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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：
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importandroid.support.v4.app.Fragment;
publicabstractclassBaseFragmentextendsFragment {
/** Fragment当前状态是否可见 */
protectedbooleanisVisible;
@Override
publicvoidsetUserVisibleHint(booleanisVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(getUserVisibleHint()) {
isVisible =true;
onVisible();
}else{
isVisible =false;
onInvisible();
}
}
/**
* 可见
*/
protectedvoidonVisible() {
lazyLoad();
}
/**
* 不可见
*/
protectedvoidonInvisible() {
}
/**
* 延迟加载
* 子类必须重写此方法
*/
protectedabstractvoidlazyLoad();
}
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importandroid.os.AsyncTask;
importandroid.os.Bundle;
importandroid.view.LayoutInflater;
importandroid.view.View;
importandroid.view.ViewGroup;
importandroid.widget.TextView;
publicclassCustomListFragmentextendsBaseFragment {
privatestaticfinalString FRAGMENT_INDEX = fragment_index;
privatefinalintFIRST_FRAGMENT =0;
privatefinalintSECOND_FRAGMENT =1;
privatefinalintTHIRD_FRAGMENT =2;
privateTextView mFragmentView;
privateintmCurIndex = -1;
/** 标志位，标志已经初始化完成 */
privatebooleanisPrepared;
/** 是否已被加载过一次，第二次就不再去请求数据了 */
privatebooleanmHasLoadedOnce;
/**
* 创建新实例
*
*
@param index
*
@return
*/
publicstaticCustomListFragment newInstance(intindex) {
Bundle bundle =newBundle();
bundle.putInt(FRAGMENT_INDEX, index);
CustomListFragment fragment =newCustomListFragment();
fragment.setArguments(bundle);
returnfragment;
}
@Override
publicView 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Activity
ViewGroup parent = (ViewGroup)mFragmentView.getParent();
if(parent !=null) {
parent.removeView(mFragmentView);
}
returnmFragmentView;
}
@Override
protectedvoidlazyLoad() {
if(!isPrepared || !isVisible || mHasLoadedOnce) {
return;
}
newAsyncTask<void,boolean="">() {
@Override
protectedvoidonPreExecute() {
super.onPreExecute();
//显示加载进度对话框
UIHelper.showDialogForLoading(getActivity(), 正在加载...,true);
}
@Override
protectedBoolean 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();
}
returntrue;
}
@Override
protectedvoidonPostExecute(Boolean isSuccess) {
if(isSuccess) {
// 加载成功
setView();
mHasLoadedOnce =true;
}else{
// 加载失败
}
//关闭对话框
UIHelper.hideDialogForLoading();
}
}.execute();
}
privatevoidsetView() {
// 根据索引加载不同视图
switch(mCurIndex) {
caseFIRST_FRAGMENT:
mFragmentView.setText(第一个);
break;
caseSECOND_FRAGMENT:
mFragmentView.setText(第二个);
break;
caseTHIRD_FRAGMENT:
mFragmentView.setText(第三个);
break;
}
}
}</void,>
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importjava.util.ArrayList;
importjava.util.List;
importandroid.support.v4.app.Fragment;
importandroid.support.v4.app.FragmentActivity;
importandroid.support.v4.view.ViewPager;
importandroid.support.v4.view.ViewPager.OnPageChangeListener;
importandroid.view.View;
importandroid.view.View.OnClickListener;
importandroid.widget.RadioButton;
importandroid.os.Bundle;
publicclassMainActivityextendsFragmentActivityimplementsOnClickListener{
privateRadioButton mFstBtn;
privateRadioButton mSndBtn;
privateRadioButton mThdBtn;
privateViewPager mViewPager;
privateListFragmentPagerAdapter mPagerAdapter;
privateList<fragment> mFragments =newArrayList<fragment>();
privatefinalintFIRST_FRAGMENT =0;
privatefinalintSECOND_FRAGMENT =1;
privatefinalintTHIRD_FRAGMENT =2;
@Override
protectedvoidonCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initButton();
initViewPager();
}
/**
* 初始化按钮
*/
privatevoidinitButton() {
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.setOnClickListener(this);
}
/**
* 初始化ViewPager控件
*/
privatevoidinitViewPager() {
mViewPager = (ViewPager)findViewById(R.id.id_vp_viewpager);
//关闭预加载，默认一次只加载一个Fragment
mViewPager.setOffscreenPageLimit(1);
//添加Fragment
mFragments.add(CustomListFragment.newInstance(FIRST_FRAGMENT));
mFragments.add(CustomListFragment.newInstance(SECOND_FRAGMENT));
mFragments.add(CustomListFragment.newInstance(THIRD_FRAGMENT));
//适配器
mPagerAdapter =newListFragmentPagerAdapter(getSupportFragmentManager(), mFragments);
mViewPager.setAdapter(mPagerAdapter);
mViewPager.setOnPageChangeListener(onPageChangeListener);
}
privateOnPageChangeListener onPageChangeListener =newOnPageChangeListener() {
@Override
publicvoidonPageSelected(intposition) {
//根据用户选中的按钮修改按钮样式
switch(position) {
caseFIRST_FRAGMENT:
mFstBtn.setChecked(true);
mSndBtn.setChecked(false);
mThdBtn.setChecked(false);
break;
caseSECOND_FRAGMENT:
mFstBtn.setChecked(false);
mSndBtn.setChecked(true);
mThdBtn.setChecked(false);
break;
caseTHIRD_FRAGMENT:
mFstBtn.setChecked(false);
mSndBtn.setChecked(false);
mThdBtn.setChecked(true);
break;
}
}
@Override
publicvoidonPageScrolled(intarg0,floatarg1,intarg2) {}
@Override
publicvoidonPageScrollStateChanged(intarg0) {}
};
@Override
publicvoidonClick(View v) {
switch(v.getId()) {
caseR.id.id_rb_fst:
mViewPager.setCurrentItem(FIRST_FRAGMENT);
break;
caseR.id.id_rb_snd:
mViewPager.setCurrentItem(SECOND_FRAGMENT);
break;
caseR.id.id_rb_thd:
mViewPager.setCurrentItem(THIRD_FRAGMENT);
break;
}
}
}
</fragment></fragment>
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原文地址：http://my.oschina.net/yuanxulong/blog/494067
