ViewPager2 使用

灰太狼 2024-02-19 11:01 36阅读 0赞

## 一、什么是ViewPager？ ##

布局管理器允许左右翻转带数据的页面，你想要显示的视图可以通过实现PagerAdapter来显示。这个类其实是在早期设计和开发的，它的API在后面的更新之中可能会被改变，当它们在新版本之中编译的时候可能还会改变源码。

ViewPager经常用来连接Fragment，它很方便管理每个页面的生命周期，使用ViewPager管理Fragment是标准的适配器实现。最常用的实现一般有FragmentPagerAdapter和FragmentStatePagerAdapter。

ViewPager是android扩展包v4包中的类，这个类可以让我们左右切换当前的view。我们先来聊聊ViewPager的几个相关知识点：

*  1、ViewPager类直接继承了ViewGroup类，因此它一个容器类，可以添加其他的view类
 *  2、ViewPager类需要一个PagerAdapter适配器类给它提供数据（这点跟ListView一样需要数据适配器Adater）
 *  3、ViewPager经常和Fragment一起使用，并且官方还提供了专门的FragmentPagerAdapterFragmentStatePagerAdapter类供Fragment中的ViewPager使用

## 二、什么是ViewPager2 ##

原理: viewpager2 内部实现原理是使用recycleview加LinearLayoutManager实现竖直滚动,其实可以理解为对recyclerview的二次封装

private void initialize(Context context, AttributeSet attrs) {
        mAccessibilityProvider = sFeatureEnhancedA11yEnabled
        ? new PageAwareAccessibilityProvider()
        : new BasicAccessibilityProvider();
    
        // 实例化recycleview对象
        mRecyclerView = new RecyclerViewImpl(context);
        mRecyclerView.setId(ViewCompat.generateViewId());
        mRecyclerView.setDescendantFocusability(FOCUS_BEFORE_DESCENDANTS);
    
        // 实例化LinearLayoutManager对象
        mLayoutManager = new LinearLayoutManagerImpl(context);
        mRecyclerView.setLayoutManager(mLayoutManager);
        mRecyclerView.setScrollingTouchSlop(RecyclerView.TOUCH_SLOP_PAGING);
        setOrientation(context, attrs);
    
        mRecyclerView.setLayoutParams(
            new ViewGroup.LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
        mRecyclerView.addOnChildAttachStateChangeListener(enforceChildFillListener());
    
        // Create ScrollEventAdapter before attaching PagerSnapHelper to RecyclerView, because the
        // attach process calls PagerSnapHelperImpl.findSnapView, which uses the mScrollEventAdapter
        mScrollEventAdapter = new ScrollEventAdapter(this);
        // Create FakeDrag before attaching PagerSnapHelper, same reason as above
        mFakeDragger = new FakeDrag(this, mScrollEventAdapter, mRecyclerView);
        mPagerSnapHelper = new PagerSnapHelperImpl();
        mPagerSnapHelper.attachToRecyclerView(mRecyclerView);
        // Add mScrollEventAdapter after attaching mPagerSnapHelper to mRecyclerView, because we
        // don't want to respond on the events sent out during the attach process
        mRecyclerView.addOnScrollListener(mScrollEventAdapter);
    
        mPageChangeEventDispatcher = new CompositeOnPageChangeCallback(3);
        mScrollEventAdapter.setOnPageChangeCallback(mPageChangeEventDispatcher);
    
        // Callback that updates mCurrentItem after swipes. Also triggered in other cases, but in
        // all those cases mCurrentItem will only be overwritten with the same value.
        final OnPageChangeCallback currentItemUpdater = new OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                if (mCurrentItem != position) {
                    mCurrentItem = position;
                    mAccessibilityProvider.onSetNewCurrentItem();
                }
            }
    
            @Override
            public void onPageScrollStateChanged(int newState) {
                if (newState == SCROLL_STATE_IDLE) {
                    updateCurrentItem();
                }
            }
        };
    
        // Prevents focus from remaining on a no-longer visible page
        final OnPageChangeCallback focusClearer = new OnPageChangeCallback() {
            @Override
            public void onPageSelected(int position) {
                clearFocus();
                if (hasFocus()) { // if clear focus did not succeed
                    mRecyclerView.requestFocus(View.FOCUS_FORWARD);
                }
            }
        };
    
        // Add currentItemUpdater before mExternalPageChangeCallbacks, because we need to update
        // internal state first
        mPageChangeEventDispatcher.addOnPageChangeCallback(currentItemUpdater);
        mPageChangeEventDispatcher.addOnPageChangeCallback(focusClearer);
        // Allow a11y to register its listeners after currentItemUpdater (so it has the
        // right data). TODO: replace ordering comments with a test.
        mAccessibilityProvider.onInitialize(mPageChangeEventDispatcher, mRecyclerView);
        mPageChangeEventDispatcher.addOnPageChangeCallback(mExternalPageChangeCallbacks);
    
        // Add mPageTransformerAdapter after mExternalPageChangeCallbacks, because page transform
        // events must be fired after scroll events
        mPageTransformerAdapter = new PageTransformerAdapter(mLayoutManager);
        mPageChangeEventDispatcher.addOnPageChangeCallback(mPageTransformerAdapter);
    
        attachViewToParent(mRecyclerView, 0, mRecyclerView.getLayoutParams());
    }

ViewPager2的出现是为了替代ViewPager，它有以下几个优势：

*  支持RTL布局,
 *  支持竖向滚动
 *  支持notifyDataSetChanged

RTL布局是Right To Left布局也就是从右往左的布局，大家知道我们平常写的布局都是从左往右，但是如果你适配阿拉伯语等环境的UI布局，他们的写法是从右往左的，具体这里不做研究。

