RecyclerView定制:通用ItemDecoration及全展开RecyclerView的实现

suowed

suowed

发表于 2016-12-29 17:43:22
内容来源: 网络

Android L面世之后,Google就推荐在开发项目中使用RecyclerView来取代ListView,因为RecyclerView的灵活性跟性能都要比ListView更强,但是,带来的问题也不少,比如:列表分割线都要开发者自己控制,再者,RecyclerView的测量与布局的逻辑都委托给了自己LayoutManager来处理,如果需要对RecyclerView进行改造,相应的也要对其LayoutManager进行定制。本文主要就以以下场景给出RecyclerView使用参考:

RecyclerView的几种常用场景

  • 如何实现带分割线的列表式RecyclerView

  • 如何实现带分割线网格式RecyclerView

  • 如何实现全展开的列表式RecyclerView(比如:嵌套到ScrollView中使用)

  • 如何实现全展开的网格式RecyclerView(比如:嵌套到ScrollView中使用)

先看一下实现样式,为了方便控制,边界的均不设置分割线,方便定制,如果需要可以采用Padding或者Margin来实现。Github连接 RecyclerItemDecoration


不同场景RecyclerView实现

默认的纵向列表式RecyclerView

首先看一下最简单的纵向线性RecyclerView,一般用以下代码:

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

以上就是最简单的线性RecyclerView的实现,但默认不带分割线,如果想要使用比如20dp的黑色作为分割线,就需要自己定制,Google为RecyclerView提供了ItemDecoration,它的作用就是为Item添加一些附属信息,比如:分割线,浮层等。

带分割线的列表式RecyclerView--LinearItemDecoration

RecyclerView提供了addItemDecoration接口与ItemDecoration类用来定制分割线样式,那么,在RecyclerView源码中,是怎么用使用ItemDecoration的呢。与普通View的绘制流程一致,RecyclerView也要经过measure->layout->draw,并且在measure、layout之后,就应该按照ItemDecoration的限制,为RecyclerView的分割线挪出空间。RecyclerView的measure跟Layout其实都是委托给自己的LayoutManager的,在LinearLayoutManager测量或者布局时都会直接或者间接调用RecyclerView的measureChildWithMargins函数,而measureChildWithMargins函数会进一步找到addItemDecoration添加的ItemDecoration,通过其getItemOffsets函数获取所需空间信息,源码如下:

  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);
      }
  }
    
    

可见measureChildWithMargins会首先通过getItemDecorInsetsForChild计算出每个child的ItemDecoration所限制的边界信息,之后将边界所需的空间作为已用空间为child构造MeasureSpec,最后用MeasureSpec对child进行尺寸测量:child.measure(widthSpec, heightSpec);来看一下getItemDecorInsetsForChild函数:

   
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);
        <!--通过这里知道,需要绘制的空间位置-->
        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;
}

一般而言,不会同时设置多类ItemDecoration,太麻烦,对于普通的线性布局列表,其实就简单设定一个自定义ItemDecoration即可,其中outRect参数主要是控制每个Item上下左右的分割线所占据的宽度跟高度,这个尺寸跟绘制的时候的尺寸应该对应(如果需要绘制的话),看一下LinearItemDecoration的getItemOffsets实现:


@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
    if (mOrientation == VERTICAL_LIST) {
    
    <!--垂直方向 ,最后一个不设置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 ,最后一个不设置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之后,再来看一下RecyclerView的onDraw函数, RecyclerView在onDraw函数中会调用ItemDecoration的onDraw,绘制分割线或者其他辅助信息,ItemDecoration 支持上下左右四个方向定制占位分割线等信息,具体要绘制的样式跟位置都完全由开发者确定,所以自由度非常大,其实如果不是太特殊的需求的话,onDraw函数完全可以不做任何处理,仅仅用背景色就可以达到简单的分割线的目的,当然,如果想要定制一些特殊的图案之类的需话,就需要自己绘制,来看一下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 {
       ...
    }
}

