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：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 | 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(); } |
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 | 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 * * index * */ 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,> |
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 | 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> |