ViewPager+Fragment取消预加载（延迟加载）（转）

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原文：http://www.2cto.com/kf/201501/368954.html

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..

技术分享

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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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：

 

 

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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。

 

 

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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;
        }
    }
}
</fragment></fragment>

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