RecyclerView区分视图类型的Divider

我们都知道support库有一个DividerItemDecoration, 可以作为Item之间的分隔线(divider)。但它作用以后所有的item之间都会有这个分隔线，实际情况往往是：recyclerView中存在多种的视图类型(viewType), 我们只是需要在某一类型的视图后添加分隔线。
要实现这种分隔线效果并不是什么难事，既然是某一类型有这个分隔线，那在直接在这种视图的layout文件上增加如下一个bottomLine界面元素妥妥的:

<View
    android:layout_width="match_parent"
    android:layout_height="0.5dp"
    android:background="@color/colorGrayLight" />

但如果添加这样一个界面元素导致新增一层布局那感觉代价有点大。另外一种情况，如果当前这个视图由于某种原因存在padding，而期望的分隔线是穿透整个布局的，那添加bottomLine的做法也是行不通的；还有一种情况同一类型布局在一个页面有分隔线，在另一个页面没有分隔线；总之就是希望分隔线和视图内容无关。所以我们需要一种类似DividerItemDecoration的decoration，它能针对某些viewType起作用。

先要了解一下RecyclerView.ItemDecoration，它有2个重要的回调方法：onDrawOver和getItemOffsets。onDrawOver其实是一种应用多个item的方法，所以无论如何都需要一个遍历操作。需要理解的是getItemOffsets中outRect这个输出型参数，虽然是一个Rect类型，但并不表示任何范围，而只是一个item四周的间隔距离:

我们思路已经很清楚了: 在getItemOffsets中判断当前view的类型(parent.getChildViewHolder(view)), 如果是我们需要的类型设置对应的bottom；在onDrawOver中我们遍历recyclerView的child，同样如果是我们需要的类型将分隔线画在对应位置上就行了；这个decoration可以针对任意一种或者几种类型设置不种的drawable，我们当前用SparseArray存储，key就是视图的viewType
private final SparseArrayCompat<Drawable> mDividers = new SparseArrayCompat<>(2);
同时我们可以设置这个item的高度，当Drawable为null时相当于一个透明的间隙，不为null时具有强制指定的高度。
同样我们还要考虑方向：layoutManager为横向和竖向的情况，无它，只是画的位置不同而已。
话不多说了，上代码:

import android.graphics.drawable.Drawable;
import android.support.annotation.Nullable;
import android.support.v7.widget.RecyclerView;

public class ViewTypeDivider extends ItemHolderDivider {

    @Override
    protected int keyFrom(RecyclerView.ViewHolder holder) {
        return holder.getItemViewType();
    }

    public ViewTypeDivider put(int viewType, Drawable drawable) {
        putDrawable(viewType, drawable);
        return this;
    }

    public ViewTypeDivider put(int viewType, @Nullable Drawable drawable, int height) {
        putHeight(viewType, drawable, height);
        return this;
    }
}

import android.graphics.Canvas;
import android.graphics.Rect;
import android.graphics.drawable.Drawable;
import android.support.annotation.Nullable;
import android.support.v4.util.SparseArrayCompat;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.View;

abstract class ItemHolderDivider extends RecyclerView.ItemDecoration {
    private final SparseArrayCompat<Drawable> mDividers = new SparseArrayCompat<>(2);
    private final SparseArrayCompat<Integer> mHeights = new SparseArrayCompat<>(2);

    protected abstract int keyFrom(RecyclerView.ViewHolder holder);

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        final int childCount = parent.getChildCount();
        final int width = parent.getWidth();
        final int height = parent.getHeight();
        for (int childViewIndex = 0; childViewIndex < childCount; childViewIndex++) {
            final View view = parent.getChildAt(childViewIndex);
            RecyclerView.ViewHolder holder = parent.getChildViewHolder(view);
            int key = keyFrom(holder);
            if (isVertical(parent)) {
                drawBottom(c, key, (int) view.getY() + view.getHeight(), width);
                drawTop(c, -key, (int) view.getY(), width);
            } else {
                drawRight(c, key, (int) view.getX() + view.getWidth(), height);
                drawLeft(c, -key, (int) view.getX(), height);
            }
        }
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
                               RecyclerView.State state) {
        RecyclerView.ViewHolder holder = parent.getChildViewHolder(view);
        int key = keyFrom(holder);
        if (isVertical(parent)) {
            outRect.bottom = getHeight(key);
            outRect.top = getHeight(-key);
        } else {
            outRect.right = getHeight(key);
            outRect.left = getHeight(-key);
        }
    }

    private void drawBottom(Canvas c, int key, int y, int width) {
        Drawable d = mDividers.get(key);
        if (d != null) {
            d.setBounds(0, y, width, y + getHeight(key, d));
            d.draw(c);
        }
    }

    private void drawTop(Canvas c, int key, int y, int width) {
        Drawable d = mDividers.get(key);
        if (d != null) {
            d.setBounds(0, y - getHeight(key, d), width, y);
            d.draw(c);
        }
    }

    private void drawRight(Canvas c, int key, int x, int height) {
        Drawable d = mDividers.get(key);
        if (d != null) {
            d.setBounds(x, 0, x + getHeight(key, d), height);
            d.draw(c);
        }
    }

    private void drawLeft(Canvas c, int key, int x, int height) {
        Drawable d = mDividers.get(key);
        if (d != null) {
            d.setBounds(x - getHeight(key, d), 0, x, height);
            d.draw(c);
        }
    }

    final void putDrawable(int key, Drawable drawable) {
        mDividers.put(key, drawable);
    }

    final void putHeight(int key, @Nullable Drawable drawable, int height) {
        if (drawable != null) {
            mDividers.put(key, drawable);
        }
        mHeights.put(key, height);
    }

    private int getHeight(int key) {
        Drawable d = mDividers.get(key);
        return getHeight(key, d);
    }

    private int getHeight(int key, @Nullable Drawable d) {
        int index = mHeights.indexOfKey(key);
        return index < 0 ? d == null ? 0 : d.getIntrinsicHeight() : mHeights.valueAt(index);
    }

    private boolean isVertical(RecyclerView parent) {
        RecyclerView.LayoutManager layoutManager = parent.getLayoutManager();
        return !(layoutManager instanceof LinearLayoutManager) ||
                ((LinearLayoutManager) layoutManager).getOrientation() == LinearLayoutManager.VERTICAL;
    }
}