ViewPager+Fragment取消预加载（延迟加载）

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

import android.support.v4.app.Fragment;   public abstract class BaseFragment extends Fragment {           /** Fragment当前状态是否可见 */     protected boolean isVisible;                 @Override     public 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：

?

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

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;     }       @Override     public 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Activity         ViewGroup parent = (ViewGroup)mFragmentView.getParent();         if(parent != null) {             parent.removeView(mFragmentView);         }         return mFragmentView;     }       @Override     protected void lazyLoad() {         if (!isPrepared || !isVisible || mHasLoadedOnce) {             return;         }           new AsyncTask<void, boolean="">() {               @Override             protected void onPreExecute() {                 super.onPreExecute();                 //显示加载进度对话框                 UIHelper.showDialogForLoading(getActivity(), 正在加载..., true);             }               @Override             protected 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;             }               @Override             protected 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;         }     } }</void,>

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(int limit)，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。

?

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

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<fragment> mFragments = new ArrayList<fragment>();           private final int FIRST_FRAGMENT = 0;     private final int SECOND_FRAGMENT = 1;     private final int THIRD_FRAGMENT = 2;                 @Override     protected 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.setOnClickListener(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 = new ListFragmentPagerAdapter(getSupportFragmentManager(), mFragments);         mViewPager.setAdapter(mPagerAdapter);         mViewPager.setOnPageChangeListener(onPageChangeListener);     }             private OnPageChangeListener onPageChangeListener = new OnPageChangeListener() {                   @Override         public 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;             }         }                   @Override         public void onPageScrolled(int arg0, float arg1, int arg2) {}                   @Override         public void onPageScrollStateChanged(int arg0) {}     };                 @Override     public 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;         }     } }