其实,如果不是特殊的绘制需求,比如显示七彩的,或者图片,完全不需要任何绘制,如果一定要绘制,注意绘制的尺寸区域跟原来getItemOffsets所限制的区域一致,绘制的区域过大不仅不会显示出来,还会引起过度绘制的问题:


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);            }        
    }        
}

简单看一下真个流程图

带分割线的网格式RecyclerView--GridLayoutItemDecoration

网格式RecyclerView的处理流程跟上面的线性列表类似,不过网格式的需要根据每个Item的位置为其设置好边距,比如最左面的不需要左边占位,最右面的不需要右面的占位,最后一行不需要底部的占位,如下图所示

RecyclerView的每个childView都会通过getItemOffsets来设置自己ItemDecoration,对于网格式的RecyclerView,需要在四个方向上对其ItemDecoration进行限制,来看一下其实现类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);
        }
    }
}

其实上面的代码就是根据RecyclerView滑动方向(横向或者纵向)以及child的位置(是不是最后一行或者最后一列),对附属区域进行限制,同样,如果不是特殊的分割线样式,通过背景就基本可以实现需求,不用特殊draw。

全展开的列表式RecyclerView--ExpandedLinearLayoutManager

RecyclerView全展开的逻辑跟分割线不同,全展开主要是跟measure逻辑相关,简单看一下RecyclerView(v-22版本,相对简单)的measure源码:

@Override
protected void onMeasure(int widthSpec, int heightSpec) {
        ...
        
        <!--关键代码,如果mLayout(LayoutManager)非空,就采用LayoutManager的mLayout.onMeasure-->
    if (mLayout == null) {
        defaultOnMeasure(widthSpec, heightSpec);
    } else {
        mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
    }

    mState.mInPreLayout = false; // clear
}

由以上代码可以看出,在为RecyclerView设置了LayoutManager之后,RecyclerView的measure逻辑其实就是委托给了它的LayoutManager,这里以LinearLayoutManager为例,不过LinearLayoutManager源码里面并没有重写onMeasure函数,也就是说,对于RecyclerView的线性样式,对于尺寸的处理采用的是跟ViewGroup一样的处理,完全由父控件限制,不过对于v-23里面有了一些修改,就是增加了对wrap_content的支持。既然这样,我们就可以把设置尺寸的时机放到LayoutManager的onMeasure中,对全展开的RecyclerView来说,其实就是将所有child测量一遍,之后将每个child需要高度或者宽度累加,看一下ExpandedLinearLayoutManager的实现:在测量child的时候,采用RecyclerView的measureChildWithMargins,该函数已经将ItemDecoration的占位考虑进去,之后通过getDecoratedMeasuredWidth获取真正需要占用的尺寸。

@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];
           <!--获取最大高度-->
            measureHeight = Math.max(measureHeight, measuredDimension[1]);
        } else {
            measureHeight = measureHeight + measuredDimension[1];
            <!--获取最大宽度-->
            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);
        //测量childView,以便获得宽高(包括ItemDecoration的限制)
        super.measureChildWithMargins(view, 0, 0);
        //获取childView,以便获得宽高(包括ItemDecoration的限制),以及边距
        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;
}

全展开的网格式RecyclerView--ExpandedGridLayoutManager

全展开的网格式RecyclerView的实现跟线性的十分相似,唯一不同的就是在确定尺寸的时候,不是将每个child的尺寸叠加,而是要将每一行或者每一列的尺寸叠加,这里假定行高或者列宽都是相同的,其实在使用中这两种场景也是最常见的,看如下代码,其实除了加了行与列判断逻辑,其他基本跟上面的全展开线性的类似。

 @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);
}

最后附上横向滑动效果图:

以上就是比较通用的RecyclerView使用场景及所做的兼容 ,最后附上Github链接RecyclerItemDecoration,欢迎star,fork。

内容来源:https://segmentfault.com/a/1190000007949504

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