续前节，接下来我们分析ViewRootImpl#performTraversals方法，代码如下：
/frameworks/base/core/java/android/view/ViewRootImpl.java
private void performTraversals() {
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
// Ask host how big it wants to be
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
if (measureAgain) {
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
...
performLayout(lp, mWidth, mHeight);
...
performDraw();
...
}
这个函数做了很多的事情，但最重要的是调用Measuer、Layout、Draw三个过程。这里首先看到有获取子View的MeasureSpec的方法，代码如下：
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
// Window can't resize. Force root view to be windowSize.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
// Window can resize. Set max size for root view.
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
// Window wants to be an exact size. Force root view to be that size.
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
可以看到，如果是WRAP_CONTENT对应的模式就是AT_MOST，MATCH_PARENT或其他值，也就是具体数值，对应的模式是EXACTLY。这个MeasureSpec想必大家都了解过，它的高两位用来表示模式SpecMode，低30位用来表示大小SpecSize。SpecMode共有以下三种类型：
- UNSPECIFIED：父容器不作限制，子View想多大就多大，一般用于系统内部。
- EXACTLY：精确模式，大小为SpecSize，父容器完全决定子View的大小，对应LayoutParams中的match_parent和具体数值。
- AT_MOST：最大模式，大小不能大于SpecSize，也就是子View的大小有上限，对应于LayoutParams中的warp_content。
接下来我们分析下测量的过程，代码如下：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
这里Measure交给View来完成，代码如下：
/frameworks/base/core/java/android/view/View.java
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
measure主要的工作是回调onMeasure，在自定义View时也经常会重写此方法，代码如下：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
可以看到，如果不重写此方法，系统会设置一个默认的大小给子View，这里先看下这个默认大小的实现，代码如下：
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
也就是说，无论是EXACTLY还是AT_MOST，都按照测量结果进行设置。继续来看setMeasuredDimension方法，代码如下：
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
...
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
也就是最终将测量的结果保存在mMeasuredWidth和mMeasuredHeight变量中。ViewGroup的测量流程和此一致，只是其在onMeasure时需要测量子View，我们结合DecorView来分析，它继承自FrameLayout，以下是它的onMeasure方法实现：
/frameworks/base/core/java/com/android/internal/policy/DecorView.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
final boolean isPortrait =
getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
boolean fixedWidth = false;
mApplyFloatingHorizontalInsets = false;
if (widthMode == AT_MOST) {
final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
final int w;
if (tvw.type == TypedValue.TYPE_DIMENSION) {
w = (int) tvw.getDimension(metrics);
} else if (tvw.type == TypedValue.TYPE_FRACTION) {
w = (int) tvw.getFraction(metrics.widthPixels, metrics.widthPixels);
} else {
w = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed width: " + w);
final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
if (w > 0) {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.min(w, widthSize), EXACTLY);
fixedWidth = true;
} else {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
widthSize - mFloatingInsets.left - mFloatingInsets.right,
AT_MOST);
mApplyFloatingHorizontalInsets = true;
}
}
}
mApplyFloatingVerticalInsets = false;
if (heightMode == AT_MOST) {
final TypedValue tvh = isPortrait ? mWindow.mFixedHeightMajor
: mWindow.mFixedHeightMinor;
if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
final int h;
if (tvh.type == TypedValue.TYPE_DIMENSION) {
h = (int) tvh.getDimension(metrics);
} else if (tvh.type == TypedValue.TYPE_FRACTION) {
h = (int) tvh.getFraction(metrics.heightPixels, metrics.heightPixels);
} else {
h = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Fixed height: " + h);
final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if (h > 0) {
