RecyclerView中itemDecoration

ItemDecoration

我们可以通过该方法添加分割线： 
mRecyclerView.addItemDecoration() 
该方法的参数为RecyclerView.ItemDecoration，该类为抽象类，官方目前并没有提供默认的实现类（我觉得最好能提供几个）。 
该类的源码：

public static abstract class ItemDecoration {public void onDraw(Canvas c, RecyclerView parent, State state) {            onDraw(c, parent); }public void onDrawOver(Canvas c, RecyclerView parent, State state) {            onDrawOver(c, parent); }public void getItemOffsets(Rect outRect, View view, RecyclerView parent, State state) {            getItemOffsets(outRect, ((LayoutParams) view.getLayoutParams()).getViewLayoutPosition(),                    parent);}@Deprecatedpublic void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {            outRect.set(0, 0, 0, 0); }

当我们调用mRecyclerView.addItemDecoration()方法添加decoration的时候，RecyclerView在绘制的时候，去会绘制decorator，

即调用该类的onDraw和onDrawOver方法，

• onDraw方法先于drawChildren
• onDrawOver在drawChildren之后，一般我们选择复写其中一个即可。
• getItemOffsets 可以通过outRect.set()为每个Item设置一定的偏移量，主要用于绘制Decorator。

接下来我们看一个RecyclerView.ItemDecoration的实现类，该类很好的实现了RecyclerView添加分割线（当使用LayoutManager为LinearLayoutManager时）。 

/* * Copyright (C) 2014 The Android Open Source Project * * Licensed under the Apache License, Version 2.0 (the "License"); * limitations under the License. */import android.content.Context;import android.content.res.TypedArray;import android.graphics.Canvas;import android.graphics.Rect;import android.graphics.drawable.Drawable;import android.support.v7.widget.LinearLayoutManager;import android.support.v7.widget.RecyclerView;import android.support.v7.widget.RecyclerView.State;import android.util.Log;import android.view.View;/** * This class is from the v7 samples of the Android SDK. It's not by me! * <p/> * See the license above for details. */public class DividerItemDecoration extends RecyclerView.ItemDecoration {    private static final int[] ATTRS = new int[]{            android.R.attr.listDivider    };    public static final int HORIZONTAL_LIST = LinearLayoutManager.HORIZONTAL;    public static final int VERTICAL_LIST = LinearLayoutManager.VERTICAL;    private Drawable mDivider;    private int mOrientation;    public DividerItemDecoration(Context context, int orientation) {        final TypedArray a = context.obtainStyledAttributes(ATTRS);        mDivider = a.getDrawable(0);        a.recycle();        setOrientation(orientation);    }    public void setOrientation(int orientation) {        if (orientation != HORIZONTAL_LIST && orientation != VERTICAL_LIST) {            throw new IllegalArgumentException("invalid orientation");        }        mOrientation = orientation;    }    @Override    public void onDraw(Canvas c, RecyclerView parent) {        Log.v("recyclerview - itemdecoration", "onDraw()");        if (mOrientation == VERTICAL_LIST) {            drawVertical(c, parent);        } else {            drawHorizontal(c, parent);        }    }    public void drawVertical(Canvas c, RecyclerView parent) {        final int left = parent.getPaddingLeft();        final int right = parent.getWidth() - parent.getPaddingRight();        final int childCount = parent.getChildCount();        for (int i = 0; i < childCount; i++) {            final View child = parent.getChildAt(i);            android.support.v7.widget.RecyclerView v = new android.support.v7.widget.RecyclerView(parent.getContext());            final RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child                    .getLayoutParams();            final int top = child.getBottom() + params.bottomMargin;            final int bottom = top + mDivider.getIntrinsicHeight();            mDivider.setBounds(left, top, right, bottom);            mDivider.draw(c);        }    }    public void drawHorizontal(Canvas c, RecyclerView parent) {        final int top = parent.getPaddingTop();        final int bottom = parent.getHeight() - parent.getPaddingBottom();        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 left = child.getRight() + params.rightMargin;            final int right = left + mDivider.getIntrinsicHeight();            mDivider.setBounds(left, top, right, bottom);            mDivider.draw(c);        }    }    @Override    public void getItemOffsets(Rect outRect, int itemPosition, RecyclerView parent) {        if (mOrientation == VERTICAL_LIST) {            outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());        } else {            outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);        }    }}

该实现类可以看到通过读取系统主题中的 Android.R.attr.listDivider作为Item间的分割线，并且支持横向和纵向。

如果你不清楚它是怎么做到的读取系统的属性用于自身，请参考我的另一篇博文：Android 深入理解Android中的自定义属性

获取到listDivider以后，该属性的值是个Drawable，在getItemOffsets中，outRect去设置了绘制的范围。onDraw中实现了真正的绘制。

我们在原来的代码中添加一句：

mRecyclerView.addItemDecoration(new DividerItemDecoration(this,DividerItemDecoration.VERTICAL_LIST));

ok，现在再运行，就可以看到分割线的效果了。

该分割线是系统默认的，你可以在theme.xml中找到该属性的使用情况。那么，使用系统的listDivider有什么好处呢？

就是方便我们去随意的改变，该属性我们可以直接声明在：

 <!-- Application theme. -->    <style name="AppTheme" parent="AppBaseTheme">      <item name="android:listDivider">@drawable/divider_bg</item>      </style>

然后自己写个drawable即可，下面我们换一种分隔符：

<?xml version="1.0" encoding="utf-8"?><shape xmlns:android="http://schemas.android.com/apk/res/android"    android:shape="rectangle" >    <gradient        android:centerColor="#ff00ff00"        android:endColor="#ff0000ff"        android:startColor="#ffff0000"        android:type="linear" />    <size android:height="4dp"/></shape>

现在的样子是：

当然了，你可以根据自己的需求，去随意的绘制，反正是画出来的，随便玩~~

节选自：
http://blog.csdn.net/lmj623565791/article/details/45059587

hongyang的博文，多谢！！