Get center visible item of RecycleView when scrolling

Question

This is what I want:

As image above, I want to draw a center line on RecycleView, then get the center item when scrolling (as well as move left or right)
Here is my try to draw a horizontal RecycleView:

    HorizontalAdapter adapter = new HorizontalAdapter(data);
    LinearLayoutManager layoutManager
            = new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
    recycleView.setLayoutManager(layoutManager);
    recycleView.setAdapter(adapter);

Is there any way to know which item is moved to the center of RecycleView? And how can I scroll RecycleView to left or right just one position?

Update: I tried to use a scroll listener to get the middle position, but it doesn't work as an aspect.

  @Override
        public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
            super.onScrolled(recyclerView, dx, dy);
            int firstPos = layoutManager.findFirstVisibleItemPosition();
            int lastPos = layoutManager.findLastVisibleItemPosition();
            int middle = Math.abs(lastPos - firstPos) / 2 + firstPos;

            int selectedPos = -1;
            for (int i = 0; i < adapter.getItemCount(); i++) {
                if (i == middle) {
                    adapter.getItem(i).setSelected(true);
                    selectedPos = i;
                } else {
                    adapter.getItem(i).setSelected(false);
                }
            }

            adapter.notifyDataSetChanged();
        }

And get the result:

I only want to change the selected item (make text to white color) when it is on the blue Rect

Answer 1

I made something just like this. I can do exactly what you need. First of all, this is how is my alogrithm work

This is my recyclerView Adapter

public class DateAdapter extends RecyclerView.Adapter<DateAdapter.DateViewHolder> {
private ArrayList<LabelerDate> dateDataList;


private static final int VIEW_TYPE_PADDING = 1;
private static final int VIEW_TYPE_ITEM = 2;
private int paddingWidthDate = 0;

private int selectedItem = -1;

public DateAdapter(ArrayList<LabelerDate> dateData, int paddingWidthDate) {
    this.dateDataList = dateData;
    this.paddingWidthDate = paddingWidthDate;

}


@Override
public DateViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    if (viewType == VIEW_TYPE_ITEM) {
        final View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item_date,
                parent, false);
        return new DateViewHolder(view);
    } else {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.list_item_padding,
                parent, false);

        RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) view.getLayoutParams();
        layoutParams.width = paddingWidthDate;
        view.setLayoutParams(layoutParams);
        return new DateViewHolder(view);
    }
}

@Override
public void onBindViewHolder(DateViewHolder holder, int position) {
    LabelerDate labelerDate = dateDataList.get(position);
    if (getItemViewType(position) == VIEW_TYPE_ITEM) {
        if(labelerDate.dateType.equals(BirthDayActivity.DateType.C31))
                holder.tvDate.setText(String.valueOf(labelerDate.valueDate));
                holder.tvDate.setVisibility(View.VISIBLE);
                holder.imgSmall.setVisibility(View.VISIBLE);

        if (position == selectedItem) {
            holder.tvDate.setTextColor(Color.parseColor("#094673"));
            holder.tvDate.setTextSize(35);
            holder.imgSmall.setBackgroundResource(R.color.textviewbold);

        } else {
            holder.tvDate.setTextColor(Color.GRAY);
            holder.tvDate.setTextSize(35);
            holder.imgSmall.setBackgroundResource(R.color.gray);
        }
    }
}

public void setSelecteditem(int selecteditem) {
    this.selectedItem = selecteditem;
    notifyDataSetChanged();
}

@Override
public int getItemCount() {
    return dateDataList.size();
}

@Override
public int getItemViewType(int position) {
    LabelerDate labelerDate = dateDataList.get(position);
    if (labelerDate.dateType.equals(BirthDayActivity.DateType.NONE)) {
        return VIEW_TYPE_PADDING;
    }
    return VIEW_TYPE_ITEM;
}


public class DateViewHolder extends RecyclerView.ViewHolder {
    public TextView tvDate;
    public ImageView imgSmall;

    public DateViewHolder(View itemView) {
        super(itemView);
        tvDate = (TextView) itemView.findViewById(R.id.tvNumberDate);
        imgSmall = (ImageView) itemView.findViewById(R.id.small_marked_dob);
    }
}}

