NavHostFragment生命周期方法

1.create—NavHostFragment的创建

在NavHostFragment.create方法

1. 
   
2. 初始化Bundle，并且将graphResId，startDestinationArgs存储在Bundle中。
   
3. new NavHostFragment()返回NavHostFragment实例。
   

    //NavHostFragment.java
@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;

    }

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2.onInflate—XML文件的解析

主要是解析布局文件的两个属性：defaultNavHost和navGraph，并且初始化全局变量。

NavHostFragment.onInflate方法 当Fragment以XML的方式静态加载时，最先会调用onInflate的方法（调用时机：Fragment所关联的Activity在执行setContentView时）。

    //NavHostFragment.java
@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;   //navigation的图布局

        }

        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;    //是否监听物理返回键

        }

        a.recycle();

    }

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3.onCreateNavController—创建Navigator

在实现导航的时候，我们需要根据navigation配置文件生成NavGraph类，然后在根据每个不同的actionid，找到对应的NavDestination就可以实现页面导航跳转了。

创建Navigator

Navigator类的作用是：能够实例化对应的NavDestination，并且能够实现导航功能，拥有自己的回退栈。

    @CallSuper

    protected void onCreateNavController(@NonNull NavController navController) {
navController.getNavigatorProvider().addNavigator(

                new DialogFragmentNavigator(requireContext(), getChildFragmentManager()));

//创建Navigator并绑定到NavigatorProvider中。
//mNavigatorProvider是NavController中的全局变量，内部通过HashMap键值对的形式保存Navigator类。
navController.getNavigatorProvider().addNavigator(createFragmentNavigator());

    }

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1. 
   
2. 其中mNavigatorProvider是NavController中的全局变量，内部通过HashMap键值对的形式保存Navigator类
   
3. createFragmentNavigator方法，构建了FragmentNavigator对象，其中抽象类Navigator还有个重要的实现类ActivityNavigator和NavGraphNavigator。这个两个类的对象在NavController的构造方法中被添加。
   

//NavController.java
public NavController(@NonNull Context context) {

        mContext = context;
while (context instanceof ContextWrapper) {
if (context instanceof Activity) {

                mActivity = (Activity) context;
break;

            }

            context = ((ContextWrapper) context).getBaseContext();

        }

        mNavigatorProvider.addNavigator(new NavGraphNavigator(mNavigatorProvider));

        mNavigatorProvider.addNavigator(new ActivityNavigator(mContext));

}

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4.onCreate—导航初始化

无论是XML实现还是代码实现，都会执行Fragment的onCreate方法。NavController在这里被创建，并且NavHostFragment中有一个NavController对象。

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

//1.初始化NavController，NavController为导航的控制类，核心类

        mNavController = new NavHostController(context);

        mNavController.setLifecycleOwner(this);

        mNavController.setOnBackPressedDispatcher(requireActivity().getOnBackPressedDispatcher());


        mNavController.enableOnBackPressed(

                mIsPrimaryBeforeOnCreate != null && mIsPrimaryBeforeOnCreate);

        mIsPrimaryBeforeOnCreate = null;

        mNavController.setViewModelStore(getViewModelStore());

        onCreateNavController(mNavController);


        Bundle navState = null;
//2.开始恢复状态
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) {

            mNavController.restoreState(navState);

        }

//3.设置导航图信息
if (mGraphId != 0) {

            mNavController.setGraph(mGraphId);

        } else {
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);

            }

        }
super.onCreate(savedInstanceState);

    }

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5.onCreateView

NavHostFragment的视图就只有一个FragmentContainerView 继承 FrameLayout

    //NavHostFragment.java
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {

        FragmentContainerView containerView = new FragmentContainerView(inflater.getContext());

//这行主要用于以代码方式添加fragment

        containerView.setId(getContainerId());
return containerView;

    }

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6.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");

        }
//把mNavController记录在view的tag中

        Navigation.setViewNavController(view, mNavController);

if (view.getParent() != null) {

            mViewParent = (View) view.getParent();
if (mViewParent.getId() == getId()) {
//把mNavController记录在view的tag中

                Navigation.setViewNavController(mViewParent, mNavController);

            }

        }

    }

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获取NavController

1.获取NavController

NavHostFragment.findNavController(fragment)

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    //NavHostFragment.java
@NonNull
public static NavController findNavController(@NonNull Fragment fragment) {

        Fragment findFragment = fragment;
while (findFragment != null) {
if (findFragment instanceof NavHostFragment) {
return ((NavHostFragment) findFragment).getNavController();

            }

            Fragment primaryNavFragment = findFragment.getParentFragmentManager()

                    .getPrimaryNavigationFragment();
if (primaryNavFragment instanceof NavHostFragment) {
return ((NavHostFragment) primaryNavFragment).getNavController();

            }

            findFragment = findFragment.getParentFragment();

        }


        View view = fragment.getView();
if (view != null) {
return Navigation.findNavController(view);

        }


        Dialog dialog = fragment instanceof DialogFragment

                ? ((DialogFragment) fragment).getDialog()

                : null;
if (dialog != null && dialog.getWindow() != null) {
return Navigation.findNavController(dialog.getWindow().getDecorView());

        }

throw new IllegalStateException("Fragment " + fragment

                + " does not have a NavController set");

    }

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2.Navigation中findNavController

3.findViewNavController

通过view.tag查找NavController。内部调用了getViewNavController方法。

4.getViewNavController

getViewNavController方法 通过获取view的Tag，获取NavController对象，这里的tag ID和setViewNavController都是nav_controller_view_tag。

    //Navigation.java
@NonNull
public static NavController findNavController(@NonNull View view) {
//3.