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.min(h, heightSize), EXACTLY);
} else if ((mWindow.getAttributes().flags & FLAG_LAYOUT_IN_SCREEN) == 0) {
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
heightSize - mFloatingInsets.top - mFloatingInsets.bottom, AT_MOST);
mApplyFloatingVerticalInsets = true;
}
}
}
getOutsets(mOutsets);
if (mOutsets.top > 0 || mOutsets.bottom > 0) {
int mode = MeasureSpec.getMode(heightMeasureSpec);
if (mode != MeasureSpec.UNSPECIFIED) {
int height = MeasureSpec.getSize(heightMeasureSpec);
heightMeasureSpec = MeasureSpec.makeMeasureSpec(
height + mOutsets.top + mOutsets.bottom, mode);
}
}
if (mOutsets.left > 0 || mOutsets.right > 0) {
int mode = MeasureSpec.getMode(widthMeasureSpec);
if (mode != MeasureSpec.UNSPECIFIED) {
int width = MeasureSpec.getSize(widthMeasureSpec);
widthMeasureSpec = MeasureSpec.makeMeasureSpec(
width + mOutsets.left + mOutsets.right, mode);
}
}
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = getMeasuredWidth();
boolean measure = false;
widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);
if (!fixedWidth && widthMode == AT_MOST) {
final TypedValue tv = isPortrait ? mWindow.mMinWidthMinor : mWindow.mMinWidthMajor;
if (tv.type != TypedValue.TYPE_NULL) {
final int min;
if (tv.type == TypedValue.TYPE_DIMENSION) {
min = (int)tv.getDimension(metrics);
} else if (tv.type == TypedValue.TYPE_FRACTION) {
min = (int)tv.getFraction(mAvailableWidth, mAvailableWidth);
} else {
min = 0;
}
if (DEBUG_MEASURE) Log.d(mLogTag, "Adjust for min width: " + min + ", value::"
+ tv.coerceToString() + ", mAvailableWidth=" + mAvailableWidth);
if (width < min) {
widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
measure = true;
}
}
}
// TODO: Support height?
if (measure) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
可以看到，主要是对AT_MOST模式下的宽高进行了修正，然后调用父类的方法，我们的关注点在后者，下面是FrameLayout的相关实现：
/frameworks/base/core/java/android/widget/FrameLayout.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
final boolean measureMatchParentChildren =
MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
mMatchParentChildren.clear();
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 对child进行测量
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
// 一些值的修正
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
// 设置测量结果
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
// 根据不同的LP，给child设置不同的测量宽度
if (lp.width == LayoutParams.MATCH_PARENT) {
// 使用剩余的全部空间
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
// 重新指定
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
// 调用child的measure方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
这里首先测量child，根据此来设置ViewGroup的宽高，然后再设定所有的设置属性为MATCH_PARENT的child的宽高，这是因为ViewGroup后来处理了padding与Foreground的值，导致可用空间和测量结果不一致了。我们先看它是如何测量child的，代码如下：
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
可以看到，这里也调用了getChildMeasureSpec方法，和后边处理MATCH_PARENT模式的child唯一不同之处在于padding值的大小，也就是将Foreground的padding值加了进来。现在我们看下这个getChildMeasureSpec的实现：
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
这里根据ViewGroup的SpecMode不同，给child设置了不同的模式和大小，以保证child能正确完成测量的过程。
接下来我们分析Layout的流程，代码如下：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
...
try {
// 先调用mView的layout方法
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
int numViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
// requestLayout() was called during layout.
// If no layout-request flags are set on the requesting views, there is no problem.
// If some requests are still pending, then we need to clear those flags and do
// a full request/measure/layout pass to handle this situation.
