Andrey Rankov Andrey Rankov - 1 year ago 88
Android Question

Zoom central image recycler view

I have RecyclerView with images. It based on this solution. Images are lazy loaded into view with Glide. I need to add zoom on central image (like on this example). How can i do it?

Answer Source

The most direct way to affect what you want is to extend LinearLayoutManager. As you've no doubt discovered, hooking into the scroll events properly is a pain:

So let's extend the manager. We'll create a few parameters that you might expose.

 public class ZoomCenterCardLayoutManager extends LinearLayoutManager {
   // Shrink the cards around the center up to 50%
   private final float mShrinkAmount = 0.5f;
   // The cards will be at 50% when they are 75% of the way between the
   // center and the edge.
   private final float mShrinkDistance = 0.75f;

Fill out your constructors, and then override scrollHorizontallyBy:

   public int scrollHorizontallyBy(int dx, 
      RecyclerView.Recycler recycler, RecyclerView.State state) {

Call the parent's version and save the distance travelled. We'll need to return this at the end of the method:

      int scrolled = super.scrollHorizontallyBy(dx, recycler, state);

We are going to set up a simple linear interpolation. It looks nice enough.

      float midpoint = getWidth() / 2.f;
      float d0 = 0.f;
      float d1 = mShrinkDistance * midpoint;
      float s0 = 1.f;
      float s1 = 1.f - mShrinkAmount;

Loop through all of the active children of the control, run the interpolation, and set the scale of the child.

      for (int i = 0; i < getChildCount(); i++) {
        View child = getChildAt(i);
        float childMidpoint = 
           (getDecoratedRight(child) + getDecoratedLeft(child)) / 2.f;
        float d = Math.min(d1, Math.abs(midpoint - childMidpoint));
        float scale = s0 + (s1 - s0) * (d - d0) / (d1 - d0);

      return scrolled;

This is almost all you need. One final step is to make sure that this adjustment is called after initialization -- otherwise the zooming won't take effect until the first time the control is moved:

   public void onLayoutChildren(Recycler recycler, State state) {
     super.onLayoutChildren(recycler, state);
     scrollVerticallyBy(0, recycler, state);


And that's all there is to it. Super responsive, and you can drop this new layout manager into any horizontal recycler.

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