Android開(kāi)發(fā)學(xué)習(xí)教程（27）- ViewPager+Fragment+FragmentPagerAdapter+Fragm
—— 我從未見(jiàn)過(guò)一個(gè)早起勤奮謹(jǐn)慎誠(chéng)實(shí)的人抱怨命運(yùn)不好，良好的品格，堅(jiān)強(qiáng)的意志，是不會(huì)被所謂的命運(yùn)擊敗的。
ViewPager是什么
ViewPager是一個(gè)布局管理器，用戶通過(guò)左右移動(dòng)來(lái)滑動(dòng)頁(yè)面，其中每個(gè)頁(yè)面使用Fragment而不是使用Activity。它還可以用在用戶首次啟動(dòng)應(yīng)用程序時(shí)的引導(dǎo)頁(yè)。
ViewPager的用法
實(shí)現(xiàn)viewpager的步驟：
1. 將 ViewPager 小部件添加到 XML 布局中。
2. 通過(guò)擴(kuò)展 FragmentPagerAdapter 或 FragmentStatePagerAdapter 類(lèi)來(lái)創(chuàng)建適配器。
適配器用來(lái)填充 Viewpager 內(nèi)的頁(yè)面。PagerAdapter 是由 FragmentPagerAdapter 和 FragmentStatePagerAdapter 擴(kuò)展的基類(lèi)，讓我們看一下這兩個(gè)類(lèi)之間的區(qū)別。
FragmentPagerAdapter 和 FragmentStatePagerAdapter 的區(qū)別：
（1）fragments對(duì)象的處理：FragmentPagerAdapter范圍外fragments會(huì)保存在內(nèi)存中(detach)，但是fragment對(duì)應(yīng)的View會(huì)被銷(xiāo)毀；FragmentStatePagerAdapter范圍外fragments不會(huì)保存在內(nèi)存中(remove)，View也會(huì)被銷(xiāo)毀。
（2）狀態(tài)的處理：FragmentPagerAdapter范圍外fragments對(duì)應(yīng)的SavedState會(huì)保存；FragmentStatePagerAdapter只保存范圍內(nèi)fragments對(duì)應(yīng)的SavedState。這個(gè)SavedState在Fragment的生命周期回調(diào)中供外部傳參數(shù)，和Activity類(lèi)似。
（3）適用場(chǎng)景：相同數(shù)量的fragments，F(xiàn)ragmentPagerAdapter內(nèi)存較大，但頁(yè)面切換更友好；FragmentStatePagerAdapter內(nèi)存占用少，頁(yè)面切換稍差。因此FragmentPagerAdapter適用于Fragment數(shù)量少的情況，F(xiàn)ragmentStatePagerAdapter適用于Fragment數(shù)量多的情況。
FragmentPagerAdapter的用法
第 1 步：將 ViewPager 小部件添加到 XML 布局中：
<?xml?version="1.0"?encoding="utf-8"?><LinearLayout?xmlns:android="http://schemas.android.com/apk/res/android"????xmlns:app="http://schemas.android.com/apk/res-auto"????android:layout_width="fill_parent"????android:layout_height="fill_parent"????android:orientation="vertical">????<com.google.android.material.tabs.TabLayout????????android:id="@+id/tab_layout"????????android:layout_width="match_parent"????????android:layout_height="wrap_content"????????app:tabGravity="fill"????????app:tabMode="fixed"?/>????<androidx.viewpager.widget.ViewPager????????android:id="@+id/viewpager"????????android:layout_width="match_parent"????????android:layout_height="match_parent"?/></LinearLayout>第 2 步：創(chuàng)建Fragment：
import?android.os.Bundle;import?android.view.LayoutInflater;import?android.view.View;import?android.view.ViewGroup;import?androidx.annotation.NonNull;import?androidx.annotation.Nullable;import?androidx.fragment.app.Fragment;public?class?Page1Fragment?extends?Fragment {????public?Page1Fragment() {????????// required empty public constructor.????}????@Override????public?void?onCreate(@Nullable?Bundle savedInstanceState) {????????super.onCreate(savedInstanceState);????}????@Nullable????@Override????public?View onCreateView(@NonNull?LayoutInflater inflater,?@Nullable?ViewGroup container,?@Nullable?Bundle savedInstanceState) {????????return?inflater.inflate(R.layout.fragment_page1, container,?false);????}}fragment_page1.xml文件：
<?xml?version="1.0"?encoding="utf-8"?><FrameLayout????xmlns:android="http://schemas.android.com/apk/res/android"????xmlns:tools="http://schemas.android.com/tools"????android:layout_width="match_parent"????android:layout_height="match_parent"????android:background="#0F9D58"????tools:context=".Page1Fragment">????
