FragmentPagerAdapter和FragmentStatePagerAdapter

Question

我目前正在考虑应该使用PagerAdapter的哪个实现。我对他们俩都有两难的选择。让我给你看看这些是什么。

1#FragmentPagerAdapter

工作良好，当没有以前的实例可用时，它会创建片段的新实例，当有这样的机会时，它会检索以前的片段。

那不是很低效吗？我在Google I/O materials上看到Romain Guy说，创建新的视图效率不高，特别是当我们创建大量视图时，就像在ListView中那样，因此我们使用ConvertView检索现有的视图并根据需要多次更改它。因此页面之间的翻转非常相似--许多新的视图--因为fragment是某种视图

在两个 PagerAdapter中，我尝试了重写 destroyItem()方法，但根本不起作用。

我的问题是。

我该怎么办？

我应该使用SharedPreference和选项#1来配合FragmentPagerAdapter还是使用选项#2来配合FragmentStatePagerAdapter？

有没有可能，我在这些适配器中做错了什么，使它们的行为不像我们预期的那样？

/**
 * Adapter class to {@link WizardPager}
 */
public static class WizardCrazyAdapter extends FragmentStatePagerAdapter
    implements OnPageChangeListener{

    public static final String tag = "android:switcher:"+R.id.pager_w+":";

    /**
     * Refernece to root activity
     */
    WizardActivity wizardActivity;

    /**
     * list of fragments
     */
    private final ArrayList<FragmentInfo> fInfos = new ArrayList<FragmentInfo>();

    private short prevPageNumber = 0;

    /**
     * Constructor of adapter
     * @param wizardActivity
     *            {@link WizardActivity} as reference to activity root
     */
    public WizardCrazyAdapter(WizardActivity wizardActivity) {
        super(wizardActivity.getSupportFragmentManager());
        this.wizardActivity = wizardActivity;           
    }

    static final class FragmentInfo {
        private final Class<?> _clss;
        private Bundle _args;

        public FragmentInfo(Class<?> clss, Bundle args) {
            _clss   =clss;
            _args   =args;
        }
    }

    public void addPage(Class<?> clss, Bundle args){
        FragmentInfo fi = new FragmentInfo(clss, args);         
        fInfos.add(fi);
    }

    /**
     * Return number of pages
     */
    public int getCount() {
        return fInfos.size();
    }

    /**
     * Searches in {@link FragmentManager} for {@link Fragment} at specified position
     * @param position
     * @return
     */
    private AbstractWizardFragment getFragmentAt(int position){
        FragmentManager fm = wizardActivity.getSupportFragmentManager();

        AbstractWizardFragment awf = (AbstractWizardFragment) fm.findFragmentByTag(tag+position);

        return awf;         
    }

    /**
     * Return page of view pager
     */
    @Override
    public Fragment getItem(int position) {
        /*finding existing instance of fragment*/
        AbstractWizardFragment awf = getFragmentAt(position);

        if(awf == null){
            /*creating new instance if no instance exist*/
            Log.v("WizardActivity", "creating new Fragment");
            FragmentInfo fi = fInfos.get(position);
            awf = (AbstractWizardFragment) Fragment.instantiate(wizardActivity, fi._clss.getName());
        }else{
            Log.v("WizardActivity", "found existing Fragment");
        }

        return awf;
    }

    @Override
    public void onPageSelected(int pageNumber) {
        wizardActivity.stepFragment.setCurrentStepAndChangeText(pageNumber);

        if(pageNumber != prevPageNumber){
            AbstractWizardFragment prevFragment = (AbstractWizardFragment) getItem(prevPageNumber);//TODO change if any problems
            prevFragment.onDetachedFromViewPager(wizardActivity.mForm);
        }

        AbstractWizardFragment currFragment = (AbstractWizardFragment) getItem(pageNumber);//TODO change if any problems
        currFragment.onAttachedToViewPager(wizardActivity.mForm);

        prevPageNumber = (short) pageNumber;


        Log.d("WizardActivity", "onPageSelected");
    }

    @Override
    public Object instantiateItem(ViewGroup arg0, int arg1) {
        Log.d("WizardActivity", "instantiateItem "+arg1);
        return super.instantiateItem(arg0, arg1);
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
   //           super.destroyItem(container, position, object);

        Log.v("WizardActivity", "that would be destroy");
    }
}

ViewPager代码：

public class WizardPager extends ViewPager{

/**
 * Flag to check if view pager must be scrolled 
 */
protected boolean isScrollable;

/**
 * Default constructor
 * @param context {@link Context}
 */
public WizardPager(Context context) {
    super(context);
    isScrollable = true;
}

/**
 * Standard constructor
 * @param context {@link Context}
 * @param attrs {@link AttributeSet}
 */
public WizardPager(Context context, AttributeSet attrs) {
    super(context, attrs);
    isScrollable = true;
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    if (this.isScrollable) {
        return super.onTouchEvent(event);
    }

    return false;
}

@Override
public boolean onInterceptTouchEvent(MotionEvent event) {
    if (this.isScrollable) {
        return super.onInterceptTouchEvent(event);
    }

    return false;
}

/**
 * Enable scroll of pages
 */
public void enableScroll(){
    this.isScrollable = true;
}

/**
 * Disable scroll of pages
 */
public void disableScroll(){
    this.isScrollable = false;
}

/**
 * Check if pages can be scrolled
 * @return
 */
public boolean isScrollable(){
    return isScrollable;
}
}

Answer 1

我在OnPageSelectedMethod中搞砸了--我强制FragmentStatePagerAdapter调用GetItem--每当页面发生更改时，它至少实例化片断一次--方法。这就是为什么我抱怨每次更改页面时都要实例化Fragment：）

而不是getItem()，我应该在那里调用我的getFragmentat()方法，整个回调应该是这样的。

@Override
public void onPageSelected(int pageNumber) {
    wizardActivity.stepFragment.setCurrentStepAndChangeText(pageNumber);

    if(pageNumber != prevPageNumber){
        AbstractWizardFragment prevFragment = (AbstractWizardFragment) getFragmentAt(prevPageNumber);
        prevFragment.onDetachedFromViewPager(wizardActivity.mForm);
    }

    AbstractWizardFragment currFragment = (AbstractWizardFragment) getFragmentAt(pageNumber);
    currFragment.onAttachedToViewPager(wizardActivity.mForm);

    prevPageNumber = (short) pageNumber;


    Log.d("WizardActivity", "onPageSelected");
}

虽然它工作得很好。仍然可能存在这样的风险，即找不到fragment并且方法将返回null，然后将引发NPE。