ViewPager+Fragment取消预加载(延迟加载)

Posted brilliantzzk

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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() 
            @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 mFragments = new ArrayList();

    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;
        
    

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