一、装载fragment的Activity：
FragActivity.java
package com.wei.wanandroid.activity.fragment;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import com.wei.wanandroid.R;
import com.wei.wanandroid.activity.BaseActivity;
import java.util.ArrayList;
import java.util.List;
public class FragActivity extends BaseActivity
{
public static final int TAB_ONE = 0;
public static final int TAB_TWO = 1;
public static final int TAB_THREE = 2;
private ViewPager mViewPager;
private MainTabPagerAdapter mMainTabPagerAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_frag);
}
@Override
public void initView() {
mViewPager = findViewById(R.id.viewPager);
mMainTabPagerAdapter = new MainTabPagerAdapter(getSupportFragmentManager());
mViewPager.setAdapter(mMainTabPagerAdapter);
mViewPager.setCurrentItem(TAB_ONE);
mViewPager.setOffscreenPageLimit(mMainTabPagerAdapter.getCount());
}
private class MainTabPagerAdapter extends FragmentPagerAdapter
{
private List<Fragment> mFragments;
private FragmentManager mFragmentManager;
public MainTabPagerAdapter(FragmentManager fm) {
super(fm);
mFragmentManager = fm;
mFragments = new ArrayList<>();
}
@Override
public Fragment getItem(int position)
{
Fragment fragment = null;
switch (position)
{
case TAB_ONE:
fragment = new Fragment1();
break;
case TAB_TWO:
fragment = new Fragment2();
break;
case TAB_THREE:
fragment = new Fragment3();
break;
default:
}
mFragments.add(fragment);
return fragment;
}
@Override
public int getCount()
{
return 3;
}
}
}
二、各fragment的父类，LazyFragment.java。
package com.wei.wanandroid.activity.fragment;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.util.Log;
import com.wei.wanandroid.activity.BaseFragment;
/**
* 懒加载（延迟加载）
*
* @author: WEI
* @date: 2018/6/5
*/
public abstract class LazyFragment extends BaseFragment {
private boolean hasCreated = false, isFirstVisible = true, isFirstResume = true;
/**
* setUserVisibleHint()在Fragment实例化时会先调用一次，并且默认值是false，当选中当前
显示的Fragment时还会再调用一次。
* 此方法在fragment生命周期开始之前便被调用，运行在onAttach()之前。所以，它运行的时候，
fragment都还没创建。
* @param isVisibleToUser
*/
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
Log.e(TAG, "--- setUserVisibleHint ---, isVisibleToUser = "
+ isVisibleToUser + ", hasCreated = " + hasCreated);
super.setUserVisibleHint(isVisibleToUser);
if (isVisibleToUser)
{
if (isFirstVisible) {
isFirstVisible = false;
// 之前还没初始化
initPrepare();
} else {
// 已经初始化
onLazyResume();
}
} else {
if (hasCreated)
{
onLazyPause();
}
}
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
Log.e(TAG, "--- onActivityCreated ---");
super.onActivityCreated(savedInstanceState);
hasCreated = true;
initPrepare();
}
@Override
public void onResume()
{
Log.e(TAG, "--- onResume ---");
super.onResume();
if (isFirstResume)
{ // 页面一加载的时候，所有fragment的生命周期都会执行到onResume。所以，第一次到
// 这里的都停止，不再往后操作。
isFirstResume = false;
return;
}
Log.e(TAG, "onResume(), getUserVisibleHint() = " + getUserVisibleHint());
if (getUserVisibleHint()) {
// 用户可见状态
onLazyResume();
}
}
@Override
public void onPause() {
Log.e(TAG, "--- onPause ---");
super.onPause();
Log.e(TAG, "onPause(), getUserVisibleHint() = " + getUserVisibleHint());
if (getUserVisibleHint()) {
onLazyPause();
}
}
private void initPrepare()
{
// 必须是用户可见状态并且fragment已经创建
if (getUserVisibleHint() && hasCreated)
{
Log.e(TAG, "--- initPrepare ---");
onFirstInit();
onLazyResume();
}
}
/**
* 第一次fragment可见（进行初始化工作）
*/
public abstract void onFirstInit();
/**