This is most important alogrithm:

public void getRecyclerviewDate() {
    recyclerViewDate = (RecyclerView) findViewById(R.id.recyclerViewDay);
    ViewTreeObserver vtoDate = recyclerViewDate.getViewTreeObserver();
    vtoDate.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
        @Override
        public boolean onPreDraw() {
            recyclerViewDate.getViewTreeObserver().removeOnPreDrawListener(this);
            finalWidthDate = recyclerViewDate.getMeasuredWidth();
            itemWidthDate = getResources().getDimension(R.dimen.item_dob_width);
            paddingDate = (finalWidthDate - itemWidthDate) / 2;
            firstItemWidthDate = paddingDate ;
            allPixelsDate = 0;

            final LinearLayoutManager dateLayoutManager = new LinearLayoutManager(getApplicationContext());
            dateLayoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
            recyclerViewDate.setLayoutManager(dateLayoutManager);
            recyclerViewDate.addOnScrollListener(new RecyclerView.OnScrollListener() {
                @Override
                public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                    super.onScrollStateChanged(recyclerView, newState);
                    synchronized (this) {
                         if(newState == RecyclerView.SCROLL_STATE_IDLE){           
                            calculatePositionAndScrollDate(recyclerView);
                        }
                    }

                }

                @Override
                public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                    super.onScrolled(recyclerView, dx, dy);
                    allPixelsDate += dx;
                }
            });
            if (labelerDates == null)
                labelerDates = new ArrayList<>();
            labelerDates.addAll(genLabelerDate(currentMonth, currentYear));
            dateAdapter = new DateAdapter(labelerDates, (int) firstItemWidthDate);
            recyclerViewDate.setAdapter(dateAdapter);
            return true;
        }
    });
}
/* this if most important, if expectedPositionDate < 0 recyclerView will return to nearest item*/

private void calculatePositionAndScrollDate(RecyclerView recyclerView) {
    int expectedPositionDate = Math.round((allPixelsDate + paddingDate - firstItemWidthDate) / itemWidthDate);

    if (expectedPositionDate == -1) {
        expectedPositionDate = 0;
    } else if (expectedPositionDate >= recyclerView.getAdapter().getItemCount() - 2) {
        expectedPositionDate--;
    }
    scrollListToPositionDate(recyclerView, expectedPositionDate);

}
/* this if most important, if expectedPositionDate < 0 recyclerView will return to nearest item*/
private void scrollListToPositionDate(RecyclerView recyclerView, int expectedPositionDate) {
    float targetScrollPosDate = expectedPositionDate * itemWidthDate + firstItemWidthDate - paddingDate;
    float missingPxDate = targetScrollPosDate - allPixelsDate;
    if (missingPxDate != 0) {
        recyclerView.smoothScrollBy((int) missingPxDate, 0);
    }
}
private void setDateValue() {
    int expectedPositionDateColor = Math.round((allPixelsDate + paddingDate - firstItemWidthDate) / itemWidthDate);
    setColorDate = expectedPositionDateColor + 1;
    //set color here
    dateAdapter.setSelecteditem(setColorDate);
}
 @Override
protected void onRestoreInstanceState(@NonNull Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);   
    allPixelsDate = savedInstanceState.getFloat(BUNDLE_LIST_PIXELS_DATE);
    allPixelsDateChanged = savedInstanceState.getFloat(BUNDLE_LIST_PIXELS_DATE_CHANGED);
}

@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putFloat(BUNDLE_LIST_PIXELS_DATE, allPixelsDate);
    outState.putFloat(BUNDLE_LIST_PIXELS_DATE_CHANGED, allPixelsDateChanged);
}

And this is my result:

Look at this video link, this is my app demo

Answer 2

Sometimes is needed the entire example code block together, because we may miss something. Here is what I have, feel free to correct anything since I may be doing some little mistake somewhere. And Yes, this answer is an extension of @tranhieu answer. Thanks @tranhieu.

