Viewpager“ ViewGroup”更改背景颜色运行时

Question

I have got a simple viewpager with a few views. I have a button in each view that once clicked it should change the background color of the view to a pre-determined color.

In my instantiateItem I have the following:

public CustomPagerAdapter(Context context) {
    mContext = context;
}

@Override
public Object instantiateItem(ViewGroup collection, int position) {
    LayoutInflater inflater = LayoutInflater.from(mContext);
    ViewGroup layout = (ViewGroup) inflater.inflate(R.layout.child_item, collection, false);

    PopulateView(position);
    collection.addView(layout);
    return layout;
}

I have another function (populateView) that takes care of drawing my view and creating the button.

I was thinking of holding on to an instance of "ViewGroup layout" as class variable and use it but clicking the button in view 1 would cause a random view (say view 2)'s background color to change.

So I kept a copy of "ViewGroup collection" and used

collection.getChildAt(position).setBackgroundColor(mContext.getResources().getColor(sky));

but the app would crash on the last view color change with the error java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setBackgroundColor(int)' on a null object reference

collection = {ViewPage@4834} "android.support.v4.view.ViewPager{e1f06a0 VFED..... ......ID 0,0-1080,1536 #7f0d0091 app:id/viewpager}"
mContext = {MainActivity@4835}
position = 2

The strangest thing is when I look inside "collection", and expand the "Children" there are only 2 children listed. There should be 3. I can't understand why this is.

Can someone please help me?

Answer 1

Well you need to give the position at which to add the child. See here

@Override
public Object instantiateItem(ViewGroup collection, int position) {

    .....


    collection.addView(layout, 0);
    return layout;
}