SnapHelper

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In this blog, we will learn how to use SnapHelper in RecyclerView in Android Application.

What is SnapHelper?

SnapHelper is a helper class that helps in snapping any child view of the RecyclerView. For example, you can snap the firstVisibleItem of the RecyclerView as you must have seen in the play store application that the firstVisibleItem will be always completely visible when scrolling comes to the idle position.

View the GIF here.

Android already provides a helper class to do the same. The class is LinearSnapHelper which will only enable the center snapping.

You just need to do the following changes in your code:

SnapHelper snapHelper = new LinearSnapHelper();
snapHelper.attachToRecyclerView(yourRecyclerView);

View the GIF here.

Now, how to do start snapping like below?

View the GIF here.

First of all, you will have to create a class let say StartSnapHelper which will be extending LinearSnapHelper and mainly override the following methods:

1. calculateDistanceToFinalSnap

Override this method to snap to a particular point within the target view or the container view on any axis. This method is called when the SnapHelper has intercepted a fling and it needs to know the exact distance required to scroll by in order to snap to the target view.

2. findSnapView

Override this method to provide a particular target view for snapping. This method is called when the SnapHelper is ready to start snapping and requires a target view to snap to. It will be explicitly called when the scroll state becomes idle after a scroll. It will also be called when the SnapHelper is preparing to snap after a fling and requires a reference view from the current set of child views. If this method returns null, SnapHelper will not snap to any view.

import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.LinearSnapHelper;
import android.support.v7.widget.OrientationHelper;
import android.support.v7.widget.RecyclerView;
import android.view.View;

/**
 * Created by amitshekhar on 15/01/17.
 */

public class StartSnapHelper extends LinearSnapHelper {

    private OrientationHelper mVerticalHelper, mHorizontalHelper;

    public StartSnapHelper() {

    }

    @Override
    public void attachToRecyclerView(@Nullable RecyclerView recyclerView)
            throws IllegalStateException {
        super.attachToRecyclerView(recyclerView);
    }

    @Override
    public int[] calculateDistanceToFinalSnap(@NonNull RecyclerView.LayoutManager layoutManager,
                                              @NonNull View targetView) {
        int[] out = new int[2];

        if (layoutManager.canScrollHorizontally()) {
            out[0] = distanceToStart(targetView, getHorizontalHelper(layoutManager));
        } else {
            out[0] = 0;
        }

        if (layoutManager.canScrollVertically()) {
            out[1] = distanceToStart(targetView, getVerticalHelper(layoutManager));
        } else {
            out[1] = 0;
        }
        return out;
    }

    @Override
    public View findSnapView(RecyclerView.LayoutManager layoutManager) {

        if (layoutManager instanceof LinearLayoutManager) {

            if (layoutManager.canScrollHorizontally()) {
                return getStartView(layoutManager, getHorizontalHelper(layoutManager));
            } else {
                return getStartView(layoutManager, getVerticalHelper(layoutManager));
            }
        }

        return super.findSnapView(layoutManager);
    }

    private int distanceToStart(View targetView, OrientationHelper helper) {
        return helper.getDecoratedStart(targetView) - helper.getStartAfterPadding();
    }

    private View getStartView(RecyclerView.LayoutManager layoutManager,
                              OrientationHelper helper) {

        if (layoutManager instanceof LinearLayoutManager) {
            int firstChild = ((LinearLayoutManager) layoutManager).findFirstVisibleItemPosition();

            boolean isLastItem = ((LinearLayoutManager) layoutManager)
                    .findLastCompletelyVisibleItemPosition()
                    == layoutManager.getItemCount() - 1;

            if (firstChild == RecyclerView.NO_POSITION || isLastItem) {
                return null;
            }

            View child = layoutManager.findViewByPosition(firstChild);

            if (helper.getDecoratedEnd(child) >= helper.getDecoratedMeasurement(child) / 2
                    && helper.getDecoratedEnd(child) > 0) {
                return child;
            } else {
                if (((LinearLayoutManager) layoutManager).findLastCompletelyVisibleItemPosition()
                        == layoutManager.getItemCount() - 1) {
                    return null;
                } else {
                    return layoutManager.findViewByPosition(firstChild + 1);
                }
            }
        }

        return super.findSnapView(layoutManager);
    }

    private OrientationHelper getVerticalHelper(RecyclerView.LayoutManager layoutManager) {
        if (mVerticalHelper == null) {
            mVerticalHelper = OrientationHelper.createVerticalHelper(layoutManager);
        }
        return mVerticalHelper;
    }

    private OrientationHelper getHorizontalHelper(RecyclerView.LayoutManager layoutManager) {
        if (mHorizontalHelper == null) {
            mHorizontalHelper = OrientationHelper.createHorizontalHelper(layoutManager);
        }
        return mHorizontalHelper;
    }
}

Now, attach this to your RecyclerView.

SnapHelper startSnapHelper = new StartSnapHelper();
startSnapHelper.attachToRecyclerView(yourRecyclerView);

Here, you can find the complete sample app.

Prepare yourself for Android Interview: Android Interview Questions

That's it for now.

Thanks

Amit Shekhar

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What is SnapHelper?

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