Android L面世之後，Google就推薦在開發專案中使用RecyclerView來取代ListView，因為RecyclerView的靈活性跟效能都要比ListView更強，但是，帶來的問題也不少，比如：列表分割線都要開發者自己控制，再者，RecyclerView的測量與佈局的邏輯都委託給了自己LayoutManager來處理，如果需要對RecyclerView進行改造，相應的也要對其LayoutManager進行定製。本文主要就以以下場景給出RecyclerView使用參考：

RecyclerView的幾種常用場景

• 如何實現帶分割線的列表式RecyclerView
• 如何實現帶分割線網格式RecyclerView
• 如何實現全展開的列表式RecyclerView(比如：巢狀到ScrollView中使用)
• 如何實現全展開的網格式RecyclerView(比如：巢狀到ScrollView中使用)

先看一下實現樣式，為了方便控制，邊界的均不設定分割線，方便定製，如果需要可以採用Padding或者Margin來實現。 Github連線RecyclerItemDecoration

不同場景RecyclerView實現

預設的縱向列表式RecyclerView

首先看一下最簡單的縱向線性RecyclerView，一般用以下程式碼：

    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    mRecyclerView.setLayoutManager(linearLayoutManager);複製程式碼

以上就是最簡單的線性RecyclerView的實現，但預設不帶分割線，如果想要使用比如20dp的黑色作為分割線，就需要自己定製，Google為RecyclerView提供了ItemDecoration，它的作用就是為Item新增一些附屬資訊，比如：分割線，浮層等。

帶分割線的列表式RecyclerView–LinearItemDecoration

RecyclerView提供了addItemDecoration介面與ItemDecoration類用來定製分割線樣式，那麼，在RecyclerView原始碼中，是怎麼用使用ItemDecoration的呢。與普通View的繪製流程一致，RecyclerView也要經過measure->layout->draw，並且在measure、layout之後，就應該按照ItemDecoration的限制，為RecyclerView的分割線挪出空間。RecyclerView的measure跟Layout其實都是委託給自己的LayoutManager的，在LinearLayoutManager測量或者佈局時都會直接或者間接呼叫RecyclerView的measureChildWithMargins函式，而measureChildWithMargins函式會進一步找到addItemDecoration新增的ItemDecoration，通過其getItemOffsets函式獲取所需空間資訊，原始碼如下：

  public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
      final LayoutParams lp = (LayoutParams) child.getLayoutParams();

      final Rect insets = mRecyclerView.getItemDecorInsetsForChild(child);
      widthUsed += insets.left + insets.right;
      heightUsed += insets.top + insets.bottom;

      final int widthSpec = getChildMeasureSpec(getWidth(), getWidthMode(),
              getPaddingLeft() + getPaddingRight() +
                      lp.leftMargin + lp.rightMargin + widthUsed, lp.width,
              canScrollHorizontally());
      final int heightSpec = getChildMeasureSpec(getHeight(), getHeightMode(),
              getPaddingTop() + getPaddingBottom() +
                      lp.topMargin + lp.bottomMargin + heightUsed, lp.height,
              canScrollVertically());
      if (shouldMeasureChild(child, widthSpec, heightSpec, lp)) {
          child.measure(widthSpec, heightSpec);
      }
  }複製程式碼

可見measureChildWithMargins會首先通過getItemDecorInsetsForChild計算出每個child的ItemDecoration所限制的邊界資訊，之後將邊界所需的空間作為已用空間為child構造MeasureSpec，最後用MeasureSpec對child進行尺寸測量：child.measure(widthSpec, heightSpec);來看一下getItemDecorInsetsForChild函式：

Rect getItemDecorInsetsForChild(View child) {
    final LayoutParams lp = (LayoutParams) child.getLayoutParams();
    if (!lp.mInsetsDirty) {
        return lp.mDecorInsets;
    }

    final Rect insets = lp.mDecorInsets;
    insets.set(0, 0, 0, 0);
    final int decorCount = mItemDecorations.size();
    for (int i = 0; i < decorCount; i++) {
        mTempRect.set(0, 0, 0, 0);
        <!--通過這裡知道，需要繪製的空間位置-->
        mItemDecorations.get(i).getItemOffsets(mTempRect, child, this, mState);
        insets.left += mTempRect.left;
        insets.top += mTempRect.top;
        insets.right += mTempRect.right;
        insets.bottom += mTempRect.bottom;
    }
    lp.mInsetsDirty = false;
    return insets;
}複製程式碼

