实现GridLayoutManager 平均间距的ItemDecoration

计算原理查看文章：RecyclerView GridLayoutManger平分间距问题

package com.trs.v8.main.module.providers.mutil_line;

import android.graphics.Canvas;
import android.graphics.Rect;
import android.util.Log;
import android.view.View;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

/**
 * 
 * Created by zhuguohui
 * Date: 2023/11/23
 * Time: 17:51
 * Desc:
 * 和GridLayoutManager搭配，可以实现所有item。前后左右的间距都是一样的
 * 计算原理查看文章：https://juejin.cn/post/6989596537147883528
 * 
 */
class AverageGridSpacingItemDecoration extends RecyclerView.ItemDecoration {
    private static final String TAG = AverageGridSpacingItemDecoration.class.getSimpleName();
    int spanCount;
    /**
     *
     * @param spanCount 每一行有多少个item
     * @param itemSpace 每个item之间的间距。为什么要传入固定值，
     *                  因为在{@link #getItemOffsets(Rect, View, RecyclerView, RecyclerView.State)}方法中，
     *                  不一定能获取到recycleView的宽度，导致间距不一定能计算出来。
     *                  间距的计算方法是（RecycleView的宽度-spanCount*item的宽度）/(spanCount+1)
     *
     */
    public AverageGridSpacingItemDecoration(int spanCount, int itemSpace) {
        this.spanCount = spanCount;
        this.itemSpace = itemSpace;
        //要计算left边距，和right边距，在GridLayoutManager中item的宽度
        int remainWidth =itemSpace *( spanCount+1);
        // p为每个Item都需要减去的间距
        p = remainWidth / spanCount;
    }

    int itemSpace = -1;
    int p;

    @Override
    public void getItemOffsets(@NonNull Rect outRect, @NonNull View view, @NonNull RecyclerView parent, @NonNull RecyclerView.State state) {


        int position = parent.getChildAdapterPosition(view);

        // 获取第几列
        int column = position % spanCount;
        // 第几行
        int row = position / spanCount;
        if (row == 0) { // 设置top
            outRect.top = itemSpace;
        }


        int left = itemSpace + column * (itemSpace - p);
        int right = p - left;

        outRect.left = Math.round(left);
        outRect.right = Math.round(right);
        outRect.bottom=itemSpace;
        Log.i(TAG, "getItemOffsets: position="+position+" left="+left+" right="+right);


    }

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