ViewPager+Fragment取消预加载（延迟加载）

用过ViewPage+Fragment组合的童鞋自然知道这个问题，没有遇到的同学祝愿你们永远不会遇到，呵呵。在项目中，都或多或少地使用的Tab布局，所以大都会用到ViewPager+Fragment，但是 Fragment有个不好或者太好的地方。例如你在ViewPager中添加了三个Fragment，当加载ViewPager中第一个Fragment 时，它会默认帮你预先加载了第二个Fragment，当你加载第二个Fragment时，它会帮你加载第三个Fragment。这样虽然有时很好，但是用户只需看一个Fragment时，我们就做了一些多余工作加载了第二个Fragment。在这只需要取消Fragment的预加载即可，只有当用户切换到 某个Fragment才加载..首先，介绍两个方法void setUserVisibleHint(boolean isVisibleToUser)、boolean getUserVisibleHint()，它们分别用作设置/获得Fragment可见状态，我们可以重写Fragment在其中做判断，代码如下：import android.support.v4.app.Fragment;public abstract class BaseFragment extends Fragment {/** Fragment当前状态是否可见 */protected boolean isVisible;@Overridepublic void setUserVisibleHint(boolean isVisibleToUser) {super.setUserVisibleHint(isVisibleToUser);if(getUserVisibleHint()) {isVisible = true;onVisible();} else {isVisible = false;onInvisible();}}/*** 可见*/protected void onVisible() {lazyLoad();}/*** 不可见*/protected void onInvisible() {}/*** 延迟加载* 子类必须重写此方法*/protected abstract void lazyLoad();}在我们的Fragment中，只需要继承这个类，然后重写其中的lazyLoad()方法，当Fragment对用户可见（即用户切换到此Fragment时）我们在lazyLoad()中加载所需数据，详细代码看下面，我写了个假的获取数据线程：import android.os.AsyncTask;import android.os.Bundle;import android.view.LayoutInflater;import android.view.View;import android.view.ViewGroup;import android.widget.TextView;public class CustomListFragment extends BaseFragment {private static final String FRAGMENT_INDEX = fragment_index;private final int FIRST_FRAGMENT = 0;private final int SECOND_FRAGMENT = 1;private final int THIRD_FRAGMENT = 2;private TextView mFragmentView;private int mCurIndex = -1;/** 标志位，标志已经初始化完成 */private boolean isPrepared;/** 是否已被加载过一次，第二次就不再去请求数据了 */private boolean mHasLoadedOnce;/*** 创建新实例** @param index* @return*/public static CustomListFragment newInstance(int index) {Bundle bundle = new Bundle();bundle.putInt(FRAGMENT_INDEX, index);CustomListFragment fragment = new CustomListFragment();fragment.setArguments(bundle);return fragment;}@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {if(mFragmentView == null) {mFragmentView = (TextView) inflater.inflate(R.layout.fragment, container, false);//获得索引值Bundle bundle = getArguments();if (bundle != null) {mCurIndex = bundle.getInt(FRAGMENT_INDEX);}isPrepared = true;lazyLoad();}//因为共用一个Fragment视图，所以当前这个视图已被加载到Activity中，必须先清除后再加入ActivityViewGroup parent = (ViewGroup)mFragmentView.getParent();if(parent != null) {parent.removeView(mFragmentView);}return mFragmentView;}@Overrideprotected void lazyLoad() {if (!isPrepared || !isVisible || mHasLoadedOnce) {return;}new AsyncTask() {@Overrideprotected void onPreExecute() {super.onPreExecute();//显示加载进度对话框UIHelper.showDialogForLoading(getActivity(), 正在加载..., true);}@Overrideprotected Boolean doInBackground(Void... params) {try {Thread.sleep(2000);//在这里添加调用接口获取数据的代码//doSomething()} catch (Exception e) {e.printStackTrace();}return true;}@Overrideprotected void onPostExecute(Boolean isSuccess) {if (isSuccess) {// 加载成功setView();mHasLoadedOnce = true;} else {// 加载失败}//关闭对话框UIHelper.hideDialogForLoading();}}.execute();}private void setView() {// 根据索引加载不同视图switch (mCurIndex) {case FIRST_FRAGMENT:mFragmentView.setText(第一个);break;case SECOND_FRAGMENT:mFragmentView.setText(第二个);break;case THIRD_FRAGMENT:mFragmentView.setText(第三个);break;}}}到这里我们只是写好了Fragment，在FragmentActivity中还需要对ViewPager设置一下，让它每次只加载一个Fragment，ViewPager.setOffscreenPageLimit(intlimit)，其中参数可以设为0或者1，参数小于1时，会默认用1来作为参数，未设置之前，ViewPager会默认加载两个Fragment。