ViewPager2和Fragment可见性及懒加载解决方案

ViewPager2和Fragment新的可见性及懒加载解决方案，旨在解决ViewPager（ViewPager2）只有当前Fragment唯一可见时候网络刷新或绘图。注意：过去的setUserVisibleHint()已被废弃，开发者不要再使用基于setUserVisibleHint()解决Fragment可见性和懒加载（或延迟加载）的场景。新方案用最新的ViewPager2替代过去旧版的ViewPager，再结合Fragment的resume和pause生命周期实现。

写一个demo例子，测试的主类MainActivity：

package zhangphil.app;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentActivity;
import androidx.viewpager2.adapter.FragmentStateAdapter;
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 {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final List<Fragment> fragments = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            Fragment testFrgment = new TestFrgment("卡" + i);
            fragments.add(testFrgment);
        }

        ViewPager2 vp = findViewById(R.id.vp2);
        vp.setOffscreenPageLimit(1);
        MyFragmentAdapter adapter = new MyFragmentAdapter(this, fragments);
        vp.setAdapter(adapter);

        TabLayout tabLayout = findViewById(R.id.tablayout);
        TabLayoutMediator tabLayoutMediator = new TabLayoutMediator(tabLayout, vp, new TabLayoutMediator.TabConfigurationStrategy() {
            @Override
            public void onConfigureTab(@NonNull TabLayout.Tab tab, int position) {
                TestFrgment f = (TestFrgment) fragments.get(position);
                tab.setText(f.getTabTag());
            }
        });
        tabLayoutMediator.attach();
    }

    private class MyFragmentAdapter extends FragmentStateAdapter {
        private List<Fragment> fragments;

        public MyFragmentAdapter(@NonNull FragmentActivity fragmentActivity, List<Fragment> list) {
            super(fragmentActivity);
            this.fragments = list;
        }

        @NonNull
        @Override
        public Fragment createFragment(int position) {
            return fragments.get(position);
        }

        @Override
        public int getItemCount() {
            return fragments.size();
        }
    }
}

测试的Fragment：

package zhangphil.app;

import android.os.Bundle;
import android.util.Log;
import android.util.TypedValue;
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.core.content.ContextCompat;
import androidx.fragment.app.Fragment;

public class TestFrgment extends Fragment {
    private boolean bUserVisible = false;

    private String tag = null;
    private String LOG = "生命周期->";

    public TestFrgment(String tag) {
        this.tag = tag;
    }

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(LOG + tag, "onCreate");
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        TextView text = new TextView(getContext());
        text.setText(tag);
        text.setTextSize(TypedValue.COMPLEX_UNIT_DIP, 80);
        text.setTextColor(ContextCompat.getColor(getContext(), android.R.color.holo_red_dark));
        return text;
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        Log.d(LOG + tag, "onViewCreated");
    }

    @Override
    public void onStart() {
        super.onStart();
        Log.d(LOG + tag, "onStart");
    }

    @Override
    public void onResume() {
        super.onResume();
        Log.d(LOG + tag, "onResume");

        //当前Fragment用户可见了
        //可以启动网络加载或绘图工作了。
        bUserVisible = true;
    }

    @Override
    public void onPause() {
        super.onPause();
        Log.d(LOG + tag, "onPause");

        // 当前Fragment用户不可见了，
        // 如果有网络数据加载或绘图工作，可以停止了。
        bUserVisible = false;
    }

    @Override
    public void onStop() {
        super.onStop();
        Log.d(LOG + tag, "onStop");
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.d(LOG + tag, "onDestroy");
    }

    public String getTabTag() {
        return tag;
    }

    /**
     * 当前Fragment是否可见（用户在手机屏幕看得见？）。
     *
     * @return
     */
    public boolean isCurFragmentVisible() {
        return bUserVisible;
    }
}

xml布局文件：

<?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:orientation="vertical">

    <androidx.viewpager2.widget.ViewPager2
        android:id="@+id/vp2"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal" />

    <com.google.android.material.tabs.TabLayout
        android:id="@+id/tablayout"
        android:layout_width="match_parent"
        android:layout_height="50dp" />

</LinearLayout>

当初次启动app：

2020-10-01 09:32:52.843 5123-5123/zhangphil.app D/生命周期->卡0: onCreate
2020-10-01 09:32:52.846 5123-5123/zhangphil.app D/生命周期->卡0: onViewCreated
2020-10-01 09:32:52.846 5123-5123/zhangphil.app D/生命周期->卡0: onStart
2020-10-01 09:32:52.846 5123-5123/zhangphil.app D/生命周期->卡0: onResume
2020-10-01 09:32:52.850 5123-5123/zhangphil.app D/生命周期->卡1: onCreate
2020-10-01 09:32:52.851 5123-5123/zhangphil.app D/生命周期->卡1: onViewCreated
2020-10-01 09:32:52.852 5123-5123/zhangphil.app D/生命周期->卡1: onStart

当按home键使得整个app退回后台，消失在屏幕：

2020-10-01 09:33:55.724 5123-5123/zhangphil.app D/生命周期->卡0: onPause
2020-10-01 09:33:56.295 5123-5123/zhangphil.app D/生命周期->卡0: onStop
2020-10-01 09:33:56.295 5123-5123/zhangphil.app D/生命周期->卡1: onStop

当重新调起app：

2020-10-01 09:34:48.502 5123-5123/zhangphil.app D/生命周期->卡0: onStart
2020-10-01 09:34:48.502 5123-5123/zhangphil.app D/生命周期->卡1: onStart
2020-10-01 09:34:48.504 5123-5123/zhangphil.app D/生命周期->卡0: onResume

当切换到Fragment1( 即卡1 ):

2020-10-01 09:35:44.849 5123-5123/zhangphil.app D/生命周期->卡2: onCreate
2020-10-01 09:35:44.852 5123-5123/zhangphil.app D/生命周期->卡2: onViewCreated
2020-10-01 09:35:44.852 5123-5123/zhangphil.app D/生命周期->卡2: onStart
2020-10-01 09:35:45.232 5123-5123/zhangphil.app D/生命周期->卡0: onPause
2020-10-01 09:35:45.232 5123-5123/zhangphil.app D/生命周期->卡1: onResume

当切换回卡0（即Fragment0）:

2020-10-01 09:36:43.224 5123-5123/zhangphil.app D/生命周期->卡1: onPause
2020-10-01 09:36:43.224 5123-5123/zhangphil.app D/生命周期->卡0: onResume

来源：oschina

链接：https://my.oschina.net/u/4418236/blog/4658243

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