ViewPager2和Fragment可见性及懒加载解决方案

我不是女神ヾ 2022-12-11 13:33 675阅读 0赞

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