Android L面世之后九巡，Google就推薦在開發(fā)項目中使用RecyclerView來取代ListView倦畅，因為RecyclerView的靈活性跟性能都要比ListView更強(qiáng)，但是麻汰，帶來的問題也不少速客，比如：列表分割線都要開發(fā)者自己控制，再者五鲫，RecyclerView的測量與布局的邏輯都委托給了自己LayoutManager來處理溺职，如果需要對RecyclerView進(jìn)行改造，相應(yīng)的也要對其LayoutManager進(jìn)行定制。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(xiàn)帶分割線的列表式RecyclerView
- 如何實現(xiàn)帶分割線網(wǎng)格式RecyclerView
- 如何實現(xiàn)全展開的列表式RecyclerView(比如：嵌套到ScrollView中使用)
- 如何實現(xiàn)全展開的網(wǎng)格式RecyclerView(比如：嵌套到ScrollView中使用)
先看一下實現(xiàn)樣式浪耘，為了方便控制乱灵，邊界的均不設(shè)置分割線，方便定制七冲，如果需要可以采用Padding或者M(jìn)argin來實現(xiàn)痛倚。Github連接 RecyclerItemDecoration
不同場景RecyclerView實現(xiàn)
默認(rèn)的縱向列表式RecyclerView
首先看一下最簡單的縱向線性RecyclerView，一般用以下代碼：
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(xiàn)癞埠，但默認(rèn)不帶分割線状原，如果想要使用比如20dp的黑色作為分割線聋呢，就需要自己定制苗踪，Google為RecyclerView提供了ItemDecoration，它的作用就是為Item添加一些附屬信息削锰，比如：分割線通铲，浮層等。
帶分割線的列表式RecyclerView--LinearItemDecoration
RecyclerView提供了addItemDecoration接口與ItemDecoration類用來定制分割線樣式器贩，那么颅夺，在RecyclerView源碼中，是怎么用使用ItemDecoration的呢蛹稍。與普通View的繪制流程一致吧黄，RecyclerView也要經(jīng)過measure->layout->draw，并且在measure唆姐、layout之后拗慨，就應(yīng)該按照ItemDecoration的限制，為RecyclerView的分割線挪出空間奉芦。RecyclerView的measure跟Layout其實都是委托給自己的LayoutManager的赵抢，在LinearLayoutManager測量或者布局時都會直接或者間接調(diào)用RecyclerView的measureChildWithMargins函數(shù)，而measureChildWithMargins函數(shù)會進(jìn)一步找到addItemDecoration添加的ItemDecoration声功，通過其getItemOffsets函數(shù)獲取所需空間信息烦却，源碼如下：
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);
}
}
可見measureChildWithMargins會首先通過getItemDecorInsetsForChild計算出每個child的ItemDecoration所限制的邊界信息，之后將邊界所需的空間作為已用空間為child構(gòu)造MeasureSpec先巴，最后用MeasureSpec對child進(jìn)行尺寸測量：child.measure(widthSpec, heightSpec);來看一下getItemDecorInsetsForChild函數(shù)：
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);
<!--通過這里知道其爵，需要繪制的空間位置-->
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;
}
一般而言，不會同時設(shè)置多類ItemDecoration伸蚯，太麻煩醋闭，對于普通的線性布局列表，其實就簡單設(shè)定一個自定義ItemDecoration即可朝卒，其中outRect參數(shù)主要是控制每個Item上下左右的分割線所占據(jù)的寬度跟高度证逻，這個尺寸跟繪制的時候的尺寸應(yīng)該對應(yīng)（如果需要繪制的話），看一下LinearItemDecoration的getItemOffsets實現(xiàn)：
@Override
public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
if (mOrientation == VERTICAL_LIST) {
<!--垂直方向 ，最后一個不設(shè)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 囚企，最后一個不設(shè)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之后丈咐，再來看一下RecyclerView的onDraw函數(shù)， RecyclerView在onDraw函數(shù)中會調(diào)用ItemDecoration的onDraw龙宏，繪制分割線或者其他輔助信息棵逊，ItemDecoration 支持上下左右四個方向定制占位分割線等信息，具體要繪制的樣式跟位置都完全由開發(fā)者確定银酗，所以自由度非常大辆影，其實如果不是太特殊的需求的話，onDraw函數(shù)完全可以不做任何處理黍特，僅僅用背景色就可以達(dá)到簡單的分割線的目的蛙讥，當(dāng)然，如果想要定制一些特殊的圖案之類的需話灭衷，就需要自己繪制次慢，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，如果不是特殊的繪制需求翔曲，比如顯示七彩的迫像，或者圖片，完全不需要任何繪制瞳遍，如果一定要繪制闻妓，注意繪制的尺寸區(qū)域跟原來getItemOffsets所限制的區(qū)域一致，繪制的區(qū)域過大不僅不會顯示出來掠械，還會引起過度繪制的問題：