MainActivity.java

package com.test;

import android.app.Activity;
import android.graphics.Color;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.ViewTreeObserver;
import android.widget.TextView;

import java.util.ArrayList;

public class MainActivity extends Activity {

    private static final String TAG = MainActivity.class.getSimpleName();

    public float firstItemWidthDate;
    public float paddingDate;
    public float itemWidthDate;
    public int allPixelsDate;
    public int finalWidthDate;
    private DateAdapter dateAdapter;
    private ArrayList<LabelerDate> labelerDates = new ArrayList<>();

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getRecyclerviewDate();

    }


    public void getRecyclerviewDate() {
        final RecyclerView recyclerViewDate = (RecyclerView) findViewById(R.id.rv_tasks_date);
        if (recyclerViewDate != null) {
            recyclerViewDate.postDelayed(new Runnable() {
                @Override
                public void run() {
                    setDateValue();
                }
            }, 300);
            recyclerViewDate.postDelayed(new Runnable() {
                @Override
                public void run() {
                    recyclerViewDate.smoothScrollToPosition(dateAdapter.getItemCount()-1);
                    setDateValue();
                }
            }, 5000);
        }
        ViewTreeObserver vtoDate = recyclerViewDate.getViewTreeObserver();
        vtoDate.addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {


            @Override
            public boolean onPreDraw() {
                recyclerViewDate.getViewTreeObserver().removeOnPreDrawListener(this);
                finalWidthDate = recyclerViewDate.getMeasuredWidth();
                itemWidthDate = getResources().getDimension(R.dimen.item_dob_width);
                paddingDate = (finalWidthDate - itemWidthDate) / 2;
                firstItemWidthDate = paddingDate;
                allPixelsDate = 0;

                final LinearLayoutManager dateLayoutManager = new LinearLayoutManager(getApplicationContext());
                dateLayoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
                recyclerViewDate.setLayoutManager(dateLayoutManager);
                recyclerViewDate.addOnScrollListener(new RecyclerView.OnScrollListener() {
                    @Override
                    public void onScrollStateChanged(RecyclerView recyclerView, int newState) {
                        super.onScrollStateChanged(recyclerView, newState);
                        synchronized (this) {
                            if (newState == RecyclerView.SCROLL_STATE_IDLE) {
                                calculatePositionAndScrollDate(recyclerView);
                            }
                        }

                    }

                    @Override
                    public void onScrolled(RecyclerView recyclerView, int dx, int dy) {
                        super.onScrolled(recyclerView, dx, dy);
                        allPixelsDate += dx;
                    }
                });
                if (labelerDates == null) {
                    labelerDates = new ArrayList<>();
                }
                genLabelerDate();
                dateAdapter = new DateAdapter(labelerDates, (int) firstItemWidthDate);
                recyclerViewDate.setAdapter(dateAdapter);
                dateAdapter.setSelecteditem(dateAdapter.getItemCount() - 1);
                return true;
            }
        });
    }

    private void genLabelerDate() {
        for (int i = 0; i < 32; i++) {
            LabelerDate labelerDate = new LabelerDate();
            labelerDate.setNumber(Integer.toString(i));
            labelerDates.add(labelerDate);

            if (i == 0 || i == 31) {
                labelerDate.setType(DateAdapter.VIEW_TYPE_PADDING);
            } else {
                labelerDate.setType(DateAdapter.VIEW_TYPE_ITEM);
            }
        }
    }
/* this if most important, if expectedPositionDate < 0 recyclerView will return to nearest item*/

    private void calculatePositionAndScrollDate(RecyclerView recyclerView) {
        int expectedPositionDate = Math.round((allPixelsDate + paddingDate - firstItemWidthDate) / itemWidthDate);

        if (expectedPositionDate == -1) {
            expectedPositionDate = 0;
        } else if (expectedPositionDate >= recyclerView.getAdapter().getItemCount() - 2) {
            expectedPositionDate--;
        }
        scrollListToPositionDate(recyclerView, expectedPositionDate);

    }

    /* this if most important, if expectedPositionDate < 0 recyclerView will return to nearest item*/
    private void scrollListToPositionDate(RecyclerView recyclerView, int expectedPositionDate) {
        float targetScrollPosDate = expectedPositionDate * itemWidthDate + firstItemWidthDate - paddingDate;
        float missingPxDate = targetScrollPosDate - allPixelsDate;
        if (missingPxDate != 0) {
            recyclerView.smoothScrollBy((int) missingPxDate, 0);
        }
        setDateValue();
    }

