FragmentPagerAdapter getItem() 的返回值超过了应有的值

Question

我有一个MainActivity控制四个片段，每个片段都是一个选项卡。当我的 main activity 启动时，我在日志中打印了一行，以显示正在实例化的片段。这是我的 FragmentPagerAdapter：

public class TabsPagerAdapter extends FragmentPagerAdapter {

public TabsPagerAdapter(FragmentManager fm) {
    super(fm);
}

@Override
public Fragment getItem(int index) {

    switch (index) {
        case 0:
            System.out.println("Returning new PrearrivalPlan()");
            return new PrearrivalPlan();
        case 1:
            System.out.println("Returning new PrimarySurvey()");
            return new PrimarySurvey();
        case 2:
            System.out.println("Returning new SecondarySurvey()");
            return new SecondarySurvey();
        case 3:
            System.out.println("Returning new PrepareForTravel()");
            return new PrepareForTravel();
    }

    return null;
}

@Override
public int getCount() {
    // get item count - equal to number of tabs
    return 4;
}

}

标签栏依次有以下选项：

Prearrival Plan | Primary Survey | Secondary Survey | Prepare for Travel

当我的 main activity 启动时，屏幕上会打印以下内容：

Returning new PrearrivalPlan() Returning new PrimarySurvey()

它似乎在做的是在我选择的选项卡之前加载一个选项卡。由于 PrearrivalPlan 是第一个选项卡，我认为它应该只是 return 一个新的 PrearrivalPlan() 除了它 return 两者。另一个例子，当我点击 Primary Survey 选项卡（第二个选项卡）时，屏幕上会打印以下内容：

Returning new SecondarySurvey() // <- This is the third tab!?

因为 PrimarySurvey 在 activity 第一次启动时已经实例化（见上面的输出），它像以前一样跳到前面并加载了第三个选项卡，即使我只点击了第二个选项卡。

这是我的 MainActivity:

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {

private CustomViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;

private String[] tabTitles = {"Pre-arrival Plan", "Primary Survey", "Secondary Survey", "Prepare for Travel"};

@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    viewPager = (CustomViewPager) findViewById(R.id.pager);
    actionBar = getActionBar();
    mAdapter = new TabsPagerAdapter(getSupportFragmentManager());

    viewPager.setAdapter(mAdapter);
    viewPager.setPagingEnabled(false);

    actionBar.setHomeButtonEnabled(true);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);


    viewPager.setOnPageChangeListener(new CustomViewPager.OnPageChangeListener() {


        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int position) {
            actionBar.setSelectedNavigationItem(position);
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }

    });

    // Remove Android icon from Action Bar
    getActionBar().setIcon(new ColorDrawable(getResources().getColor(android.R.color.transparent)));
    getActionBar().setDisplayShowHomeEnabled(false);

    // Add tabs to Action Bar
    for (String tab_name : tabTitles) {
        actionBar.addTab(actionBar.newTab().setText(tab_name).setTabListener(this));
    }

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

public void updateTabTitles(int tabNumber, int checkBoxesRemaining) {
    String text = tabTitles[tabNumber] + " \n             (" + checkBoxesRemaining + ")";
    actionBar.getTabAt(tabNumber).setText(text);
}

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
    viewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case android.R.id.home:
            Intent intent = new Intent(this, MainMenu.class);
            startActivity(intent);
            return true;
        case R.id.complete:
            DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    switch (which){
                        case DialogInterface.BUTTON_POSITIVE:
                            goToReport();
                            break;
                        case DialogInterface.BUTTON_NEGATIVE:
                            //No button clicked
                            break;
                    }
                }
            };
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setMessage("Are you sure you want to complete the checklist?").setPositiveButton("Yes", dialogClickListener).setNegativeButton("No", dialogClickListener).show();
            return true;
        default:
            return super.onOptionsItemSelected(item);
    }
}

public void goToReport() {
    Intent intent = new Intent(this, Report.class);
    startActivity(intent);
}

Answer 1

这是 ViewPager 的预期行为。 ViewPager 始终在内存中前后保留一个选项卡以便显示滑块动画。如果您没有内存中的下一个或上一个选项卡（或片段），则尝试在滑块过渡期间启动片段将导致性能滞后，或者更糟糕的是，您将使滑动片段为空并稍后加载.

ViewPager 还允许您通过 setOffscreenPageLimit() 设置页面偏移限制，这将控制片段数量以将它们保存在内存中。默认情况下，这是 1，这意味着前后一个片段将始终存在于内存中

FragmentPagerAdapter getItem() 的返回值超过了应有的值

FragmentPagerAdapter getItem() is returning more than it should

android

android-fragments

tabs

android-tabs