Android性能優化之ViewPagers + Fragment緩存優化
前言
大傢看標題,可能會有點兒懵,什麼是ViewPagers,因為在很久之前,我們使用的都是ViewPager,但是現在更多的是在用ViewPager2,因此用ViewPagers(ViewPager、ViewPager2)來代替兩者,主要介紹兩者的區別。
ViewPagers嵌套Fragment架構,在我們常用的App中隨處可見,抖音的首頁、各大電商app首頁(淘寶、京東、拼多多)等,通過左右滑動切換Tab;但因為ViewPager的預加載機制存在,
我們先看下ViewPager的源碼:
public void setOffscreenPageLimit(int limit) { if (limit < DEFAULT_OFFSCREEN_PAGES) { Log.w(TAG, "Requested offscreen page limit " + limit + " too small; defaulting to " + DEFAULT_OFFSCREEN_PAGES); limit = DEFAULT_OFFSCREEN_PAGES; } if (limit != mOffscreenPageLimit) { mOffscreenPageLimit = limit; populate(); } }
當我們設置offscreenPageLimit(離屏加載)的數值時,我們可以看到limit的值是有限制,不能小於DEFAULT_OFFSCREEN_PAGES
private static final int DEFAULT_OFFSCREEN_PAGES = 1;
那麼就意味著ViewPager默認支持預加載,我們看下面這張圖
如果紅色區域默認為首頁,根據ViewPager默認預加載的閾值,那麼左右兩邊的頁面同樣也會被加載,如果有網絡請求,也就是說,我們沒有打開左邊的頁面,它已經默認進行瞭網絡請求,這種體驗是非常差的,因為會在暗地裡消耗流量。
理想情況下,我們需要的是打開某個頁面的時候才去加載,這裡就需要通過懶加載的方式優化。
1 ViewPager懶加載優化
1.1 ViewPager的緩存機制
很多時候,我們在使用Fragment的時候,發現打開過的頁面再回來,頁面沒有重建刷新,很多人覺得是Fragment是有緩存的,其實並不是Fragment有緩存,而是ViewPager具備緩存能力;
如果有小夥伴使用過單Activity + 多Fragment架構的時候就會發現,打開過的頁面再次返回的時候,Fragment會被重建,所以兩種架構都有利弊,關鍵看我們怎麼選擇,下面我們看下ViewPager的緩存機制。
public void setAdapter(@Nullable PagerAdapter adapter) { if (mAdapter != null) { ① mAdapter.setViewPagerObserver(null); mAdapter.startUpdate(this); for (int i = 0; i < mItems.size(); i++) { final ItemInfo ii = mItems.get(i); mAdapter.destroyItem(this, ii.position, ii.object); } mAdapter.finishUpdate(this); mItems.clear(); removeNonDecorViews(); mCurItem = 0; scrollTo(0, 0); } ② final PagerAdapter oldAdapter = mAdapter; mAdapter = adapter; mExpectedAdapterCount = 0; ③ if (mAdapter != null) { if (mObserver == null) { mObserver = new PagerObserver(); } mAdapter.setViewPagerObserver(mObserver); mPopulatePending = false; final boolean wasFirstLayout = mFirstLayout; mFirstLayout = true; mExpectedAdapterCount = mAdapter.getCount(); if (mRestoredCurItem >= 0) { mAdapter.restoreState(mRestoredAdapterState, mRestoredClassLoader); setCurrentItemInternal(mRestoredCurItem, false, true); mRestoredCurItem = -1; mRestoredAdapterState = null; mRestoredClassLoader = null; } else if (!wasFirstLayout) { ④ populate(); } else { ⑤ requestLayout(); } } // Dispatch the change to any listeners if (mAdapterChangeListeners != null && !mAdapterChangeListeners.isEmpty()) { for (int i = 0, count = mAdapterChangeListeners.size(); i < count; i++) { mAdapterChangeListeners.get(i).onAdapterChanged(this, oldAdapter, adapter); } } }
核心方法就是setAdapter,像RecyclerView一樣,因為會有緩存,所以當頁面滑動的時候,如果緩存中存在頁面,那麼就會從緩存中取,如果沒有,就需要去創建新的頁面,所以我們先來關註一下PagerAdapter
public abstract class PagerAdapter { private final DataSetObservable mObservable = new DataSetObservable(); private DataSetObserver mViewPagerObserver; public static final int POSITION_UNCHANGED = -1; public static final int POSITION_NONE = -2; public abstract int getCount(); //開始更新 public void startUpdate(@NonNull ViewGroup container) { startUpdate((View) container); } //初始化頁面 @NonNull public Object instantiateItem(@NonNull ViewGroup container, int position) { return instantiateItem((View) container, position); } //銷毀頁面 public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) { destroyItem((View) container, position, object); } //結束刷新 public void finishUpdate(@NonNull ViewGroup container) { finishUpdate((View) container); } }