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(wǎng)格式RecyclerView--GridLayoutItemDecoration
網(wǎng)格式RecyclerView的處理流程跟上面的線性列表類似由缆，不過網(wǎng)格式的需要根據(jù)每個Item的位置為其設(shè)置好邊距，比如最左面的不需要左邊占位份蝴，最右面的不需要右面的占位犁功，最后一行不需要底部的占位，如下圖所示
RecyclerView的每個childView都會通過getItemOffsets來設(shè)置自己ItemDecoration婚夫，對于網(wǎng)格式的RecyclerView浸卦，需要在四個方向上對其ItemDecoration進(jìn)行限制，來看一下其實現(xiàn)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(jù)RecyclerView滑動方向（橫向或者縱向）以及child的位置（是不是最后一行或者最后一列）案糙，對附屬區(qū)域進(jìn)行限制限嫌，同樣，如果不是特殊的分割線樣式时捌，通過背景就基本可以實現(xiàn)需求怒医，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奢讨，全展開主要是跟measure邏輯相關(guān)稚叹，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(guān)鍵代碼，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
}
由以上代碼可以看出扒袖，在為RecyclerView設(shè)置了LayoutManager之后塞茅，RecyclerView的measure邏輯其實就是委托給了它的LayoutManager，這里以LinearLayoutManager為例季率，不過LinearLayoutManager源碼里面并沒有重寫onMeasure函數(shù)野瘦，也就是說，對于RecyclerView的線性樣式飒泻，對于尺寸的處理采用的是跟ViewGroup一樣的處理鞭光，完全由父控件限制，不過對于v-23里面有了一些修改泞遗，就是增加了對wrap_content的支持惰许。既然這樣，我們就可以把設(shè)置尺寸的時機(jī)放到LayoutManager的onMeasure中刹孔，對全展開的RecyclerView來說啡省，其實就是將所有child測量一遍娜睛，之后將每個child需要高度或者寬度累加髓霞，看一下ExpandedLinearLayoutManager的實現(xiàn)：在測量child的時候，采用RecyclerView的measureChildWithMargins畦戒，該函數(shù)已經(jīng)將ItemDecoration的占位考慮進(jìn)去方库，之后通過getDecoratedMeasuredWidth獲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];
<!--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障斋，以便獲得寬高（包括ItemDecoration的限制）
super.measureChildWithMargins(view, 0, 0);
//獲取childView纵潦，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;
}
全展開的網(wǎng)格式RecyclerView--ExpandedGridLayoutManager
全展開的網(wǎng)格式RecyclerView的實現(xiàn)跟線性的十分相似垃环，唯一不同的就是在確定尺寸的時候邀层，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遂庄，這里假定行高或者列寬都是相同的寥院，其實在使用中這兩種場景也是最常見的，看如下代碼涛目，其實除了加了行與列判斷邏輯秸谢，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 = heightMode == View.MeasureSpec.EXACTLY ? heightSize : measureHeight;
measureWidth = widthMode == View.MeasureSpec.EXACTLY ? widthSize : measureWidth;
setMeasuredDimension(measureWidth, measureHeight);
}
最后附上橫向滑動效果圖：
以上就是比較通用的RecyclerView使用場景及所做的兼容 霹肝，最后附上Github鏈接RecyclerItemDecoration估蹄，歡迎star，fork沫换。
作者：看書的小蝸牛
原文鏈接: RecyclerView定制：通用ItemDecoration及全展開RecyclerView的實現(xiàn)