    //
    private void setDateValue() {
        int expectedPositionDateColor = Math.round((allPixelsDate + paddingDate - firstItemWidthDate) / itemWidthDate);
        int setColorDate = expectedPositionDateColor + 1;
//        set color here
        dateAdapter.setSelecteditem(setColorDate);
    }


    public class DateAdapter extends RecyclerView.Adapter<DateAdapter.DateViewHolder> {
        private ArrayList<LabelerDate> dateDataList;


        private static final int VIEW_TYPE_PADDING = 1;
        private static final int VIEW_TYPE_ITEM = 2;
        private int paddingWidthDate = 0;

        private int selectedItem = -1;

        public DateAdapter(ArrayList<LabelerDate> dateData, int paddingWidthDate) {
            this.dateDataList = dateData;
            this.paddingWidthDate = paddingWidthDate;

        }


        @Override
        public DateViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            if (viewType == VIEW_TYPE_ITEM) {
                final View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item,
                    parent, false);
                return new DateViewHolder(view);
            } else {
                View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.item,
                    parent, false);

                RecyclerView.LayoutParams layoutParams = (RecyclerView.LayoutParams) view.getLayoutParams();
                layoutParams.width = paddingWidthDate;
                view.setLayoutParams(layoutParams);
                return new DateViewHolder(view);
            }
        }

        @Override
        public void onBindViewHolder(DateViewHolder holder, int position) {
            LabelerDate labelerDate = dateDataList.get(position);
            if (getItemViewType(position) == VIEW_TYPE_ITEM) {
                holder.tvDate.setText(labelerDate.getNumber());
                holder.tvDate.setVisibility(View.VISIBLE);

                Log.d(TAG, "default " + position + ", selected " + selectedItem);
                if (position == selectedItem) {
                    Log.d(TAG, "center" + position);
                    holder.tvDate.setTextColor(Color.parseColor("#76FF03"));
                    holder.tvDate.setTextSize(35);

                } else {
                    holder.tvDate.setTextColor(Color.WHITE);
                    holder.tvDate.setTextSize(18);
                }
            } else {
                holder.tvDate.setVisibility(View.INVISIBLE);
            }
        }

        public void setSelecteditem(int selecteditem) {
            this.selectedItem = selecteditem;
            notifyDataSetChanged();
        }

        @Override
        public int getItemCount() {
            return dateDataList.size();
        }

        @Override
        public int getItemViewType(int position) {
            LabelerDate labelerDate = dateDataList.get(position);
            if (labelerDate.getType() == VIEW_TYPE_PADDING) {
                return VIEW_TYPE_PADDING;
            } else {
                return VIEW_TYPE_ITEM;
            }

        }


        public class DateViewHolder extends RecyclerView.ViewHolder {
            public TextView tvDate;

            public DateViewHolder(View itemView) {
                super(itemView);
                tvDate = (TextView) itemView.findViewById(R.id.txt_date);
            }
        }
    }

    private class LabelerDate {
        private int type;
        private String number;

        public String getNumber() {
            return number;
        }

        public void setNumber(String number) {
            this.number = number;
        }

        public int getType() {
            return type;
        }

        public void setType(int type) {
            this.type = type;
        }
    }
}

activity_main.xml

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

    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <android.support.v7.widget.RecyclerView
            android:id="@+id/rv_tasks_date"
            android:layout_width="match_parent"
            android:layout_height="48dp" />

        <ImageView
            android:layout_width="48dp"
            android:layout_height="48dp"
            android:layout_gravity="center"
            android:layout_marginTop="48dp"
            android:src="@android:drawable/ic_dialog_info" />
    </FrameLayout>

</LinearLayout>

item.xml

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

    <TextView
        android:id="@+id/txt_date"
        android:layout_width="@dimen/item_dob_width"
        android:layout_height="48dp"
        android:text="32"
        android:textColor="@android:color/white"
        android:background="@android:color/darker_gray"
        android:textSize="28sp"
        android:gravity="center"/>

</LinearLayout>

dimens.xml

<resources>
    <dimen name="item_dob_width">100dp</dimen>
</resources>

Answer 3

Oh boy. I've been searching for this answer for almost a week and then found out the solution. Custom LayoutManagers? No. ItemDecorator? Nope.