一般而言，不會同時設定多類ItemDecoration，太麻煩，對於普通的線性佈局列表，其實就簡單設定一個自定義ItemDecoration即可，其中outRect引數主要是控制每個Item上下左右的分割線所佔據的寬度跟高度，這個尺寸跟繪製的時候的尺寸應該對應（如果需要繪製的話），看一下LinearItemDecoration的getItemOffsets實現：

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    if (mOrientation == VERTICAL_LIST) {

    <!--垂直方向 ，最後一個不設定padding-->
        if (parent.getChildAdapterPosition(view) < parent.getAdapter().getItemCount()1) {
            outRect.set(0, 0, 0, mSpanSpace);
        } else {
            outRect.set(0, 0, 0, 0);
        }
    } else {
     <!--水平方向 ，最後一個不設定padding-->
        if (parent.getChildAdapterPosition(view) < parent.getAdapter().getItemCount()1) {
            outRect.set(0, 0, mSpanSpace, 0);
        } else {
            outRect.set(0, 0, 0, 0);
        }
    }
}   複製程式碼

measure跟layout之後，再來看一下RecyclerView的onDraw函式， RecyclerView在onDraw函式中會呼叫ItemDecoration的onDraw，繪製分割線或者其他輔助資訊，ItemDecoration 支援上下左右四個方向定製佔位分割線等資訊，具體要繪製的樣式跟位置都完全由開發者確定，所以自由度非常大，其實如果不是太特殊的需求的話，onDraw函式完全可以不做任何處理，僅僅用背景色就可以達到簡單的分割線的目的，當然，如果想要定製一些特殊的圖案之類的需話，就需要自己繪製，來看一下LinearItemDecoration的onDraw（只看Vertical的）

@Override
public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
    if (mOrientation == VERTICAL_LIST) {
        drawVertical(c, parent);
    } else {
       ...
    }
}複製程式碼

其實，如果不是特殊的繪製需求，比如顯示七彩的，或者圖片，完全不需要任何繪製，如果一定要繪製，注意繪製的尺寸區域跟原來getItemOffsets所限制的區域一致，繪製的區域過大不僅不會顯示出來，還會引起過度繪製的問題：

public void drawVertical(Canvas c, RecyclerView parent) {                               int totalCount = parent.getAdapter().getItemCount();               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 top = child.getBottom() + params.bottomMargin +                           Math.round(ViewCompat.getTranslationY(child));                   final int bottom = top + mVerticalSpan;                           final int left = child.getLeft() + params.leftMargin;                   final int right = child.getRight() + params.rightMargin;                           if (!isLastRaw(parent, i, mSpanCount, totalCount))                       if (childCounti > mSpanCount) {                           drawable.setBounds(left, top, right, bottom);                           drawable.draw(c);            }        
    }        
}複製程式碼

帶分割線的網格式RecyclerView–GridLayoutItemDecoration

網格式RecyclerView的處理流程跟上面的線性列表類似，不過網格式的需要根據每個Item的位置為其設定好邊距，比如最左面的不需要左邊佔位，最右面的不需要右面的佔位，最後一行不需要底部的佔位，如下圖所示

RecyclerView的每個childView都會通過getItemOffsets來設定自己ItemDecoration，對於網格式的RecyclerView，需要在四個方向上對其ItemDecoration進行限制，來看一下其實現類GridLayoutItemDecoration的getItemOffsets：

