本文将简单分析下RecyclerView的绘制流程。既然是一个View，就从onMeasure开始。
onMeasure
protected void onMeasure(int widthSpec, int heightSpec) {
if (mLayout.mAutoMeasure) {
final int widthMode = MeasureSpec.getMode(widthSpec);
final int heightMode = MeasureSpec.getMode(heightSpec);
final boolean skipMeasure = widthMode == MeasureSpec.EXACTLY
&& heightMode == MeasureSpec.EXACTLY;
mLayout.onMeasure(mRecycler, mState, widthSpec, heightSpec);
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
}
mLayout.setMeasureSpecs(widthSpec, heightSpec);
dispatchLayoutStep2();
mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
}
}在onMeasure中，mLayout就是一个LayoutManager对象，RecyclerView将onMeasure的计算交给了LayoutManager，在LayoutManager#onMeasure中又调用了RecyclerView#defaultOnMeasure方法，在其中调用了LayoutManager#chooseSize方法。
public static int chooseSize(int spec, int desired, int min) {
final int mode = View.MeasureSpec.getMode(spec);
final int size = View.MeasureSpec.getSize(spec);
switch (mode) {
case View.MeasureSpec.EXACTLY:
return size;
case View.MeasureSpec.AT_MOST:
return Math.min(size, Math.max(desired, min));
case View.MeasureSpec.UNSPECIFIED:
default:
return Math.max(desired, min);
}
}可以很明显的看出此方法，根据MessureSpec类型计算了View的宽高尺寸，之后将会调用dispatchLayoutStep2方法对item以及子view进行测量。onLayout中也将调用dispatchLayoutStep2方法，我们将在其中一起说明。
onLayout
这里看下onLayout方法。
protected void onLayout(boolean changed, int l, int t, int r, int b) {
dispatchLayout();
}
void dispatchLayout() {
if (mState.mLayoutStep == State.STEP_START) {
dispatchLayoutStep1();
mLayout.setExactMeasureSpecsFrom(this);
dispatchLayoutStep2();
}
dispatchLayoutStep3();
}dispatchLayoutStep2()方法中又调用LayoutManager#onLayoutChildren方法进行布局。这里以LinearLayoutManager为例。
public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
// layout algorithm:
// 1) by checking children and other variables, find an anchor coordinate and an anchor
// item position.
// 2) fill towards start, stacking from bottom
// 3) fill towards end, stacking from top
// 4) scroll to fulfill requirements like stack from bottom.
// create layout state
int startOffset;
int endOffset;
final int firstLayoutDirection;
onAnchorReady(recycler, state, mAnchorInfo, firstLayoutDirection);
detachAndScrapAttachedViews(recycler);
mLayoutState.mInfinite = resolveIsInfinite();
mLayoutState.mIsPreLayout = state.isPreLayout();
if (mAnchorInfo.mLayoutFromEnd) {
} else {
// 向下布局
updateLayoutStateToFillEnd(mAnchorInfo);
mLayoutState.mExtra = extraForEnd;
// 填充Item
fill(recycler, mLayoutState, state, false);
endOffset = mLayoutState.mOffset;
final int lastElement = mLayoutState.mCurrentPosition;
if (mLayoutState.mAvailable > 0) {
extraForStart += mLayoutState.mAvailable;
}
// 向上布局
updateLayoutStateToFillStart(mAnchorInfo);
mLayoutState.mExtra = extraForStart;
mLayoutState.mCurrentPosition += mLayoutState.mItemDirection;
fill(recycler, mLayoutState, state, false);
startOffset = mLayoutState.mOffset;
}
}布局的流程在开头的注释中已经清楚的说明了，这里不再赘述。这里的关注点在LinearLayoutManager#fill方法。
int fill(RecyclerView.Recycler recycler, LayoutState layoutState,
RecyclerView.State state, boolean stopOnFocusable) {
// 存储当前可见空间
final int start = layoutState.mAvailable;
// 计算可用布局的宽高