PagerAdapter是一個抽象類,那麼這些方法肯定是具體實現類實現,如果我們在使用ViewPager嵌套Fragment的時候,使用的是FragmentPageAdapter
接著回到setAdapter方法中:
- ①:有一個全局變量 mAdapter,如果是第一個加載進來,那麼mAdapter是空的,走到②
- ②:這裡就是將我們傳入的adapter給mAdapter賦值
- ③:這個時候mAdapter不為空,這裡需要關註幾個參數:
wasFirstLayout = true mRestoredCurItem = -1
所以這裡直接走到⑤,調用requestLayout方法,會執行到onMeasure,在這個方法中,會執行populate方法(這個大傢自己去爬樓)
populate幹瞭什麼呢?代碼太多瞭就不貼出來瞭,直接上圖:
如果是默認緩存(mOffscreenPageLimit = 1),那麼在mItems就會緩存3個Fragment
private final ArrayList<ItemInfo> mItems = new ArrayList<ItemInfo>();
當頁面滑動時,page2成為瞭當前頁,那麼ViewPager的populate做瞭什麼操作呢?
- (1)首先page3會被預加載,這個時候調用瞭PagerAdapter的instantiateItem方法新建頁面,並放在mItems集合中,並且設置為不可見的狀態(setUserVisibleHint(false)),所有緩存中不可見的頁面同理(2)page1就會從緩存中移除,調用瞭PagerAdapter的destroyItem方法,curPage會成為mItems中第一個緩存對象;
- (3)將page2設置為當前展示的Fragment
因此populate幹的主要工作就是,隨著頁面的滑動,將Page從緩存中移除銷毀,或者將新頁面新建加入緩存中。
1.2 ViewPager懶加載方案
如上所述,ViewPager默認就是開啟預加載的,而且默認最多能夠緩存3個Fragment頁面,那麼為瞭避免流量的消耗,需要我們針對預加載這種情況進行頁面懶加載,隻有當頁面可見的時候,才能加載數據。
class MainLazyLoadAdapter( fragmentManager: FragmentManager, val fragments:MutableList<Fragment> ) : FragmentPagerAdapter(fragmentManager) { override fun getCount(): Int { return fragments.size } override fun getItem(position: Int): Fragment { return fragments[position] } }
class LazyFragment(val index:Int) : Fragment() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) Log.e("TAG","LazyFragment $index onCreate") } override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { Log.e("TAG","LazyFragment $index onCreateView") return inflater.inflate(R.layout.fragment_lazy, container, false) } }
val fragments = mutableListOf<Fragment>() for (index in 0..5) { fragments.add(LazyFragment(index)) } vp_lazy_load.adapter = MainLazyLoadAdapter(supportFragmentManager, fragments)
首先我們先看默認預加載狀態,驗證之前源碼中的原理:
//第一次進來 2022-08-28 13:41:15.759 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 13:41:15.760 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 13:41:15.783 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 13:41:15.784 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreateView
我們看到第一次進來,第二個Fragment被加載進來,然後右滑,第三個Fragment被加載
2022-08-28 13:41:15.759 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 13:41:15.760 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 13:41:15.783 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 13:41:15.784 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreateView 2022-08-28 13:48:45.248 12677-12677/com.lay.image_process E/TAG: LazyFragment 2 onCreate 2022-08-28 13:48:45.250 12677-12677/com.lay.image_process E/TAG: LazyFragment 2 onCreateView
當我們滑到第4個Fragment的時候,左滑回到第3個Fragment,發現並沒有重建是因為緩存的原因,因為滑到第4個Fragment的時候,第2個Fragment已經被銷毀瞭,再回到第3個Fragment的時候,第2個Fragment被重建,走瞭onCreateView方法