* fragment可见（onResume 回调）
*/
public abstract void onLazyResume();
/**
* fragment不可见（onPause 回调）
*/
public abstract void onLazyPause();
}
三、流程打印：
1.刚进入FragActivity ：
10:56:24.271 6061-6061/com.wei.wanandroid E/MainActivity: onPause
10:56:24.298 6061-6061/com.wei.wanandroid E/FragActivity: onCreate
10:56:24.356 6061-6061/com.wei.wanandroid E/FragActivity: onStart
10:56:24.360 6061-6061/com.wei.wanandroid E/FragActivity: onResume
10:56:24.379 6061-6061/com.wei.wanandroid E/Fragment1: --- setUserVisibleHint ---, isVisibleToUser = false, hasCreated = false
10:56:24.380 6061-6061/com.wei.wanandroid E/Fragment2: --- setUserVisibleHint ---, isVisibleToUser = false, hasCreated = false
10:56:24.380 6061-6061/com.wei.wanandroid E/Fragment3: --- setUserVisibleHint ---, isVisibleToUser = false, hasCreated = false
10:56:24.380 6061-6061/com.wei.wanandroid E/Fragment1: --- setUserVisibleHint ---, isVisibleToUser = true, hasCreated = false
10:56:24.381 6061-6061/com.wei.wanandroid E/Fragment1: --- onAttach ---
- onCreate ---
10:56:24.381 6061-6061/com.wei.wanandroid E/Fragment2: --- onAttach ---
- onCreate ---
10:56:24.381 6061-6061/com.wei.wanandroid E/Fragment3: --- onAttach ---
- onCreate ---
10:56:24.382 6061-6061/com.wei.wanandroid E/Fragment1: --- onCreateView ---
10:56:24.396 6061-6061/com.wei.wanandroid E/Fragment1: --- onActivityCreated ---
- initPrepare ---
- onFirstInit() ---
- onLazyResume() ---
- onResume ---
10:56:24.396 6061-6061/com.wei.wanandroid E/Fragment2: --- onCreateView ---
10:56:24.400 6061-6061/com.wei.wanandroid E/Fragment2: --- onActivityCreated ---
10:56:24.400 6061-6061/com.wei.wanandroid E/Fragment3: --- onCreateView ---
10:56:24.403 6061-6061/com.wei.wanandroid E/Fragment3: --- onActivityCreated ---
10:56:24.403 6061-6061/com.wei.wanandroid E/Fragment2: --- onResume ---
10:56:24.403 6061-6061/com.wei.wanandroid E/Fragment3: --- onResume ---
10:56:24.476 6061-6113/com.wei.wanandroid D/OpenGLRenderer: endAllActiveAnimators on 0xcb3c1c00 (MenuPopupWindow$MenuDropDownListView) with handle 0xc9fc7340
10:56:24.749 6061-6061/com.wei.wanandroid E/MainActivity: onSaveInstanceState
10:56:24.752 6061-6061/com.wei.wanandroid E/MainActivity: onStop
2.切换到第二个fragment：
11:00:49.654 6061-6061/com.wei.wanandroid E/Fragment1: --- setUserVisibleHint ---, isVisibleToUser = false, hasCreated = true
11:00:49.655 6061-6061/com.wei.wanandroid E/Fragment1: --- onLazyPause() ---
11:00:49.655 6061-6061/com.wei.wanandroid E/Fragment2: --- setUserVisibleHint ---, isVisibleToUser = true, hasCreated = true
- initPrepare ---
11:00:49.656 6061-6061/com.wei.wanandroid E/Fragment2: --- onFirstInit() ---
- onLazyResume() ---
3.再回到第一个fragment：
11:03:02.823 6061-6061/com.wei.wanandroid E/Fragment2: --- setUserVisibleHint ---, isVisibleToUser = false, hasCreated = true
- onLazyPause() ---
11:03:02.824 6061-6061/com.wei.wanandroid E/Fragment1: --- setUserVisibleHint ---, isVisibleToUser = true, hasCreated = true
- onLazyResume() ---
四、总结：
懒加载技术要点在于对setUserVisibleHint(boolean isVisibleToUser)方法的处理时机的把握。该方法第一次调用时，Fragment的onAttach(Context context)方法都还未执行，也就是说此时Fragment还没创建，此时什么事情也不能做。直到onActivityCreated(@Nullable Bundle savedInstanceState)执行完毕，且getUserVisibleHint()为真（Fragment可见）时才进行数据的加载，此时只加载当前可见的Fragment界面的数据，其它不可见的Fragment并未加载数据，从而提高数据展示速度，展示更好的用户体验效果。以后在各个Fragment之间切换时，都会在onLazyPause()与onLazyResume()之间回调。
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