1 Introduction
In the previous chapter [Android Framework Series] Chapter 14 Core Principles of Fragment (AndroidX version), we learned the core principles of Fragment. In this chapter, we learn the commonly used Fragment+ViewPagerand Fragment+ViewPager2related uses and some basic source code analysis.
2 Fragment+ViewPager
We commonly use two PagerAdapterimplementation classes, namely FragmentStatePagerAdapterand FragmentPagerAdapter. Today, we will learn how to use them and compare them.
2.1 The difference between FragmentPagerAdapter and FragmentStatePagerAdapter
1. fragmentsObject processing:
FragmentPagerAdapter: Out of scope fragmentswill be saved in memory ( detach), but fragmentthe corresponding object Viewwill be destroyed
FragmentStatePagerAdapter: Out of scope fragmentswill not be saved in memory ( remove), Viewand will also be destroyed.
2. Status processing: : The corresponding ones
FragmentPagerAdapteroutside the range will be saved : Only the corresponding ones within the range will be saved . This is used for external parameters to be passed in the life cycle callback, and is similar. 3. Applicable scenarios: The same amount , the memory is larger, but the page switching is more friendly; the memory usage is small, the page switching is slightly worse.fragmentsSavedState
FragmentStatePagerAdapterfragmentsSavedStateSavedStateFragmentActivityfragmentsFragmentPagerAdapterFragmentStatePagerAdapter
Therefore, FragmentPagerAdapterit is suitable for Fragmentsituations where the number is small, and FragmentStatePagerAdapterit is suitable for situations where the number of Fragments is large.
Let’s take a look first FragmentPagerAdapter:
@Override
public Object instantiateItem(ViewGroup container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);
// Do we already have this fragment?
//判断请求的Fragment是否已经被生成过
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
// 当前缓存有则直接使用
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
//调用这个方法来生成新的Fragment
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
// 将新生成的Fragment存储起来，以便以后再次用到时，直接attach()
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
} else {
fragment.setUserVisibleHint(false);
}
}
return fragment;
}
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment) object;
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
+ " v=" + fragment.getView());
mCurTransaction.detach(fragment);
if (fragment.equals(mCurrentPrimaryItem)) {
mCurrentPrimaryItem = null;
}
}
FragmentPagerAdapterThe destroyItemmethod called detach()only changes the Fragment state, which means that only by eliminating the entire adaptertime can all the generated ones Fragmentbe eliminated, otherwise they will be directly in the memory.
Next we compare FragmentStatePagerAdapter:
@Override
public Object instantiateItem(@NonNull ViewGroup container, int position) {
// If we already have this item instantiated, there is nothing
// to do. This can happen when we are restoring the entire pager
// from its saved state, where the fragment manager has already
// taken care of restoring the fragments we previously had instantiated.
// mFragments中对应位置有Fragment的情况下直接返回
if (mFragments.size() > position) {
Fragment f = mFragments.get(position);
if (f != null) {
return f;
}
}
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
Fragment fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
if (mSavedState.size() > position) {
Fragment.SavedState fss = mSavedState.get(position);
if (fss != null) {
fragment.setInitialSavedState(fss);
}
}
while (mFragments.size() <= position) {
mFragments.add(null);
}
fragment.setMenuVisibility(false);
if (mBehavior == BEHAVIOR_SET_USER_VISIBLE_HINT) {
fragment.setUserVisibleHint(false);
}
mFragments.set(position, fragment);
mCurTransaction.add(container.getId(), fragment);
if (mBehavior == BEHAVIOR_RESUME_ONLY_CURRENT_FRAGMENT) {
mCurTransaction.setMaxLifecycle(fragment, Lifecycle.State.STARTED);
}
return fragment;
}
@Override
public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
Fragment fragment = (Fragment) object;
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG) Log.v(TAG, "Removing item #" + position + ": f=" + object
+ " v=" + ((Fragment)object).getView());
while (mSavedState.size() <= position) {
mSavedState.add(null);
}
mSavedState.set(position, fragment.isAdded()
? mFragmentManager.saveFragmentInstanceState(fragment) : null);
mFragments.set(position, null);
// 缓存中移除fragment，下次使用得重新创建