2022-08-28 13:41:15.759 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 13:41:15.760 12677-12677/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 13:41:15.783 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 13:41:15.784 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreateView 2022-08-28 13:48:45.248 12677-12677/com.lay.image_process E/TAG: LazyFragment 2 onCreate 2022-08-28 13:48:45.250 12677-12677/com.lay.image_process E/TAG: LazyFragment 2 onCreateView 2022-08-28 13:50:00.439 12677-12677/com.lay.image_process E/TAG: LazyFragment 3 onCreate 2022-08-28 13:50:00.440 12677-12677/com.lay.image_process E/TAG: LazyFragment 3 onCreateView 2022-08-28 13:50:01.344 12677-12677/com.lay.image_process E/TAG: LazyFragment 4 onCreate 2022-08-28 13:50:01.345 12677-12677/com.lay.image_process E/TAG: LazyFragment 4 onCreateView 2022-08-28 13:50:03.315 12677-12677/com.lay.image_process E/TAG: LazyFragment 1 onCreateView
首先我們先看下,Adapter重建Fragment的時候的核心代碼
public Object instantiateItem(@NonNull ViewGroup container, int position) { if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } final long itemId = getItemId(position); // Do we already have this fragment? String name = makeFragmentName(container.getId(), itemId); Fragment fragment = mFragmentManager.findFragmentByTag(name); if (fragment != null) { if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment); mCurTransaction.attach(fragment); } else { fragment = getItem(position); if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment); 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 { //關鍵代碼 fragment.setUserVisibleHint(false); } } return fragment; }
我們可以看到,當前Fragment如果被創建但是沒有在當前頁面展示的時候,調用瞭fragment.setUserVisibleHint(false),也就是說setUserVisibleHint能夠監聽當前Fragment是否可見
所以我們對Fragment進行改造:
class LazyFragment(val index:Int) : Fragment() { //判斷當前頁面是否可見 private var isShow = false //判斷頁面是否創建成功 private var isViewCreated = false override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) Log.e("TAG","LazyFragment $index onCreate") } override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { Log.e("TAG","LazyFragment $index onCreateView") return inflater.inflate(R.layout.fragment_lazy, container, false) } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) isViewCreated = true lazyLoad() } override fun setUserVisibleHint(isVisibleToUser: Boolean) { super.setUserVisibleHint(isVisibleToUser) Log.e("TAG","LazyFragment $index isVisibleToUser $isVisibleToUser") if(isVisibleToUser){ isShow = true //才有資格去懶加載 lazyLoad() }else{ isShow = false } } private fun lazyLoad() { if(isViewCreated && isShow){ Log.e("TAG","Fragment $index loadData") } } override fun onDestroy() { super.onDestroy() isViewCreated = false isShow = false } }
如果按照之前的方式,當調用onViewCreated方法的時候,我們就會加載數據;做瞭懶加載處理之後,重寫瞭setUserVisibleHint方法,當前頁面可見的時候,才有資格去加載數據,這樣即便創建瞭Fragment,但是如果不可見就不會加載數據
