问题描述

我有一个带有 ViewPager 的 Activity，它显示一堆图片.当它启动时，ViewPager 的位置是根据用户在前一个 Activity 中选择的内容设置的.类似于画廊.

I have an Activity with a ViewPager which displays a bunch of pictures. When it starts the ViewPager's position is set based on what the user selected in a previous Activity. Similar to a gallery.

我希望每次选择新页面时都调用 onPageSelected，即首次打开 Activity 或用户滑动到新页面时.

I want the onPageSelected to be called every time a new page is selected, i.e. when the Activity is first opened or when the user slides to a new page.

我这样设置起点:

 mPager.setCurrentItem(index);

一切正常，除非调用 setCurrentItem 并将索引设置为 0，因为这不会触发 onPageSelected.

Everything works, except when setCurrentItem called with index set to 0 as this will not trigger onPageSelected.

mPager.setOnPageChangeListener(new OnPageChangeListener() {
  @Override
  public void onPageSelected(int index) {
    Log.d(TAG, "onPageSelected " + index);
  }
  ...
}

所以我的问题是；这是一个错误吗?如果是，我该怎么办?

So my question is; is this a bug, and if so what can I do about it?

推荐答案

到目前为止我找到的最干净的解决方案是引用您在 ViewPager 上设置的 onPageChangeListener(因为我认为没有ViewPager.getOnPageChangeListener() 方法)，然后在设置好 ViewPager 的适配器后，调用:

The cleanest solution I've found to this so far is to take a reference to the onPageChangeListener you set on the ViewPager (since I don't think there's a ViewPager.getOnPageChangeListener() method), then after you've set the ViewPager's adapter, call:

onPageChangeListener.onPageSelected(viewPager.getCurrentItem());

但是，当前索引处的页面片段还没有被实例化(至少如果您使用的是 FragmentStatePagerAdapter)，因此您可能需要将它包装在一个可运行的文件中，ala:

However, the fragment for the page at the current index won't have been instantiated yet (at least if you're using FragmentStatePagerAdapter), so you may need to wrap it in a runnable, ala:

viewPager.post(new Runnable(){
@Override
    public void run() {
        onPageChangeListener.onPageSelected(viewPager.getCurrentItem());
    }
});

此外，如果在 onPageSelected 处理程序中需要对片段的引用，则必须自己进行.我为我的 FragmentStatePagerAdapter 使用了一个抽象基类，它覆盖了实例化和销毁方法，并从 SparseArray 添加/删除了片段.

Also, if within the onPageSelected handler you need a reference to the fragment, you'll have to do that yourself. I use an abstract base class for my FragmentStatePagerAdapter which overrides the instantiate and destroy methods, and adds/removes the fragments from a SparseArray.

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