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:
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:
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; } } }
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。
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; } } }