mCurTransaction.remove(fragment);
if (fragment.equals(mCurrentPrimaryItem)) {
mCurrentPrimaryItem = null;
}
}
FragmentStatePagerAdapterIt was removed when destroyItemthe method was called Fragment, so it needs to be re-created next time.Fragment
3 Fragment+ViewPager2
ViewPager2It is Android Jetpacka component in the library and is a container used to implement page switching and sliding effects in the application. In fact, ViewPager2itself is inherited fromRecyclerView . You can review our [Android Framework Series] Chapter 12 RecycleView related principles and four-level cache strategy analysis
3.1 The role and purpose of ViewPager2
3.1.1 ViewPager2 usage scenarios
It is a powerful sliding container that can be used in a variety of scenarios. It provides flexible page switching and layout customization functions, making the application interface richer and more interactive. It can be used in the following scenarios:
-
Implementing onboarding or welcome pages
ViewPager2can be used to create onboarding or welcome pages that let users swipe through to introduce app features or display welcome content. -
Create a picture browser
ViewPager2can be used to create a picture browser, allowing users to switch between different pictures by sliding, and supports zoom and gesture interaction. -
Building carousels
ViewPager2is very suitable for building carousel functions, which can dynamically load different carousel items through adapters and provide automatic cycle scrolling. -
Implementing tabbed layout
Combination can be used to create a tabbed layout, allowing users to switch between different content pages by sliding tabsTabLayout.ViewPager2 -
Create a vertical sliding page
UnlikeViewPager,ViewPager2it supports vertical sliding, so it can be used to create vertical sliding page layouts, such as vertical sliding navigation menus or vertical news lists. -
Implementing paging data display
ViewPager2can be used to display paging data, such as loading a large amount of data by page and displaying a portion of the content on each page. -
Nested sliding layout
ViewPager2can be used nested with other sliding components (such asRecyclerView) to implement complex sliding layout structures. -
Achieve customized sliding effects
By using a customized converter (Transformer), you can achieve various cool page switching effects, such as gradient, zoom, rotation, etc.
3.1.2 Improvements and advantages of ViewPager2 compared to ViewPager
ViewPager2is ViewPageran improved version of the original, offering better performance, more flexible adapters, and richer functionality. It is the preferred component for building sliding page layouts. It can implement various sliding page requirements in applications and provide a better user experience. It has the following improvements and advantages:
-
Support for vertical sliding
ViewPager2isViewPagerimproved upon, and one of the most significant improvements is支持垂直滑动. InViewPagerAndroid, only horizontal sliding is supported. This makesViewPager2the vertical sliding function more convenient and flexible when creating vertical layouts or specific scenarios. -
Better performance and stability
ViewPager2The internal implementation is usedRecyclerViewas a container and no longer relies onViewPagerthe implementation. This gives the advantageViewPager2of e.g. At the same time, some known problems and instabilities have also been solved , such as incorrect entry positions and untimely refresh of data.RecyclerView更好的性能和内存管理更流畅的滑动体验及更好的布局回收和复用机制ViewPager2ViewPager -
Supports the use of Fragment as a page.
Unlike FragmentViewPager,ViewPager2it directly supports the useFragmentof Fragment as a page without passingFragmentPagerAdapterorFragmentStatePagerAdapteradapting. This simplifies page management and lifecycle handling and provides a more intuitive and consistent usage experience. -
More flexible adapters
ViewPager2introduce new adapter interfaces,RecyclerView.Adaptersubclasses ofRecyclerView.Adapter. This makes adapter creation and management more flexible while providing more functionality and extensibility. -
Richer functions and interfaces
ViewPager2provide many new functions and interfaces, such as support for page preloading, more powerful page switching animation support, richer callback interfaces, etc. These features and interfaces give developers greater control and customizationViewPager2of behavior and appearance.
