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滚动时获取RecycleView的中心可见项

作者：大大炮 | 2023-09-08 15:25

如何解决《滚动时获取RecycleView的中心可见项》经验，为你挑选了3个好方法。

这就是我要的:

在此输入图像描述

如上图所示,我想绘制一条中心线RecycleView,然后在滚动时(以及向左或向右移动)获取中心项目.
这是我尝试绘制水平线RecycleView:

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

有没有办法知道哪个项目被移动到中心RecycleView？如何RecycleView向左或向右滚动一个位置？

更新:我试图使用滚动侦听器来获得中间位置,但它不能作为方面.

  @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();
        }

得到结果:

在此输入图像描述

我只想在蓝色时更改选择项目(使文本变为白色) Rect

1> TranHieu..：

我做了这样的事.我可以完全按照你的需要做.首先,这就是我的算法是如何运作的在此输入图像描述

这是我的recyclerView适配器

public class DateAdapter extends RecyclerView.Adapter {
private ArrayList 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 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);
    }
}}

这是最重要的算法:

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);
}

这是我的结果: 在此输入图像描述

看看这个视频链接,这是我的应用程序演示

截图来自iOS设备？这有点令人困惑.

@TranHieu你可以分享这个代码示例吗？

2> silentsudo..：

有时需要将整个示例代码块放在一起,因为我们可能会错过一些东西.这就是我所拥有的,随意纠正任何事情,因为我可能在某处犯了一些小错误.是的,这个答案是@tranhieu答案的延伸.谢谢@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 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 {
        private ArrayList 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 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中

item.xml

dimens.xml


    100dp

3> firegloves..：

我在这里使用了SnapHelper:

    // 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);

正如TranHieu所说,插入2项用于填充(在开始和结束位置)的解决方案是好的.

我不喜欢使用ViewTreeObserver,因为代码的可读性差.使用此技术,您还必须管理重新绘制项目的重新绘制.

如果您使用的是customview类,则可以将其宽度直接设置为这些类.

例如,这是我的填充类

/**
 * 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));
    }

}

在我的适配器中,我存储了所需的填充项目宽度,等于(displayWidth/2) - (realItemWidth/2)

这是我的适配器,不要看不匹配RecyclerView.Adapter的方法,注意initAdapter方法和onCreateItemView方法

@EBean
public class FooAdapterRecycler extends RecyclerViewAdapterBase {

    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;
    }
}

我正在使用AndroidAnnotations库,但这不是必需的

希望有所帮助

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