2022-08-28 14:06:29.776 25904-25904/com.lay.image_process E/TAG: LazyFragment 0 isVisibleToUser false 2022-08-28 14:06:29.776 25904-25904/com.lay.image_process E/TAG: LazyFragment 1 isVisibleToUser false 2022-08-28 14:06:29.776 25904-25904/com.lay.image_process E/TAG: LazyFragment 0 isVisibleToUser true 2022-08-28 14:06:29.782 25904-25904/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 14:06:29.783 25904-25904/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 14:06:29.796 25904-25904/com.lay.image_process E/TAG: Fragment 0 loadData 2022-08-28 14:06:29.805 25904-25904/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 14:06:29.805 25904-25904/com.lay.image_process E/TAG: LazyFragment 1 onCreateView 2022-08-28 14:06:59.395 25904-25904/com.lay.image_process E/TAG: LazyFragment 2 isVisibleToUser false 2022-08-28 14:06:59.396 25904-25904/com.lay.image_process E/TAG: LazyFragment 0 isVisibleToUser false 2022-08-28 14:06:59.396 25904-25904/com.lay.image_process E/TAG: LazyFragment 1 isVisibleToUser true 2022-08-28 14:06:59.396 25904-25904/com.lay.image_process E/TAG: Fragment 1 loadData 2022-08-28 14:06:59.399 25904-25904/com.lay.image_process E/TAG: LazyFragment 2 onCreate 2022-08-28 14:06:59.400 25904-25904/com.lay.image_process E/TAG: LazyFragment 2 onCreateView
通過日志我們可以看到,當首次進入的時候,雖然Fragment 1 被創建瞭,但是並沒有加載數據。
這裡有個問題,既然可見之後就能加載數據,那麼我隻在setUserVisibleHint的時候,判斷是否可見來去加載數據?
其實是不可以的,通過日志我們能夠發現,setUserVisibleHint是早於onCreate方法調用的,也就是說在頁面還沒有創建時,去加載數據有可能導致頁面元素找不到發生空指針異常。
2 ViewPager2與ViewPager的區別
上一小節,我們介紹瞭ViewPager的加載機制和緩存機制,那麼我們把整套頁面搬過來,唯一發生變化的就是將ViewPager轉換為ViewPager2
class MainLazyLoadAdapter2( activity: FragmentActivity, val fragments: MutableList<Fragment> ) : FragmentStateAdapter(activity) { override fun getItemCount(): Int { return fragments.size } override fun createFragment(position: Int): Fragment { return fragments[position] } }
ViewPager2的適配器使用的是FragmentStateAdapter,因為FragmentStateAdapter繼承瞭RecyclerView.Adapter,因此支持瞭橫向滑動和豎向滑動
val fragments = mutableListOf<Fragment>() for (index in 0..5) { fragments.add(LazyFragment(index)) } vp_lazy_load = findViewById(R.id.vp_lazy_load) vp_lazy_load.adapter = MainLazyLoadAdapter2(this, fragments)
用同樣的方式設置瞭適配器,我們看下日志輸出,就會發現,咦?怎麼跟ViewPager不一樣瞭
2022-08-28 14:47:11.790 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 14:47:11.792 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreateView
剛進來的時候,隻有Fragment 1 加載瞭頁面,並沒有新建緩存頁面,當我滑動到下一頁的時候,也隻有下一頁的頁面進行瞭重建
2022-08-28 14:47:11.790 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 14:47:11.792 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 14:47:13.948 15514-15514/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 14:47:13.948 15514-15514/com.lay.image_process E/TAG: LazyFragment 1 onCreateView
ViewPager2沒有預加載機制嗎?這裡我們就需要看源碼瞭,直接奔向setOffscreenPageLimit方法,我們看到跟ViewPager的setOffscreenPageLimit方法是不一樣的
public void setOffscreenPageLimit(@OffscreenPageLimit int limit) { if (limit < 1 && limit != OFFSCREEN_PAGE_LIMIT_DEFAULT) { throw new IllegalArgumentException( "Offscreen page limit must be OFFSCREEN_PAGE_LIMIT_DEFAULT or a number > 0"); } mOffscreenPageLimit = limit; // Trigger layout so prefetch happens through getExtraLayoutSize() mRecyclerView.requestLayout(); }