int remainingSpace = layoutState.mAvailable + layoutState.mExtra;
LayoutChunkResult layoutChunkResult = mLayoutChunkResult;
// 迭代填充item
while ((layoutState.mInfinite || remainingSpace > 0) && layoutState.hasMore(state)) {
layoutChunkResult.resetInternal();
// 布局item
layoutChunk(recycler, state, layoutState, layoutChunkResult);
if (layoutChunkResult.mFinished) {
break;
}
// 计算布局的偏移位置
layoutState.mOffset += layoutChunkResult.mConsumed * layoutState.mLayoutDirection;
if (!layoutChunkResult.mIgnoreConsumed || mLayoutState.mScrapList != null
|| !state.isPreLayout()) {
layoutState.mAvailable -= layoutChunkResult.mConsumed;
// 计算剩余的空间
remainingSpace -= layoutChunkResult.mConsumed;
}
}
return start - layoutState.mAvailable;
}在fill方法中将会循环调用layoutChunk方法进行布局。每次布局完成之后将计算剩余的可用空间，之后判断是否还需要继续布局Item。我们这里来看下布局的LinearLayoutManager#layoutChunk方法。
void layoutChunk(RecyclerView.Recycler recycler, RecyclerView.State state,
LayoutState layoutState, LayoutChunkResult result) {
// 获取item view
View view = layoutState.next(recycler);
// 获取布局参数
LayoutParams params = (LayoutParams) view.getLayoutParams();
if (layoutState.mScrapList == null) {
if (mShouldReverseLayout == (layoutState.mLayoutDirection
== LayoutState.LAYOUT_START)) {
// 增加item view
addView(view);
} else {
addView(view, 0);
}
}
// 测量item
measureChildWithMargins(view, 0, 0);
// 计算item使用的空间
result.mConsumed = mOrientationHelper.getDecoratedMeasurement(view);
int left, top, right, bottom;
// 按照水平或者数值方向布局，计算item坐标
if (mOrientation == VERTICAL) {
if (isLayoutRTL()) {
right = getWidth() - getPaddingRight();
left = right - mOrientationHelper.getDecoratedMeasurementInOther(view);
} else {
left = getPaddingLeft();
right = left + mOrientationHelper.getDecoratedMeasurementInOther(view);
}
if (layoutState.mLayoutDirection == LayoutState.LAYOUT_START) {
bottom = layoutState.mOffset;
top = layoutState.mOffset - result.mConsumed;
} else {
top = layoutState.mOffset;
bottom = layoutState.mOffset + result.mConsumed;
}
} else {
}
// item布局
layoutDecoratedWithMargins(view, left, top, right, bottom);
}
public void measureChildWithMargins(View child, int widthUsed, int heightUsed) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
// 测量分割线
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)) {
// 子View测量
child.measure(widthSpec, heightSpec);
}
}
public void layoutDecoratedWithMargins(View child, int left, int top, int right,
int bottom) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final Rect insets = lp.mDecorInsets;
// 子View布局
child.layout(left + insets.left + lp.leftMargin, top + insets.top + lp.topMargin,
right - insets.right - lp.rightMargin,
bottom - insets.bottom - lp.bottomMargin);
}从代码中可以看出，最后分别调用了item的measure函数与layout函数对view进行了测量和布局。下面我们来看下onDraw方法。
onDraw
public void onDraw(Canvas c) {
super.onDraw(c);
final int count = mItemDecorations.size();
for (int i = 0; i < count; i++) {
mItemDecorations.get(i).onDraw(c, this, mState);
}
}onDraw的代码比较简单，除了调用super.onDraw外，还对分割线进行了绘制。
最后
本文通过View的基本方法对RecyclerView的绘制进行了简单的分析。RecyclerView通过LayoutManager类将测量、布局、绘制等从自身中分离了出来，减少了代码的耦合，使其更加灵活、更易扩展。如果在阅读过程中，有任何疑问与问题，欢迎与我联系。
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