????<!-- TODO: Update blank fragment layout -->????<TextView????????android:layout_width="match_parent"????????android:layout_height="match_parent"????????android:gravity="center"????????android:text="Page 1"????????android:textColor="@color/white"????????android:textSize="60sp"????????android:textStyle="bold"?/></FrameLayout>和上面一樣再分別新建Page2Fragment、fragment_page2.xml、Page3Fragment、fragment_page3.xml
第 3 步：創(chuàng)建 ViewPager 適配器
??import?androidx.annotation.NonNull;??import?androidx.annotation.Nullable;??import?androidx.fragment.app.Fragment;??import?androidx.fragment.app.FragmentManager;??import?androidx.fragment.app.FragmentPagerAdapter;??import?java.util.ArrayList;??import?java.util.List;??public?class?ViewPagerAdapter?extends?FragmentPagerAdapter {??????private?final?List<fragment> fragments =?new?ArrayList<>();??????private?final?List<string> fragmentTitle =?new?ArrayList<>();??????public?ViewPagerAdapter(@NonNull?FragmentManager fm)??????{??????????super(fm);??????}??????public?void?add(Fragment fragment, String title)??????{??????????fragments.add(fragment);??????????fragmentTitle.add(title);??????}??????@NonNull?@Override?public?Fragment getItem(int?position)??????{??????????return?fragments.get(position);??????}??????@Override?public?int?getCount()??????{??????????return?fragments.size();??????}??????@Nullable??????@Override??????public?CharSequence getPageTitle(int?position)??????{??????????return?fragmentTitle.get(position);??????}??}</string></fragment>方法說(shuō)明：
getCount()：此方法返回要顯示的片段數(shù)。（需要覆蓋）getItem(int?pos)：返回 pos 索引處的片段。（需要覆蓋）ViewPagerAdapter(@NonNull?FragmentManager FM)：（必需）ViewPager Adapter 需要有一個(gè)接受 FragmentManager 實(shí)例的參數(shù)化構(gòu)造函數(shù)。它負(fù)責(zé)管理片段。FragmentManager 管理 Android 中的 Fragment，具體來(lái)說(shuō)，它處理 Fragment 之間的事務(wù)。事務(wù)是一種添加、替換或刪除片段的方法。getPageTitle(int?pos)：（可選）與 getItem() 類(lèi)似，此方法返回索引 pos 處的頁(yè)面標(biāo)題。add(Fragment fragment, String title)：此方法負(fù)責(zé)填充片段和片段標(biāo)題列表。分別持有片段和標(biāo)題。第 4 步：ViewPager設(shè)置適配器：
import?android.os.Bundle;import?androidx.appcompat.app.AppCompatActivity;import?androidx.viewpager.widget.ViewPager;import?com.google.android.material.tabs.TabLayout;public?class?MainActivity?extends?AppCompatActivity {????
????private?ViewPagerAdapter viewPagerAdapter;????private?ViewPager viewPager;????private?TabLayout tabLayout;????