        NavController navController = findViewNavController(view);
if (navController == null) {
throw new IllegalStateException("View " + view + " does not have a NavController set");

        }
return navController;

    }


@Nullable
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;

    }

@Nullable
private static NavController getViewNavController(@NonNull View view) {
//4.这里的tag ID和setViewNavController都是nav_controller_view_tag。
Object tag = view.getTag(R.id.nav_controller_view_tag);

        NavController controller = null;
if (tag instanceof WeakReference) {

            controller = ((WeakReference) tag).get();

        } else if (tag instanceof NavController) {

            controller = (NavController) tag;

        }
return controller;

    }

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导航navigate

navigate

在构建和获取到NavController对象以及NavGraph之后。下面是使用它来实现真正的导航了。下面从navigate开始分析。在navigate方法内部会查询到NavDestination，然后根据不同的Navigator实现页面导航。

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;
//2.根据id，获取对应的NavAction

        final NavAction navAction = currentNode.getAction(resId);

        Bundle combinedArgs = null;
if (navAction != null) {
if (navOptions == null) {

                navOptions = navAction.getNavOptions();

            }
//3.通过NavAction获取目的地id

            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");

        }
//4.利用目的地ID属性，通过findDestination方法，找到准备导航的目的地

        NavDestination node = findDestination(destId);
if (node == null) {

            final String dest = NavDestination.getDisplayName(mContext, destId);
if (navAction != null) {
throw new IllegalArgumentException("Navigation destination " + dest

                        + " referenced from action "

                        + NavDestination.getDisplayName(mContext, resId)

                        + " cannot be found from the current destination " + currentNode);

            } else {
throw new IllegalArgumentException("Navigation action/destination " + dest

                        + " cannot be found from the current destination " + currentNode);

            }

        }
//5.开始导航

        navigate(node, combinedArgs, navOptions, navigatorExtras);

    }


private void navigate(@NonNull NavDestination node, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {
boolean popped = false;
boolean launchSingleTop = 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);

        ...

 }

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findDestination

如果回退栈为null返回NavGraph，不为null返回回退栈中的最后一项。

  NavDestination findDestination(@IdRes int destinationId) {
if (mGraph == null) {
return null;

        }
if (mGraph.getId() == destinationId) {
return mGraph;

        }
//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。

        NavDestination currentNode = mBackStack.isEmpty()

                ? mGraph

                : mBackStack.getLast().getDestination();

        NavGraph currentGraph = currentNode instanceof NavGraph

                ? (NavGraph) currentNode

                : currentNode.getParent();
return currentGraph.findNode(destinationId);

    }

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FragmentNavigator的实现

通过以上的分析，又来到了Navigator 的子类FragmentNavigator类。下面来看看FragmentNavigator.navigate的方法。
（1）调用instantiateFragment，通过反射机制构建Fragment实例

（2）处理进出场等动画逻辑

（3）最终调用FragmentManager来处理导航逻辑。

@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;
}
//通过反射机制构建Fragment实例 
final Fragment frag = instantiateFragment(mContext, mFragmentManager,
className, args);
frag.setArguments(args);
final FragmentTransaction ft = mFragmentManager.beginTransaction();

//处理动画逻辑
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);
}

//FragmentManager来处理导航逻辑
ft.replace(mContainerId, frag);
ft.setPrimaryNavigationFragment(frag);

final @IdRes int destId = destination.getId();
final boolean initialNavigation = mBackStack.isEmpty();
final boolean isSingleTopReplacement = navOptions != null && !initialNavigation
&& navOptions.shouldLaunchSingleTop()
&& mBackStack.peekLast() == destId;

boolean isAdded;
if (initialNavigation) {
isAdded = true;
} else if (isSingleTopReplacement) {
if (mBackStack.size() > 1) {
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.EntrysharedElement : extras.getSharedElements().entrySet()) {
ft.addSharedElement(sharedElement.getKey(), sharedElement.getValue());
}
}
ft.setReorderingAllowed(true);
ft.commit();
if (isAdded) {
mBackStack.add(destId);
return destination;
} else {
return null;
}
}
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ActivityNavigator

ActivityNavigator最终也是调用了startActivity方法,请自己阅读源码。

    @Nullable
@Override
public NavDestination navigate(@NonNull Destination destination, @Nullable Bundle args,
@Nullable NavOptions navOptions, @Nullable Navigator.Extras navigatorExtras) {

           ....
if (navigatorExtras instanceof Extras) {

                Extras extras = (Extras) navigatorExtras;

                ActivityOptionsCompat activityOptions = extras.getActivityOptions();
if (activityOptions != null) {

                    ActivityCompat.startActivity(mContext, intent, activityOptions.toBundle());

                } else {

                    mContext.startActivity(intent);

                }

           } else {

               mContext.startActivity(intent);

           }

           ...

    }   

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3.总结

1. 
   
2. NavHostFragment 作为导航载体，在Activity的layout文件里被引用（或者在代码中动态），并且持有导航控制类NavController引用。
   
3. NavController 将导航任务委托给Navigator类，Navigator类有两个重要的子类FragmentNavigator和ActivityNavigator子类。NavController类持有NavInflater类引用。
   
4. NavInflater 负责解析Navgation文件，负责构建NavGraph导航图。
   
5. NavDestination 存有各个目的地信息，在FragmentNavigator和ActivityNavigator内部分别对应一个Destination类，该类继承NavDestination。
   
6. 在页面导航时，fragment的操作还是交由FragmentManager在操作，activity交由startActivity执行。