为RecyclerView添加分隔线

1.原理篇

分割线添加原理参照链接：https://www.jianshu.com/p/4eff036360da

在RecyclerView中，Google又为我们提供了什么样的方法去添加分隔线呢？通过查看官方文档，它，提供了:addItemDecoration(RecyclerView.ItemDecoration decor)这个方法了设置分隔线，那问题又来了，RecyclerView.ItemDecoration是什么东西呢？继续查：然后发现如下:它原来是一个类，里面封装了三个方法：

（1）void getItemOffsets ()（2）void onDraw ()（3）void onDrawOver ()

通过上面的三个方法，可以看出，这是要自己直接画上去，准确的说这几个方法是：添加Divider,主要是找到添加Divider的位置， 而Divider是在drawable文件中写好了的。 利用onDraw和onDrawOver都差不多，我们在创建自己的Decoration类继承RecyclerView.ItemDecoration的时候，我们只要重写getItemOffsets()，还有onDraw()和onDrawOver两者其中之一就可以了.

那getItemOffsets()方法有什么用呢？从字面意思就是Item要偏移， 由于我们在Item和Item之间加入了分隔线，线其实本质就是一个长方形，也是用户自定义的，既然线也有长宽高，就画横线来说，上面的Item加入了分隔线，那下面的Item就要往下平移，平移的量就是分隔线的高度.

2.实战篇

package com.picc.vmeng.module.coterie.atapter;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.StaggeredGridLayoutManager;
import android.view.View;

/**
 * author：zhangfeng on 2018/1/18.
 * desc：
 */

public class DividerGridItemDecoration extends RecyclerView.ItemDecoration {

    private static final int[] ATTRS = new int[]{android.R.attr.listDivider};
    private int mHorizontalDividerHight;
    private int mVerticalDividerHight;
    private Paint mColorPaint;
    private Drawable mDivider;
    int spanCount = 1;

    private DividerGridItemDecoration(Context context) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        a.recycle();
    }


    /**
     * @param context
     * @param horizontalDividerHight 水平分割线的宽度 px
     * @param verticalDividerHight 竖直分割线的宽度 px
     * @param dividerColor 分割线颜色
     */
    public DividerGridItemDecoration(Context context, int horizontalDividerHight, int verticalDividerHight, int dividerColor) {
        this(context);
        mHorizontalDividerHight = horizontalDividerHight;
        mVerticalDividerHight = verticalDividerHight;
        mColorPaint = new Paint();
        mColorPaint.setColor(dividerColor);
    }

    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {

        //画水平和垂直分割线
        drawHorizontalDivider(c, parent);
        drawVerticalDivider(c, parent);

    }

    private int getSpanCount(RecyclerView parent) {
        // 列数
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            spanCount = ((GridLayoutManager) layoutManager).getSpanCount();
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            spanCount = ((StaggeredGridLayoutManager) layoutManager)
                    .getSpanCount();
        }
        return spanCount;
    }

    public void drawVerticalDivider(Canvas c, RecyclerView parent) {
        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();

            final int top = child.getTop() - params.topMargin;
            final int bottom = child.getBottom() + params.bottomMargin;

            int left = 0;
            int right = 0;

            //左边第一列
            if ((i % spanCount) == 0) {
                //item左边分割线
                left = child.getLeft();
                right = left + mVerticalDividerHight;
                mDivider.setBounds(left, top, right, bottom);
                mDivider.draw(c);
                if (mColorPaint != null) {
                    c.drawRect(left, top, right, bottom, mColorPaint);
                }
                //item右边分割线
                left = child.getRight() + params.rightMargin - mVerticalDividerHight;
                right = left + mVerticalDividerHight;
            } else {
                //非左边第一列
                left = child.getRight() + params.rightMargin - mVerticalDividerHight;
                right = left + mVerticalDividerHight;
            }
            //画分割线
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(c);
            if (mColorPaint != null) {
                c.drawRect(left, top, right, bottom, mColorPaint);
            }

        }
    }

    public void drawHorizontalDivider(Canvas c, RecyclerView parent) {

        final int childCount = parent.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();

            final int left = child.getLeft() - params.leftMargin - mVerticalDividerHight;
            final int right = child.getRight() + params.rightMargin;
            int top = 0;
            int bottom = 0;