@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    final int position = parent.getChildAdapterPosition(view);
    final int totalCount = parent.getAdapter().getItemCount();
    int left = (position % mSpanCount == 0) ? 0 : mHorizonSpan;
    int bottom = ((position + 1) % mSpanCount == 0) ? 0 : mVerticalSpan;
    if (isVertical(parent)) {
        if (!isLastRaw(parent, position, mSpanCount, totalCount)) {
            outRect.set(left, 0, 0, mVerticalSpan);
        } else {
            outRect.set(left, 0, 0, 0);
        }
    } else {
        if (!isLastColumn(parent, position, mSpanCount, totalCount)) {
            outRect.set(0, 0, mHorizonSpan, bottom);
        } else {
            outRect.set(0, 0, 0, bottom);
        }
    }
}複製程式碼

其實上面的程式碼就是根據RecyclerView滑動方向（橫向或者縱向）以及child的位置（是不是最後一行或者最後一列），對附屬區域進行限制，同樣，如果不是特殊的分割線樣式，通過背景就基本可以實現需求，不用特殊draw。

全展開的列表式RecyclerView–ExpandedLinearLayoutManager

RecyclerView全展開的邏輯跟分割線不同，全展開主要是跟measure邏輯相關，簡單看一下RecyclerView（v-22版本，相對簡單）的measure原始碼：

@Override
protected void onMeasure(int widthSpec, int heightSpec) {
        ...

        <!--關鍵程式碼，如果mLayout（LayoutManager）非空,就採用LayoutManager的mLayout.onMeasure-->
    if (mLayout == null) {
        defaultOnMeasure(widthSpec, heightSpec);
    } else {
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
    }

    mState.mInPreLayout = false; // clear
}複製程式碼

由以上程式碼可以看出，在為RecyclerView設定了LayoutManager之後，RecyclerView的measure邏輯其實就是委託給了它的LayoutManager，這裡以LinearLayoutManager為例，不過LinearLayoutManager原始碼裡面並沒有重寫onMeasure函式，也就是說，對於RecyclerView的線性樣式，對於尺寸的處理採用的是跟ViewGroup一樣的處理，完全由父控制元件限制，不過對於v-23裡面有了一些修改，就是增加了對wrap_content的支援。既然這樣，我們就可以把設定尺寸的時機放到LayoutManager的onMeasure中，對全展開的RecyclerView來說，其實就是將所有child測量一遍，之後將每個child需要高度或者寬度累加，看一下ExpandedLinearLayoutManager的實現：在測量child的時候，採用RecyclerView的measureChildWithMargins，該函式已經將ItemDecoration的佔位考慮進去，之後通過getDecoratedMeasuredWidth獲取真正需要佔用的尺寸。

 @Override
public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state,
                      int widthSpec, int heightSpec) {
    final int widthMode = View.MeasureSpec.getMode(widthSpec);
    final int heightMode = View.MeasureSpec.getMode(heightSpec);
    final int widthSize = View.MeasureSpec.getSize(widthSpec);
    final int heightSize = View.MeasureSpec.getSize(heightSpec);
    int measureWidth = 0;
    int measureHeight = 0;
    int count;
    if (mMaxItemCount < 0 || getItemCount() < mMaxItemCount) {
        count = getItemCount();
    } else {
        count = mMaxItemCount;
    }
    for (int i = 0; i < count; i++) {
        int[] measuredDimension = getChildDimension(recycler, i);
        if (measuredDimension == null || measuredDimension.length != 2)
            return;
        if (getOrientation() == HORIZONTAL) {
            measureWidth = measureWidth + measuredDimension[0];
            measureHeight = Math.max(measureHeight, measuredDimension[1]);
        } else {
            measureHeight = measureHeight + measuredDimension[1];
            measureWidth = Math.max(measureWidth, measuredDimension[0]);
        }
    }
    measureHeight = measureHeight + getPaddingBottom() + getPaddingTop();
    measureWidth = measureWidth + getPaddingLeft() + getPaddingRight();
    measureHeight = heightMode == View.MeasureSpec.EXACTLY ? heightSize : measureHeight;
    measureWidth = widthMode == View.MeasureSpec.EXACTLY ? widthSize : measureWidth;
    if (getOrientation() == HORIZONTAL && measureWidth > widthSize) {
        if (widthMode == View.MeasureSpec.UNSPECIFIED) {
            setMeasuredDimension(measureWidth, measureHeight);
        } else {
            super.onMeasure(recycler, state, widthSize, heightSpec);
        }
    } else if (getOrientation() == VERTICAL && measureHeight > heightSize) {
        if (heightMode == View.MeasureSpec.UNSPECIFIED) {
            setMeasuredDimension(measureWidth, measureHeight);
        } else {
            setMeasuredDimension(measureWidth, heightSize);
        }
    } else {
        setMeasuredDimension(measureWidth, measureHeight);
    }
}


