RecyclerView定製：通用ItemDecoration及全展開RecyclerView的實現

時間 2019-12-04

標籤 recyclerview 定製通用 itemdecoration 展開實現简体版

原文原文鏈接

Android L面世以後，Google就推薦在開發項目中使用RecyclerView來取代ListView，由於RecyclerView的靈活性跟性能都要比ListView更強，可是，帶來的問題也很多，好比：列表分割線都要開發者本身控制，再者，RecyclerView的測量與佈局的邏輯都委託給了本身LayoutManager來處理，若是須要對RecyclerView進行改造，相應的也要對其LayoutManager進行定製。本文主要就以如下場景給出RecyclerView使用參考：git

RecyclerView的幾種經常使用場景

如何實現帶分割線的列表式RecyclerViewgithub
如何實現帶分割線網格式RecyclerViewapp
如何實現全展開的列表式RecyclerView(好比：嵌套到ScrollView中使用)ide
如何實現全展開的網格式RecyclerView(好比：嵌套到ScrollView中使用)函數

先看一下實現樣式，爲了方便控制，邊界的均不設置分割線，方便定製，若是須要能夠採用Padding或者Margin來實現。Github鏈接 RecyclerItemDecoration佈局

不一樣場景RecyclerView實現

默認的縱向列表式RecyclerView

首先看一下最簡單的縱向線性RecyclerView，通常用如下代碼：性能

LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setOrientation(LinearLayoutManager.VERTICAL);
    mRecyclerView.setLayoutManager(linearLayoutManager);

以上就是最簡單的線性RecyclerView的實現，但默認不帶分割線，若是想要使用好比20dp的黑色做爲分割線，就須要本身定製，Google爲RecyclerView提供了ItemDecoration，它的做用就是爲Item添加一些附屬信息，好比：分割線，浮層等。this

帶分割線的列表式RecyclerView--LinearItemDecoration

RecyclerView提供了addItemDecoration接口與ItemDecoration類用來定製分割線樣式，那麼，在RecyclerView源碼中，是怎麼用使用ItemDecoration的呢。與普通View的繪製流程一致，RecyclerView也要通過measure->layout->draw，而且在measure、layout以後，就應該按照ItemDecoration的限制，爲RecyclerView的分割線挪出空間。RecyclerView的measure跟Layout其實都是委託給本身的LayoutManager的，在LinearLayoutManager測量或者佈局時都會直接或者間接調用RecyclerView的measureChildWithMargins函數，而measureChildWithMargins函數會進一步找到addItemDecoration添加的ItemDecoration，經過其getItemOffsets函數獲取所需空間信息，源碼以下：spa

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);
      }
  }

可見measureChildWithMargins會首先經過getItemDecorInsetsForChild計算出每一個child的ItemDecoration所限制的邊界信息，以後將邊界所需的空間做爲已用空間爲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函數：code

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);
        <!--經過這裏知道，須要繪製的空間位置-->
        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;
}

通常而言，不會同時設置多類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便可，其中outRect參數主要是控制每一個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（若是須要繪製的話），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 ，最後一個不設置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 ，最後一個不設置padding-->
        if (parent.getChildAdapterPosition(view) < parent.getAdapter().getItemCount()1) {
            outRect.set(0, 0, mSpanSpace, 0);
        } else {
            outRect.set(0, 0, 0, 0);
        }
    }
}

measure跟layout以後，再來看一下RecyclerView的onDraw函數， RecyclerView在onDraw函數中會調用ItemDecoration的onDraw，繪製分割線或者其餘輔助信息，ItemDecoration 支持上下左右四個方向定製佔位分割線等信息，具體要繪製的樣式跟位置都徹底由開發者肯定，因此自由度很是大，其實若是不是太特殊的需求的話，onDraw函數徹底能夠不作任何處理，僅僅用背景色就能夠達到簡單的分割線的目的，固然，若是想要定製一些特殊的圖案之類的需話，就須要本身繪製，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 {
       ...
    }
}

其實，若是不是特殊的繪製需求，好比顯示七彩的，或者圖片，徹底不須要任何繪製，若是必定要繪製，注意繪製的尺寸區域跟原來getItemOffsets所限制的區域一致，繪製的區域過大不只不會顯示出來，還會引發過分繪製的問題：

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);            }        
    }        
}

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相似，不過網格式的須要根據每一個Item的位置爲其設置好邊距，好比最左面的不須要左邊佔位，最右面的不須要右面的佔位，最後一行不須要底部的佔位，以下圖所示

RecyclerView的每一個childView都會經過getItemOffsets來設置本身ItemDecoration，對於網格式的RecyclerView，須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);
        }
    }
}

其實上面的代碼就是根據RecyclerView滑動方向（橫向或者縱向）以及child的位置（是否是最後一行或者最後一列），對附屬區域進行限制，一樣，若是不是特殊的分割線樣式，經過背景就基本能夠實現需求，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一樣，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源碼：

@Override
protected void onMeasure(int widthSpec, int heightSpec) {
        ...
        
        <!--關鍵代碼，若是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
}

由以上代碼能夠看出，在爲RecyclerView設置了LayoutManager以後，RecyclerView的measure邏輯其實就是委託給了它的LayoutManager，這裏以LinearLayoutManager爲例，不過LinearLayoutManager源碼裏面並無重寫onMeasure函數，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同樣的處理，徹底由父控件限制，不過對於v-23裏面有了一些修改，就是增長了對wrap_content的支持。既然這樣，咱們就能夠把設置尺寸的時機放到LayoutManager的onMeasure中，對全展開的RecyclerView來講，其實就是將全部child測量一遍，以後將每一個child須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，該函數已經將ItemDecoration的佔位考慮進去，以後經過getDecoratedMeasuredWidth獲取真正須要佔用的尺寸。

@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];
           <!--獲取最大高度-->
            measureHeight = Math.max(measureHeight, measuredDimension[1]);
        } else {
            measureHeight = measureHeight + measuredDimension[1];
            <!--獲取最大寬度-->
            measureWidth = Math.max(measureWidth, measuredDimension[0]);
        }
    }

    measureHeight = heightMode == View.MeasureSpec.EXACTLY ? heightSize : measureHeight;
    measureWidth = widthMode == View.MeasureSpec.EXACTLY ? widthSize : measureWidth;
    if (getOrientation() == VERTICAL && measureWidth > widthSize) {
        measureWidth = widthSize;
    } else if (getOrientation() == HORIZONTAL && measureHeight > heightSize) {
        measureHeight = heightSize;
    }
    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，以便得到寬高（包括ItemDecoration的限制）
        super.measureChildWithMargins(view, 0, 0);
        //獲取childView，以便得到寬高（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;
}

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類似，惟一不一樣的就是在肯定尺寸的時候，不是將每一個child的尺寸疊加，而是要將每一行或者每一列的尺寸疊加，這裏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多見的，看以下代碼，其實除了加了行與列判斷邏輯，其餘基本跟上面的全展開線性的相似。

@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 = heightMode == View.MeasureSpec.EXACTLY ? heightSize : measureHeight;
    measureWidth = widthMode == View.MeasureSpec.EXACTLY ? widthSize : measureWidth;
    setMeasuredDimension(measureWidth, measureHeight);
}

最後附上橫向滑動效果圖：

以上就是比較通用的RecyclerView使用場景及所作的兼容，最後附上Github連接RecyclerItemDecoration，歡迎star，fork。