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Android RecyclerView区分视图类型的Divider的实现

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:

outRect参数各域的含义

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;
  }
}

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