RecyclerView之ItemDecoration

使用

RecyclerView.ItemDecoration用来装饰RecyclerView的itemView，我们常用来做分割线，但它的功能不仅是分割线。
实现ItemDecoration，需要实现子类，主要实现他的两个主要方法

/**
  这个方法主要是获取itemView的范围
  outRect就是扩展参数，0表示范围跟itemView范围一样。
  >0表示范围外扩（如果无法外扩则边界预留）。
  outRect.top=10,表示上边界外扩 10px (如果范围超出了RecyclerView区域，表示0-10px用于绘制边界，不会用于itemView的展示)
**/
   override fun getItemOffsets(
        outRect: Rect, view: View, parent: RecyclerView,state: RecyclerView.State
    )

/** 
    画装饰条目，需要绘制出全部item的装饰。一般根据itemView的LayoutParam，定位到canvas绘制的位置 
**/
    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State)

代码片

package com.me.kotlinlearn

import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.graphics.Rect
import android.view.View
import androidx.recyclerview.widget.RecyclerView

class MyDecoration(val isVertical: Boolean, val colorValue:Int, val size:Int) : RecyclerView.ItemDecoration() {
    public constructor() : this(false, Color.parseColor("#ff000000"), 100)

    private val paint = Paint().apply {
        flags = Paint.ANTI_ALIAS_FLAG
        color = colorValue
    }

   
    override fun getItemOffsets(
        outRect: Rect,
        view: View,
        parent: RecyclerView,
        state: RecyclerView.State
    ) {
        super.getItemOffsets(outRect, view, parent, state)
        if(isVertical) {
            outRect.set(0, 0, size, 0)
        } else {
            outRect.set(0, 0, 0, size)
        }
    }

    override fun onDraw(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        super.onDraw(c, parent, state)
        if (isVertical) {
            drawVertical(c, parent, state)
        } else {
            drawHorizontal(c, parent, state)
        }
    }

    /**
     * 画水平分割线
     */
    private fun drawHorizontal(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        for(i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            val param = child.layoutParams as RecyclerView.LayoutParams
            val left = child.left - param.leftMargin
            val top = child.bottom + param.bottomMargin
            val right = child.right + param.rightMargin
            val bottom = top + size
            c.drawRect(Rect(left, top, right, bottom), paint)
        }
    }

    /**
    * 画竖直分割线
    **/
    private fun drawVertical(c: Canvas, parent: RecyclerView, state: RecyclerView.State) {
        for(i in 0 until parent.childCount) {
            val child = parent.getChildAt(i)
            val param = child.layoutParams as RecyclerView.LayoutParams
            val left = child.right + param.rightMargin
            val top = child.top - param.topMargin
            val right = left + size
            val bottom = child.bottom + param.bottomMargin
            c.drawRect(Rect(left, top, right,bottom), paint)
        }
    }
}

RecyclerView提供的ItemDecoration

DividerItemDecoration

用来为线性布局提供分割线。分割线可直接在layout中实现。