private int[] getChildDimension(RecyclerView.Recycler recycler, int position) {
    try {
        int[] measuredDimension = new int[2];
        View view = recycler.getViewForPosition(position);
        //測量childView，以便獲得寬高（包括ItemDecoration的限制）
        super.measureChildWithMargins(view, 0, 0);
        //獲取childView，以便獲得寬高（包括ItemDecoration的限制），以及邊距
        RecyclerView.LayoutParams p = (RecyclerView.LayoutParams) view.getLayoutParams();
        measuredDimension[0] = getDecoratedMeasuredWidth(view) + p.leftMargin + p.rightMargin;
        measuredDimension[1] = getDecoratedMeasuredHeight(view) + p.bottomMargin + p.topMargin;
        return measuredDimension;
    } catch (Exception e) {
        Log.d("LayoutManager", e.toString());
    }
    return null;
}複製程式碼

全展開的網格式RecyclerView–ExpandedGridLayoutManager

全展開的網格式RecyclerView的實現跟線性的十分相似，唯一不同的就是在確定尺寸的時候，不是將每個child的尺寸疊加，而是要將每一行或者每一列的尺寸疊加，這裡假定行高或者列寬都是相同的，其實在使用中這兩種場景也是最常見的，看如下程式碼，其實除了加了行與列判斷邏輯，其他基本跟上面的全展開線性的類似。

  @Override
public void onMeasure(RecyclerView.Recycler recycler, RecyclerView.State state, int widthSpec, int heightSpec) {
    final int widthMode = View.MeasureSpec.getMode(widthSpec);
    final int heightMode = View.MeasureSpec.getMode(heightSpec);
    final int widthSize = View.MeasureSpec.getSize(widthSpec);
    final int heightSize = View.MeasureSpec.getSize(heightSpec);
    int measureWidth = 0;
    int measureHeight = 0;
    int count = getItemCount();
    int span = getSpanCount();
    for (int i = 0; i < count; i++) {
        measuredDimension = getChildDimension(recycler, i);
        if (getOrientation() == HORIZONTAL) {
            if (i % span == 0 ) {
                measureWidth = measureWidth + measuredDimension[0];
            }
            measureHeight = Math.max(measureHeight, measuredDimension[1]);
        } else {
            if (i % span == 0) {
                measureHeight = measureHeight + measuredDimension[1];
            }
            measureWidth = Math.max(measureWidth, measuredDimension[0]);
        }
    }
    measureHeight = measureHeight + getPaddingBottom() + getPaddingTop();
    measureWidth = measureWidth + getPaddingLeft() + getPaddingRight();
    measureHeight = heightMode == View.MeasureSpec.EXACTLY ? heightSize : measureHeight;
    measureWidth = widthMode == View.MeasureSpec.EXACTLY ? widthSize : measureWidth;
    if (getOrientation() == HORIZONTAL && measureWidth > widthSize) {
        if (widthMode == View.MeasureSpec.UNSPECIFIED) {
            setMeasuredDimension(measureWidth, measureHeight);
        } else {
            super.onMeasure(recycler, state, widthSpec, heightSpec);
        }
    } else if (getOrientation() == VERTICAL && measureHeight > heightSize) {
        if (heightMode == View.MeasureSpec.UNSPECIFIED) {
            setMeasuredDimension(measureWidth, measureHeight);
        } else {
            super.onMeasure(recycler, state, widthSpec, heightSpec);
        }
    } else {
        setMeasuredDimension(measureWidth, measureHeight);
    }
}複製程式碼

最後附上橫向滑動效果圖：

以上就是比較通用的RecyclerView使用場景及所做的相容 ，最後附上Github連結RecyclerItemDecoration，歡迎star，fork。