一、简介
玩过微信的都知道，微信用的是懒加载的模式，之所以使用懒加载是因为：当使用viewpager+adapter作为应用大的布局时，viewpager会通过setOffscreenPageLimit来设置预加载的项目，不设置setOffscreenPageLimit，则默认为1（设置0无效，可以查看该方法源码知道），也就是当我们打开应用看到的时候fragmentOne时，实际上其他fragment（例如fragmentSecond）也进行了加载，只不过没有显示出来罢了，但是这样就造成了不必要的资源浪费（例如，fragmentSecond没有显示，但是却进行了大量的网络加载操作）。
在此先要说下一下几个方法：
1.ViewPager中的setOffscreenPageLimit方法，设置预加载的页面个数，参数不能小于1因为默认最小就是1。作用是：可以让Fragment视图不会被销毁。
2.Fragment中的setUserVisibleHint方法，当参数为true时对用户可见，当为false时对用户不可见（这是懒加载的关键所在）
二、使用步骤
1、LazyFragment:
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package cn.htc.OADemo;
import android.os.Bundle;
import android.support.annotation.LayoutRes;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
/**
* A simple {@link Fragment} subclass.
* "懒加载"的Fragment的基类
* ViewPager+TabLayout+Fragment 懒加载机制
*
*/
public abstract class LazyLoadFragment extends Fragment {
private View view;
private boolean needInit; //是否需要在onCreateView中初始化组件
private boolean hasLoad; //标识是否已经加载过
private boolean hasCreated; //是否为第一次加载数据
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
//展示 & 没有加载过
if (isVisibleToUser && !hasLoad) {
//如果当前Fragment向用户展示且没有加载过，进行下一步判断
if (hasCreated) {
//如果onCreateView已经被创建，此时进行加载
initView();
} else {
//如果Fragment没有创建，那么设置标记，在onCreateView中加载
needInit = true;
}
}
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
view = inflater.inflate(getLayoutId(), container, false);
if (needInit) {
//是否要初始化，后面页不需要
initView();
needInit = false;
}
hasCreated = true;
return view;
}
private void initView() {
init(view);
hasLoad = true;
}
//获取Fragment的布局文件
@LayoutRes
protected abstract int getLayoutId();
//包含初始化数据
protected abstract void init(View view);
}
2、所有Fragment子类继承于LazyFragment，这里以一个详细的例子作为讲解
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package cn.htc.OADemo;
import android.icu.text.LocaleDisplayNames;
import android.os.Bundle;
import android.os.Handler;
import android.support.v4.widget.SwipeRefreshLayout;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import butterknife.BindView;
import butterknife.ButterKnife;
import cn.htc.OADemo.bean.TasksData;
import cn.htc.OADemo.bean.YKTDataWrapper;
import cn.htc.OADemo.utils.ToastUtilWrapper;
/*
* Tasks任务类Fragment
*/
public class TasksFragment extends LazyLoadFragment {
private static final String TAG = "TasksFragment";
private static final String ARG_TITLE = "title";
private static final String ARG_TOKEN = "param2";
private String mType = "";
private String mToken = "";
private View mView = null;
@BindView(R.id.ykt_fg_sf_container)
SwipeRefreshLayout swipeRefreshLayout;
@BindView(R.id.ykt_fg_sf_rv)
RecyclerView recyclerView;
private LinearLayoutManager linearLayoutManager = null;
private TasksRecyclerViewAdapter tasksRecyclerViewAdapter = null;
private int lastVisibleItem = 0;
private TasksJsonUtils jsonUtils = null;
private TasksData tasksData = new TasksData();
private ProgressDialogUtils utils;
public TasksFragment() {
// Required empty public constructor
}
public static TasksFragment newInstance(String title, String token) {
TasksFragment fragment = new TasksFragment();
Bundle args = new Bundle();
args.putString(ARG_TITLE, title);
args.putString(ARG_TOKEN, token);
fragment.setArguments(args);
return fragment;
}
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getArguments() != null) {
mType = getArguments().getString(ARG_TITLE);
mToken = getArguments().getString(ARG_TOKEN);
}
}
@Override
protected int getLayoutId() {
return R.layout.fragment_ykt;
}
@Override
protected void init(View view) {
//在这里做一些初始化处理
ButterKnife.bind(this, view);
getData();
//初始化View
initView();
}
private void initView() {
// 设置布局管理器
linearLayoutManager = new LinearLayoutManager(getActivity());
linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
linearLayoutManager.setReverseLayout(false);
recyclerView.setLayoutManager(linearLayoutManager);
// 设置adapter
tasksRecyclerViewAdapter = new TasksRecyclerViewAdapter();
tasksRecyclerViewAdapter.setOnItemClickListener(new TasksRecyclerViewAdapter.IOnItemClickListener() {
@Override
public void onItemClick(View view, int position) {
}
});
recyclerView.setAdapter(tasksRecyclerViewAdapter);
// 设置刷新圆圈背景