API的变动：

FragmentStateAdapter替换了原来的 FragmentStatePagerAdapter

RecyclerView.Adapter替换了原来的 PagerAdapter

registerOnPageChangeCallback替换了原来的 addPageChangeListener

FragmentStateAdapter和FragmentStatePagerAdapter作用相同, 可以用viewpager来管理fragment, 区别在于viewpager2的FragmentStateAdapter与recycleview的生命周期绑定

另外viewpager2的Adapter是继承自recyclerview的adapter, 支持除了notifyDataSetChanged()以外的notifyItemChanged(int position)等方式, 使用上更加的便捷

## 三、ViewPager2 结合 Fragment实际demo ##

在app模块的build.gradle里要加上这两个依赖

implementation "androidx.viewpager2:viewpager2:1.1.0-beta02"
    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

myfragment.xml

一个显示在屏幕正中央的文字mode1

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#FF9800"
        android:orientation="vertical"
        android:gravity="center">
    
        <TextView
            android:id="@+id/textView1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="mode1"
            android:textSize="30sp"/>
    </LinearLayout>

myfragment2.xml

一个显示在屏幕正中央的文字mode2

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#FF0000"
        android:orientation="vertical"
        android:gravity="center">
    
        <TextView
            android:id="@+id/textView1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_gravity="center_horizontal"
            android:text="mode2"
            android:textSize="30sp"/>
    </LinearLayout>

activity\_main.xml

一个TabLayout和一个ViewPager2组件

<?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"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        tools:context=".MainActivity">
        <com.google.android.material.tabs.TabLayout
            android:id="@+id/my_tab"
            android:layout_width="match_parent"
            android:layout_height="60dp"/>
        <androidx.viewpager2.widget.ViewPager2
            android:id="@+id/my_pager2"
            android:layout_width="match_parent"
            android:layout_height="match_parent"/>
    </LinearLayout>

MyAdapter.java

重写MyAdapter、createFragment、getItemCount三个方法

package com.example.viewpager2;
    
    import androidx.annotation.NonNull;
    import androidx.fragment.app.Fragment;
    import androidx.fragment.app.FragmentManager;
    import androidx.viewpager2.adapter.FragmentStateAdapter;
    import androidx.lifecycle.Lifecycle;
    import java.util.List;
    
    public class MyAdapter extends FragmentStateAdapter {
    
        List<Fragment> fragments;
    
        public MyAdapter(@NonNull FragmentManager fragmentManager, @NonNull Lifecycle lifecycle, List<Fragment> fragments) {
            super(fragmentManager, lifecycle);
            this.fragments = fragments;
        }
    
        @NonNull
        @Override
        public Fragment createFragment(int position) {
            return fragments.get(position);
        }
    
        @Override
        public int getItemCount() {
            return fragments.size();
        }
    }

MyFragment.java

实现第一个fragment页面

package com.example.viewpager2;
    
    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 MyFragment extends Fragment {
        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            return inflater.inflate(R.layout.myfragment, null);
        }
    }

MySecondFragment.java

实现第二个fragment页面

package com.example.viewpager2;
    
    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 MySecondFragment extends Fragment {
    
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            return inflater.inflate(R.layout.myfragment2, null);
        }
    
    }

MainActivity.java

进来的activity

package com.example.viewpager2;
    
    import androidx.annotation.NonNull;
    import androidx.appcompat.app.AppCompatActivity;
    import androidx.fragment.app.Fragment;
    import androidx.viewpager2.widget.ViewPager2;
    
    import android.os.Bundle;
    
    import com.google.android.material.tabs.TabLayout;
    import com.google.android.material.tabs.TabLayoutMediator;
    
    import java.util.ArrayList;
    import java.util.List;
    
    public class MainActivity extends AppCompatActivity {
    
        private TabLayout myTab;
        private ViewPager2 myPager2;
    
        List<String> titles=new ArrayList<>();
        List<Fragment> fragments=new ArrayList<>();
        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            myTab = findViewById(R.id.my_tab);
            myPager2 = findViewById(R.id.my_pager2);
    
            //添加标题
            titles.add("1");
            titles.add("2");
    
            //添加Fragment进去
            fragments.add(new MyFragment());
            fragments.add(new MySecondFragment());
    
            //实例化适配器
            MyAdapter myAdapter=new MyAdapter(getSupportFragmentManager(),getLifecycle(),fragments);
            //设置适配器
            myPager2.setAdapter(myAdapter);
            //TabLayout和Viewpager2进行关联
            new TabLayoutMediator(myTab, myPager2, new TabLayoutMediator.TabConfigurationStrategy() {
                public void onConfigureTab(@NonNull TabLayout.Tab tab, int position) {
                    tab.setText(titles.get(position));
                }
            }).attach();
    
        }
    }

ViewPager + Fragment 实现tab页切换不同页面内容的效果

![2af9f748f92c56564ed356947e832865.gif][]

参考：[【精选】Android：ViewPager \\ViewPager2 简单介绍 & 使用方法解析\_viewpager2与viewpager-CSDN博客][Android_ViewPager _ViewPager2 _ _ _viewpager2_viewpager-CSDN]

[2af9f748f92c56564ed356947e832865.gif]: https://image.dandelioncloud.cn/pgy_files/images/2024/02/19/4bc12b1252504b3c8614930b9fcfaf74.png
[Android_ViewPager _ViewPager2 _ _ _viewpager2_viewpager-CSDN]: https://blog.csdn.net/JMW1407/article/details/114273649