public static final int OFFSCREEN_PAGE_LIMIT_DEFAULT = -1;
這裡的判斷條件 limit < 1 && limit != OFFSCREEN_PAGE_LIMIT_DEFAULT,有一個數值能夠通過,就是-1,這就意味著,ViewPager2默認是不支持預加載的
但是ViewPager2的緩存策略還是存在,因為繼承瞭RecyclerView的Adapter,所以緩存復用機制是跟RecyclerView一致的,默認mViewCaches緩存池的大小是3
2022-08-28 15:30:00.579 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 15:30:00.579 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreateView 2022-08-28 15:30:03.883 15514-15514/com.lay.image_process E/TAG: LazyFragment 1 onCreate 2022-08-28 15:30:03.884 15514-15514/com.lay.image_process E/TAG: LazyFragment 1 onCreateView 2022-08-28 15:30:05.064 15514-15514/com.lay.image_process E/TAG: LazyFragment 2 onCreate 2022-08-28 15:30:05.064 15514-15514/com.lay.image_process E/TAG: LazyFragment 2 onCreateView 2022-08-28 15:30:08.997 15514-15514/com.lay.image_process E/TAG: LazyFragment 3 onCreate 2022-08-28 15:30:08.997 15514-15514/com.lay.image_process E/TAG: LazyFragment 3 onCreateView 2022-08-28 15:30:20.005 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreate 2022-08-28 15:30:20.005 15514-15514/com.lay.image_process E/TAG: LazyFragment 0 onCreateView
當我們滑動到第4個Fragment的時候,註意這裡跟ViewPager不一樣的是,ViewPager的緩存是緩存當前頁的左右兩邊,但是ViewPager2就是RecyclerView的緩存機制,順序緩存;
當滑動到第4個Fragment的時候,因為緩存池大小為3,因此LazyFragment 0 就會從緩存池中移除,當再次滑動到LazyFragment 0的時候,就會重建!
所以當我們還在思考如何針對ViewPager的預加載機制做懶加載操作時,請將項目中的ViewPager遷移至ViewPager2
附錄:
當你的項目中還在使用ViewPager時,建議使用當前這個懶加載框架
abstract class BaseLazyFragment<VM : ViewModel, VB : ViewBinding> : Fragment() { private lateinit var viewModel: VM private lateinit var binding: VB private var isShow = false private var isViewCreated = false override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) viewModel = getViewModelInstance() binding = getLayoutInflate(layoutInflater) } override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { return binding.root } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) initView() isViewCreated = true lazyLoad() } override fun setUserVisibleHint(isVisibleToUser: Boolean) { super.setUserVisibleHint(isVisibleToUser) if (isVisibleToUser) { isShow = true lazyLoad() } else { isShow = false } } override fun onDestroy() { super.onDestroy() isShow = false isViewCreated = false } private fun lazyLoad() { if (isShow && isViewCreated) { initData() } } open fun initData() {} open fun initView() {} abstract fun getViewModelInstance(): VM abstract fun getLayoutInflate(layoutInflater: LayoutInflater): VB }
使用方式:
class LazyFragment(val index:Int) : BaseLazyFragment<MainVM,FragmentLazy2Binding>() { override fun initData() { super.initData() Log.e("TAG","LazyFragment $index initData -- ") } override fun getViewModelInstance(): MainVM { return MainVM() } override fun getLayoutInflate(layoutInflater: LayoutInflater): FragmentLazy2Binding { return FragmentLazy2Binding.inflate(layoutInflater) } }
到此這篇關於Android性能優化之ViewPagers + Fragment緩存優化的文章就介紹到這瞭,更多相關Android 緩存優化內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet!
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