ViewPager with multiple visible children and selected bigger

Question

In my Activity I've added a Gallery-like View using Dave Smith's PagerContainer example and the code I'm using to instantiate it is very similar to the PagerActivity in his example, while I changed the layout because I needed to use layout weights:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <com.example.myapp.PagerContainer
        android:id="@+id/pager_container"
        android:layout_width="match_parent"
        android:overScrollMode="never"
        android:layout_marginTop="5dp"
        android:layout_marginBottom="5dp"
        android:layout_height="0dp"
        android:layout_weight="0.66"
        >
        <android.support.v4.view.ViewPager
            android:layout_width="150dp"
            android:overScrollMode="never"
            android:layout_height="match_parent"
            android:layout_gravity="center" />
    </com.example.myapp.PagerContainer>

    <ImageView
        android:layout_width="150dp"
        android:layout_height="0dp"
        android:layout_weight="0.34"
        android:id="@+id/current_selection_logo"
        android:layout_gravity="center_horizontal"/>

</LinearLayout>

Now my ViewPager works properly and looks like this (I omitted the bottom ImageView)

But I'd like to resize the unselected items, aiming to emphasize the centered item (selection), something like:

or:

I've found the accepted answer to this question, but implementing that solution means that I'll make the selection bigger than its current dimensions (which already are the maximum possible).

So I was thinking to edit instantiateItem in the PagerAdapter  to set reduced width and height, and in onPageSelected use the approach shown in the question.

The problem is that I'm not sure about which kind of LayoutParams I should call and calling getWidth() and getHeight() in instantiateItem returns 0.

Currently I'm using:

View v = container.getChildAt(position);
PagerContainer.LayoutParams params = (PagerContainer.LayoutParams)v.getLayoutParams();

but it sometimes works and sometimes throws a NPE.

Is my approach correct or should I proceed in another way? In both cases, what should I change to achieve what I want?

Answer 1

To achieve this result, I ended up using PixPlicity's MultiViewPager and coding a simple PageTransformer as suggested by @Blackbelt:

private float mScale = 0.8f;
private class ScalePageTransformer implements ViewPager.PageTransformer{
    @Override
    public void transformPage(View view, float position) {
        if(position==0){
            ViewCompat.setScaleX(view, 1);
            ViewCompat.setScaleY(view, 1);
        }
        else{
            ViewCompat.setScaleX(view, mScale);
            ViewCompat.setScaleY(view, mScale);
        }
    }
}

The result is something like:

Answer 2

You have to use a PageTransform for this purpose. The callback you have to implement receives two parameters. The former is the View object, the latter is an int. When its value is zero, it means the view objects refers to the fully visible object to the center. -1 and -1, are the fully visible views to the left and right respectively. You can use this value to implement the transformation you want. Using ViewCompat, for instance, you can change the translationX, translationY and Scale values