            // 最上面一行
            if ((i / spanCount) == 0) {
                //当前item最上面的分割线
                top = child.getTop();
                //当前item下面的分割线
                bottom = top + mHorizontalDividerHight;
                mDivider.setBounds(left, top, right, bottom);
                mDivider.draw(c);
                if (mColorPaint != null) {
                    c.drawRect(left, top, right, bottom, mColorPaint);
                }
                top = child.getBottom() + params.bottomMargin;
                bottom = top + mHorizontalDividerHight;
            } else {
                top = child.getBottom() + params.bottomMargin;
                bottom = top + mHorizontalDividerHight;
            }
            //画分割线
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(c);
            if (mColorPaint != null) {
                c.drawRect(left, top, right, bottom, mColorPaint);
            }
        }
    }

    private boolean isLastColum(RecyclerView parent, int pos, int spanCount,
                                int childCount) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            // 如果是最后一列，则不需要绘制右边
            if ((pos + 1) % spanCount == 0) {
                return true;
            }
        } else if (layoutManager instanceof LinearLayoutManager) {
            int orientation = ((LinearLayoutManager) layoutManager)
                    .getOrientation();
            if (orientation == LinearLayoutManager.VERTICAL) {
                //不需要绘制右边
                return true;
            } else {
                childCount = childCount - childCount % spanCount;
                // 如果是最后一列，则不需要绘制右边
                if (pos >= childCount) {
                    return true;
                }
            }
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            int orientation = ((StaggeredGridLayoutManager) layoutManager)
                    .getOrientation();
            if (orientation == StaggeredGridLayoutManager.VERTICAL) {
                // 如果是最后一列，则不需要绘制右边
                if ((pos + 1) % spanCount == 0) {
                    return true;
                }
            } else {
                childCount = childCount - childCount % spanCount;
                // 如果是最后一列，则不需要绘制右边
                if (pos >= childCount) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean isLastRaw(RecyclerView parent, int pos, int spanCount,
                              int childCount) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        if (layoutManager instanceof GridLayoutManager) {
            childCount = childCount - childCount % spanCount;
            // 如果是最后一行，则不需要绘制底部
            if (pos >= childCount) {
                return true;
            }
        } else if (layoutManager instanceof LinearLayoutManager) {
            int orientation = ((LinearLayoutManager) layoutManager)
                    .getOrientation();
            // LinearLayoutManager 且纵向滚动
            if (orientation == LinearLayoutManager.VERTICAL) {
                // 如果是最后一行，则不需要绘制底部
                if (pos + 1 == childCount)
                    return true;
            } else {
                // LinearLayoutManager 且横向滚动
                // 如果是最后一行，则不需要绘制底部
                return true;
            }
        } else if (layoutManager instanceof StaggeredGridLayoutManager) {
            int orientation = ((StaggeredGridLayoutManager) layoutManager)
                    .getOrientation();
            // StaggeredGridLayoutManager 且纵向滚动
            if (orientation == StaggeredGridLayoutManager.VERTICAL) {
                childCount = childCount - childCount % spanCount;
                // 如果是最后一行，则不需要绘制底部
                if (pos >= childCount)
                    return true;
            } else {
                // StaggeredGridLayoutManager 且横向滚动
                // 如果是最后一行，则不需要绘制底部
                if ((pos + 1) % spanCount == 0) {
                    return true;
                }
            }
        }
        return false;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        int spanCount = getSpanCount(parent);
        int childCount = parent.getAdapter().getItemCount();
        int itemPosition = parent.getChildLayoutPosition(view);
        // 如果是最后一行，则不需要绘制底部
        if (isLastRaw(parent, itemPosition, spanCount, childCount)) {
            outRect.set(0, 0, mVerticalDividerHight, 0);
            // 如果是最后一行的最后一列，则不需要绘制
            if (isLastColum(parent, itemPosition, spanCount, childCount)) {
                outRect.set(0, 0, 0, 0);
            }
            // 如果是最后一列的最后一行，则不需要绘制右边
        } else if (isLastColum(parent, itemPosition, spanCount, childCount)) {
            outRect.set(0, 0, 0, mHorizontalDividerHight);
            // 如果是最后一行的最后一列，则不需要绘制
            if (isLastRaw(parent, itemPosition, spanCount, childCount)) {
                outRect.set(0, 0, 0, 0);
            }
        } else {
            outRect.set(0, 0, mVerticalDividerHight, mHorizontalDividerHight);
        }
    }
}

3.调用方法

recyclerView.setLayoutManager(mLayoutManager);
recyclerView.addItemDecoration(new DividerGridItemDecoration(this,20,10, Color.parseColor("#f3f3f3")));
recyclerView.setAdapter(mAdapter);

到这里所有的工作就完成了