Android ViewPager 懒加载的简易实现方式

懒加载的Fragment

package com.luwei.constraintlayoutdemo;

import android.support.v4.app.Fragment;

/**
 * Created by Ly on 2017/7/26.
 */

public abstract class LazyFragment extends Fragment {
    protected boolean isVisible;

    @Override
    public void setUserVisibleHint(boolean isVisibleToUser) {
        super.setUserVisibleHint(isVisibleToUser);
        if (getUserVisibleHint()){
            isVisible=true;
            onVisible();
        }else {
            isVisible=false;
            onInvisible();
        }
    }

    // 懒加载的方法，在这个方法里面为fragment的各个组件去添加数据
    protected  abstract void lazyLoad();
    protected void onVisible(){
        lazyLoad();
    }
    protected void onInvisible(){}
}

放到viewpager的Fragment

package com.luwei.constraintlayoutdemo;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;

/**
 * Created by Shinelon on 2017/7/26.
 */

public class TestFragment extends LazyFragment {
    private boolean isPrepared;// 标志位 标志已经初始化完成了

    private ImageView ivTest;

    private String index;

    public static TestFragment newInstance(String index) {
        Bundle args = new Bundle();
        args.putString("index", index);
        TestFragment fragment = new TestFragment();
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    protected void lazyLoad() {
        if (isPrepared && isVisible) {
            ivTest.setImageResource(R.mipmap.ic_launcher_round);
            isPrepared = false;
            Log.e("-------------this is", index);
        }
    }

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = LayoutInflater.from(getActivity()).inflate(R.layout.fragment_test, container, false);
        isPrepared = true;
        return view;
    }

    @Override
    public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        ivTest = (ImageView) view.findViewById(R.id.iv);
        index = getArguments().getString("index");
        lazyLoad();
    }
}

加载的Activity

package com.luwei.constraintlayoutdemo;

import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by Shinelon on 2017/7/26.
 */

public class TestLazyActivity extends AppCompatActivity {
    private ViewPager vpTestLazy;
    private List<Fragment> list = new ArrayList<>();
    private TestViewPagerAdapter testViewPagerAdapter;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_lazy);
        list.add(TestFragment.newInstance("0"));
        list.add(TestFragment.newInstance("1"));
        list.add(TestFragment.newInstance("2"));
        list.add(TestFragment.newInstance("3"));
        testViewPagerAdapter = new TestViewPagerAdapter(getSupportFragmentManager(), list);
        vpTestLazy = (ViewPager) findViewById(R.id.vp_test_lazy);
        vpTestLazy.setAdapter(testViewPagerAdapter);
        vpTestLazy.setOffscreenPageLimit(list.size()-1);
    }
}

效果：

image.png