????@Override????protected?void?onCreate(Bundle savedInstanceState) {????????super.onCreate(savedInstanceState);????????setContentView(R.layout.activity_main);????????viewPager = findViewById(R.id.viewpager);????????// 實(shí)例化適配器????????viewPagerAdapter =?new?ViewPagerAdapter(getSupportFragmentManager());????????viewPagerAdapter.add(new?Page1Fragment(),?"Page 1");????????viewPagerAdapter.add(new?Page2Fragment(),?"Page 2");????????viewPagerAdapter.add(new?Page3Fragment(),?"Page 3");????????// viewPager設(shè)置適配器????????viewPager.setAdapter(viewPagerAdapter);????????tabLayout = findViewById(R.id.tab_layout);????????// 使用setupWithiewPager方法將 TabLayout 鏈接到 Viewpager????????tabLayout.setupWithViewPager(viewPager);????}}FragmentStatePagerAdapter的用法
其它步驟和上面一樣，這里Fragment我們使用同一個(gè)，只是針對(duì)不同的tab設(shè)置不同的內(nèi)容：
import?android.os.Bundle;import?android.view.LayoutInflater;import?android.view.View;import?android.view.ViewGroup;import?android.widget.TextView;import?androidx.annotation.NonNull;import?androidx.annotation.Nullable;import?androidx.fragment.app.Fragment;public?class?DynamicFragment?extends?Fragment {????public?static?DynamicFragment newInstance(String arg) {????????Bundle bundle =?new?Bundle();????????bundle.putString("arg", arg);????????DynamicFragment fragment =?new?DynamicFragment();????????fragment.setArguments(bundle);????????return?fragment;????}????@Nullable????@Override????public?View onCreateView(@NonNull?LayoutInflater inflater,?@Nullable?ViewGroup container,?@Nullable?Bundle savedInstanceState) {????????return?inflater.inflate(R.layout.fragment_page, container,?false);????}????@Override????public?void?onViewCreated(@NonNull?View view,?@Nullable?Bundle savedInstanceState) {????????super.onViewCreated(view, savedInstanceState);????????((TextView) getView().findViewById(R.id.tv)).setText("Page"?+ getArguments().getString("arg"));????}}適配器部分和上面的ViewPagerAdapter內(nèi)容一樣，區(qū)別只是繼承的是 FragmentStatePagerAdapter
??import?java.util.ArrayList;??import?java.util.List;??import?androidx.annotation.Nullable;??import?androidx.fragment.app.Fragment;??import?androidx.fragment.app.FragmentManager;??import?androidx.fragment.app.FragmentStatePagerAdapter;??public?class?DynamicFragmentAdapter?extends?FragmentStatePagerAdapter {??????private?final?List<fragment> fragments =?new?ArrayList<>();??????private?final?List<string> fragmentTitle =?new?ArrayList<>();??????public?DynamicFragmentAdapter(FragmentManager fm) {??????????super(fm);??????}??????public?void?add(Fragment fragment, String title) {??????????fragments.add(fragment);??????????fragmentTitle.add(title);??????}??????// get the current item with position number??????@Override??????public?Fragment getItem(int?position) {??????????return?fragments.get(position);??????}??????// get total number of tabs??????@Override??????public?int?getCount() {??????????return?fragmentTitle.size();??????}??????@Nullable??????@Override??????public?CharSequence getPageTitle(int?position) {??????????return?fragmentTitle.get(position);??????}??}</string></fragment>MainActivity也只是實(shí)例化適配器時(shí)改變了下：