3.2 FragmentStateAdapter
Above we have learned about some differences and optimization points of ViewPager2comparison ViewPager, let's continue to look at ViewPager2the corresponding ones Adapter.
We know that ViewPager2it inherits from RecyclerView, then its corresponding one FragmentStateAdaptermust inherit from RecyclerView.Adapter, which friends should be able to understand. Then RecyclerVieweach Itemjoin we add in Fragmentis presented as a container.
RecyclerView.AdapterThe focus is ViewHolderon the reuse, but the ones in FragmentStateAdapterare Framgentnot reused, that is, as many as there itemare should be created Fragment, so how to convert them?
Let’s take a look at the source code first FragmentStateAdapter:
public abstract class FragmentStateAdapter extends
RecyclerView.Adapter<FragmentViewHolder> implements StatefulAdapter {
// 通过FragmentStateAdapter声明中的泛型可以知道，
// ViewPager2之所以能够在RecyclerView的基础上能对外屏蔽对ViewHolder的使用，
// 其内部是借助FragmentViewHolder实现的，其内部就new了一个FrameLayout。
@Override
public final FragmentViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
return FragmentViewHolder.create(parent);
}
@Override
public final void onBindViewHolder(final @NonNull FragmentViewHolder holder, int position) {
final long itemId = holder.getItemId();
final int viewHolderId = holder.getContainer().getId();
final Long boundItemId = itemForViewHolder(viewHolderId); // item currently bound to the VH
if (boundItemId != null && boundItemId != itemId) {
removeFragment(boundItemId);
mItemIdToViewHolder.remove(boundItemId);
}
mItemIdToViewHolder.put(itemId, viewHolderId); // this might overwrite an existing entry
// 内部会最终回调到createFragment用来创建当前Fragment
ensureFragment(position);
/** Special case when {@link RecyclerView} decides to keep the {@link container}
* attached to the window, but not to the view hierarchy (i.e. parent is null) */
final FrameLayout container = holder.getContainer();
if (ViewCompat.isAttachedToWindow(container)) {
if (container.getParent() != null) {
throw new IllegalStateException("Design assumption violated.");
}
container.addOnLayoutChangeListener(new View.OnLayoutChangeListener() {
@Override
public void onLayoutChange(View v, int left, int top, int right, int bottom,
int oldLeft, int oldTop, int oldRight, int oldBottom) {
if (container.getParent() != null) {
container.removeOnLayoutChangeListener(this);
placeFragmentInViewHolder(holder);
}
}
});
}
// 回收已经不在item集合中的Fragment，节省内存开销
gcFragments();
}
private void ensureFragment(int position) {
long itemId = getItemId(position);
if (!mFragments.containsKey(itemId)) {
// TODO(133419201): check if a Fragment provided here is a new Fragment
Fragment newFragment = createFragment(position);
newFragment.setInitialSavedState(mSavedStates.get(itemId));
mFragments.put(itemId, newFragment);
}
}
void gcFragments() {
if (!mHasStaleFragments || shouldDelayFragmentTransactions()) {
return;
}
// Remove Fragments for items that are no longer part of the data-set
Set<Long> toRemove = new ArraySet<>();
for (int ix = 0; ix < mFragments.size(); ix++) {
long itemId = mFragments.keyAt(ix);
if (!containsItem(itemId)) {
toRemove.add(itemId);
mItemIdToViewHolder.remove(itemId); // in case they're still bound
}
}
// Remove Fragments that are not bound anywhere -- pending a grace period
if (!mIsInGracePeriod) {
mHasStaleFragments = false; // we've executed all GC checks
for (int ix = 0; ix < mFragments.size(); ix++) {
long itemId = mFragments.keyAt(ix);
if (!isFragmentViewBound(itemId)) {
toRemove.add(itemId);
}
}
}
for (Long itemId : toRemove) {
removeFragment(itemId);
}
}
// onViewAttachToWindow的时候调用placeFragmentInViewHolder，
// 将FragmentViewHolder的container与当前Fragment绑定
@Override
public final void onViewAttachedToWindow(@NonNull final FragmentViewHolder holder) {
placeFragmentInViewHolder(holder);
gcFragments();
}