所以，我们只需要调用下它，设置下加载Fragment个数即可。import java.util.ArrayList;import java.util.List;import android.support.v4.app.Fragment;import android.support.v4.app.FragmentActivity;import android.support.v4.view.ViewPager;import android.support.v4.view.ViewPager.OnPageChangeListener;import android.view.View;import android.view.View.OnClickListener;import android.widget.RadioButton;import android.os.Bundle;public class MainActivity extends FragmentActivity implements OnClickListener{private RadioButton mFstBtn;private RadioButton mSndBtn;private RadioButton mThdBtn;private ViewPager mViewPager;private ListFragmentPagerAdapter mPagerAdapter;private List mFragments = new ArrayList();private final int FIRST_FRAGMENT = 0;private final int SECOND_FRAGMENT = 1;private final int THIRD_FRAGMENT = 2;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);initButton();initViewPager();}/*** 初始化按钮*/private void initButton() {mFstBtn = (RadioButton)findViewById(R.id.id_rb_fst);mFstBtn.setOnClickListener(this);mSndBtn = (RadioButton)findViewById(R.id.id_rb_snd);mSndBtn.setOnClickListener(this);mThdBtn = (RadioButton)findViewById(R.id.id_rb_thd);mThdBtn.setOnClickListener4000(this);}/*** 初始化ViewPager控件*/private void initViewPager() {mViewPager = (ViewPager)findViewById(R.id.id_vp_viewpager);//关闭预加载，默认一次只加载一个FragmentmViewPager.setOffscreenPageLimit(1);//添加FragmentmFragments.add(CustomListFragment.newInstance(FIRST_FRAGMENT));mFragments.add(CustomListFragment.newInstance(SECOND_FRAGMENT));mFragments.add(CustomListFragment.newInstance(THIRD_FRAGMENT));//适配器mPagerAdapter = new ListFragmentPagerAdapter(getSupportFragmentManager(), mFragments);mViewPager.setAdapter(mPagerAdapter);mViewPager.setOnPageChangeListener(onPageChangeListener);}private OnPageChangeListener onPageChangeListener = new OnPageChangeListener() {@Overridepublic void onPageSelected(int position) {//根据用户选中的按钮修改按钮样式switch (position) {case FIRST_FRAGMENT:mFstBtn.setChecked(true);mSndBtn.setChecked(false);mThdBtn.setChecked(false);break;case SECOND_FRAGMENT:mFstBtn.setChecked(false);mSndBtn.setChecked(true);mThdBtn.setChecked(false);break;case THIRD_FRAGMENT:mFstBtn.setChecked(false);mSndBtn.setChecked(false);mThdBtn.setChecked(true);break;}}@Overridepublic void onPageScrolled(int arg0, float arg1, int arg2) {}@Overridepublic void onPageScrollStateChanged(int arg0) {}};@Overridepublic void onClick(View v) {switch (v.getId()) {case R.id.id_rb_fst:mViewPager.setCurrentItem(FIRST_FRAGMENT);break;case R.id.id_rb_snd:mViewPager.setCurrentItem(SECOND_FRAGMENT);break;case R.id.id_rb_thd:mViewPager.setCurrentItem(THIRD_FRAGMENT);break;}}}