public?class?MainActivity2?extends?AppCompatActivity {????private?DynamicFragmentAdapter dynamicFragmentAdapter;????private?ViewPager viewPager;????private?TabLayout tabLayout;????@Override????protected?void?onCreate(Bundle savedInstanceState) {????????super.onCreate(savedInstanceState);????????setContentView(R.layout.activity_main1);????????viewPager = findViewById(R.id.viewpager);????????// 實(shí)例化適配器????????dynamicFragmentAdapter =?new?DynamicFragmentAdapter(getSupportFragmentManager());????????dynamicFragmentAdapter.add(DynamicFragment.newInstance("1"),?"PAGE 1");????????dynamicFragmentAdapter.add(DynamicFragment.newInstance("2"),?"PAGE 2");????????dynamicFragmentAdapter.add(DynamicFragment.newInstance("3"),?"PAGE 3");????????// viewPager設(shè)置適配器????????viewPager.setAdapter(dynamicFragmentAdapter);????????tabLayout = findViewById(R.id.tab_layout);????????// 使用setupWithiewPager方法將 TabLayout 鏈接到 Viewpager????????tabLayout.setupWithViewPager(viewPager);????}}FragmentPagerAdapter和FragmentStatePagerAdapter不同的根本原因
任何不同的根本原因都可以從源碼中找到，我們來(lái)看下從源碼的角度兩者有何不同
Ctrl+鼠標(biāo)點(diǎn)擊跳轉(zhuǎn)到源碼內(nèi)部，我們發(fā)現(xiàn)，兩者都是繼承自PagerAdapter，整體代碼非常簡(jiǎn)短，只有200行，里面有兩個(gè)方法instantiateItem和destroyItem
FragmentPagerAdapter instantiateItem方法：
@NonNull@Overridepublic?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);????????fragment.setUserVisibleHint(false);????}????return?fragment;}此方法用來(lái)為每一個(gè)頁(yè)面創(chuàng)建頁(yè)面(這里也就是Fragment了)，源碼其實(shí)很簡(jiǎn)單，先獲取FragmentManager事務(wù)對(duì)象，然后用findFragmentByTag查找Fragment，找到了就attach上去，沒(méi)找到就調(diào)用getItem方法獲取定義的Fragment然后add上去。
FragmentStatePagerAdapter instantiateItem方法：
@NonNull@Overridepublic?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.????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);????fragment.setUserVisibleHint(false);????mFragments.set(position, fragment);????mCurTransaction.add(container.getId(), fragment);????return?fragment;}此方法一樣用來(lái)為每一個(gè)頁(yè)面創(chuàng)建頁(yè)面，源碼同樣也很簡(jiǎn)單，先在mFragments集合中查找有無(wú)對(duì)應(yīng)頁(yè)面的Fragmet，剛開(kāi)始mFragments肯定是0，繼續(xù)就往下執(zhí)行，獲取FragmentManager事務(wù)對(duì)象，然后用調(diào)用getItem方法獲取定義的Fragment然后add上去。
到這里，首先就有個(gè)很明顯的區(qū)別，上面需要先經(jīng)過(guò)findFragmentByTag而這里是直接從mFragments查找有無(wú)對(duì)應(yīng)頁(yè)面的Fragmet，效率肯定快多了吧，至于為什么這里可以直接從mFragments查找，繼續(xù)往下看
FragmentPagerAdapter destroyItem方法：
@Overridepublic?void?destroyItem(@NonNull?ViewGroup container,?int?position,?@NonNull?Object object) {????if?(mCurTransaction ==?null) {????????mCurTransaction = mFragmentManager.beginTransaction();????}????if?(DEBUG) Log.v(TAG,?"Detaching item #"?+ getItemId(position) +?": f="?+ object????????????+?" v="?+ ((Fragment)object).getView());????mCurTransaction.detach((Fragment)object);}很簡(jiǎn)單，就是detach掉某個(gè)Fragment頁(yè)面
FragmentStatePagerAdapter destroyItem方法：
@Overridepublic?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);????mCurTransaction.remove(fragment);}這里是將mFragments對(duì)應(yīng)位置設(shè)為NULL，而且從FragmentManager中remove掉Fragment。這里也解釋了上面為什么可以直接從mFragments查找而不必再使用findFragmentByTag去查找Fragment了，因?yàn)檫@里將mFragments對(duì)應(yīng)位置設(shè)為NULL了。另一方面，這里直接從FragmentManager中remove掉Fragment了，所以FragmentManager在當(dāng)前內(nèi)存中不會(huì)緩存3個(gè)以上的Fragment(假設(shè)讀者使用默認(rèn)的mOffscreenPageLimit)，這也是為什么FragmentStatePagerAdapter適用于頁(yè)面數(shù)量多的情形了。
源碼鏈接：https://yunjunet.cn/876823.html