void placeFragmentInViewHolder(@NonNull final FragmentViewHolder holder) {
Fragment fragment = mFragments.get(holder.getItemId());
if (fragment == null) {
throw new IllegalStateException("Design assumption violated.");
}
FrameLayout container = holder.getContainer();
View view = fragment.getView();
if (!fragment.isAdded() && view != null) {
throw new IllegalStateException("Design assumption violated.");
}
// { f:added, v:notCreated, v:notAttached} -> schedule callback for when created
if (fragment.isAdded() && view == null) {
scheduleViewAttach(fragment, container);
return;
}
// { f:added, v:created, v:attached } -> check if attached to the right container
if (fragment.isAdded() && view.getParent() != null) {
if (view.getParent() != container) {
addViewToContainer(view, container);
}
return;
}
// { f:added, v:created, v:notAttached} -> attach view to container
if (fragment.isAdded()) {
addViewToContainer(view, container);
return;
}
// { f:notAdded, v:notCreated, v:notAttached } -> add, create, attach
if (!shouldDelayFragmentTransactions()) {
scheduleViewAttach(fragment, container);
mFragmentManager.beginTransaction()
.add(fragment, "f" + holder.getItemId())
.setMaxLifecycle(fragment, STARTED)
.commitNow();
mFragmentMaxLifecycleEnforcer.updateFragmentMaxLifecycle(false);
} else {
if (mFragmentManager.isDestroyed()) {
return; // nothing we can do
}
mLifecycle.addObserver(new LifecycleEventObserver() {
@Override
public void onStateChanged(@NonNull LifecycleOwner source,
@NonNull Lifecycle.Event event) {
if (shouldDelayFragmentTransactions()) {
return;
}
source.getLifecycle().removeObserver(this);
if (ViewCompat.isAttachedToWindow(holder.getContainer())) {
placeFragmentInViewHolder(holder);
}
}
});
}
}
}
public final class FragmentViewHolder extends ViewHolder {
private FragmentViewHolder(@NonNull FrameLayout container) {
super(container);
}
// FragmentViewHolder实际上就创建了一个FrameLayout
@NonNull static FragmentViewHolder create(@NonNull ViewGroup parent) {
FrameLayout container = new FrameLayout(parent.getContext());
container.setLayoutParams(
new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
ViewGroup.LayoutParams.MATCH_PARENT));
container.setId(ViewCompat.generateViewId());
container.setSaveEnabled(false);
return new FragmentViewHolder(container);
}
@NonNull FrameLayout getContainer() {
return (FrameLayout) itemView;
}
}
From the above source code analysis, we can know that although Fragmentit is not reused, the alternate display is achieved ViewHolderthrough reuse.containerFramgent
4 Summary
To summarize the contents of this chapter:
Fragment+ViewPagerIt can only scroll horizontally and its performance is relatively poor. Corresponding to differentAdaptereffects are different.Fragment+ViewPagerWhen usedFragmentPagerAdapter, the outside of the scopefragmentswill be saved in the memory (detach), butfragmentthe corresponding onesViewwill be destroyed, andfragmentsthe corresponding onesSavedStatewill be saved.FragmentPagerAdapterThe memory is larger but the page switching is more friendly, suitable forFragmentsmall quantities.Fragment+ViewPagerWhen usedFragmentStatePagerAdapter,fragmentsit will not be saved in memory (remove) outside the scope andViewwill also be destroyed.fragmentsOnly the corresponding ones within the range are savedSavedState. This is used for external parameters to be passed in the life cycle callback, andSavedStateis similar. The memory usage is small and the page switching is slightly poor. Suitable for large quantities.FragmentActivityFragmentFragment+ViewPager2It can scroll horizontally or vertically, and it inheritsRecyclerViewbasically all its advantages, including memory usage, cache management, etc.FragmentStateAdapterInherited fromRecyclerView.Adapter, althoughFragmentnot reused, the alternate display is implementedViewHolderthrough reuse.containerFramgent