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：

 

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importandroid.support.v4.app.Fragment;

 

publicabstract class BaseFragment extendsFragment {

     

    /** Fragment当前状态是否可见 */

    protectedboolean isVisible;

     

     

    @Override

    publicvoid setUserVisibleHint(booleanisVisibleToUser) {

        super.setUserVisibleHint(isVisibleToUser);

         

        if(getUserVisibleHint()) {

            isVisible = true;

            onVisible();

        }else{

            isVisible = false;

            onInvisible();

        }

    }

     

     

    /**

     * 可见

     */

    protectedvoid onVisible() {

        lazyLoad();    

    }

     

     

    /**

     * 不可见

     */

    protectedvoid onInvisible() {

         

         

    }

     

     

    /**

     * 延迟加载

     * 子类必须重写此方法

     */

    protectedabstract 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：

 

 

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importandroid.os.AsyncTask;

importandroid.os.Bundle;

importandroid.view.LayoutInflater;

importandroid.view.View;

importandroid.view.ViewGroup;

importandroid.widget.TextView;

 

publicclass CustomListFragment extendsBaseFragment {

 

    privatestatic final String FRAGMENT_INDEX = fragment_index;

    privatefinal int FIRST_FRAGMENT = 0;

    privatefinal int SECOND_FRAGMENT = 1;

    privatefinal int THIRD_FRAGMENT = 2;

 

    privateTextView mFragmentView;

 

    privateint mCurIndex = -1;

    /** 标志位，标志已经初始化完成 */

    privateboolean isPrepared;

    /** 是否已被加载过一次，第二次就不再去请求数据了 */

    privateboolean mHasLoadedOnce;

 

    /**

     * 创建新实例

     *

     * @param index

     * @return

     */

    publicstatic CustomListFragment 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

    protectedvoid lazyLoad() {

        if(!isPrepared || !isVisible || mHasLoadedOnce) {

            return;

        }

 

        newAsyncTask<void,boolean="">() {

 

            @Override

            protectedvoid onPreExecute() {

                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

            protectedvoid 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();

    }

 

    privatevoid setView() {

        // 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,>

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。

 

 

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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;

 

publicclass MainActivity extendsFragmentActivity implementsOnClickListener{

 

    privateRadioButton mFstBtn;

    privateRadioButton mSndBtn;

    privateRadioButton mThdBtn;

     

    privateViewPager mViewPager;

    privateListFragmentPagerAdapter mPagerAdapter;

    privateList<fragment> mFragments = newArrayList<fragment>();

     

    privatefinal int FIRST_FRAGMENT = 0;

    privatefinal int SECOND_FRAGMENT = 1;

    privatefinal int THIRD_FRAGMENT = 2;

     

     

    @Override

    protectedvoid onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        initButton();

        initViewPager();

    }

     

     

    /**

     * 初始化按钮

     */

    privatevoid 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

     */

    privatevoid 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 = newListFragmentPagerAdapter(getSupportFragmentManager(), mFragments);

        mViewPager.setAdapter(mPagerAdapter);

        mViewPager.setOnPageChangeListener(onPageChangeListener);

    }

 

     

    privateOnPageChangeListener onPageChangeListener = newOnPageChangeListener() {

         

        @Override

        publicvoid onPageSelected(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

        publicvoid onPageScrolled(intarg0, floatarg1, intarg2) {}

         

        @Override

        publicvoid onPageScrollStateChanged(intarg0) {}

    };

     

     

    @Override

    publicvoid onClick(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>