android - ViewPager 上的 setCurrentItem()

Question

我像这样创建一个自定义 View 寻呼机:

public class TaskPagerAdapter extends FragmentStatePagerAdapter {

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

@Override
public Fragment getItem(int position) {
    Fragment fragment = null;

    switch (position) {
        case 0:
            fragment = new NotificationFragment();
            break;
        case 1:
            break;
        case 2:
            break;
        case 3:
            break;
        case 4:
            break;

    }
    return fragment;
}

@Override
public int getCount() {
    return 5;
}


@Override
public int getItemPosition(Object object) {
    return POSITION_NONE;
}

并用于主 fragment :

        mPagerAdapter = new TaskPagerAdapter(getFragmentManager());
    mViewPager.setAdapter(mPagerAdapter);
    mViewPager.setCurrentItem(-2);

在 setCurrentItem 中，我必须使用哪个数字，直到我没有收到 nullpointer 错误？我希望 NotificationFragment() 类首先在 View 寻呼机上启动

编辑

whit mViewPager.setCurrentItem(-1);我遇到错误:

 Process: safarayaneh.com.anotify.debug, PID: 21484
                                                                           java.lang.NullPointerException: Attempt to invoke virtual method 'void android.support.v4.app.Fragment.setMenuVisibility(boolean)' on a null object reference
                                                                               at android.support.v4.app.FragmentStatePagerAdapter.instantiateItem(FragmentStatePagerAdapter.java:120)
                                                                               at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1034)
                                                                               at android.support.v4.view.ViewPager.populate(ViewPager.java:1248)
                                                                               at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
                                                                               at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
                                                                               at android.view.View.measure(View.java:18788)
                                                                               at android.support.constraint.ConstraintLayout.onMeasure(ConstraintLayout.java:1028)...

Answer 1

阅读 setCurrentItem

Set the currently selected page. If the ViewPager has already been through its first layout with its current adapter there will be a smooth animated transition between the current item and the specified item.

问题

mViewPager.setCurrentItem(-1);

不要设置 -1 或 -2 。 索引从0开始 .

编辑

switch (position) {
    case 0:
        fragment = new NotificationFragment();
        break;
    case 1:
        break;
    case 2:
        break;
    case 3:
        break;
    case 4:
        break;

}
return fragment;

为什么 1,2,3,4 空白。你应该试试

 switch (position) {
    case 0:
        fragment = new NotificationFragment();
        break;

}
return fragment;

mViewPager.setCurrentItem(0);