swipeRefreshLayout.setProgressBackgroundColorSchemeResource(R.color.white);
swipeRefreshLayout.setColorSchemeResources(R.color.blue,
R.color.orange,
R.color.green,
R.color.red);
// 下拉刷新功能
swipeRefreshLayout.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
@Override
public void onRefresh() {
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
if (null != jsonUtils) {
jsonUtils.getResult(mToken, mType, Constant.LOAD_TYPE_REFRESH);
}
/// 刷新完成后标志
swipeRefreshLayout.setRefreshing(false);
ToastUtilWrapper.toastShow(getActivity(), "数据刷新", false);
}
}, 1500);
}
});
// 监听滚动事件，完成上拉加载功能
recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
@Override
public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
//滚动状态变化时回调
super.onScrollStateChanged(recyclerView, newState);
if (newState == RecyclerView.SCROLL_STATE_IDLE
&& lastVisibleItem + 1 == tasksRecyclerViewAdapter.getItemCount()) {
new Handler().postDelayed(new Runnable() {
@Override
public void run() {
if (null != jsonUtils) {
jsonUtils.getResult(mToken, mType, Constant.LOAD_TYPE_LOADMORE);
}
swipeRefreshLayout.setRefreshing(false);
}
}, 1500);
}
}
@Override
public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
//滚动时回调
super.onScrolled(recyclerView, dx, dy);
lastVisibleItem = linearLayoutManager.findLastCompletelyVisibleItemPosition();
}
});
}
private void getData() {
jsonUtils = new TasksJsonUtils(new TasksJsonUtils.CallBackListener() {
@Override
public void updateUI(TasksData dataList, boolean isAppend) {
if (null != dataList && dataList.isSuccess()) {
if (isAppend && null != tasksData) {
tasksData.appendData(dataList.getData());
} else {
tasksData = dataList;
}
//初始化数据
initData();
}
//关闭弹窗
utils.closeProgressDialog();
ToastUtilWrapper.toastShow(getActivity(),
null != dataList ? "数据更新成功" : "数据更新失败",
false);
}
});
utils = new ProgressDialogUtils(getActivity());
utils.showPorgressDialog("loading...");
if (null != jsonUtils) {
jsonUtils.getResult(mToken, mType);
}
}
//执行初始化数据以及更新UI操作
private void initData() {
if (tasksData != null) {
tasksRecyclerViewAdapter.updateData(tasksData, mType);
}
}
}
3、Activity中注意，在设置Viewpager和TabLayout绑定的时候
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//设置预加载页面的数目，避免当页面个数 > 3时，有Fragment被销毁的情况
viewPager.setOffscreenPageLimit(mList.size() - 1);
4、另一个懒加载的Fragment的基类
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public abstract class BaseFragment extends Fragment {
private boolean isVisible = false;//当前Fragment是否可见
private boolean isInitView = false;//是否与View建立起映射关系
private boolean isFirstLoad = true;//是否是第一次加载数据
private View convertView;
private SparseArray<View> mViews;
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
LogUtil.m("isVisibleToUser " + isVisibleToUser + " " + this.getClass().getSimpleName());
if (isVisibleToUser) {
isVisible = true;
lazyLoadData();
} else {
isVisible = false;
}
super.setUserVisibleHint(isVisibleToUser);
}
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
LogUtil.m(" " + this.getClass().getSimpleName());
convertView = inflater.inflate(getLayoutId(), container, false);
mViews = new SparseArray<>();
initView();
isInitView = true;
lazyLoadData();
return convertView;
}
private void lazyLoadData() {
if (isFirstLoad) {
LogUtil.m("第一次加载 " + " isInitView " + isInitView + " isVisible " + isVisible + " " + this.getClass().getSimpleName());
} else {
LogUtil.m("不是第一次加载" + " isInitView " + isInitView + " isVisible " + isVisible + " " + this.getClass().getSimpleName());
}
if (!isFirstLoad || !isVisible || !isInitView) {
LogUtil.m("不加载" + " " + this.getClass().getSimpleName());
return;
}
LogUtil.m("完成数据第一次加载"+ " " + this.getClass().getSimpleName());
initData();
isFirstLoad = false;
}
/**
* 加载页面布局文件
* @return
*/
protected abstract int getLayoutId();
/**
* 让布局中的view与fragment中的变量建立起映射
*/
protected abstract void initView();
/**
* 加载要显示的数据
*/
protected abstract void initData();
/**
* fragment中可以通过这个方法直接找到需要的view，而不需要进行类型强转
* @param viewId
* @param <E>
* @return
*/
protected <E extends View> E findView(int viewId) {
if (convertView != null) {
E view = (E) mViews.get(viewId);
if (view == null) {
view = (E) convertView.findViewById(viewId);
mViews.put(viewId, view);
}
return view;
}
return null;
}
}