Here's the easiest way to do it:

<android.support.v7.widget.RecyclerView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_gravity="center_horizontal"
    android:paddingStart="150dp"
    android:paddingEnd="150dp"
    android:clipToPadding="false" />

The critical part is:

    android:paddingStart="150dp"
    android:paddingEnd="150dp"
    android:clipToPadding="false"

And then just assign SnapHelper to your RecylcerView:

val snapHelper = LinearSnapHelper()
snapHelper.attachToRecyclerView(recyclerView)

This is it. The easiest and most perfect solution to the problem

Answer 4

I'm used the SnapHelper right here:

    // init snaphelper        
    SnapHelper snapHelper = new LinearSnapHelper();
    snapHelper.attachToRecyclerView(recyclerView)

    // init layout manager
    LinearLayoutManager layoutManager = new LinearLayoutManager(mainActivity);
    layoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
    recyclerView.setLayoutManager(layoutManager);

    // init adapter
    adatper.setSnapHelper(snapHelper);
    adatper.setLayoutManager(layoutManager);

    adatper.initAdapter(new Float((DisplayHelper.getDisplayWidth(mainActivity) / 2) - (fooViewWidth / 2)).intValue());
    recyclerView.setAdapter(adatper);

As said by TranHieu the solution of inserting 2 item for padding (at start and at end positions) is good.

I don't like the use of ViewTreeObserver because of poor readability of code. With this technique then you must also manage redrawing of the items if they are recycled.

If you are using customview classes you can set its width directly into these classes.

For example this is my padding class

/**
 * Created by firegloves on 25/09/15.
 */
@EViewGroup(R.layout.view_padding)
public class PaddingView extends FooView {

    Context mCtx;

    public PaddingView(Context context) {
        super(context);
        mCtx = context;
    }


    public void setWidth(int width) {
        setLayoutParams(new LayoutParams(width, ViewGroup.LayoutParams.WRAP_CONTENT));
    }

}

In my adapter I store the desired padding item width, that is equal to (displayWidth / 2) - (realItemWidth / 2)

This is my adapter, don't look at methods not matching RecyclerView.Adapter, pay attention to the initAdapter method and to the onCreateItemView method

@EBean
public class FooAdapterRecycler extends RecyclerViewAdapterBase<Foo, FooView> {

    private final int TYPE_PADDING_VIEW = 0;
    private final int TYPE_REAL_VIEW = 1;

    @RootContext
    Context ctx;
    @Bean(Finder.class)
    IFinder finder;

    SnapHelper snapHelper;
    RecyclerView.LayoutManager layoutManager;

    private int paddingWidth = 0;

    /**
     * preleva i dati dal finder
     */
    public void initAdapter(int paddingWidth) {

        /*******************************
         * THIS CODE IS THE IMPORTANT ONE
         ******************************/

        this.paddingWidth = paddingWidth;

        // add 1 item for initial space
        mItems = new ArrayList<>();
        Foo foo = new Foo();
        mItems.add(foo);

        // get real items from finder
        mItems.addAll(finder.findAll());

        // add 1 item for final space
        mItems = new ArrayList<>();
        Foo foo2 = new Foo();
        mItems.add(foo2);

    }


    @Override
    public int getItemViewType(int position) {
        if (position == 0 || position == getItemCount()-1) {
            return TYPE_PADDING_VIEW;
        } else {
            return TYPE_REAL_VIEW;
        }
    }

    @Override
    protected FooView onCreateItemView(ViewGroup parent, int viewType) {

        /*******************************
         * THIS CODE IS THE IMPORTANT ONE
         ******************************/

        if (viewType == TYPE_PADDING_VIEW) {
            PaddingView view = PaddingView_.build(ctx);
            view.setWidth(paddingWidth);
            return view;
        } else {
            return FooView_.build(ctx);
        }
    }

    public void setSnapHelper(SnapHelper snapHelper) {
        this.snapHelper = snapHelper;
    }

    public void setLayoutManager(RecyclerView.LayoutManager layoutManager) {
        this.layoutManager = layoutManager;
    }
}

I'm using AndroidAnnotations library but it's not required

Hope that helps