// 获取到需要进行layout的View的个数
ArrayList validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
false);
if (validLayoutRequesters != null) {
// Set this flag to indicate that any further requests are happening during
// the second pass, which may result in posting those requests to the next
// frame instead
mHandlingLayoutInLayoutRequest = true;
// Process fresh layout requests, then measure and layout
int numValidRequests = validLayoutRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = validLayoutRequesters.get(i);
Log.w("View", "requestLayout() improperly called by " + view +
" during layout: running second layout pass");
// 调用它们的requestLayout方法，
view.requestLayout();
}
// 再次进行测量
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
// 重新layout
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mHandlingLayoutInLayoutRequest = false;
// Check the valid requests again, this time without checking/clearing the
// layout flags, since requests happening during the second pass get noop'd
validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
if (validLayoutRequesters != null) {
final ArrayList finalRequesters = validLayoutRequesters;
// Post second-pass requests to the next frame
// 再次检查是否仍有需要layout的View，如果有，就到下一帧再继续
getRunQueue().post(new Runnable() {
@Override
public void run() {
int numValidRequests = finalRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = finalRequesters.get(i);
Log.w("View", "requestLayout() improperly called by " + view +
" during second layout pass: posting in next frame");
view.requestLayout();
}
}
});
}
}
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
mInLayout = false;
}
以上过程是先让DecorView进行layout，然后找到所有需要进行layout的View并调用它们的requestLayout方法，然后对整个View Tree重新测量，再次进行第二次layout过程。
DecorView并没有重写layout方法，它继承自FrameLayout，该方法的实现在ViewGroup中，代码如下：
/frameworks/base/core/java/android/view/ViewGroup.java
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
可以看到，这里最终还是调用了View的方法，所以layout的过程在View和ViewGroup间也是没有差别的，View里的相关实现如下：
/frameworks/base/core/java/android/view/View.java
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
// 需要重新测量
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 需要进行layout
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
// 回调onLayoutChange
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
可以看到，真正的layout工作是在onLayout中完成的，而这个方法在View中是空实现，在ViewGroup中是抽象的，这也是自定义ViewGroup时必须重写此方法的原因。我们再来看在DecorView中的实现：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
getOutsets(mOutsets);
if (mOutsets.left > 0) {
offsetLeftAndRight(-mOutsets.left);
}
if (mOutsets.top > 0) {
offsetTopAndBottom(-mOutsets.top);
}
if (mApplyFloatingVerticalInsets) {
offsetTopAndBottom(mFloatingInsets.top);
}
if (mApplyFloatingHorizontalInsets) {
offsetLeftAndRight(mFloatingInsets.left);
}
// If the application changed its SystemUI metrics, we might also have to adapt
// our shadow elevation.
updateElevation();
mAllowUpdateElevation = true;
if (changed && mResizeMode == RESIZE_MODE_DOCKED_DIVIDER) {
getViewRootImpl().requestInvalidateRootRenderNode();
}
}
主要工作还是交给了父类来处理，以下是FrameLayout的相关实现：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
现在我们就明白了，FrameLayout就是把子View根据其设置的Gravity放置在相应的位置，根据先后顺序进行叠加。
Layout完毕后，就到了最后一个流程：Draw。我们从入口函数看起，代码如下：
private void performDraw() {
...
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
...
if (mReportNextDraw) {
...
try {
mWindowSession.finishDrawing(mWindow);
} catch (RemoteException e) {
}
}
}
这里调用了draw函数，代码如下：
private void draw(boolean fullRedrawNeeded) {
...
if (!sFirstDrawComplete) {
synchronized (sFirstDrawHandlers) {
sFirstDrawComplete = true;
final int count = sFirstDrawHandlers.size();
for (int i = 0; i< count; i++) {
mHandler.post(sFirstDrawHandlers.get(i));
}
}
}
// 滑动到指定区域
scrollToRectOrFocus(null, false);
// 分发OnScrollChanged事件
if (mAttachInfo.mViewScrollChanged) {
mAttachInfo.mViewScrollChanged = false;
mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
}
...
// RootView滑动回调
if (mCurScrollY != curScrollY) {
mCurScrollY = curScrollY;
fullRedrawNeeded = true;
if (mView instanceof RootViewSurfaceTaker) {
((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
}
}
...
// 获取需要绘制的区域
final Rect dirty = mDirty;
...
...
// 分发onDraw
mAttachInfo.mTreeObserver.dispatchOnDraw();
...
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
// 硬件加速
} else {
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
...
}
这里主要处理了一些回调事件，以及是否设置了硬件加速，我们先不考虑硬件加速的部分，接下来看绘制的过程，代码如下：
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
try {
...
canvas = mSurface.lockCanvas(dirty);
...
// TODO: Do this in native
canvas.setDensity(mDensity);
} catch (Surface.OutOfResourcesException e) {
...
}
try {
...
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
} finally {
if (!attachInfo.mSetIgnoreDirtyState) {
// Only clear the flag if it was not set during the mView.draw() call
attachInfo.mIgnoreDirtyState = false;
}
}
} finally {
try {
surface.unlockCanvasAndPost(canvas);
} catch (IllegalArgumentException e) {
...
}
return true;
}
可以看到，这里是通过Surface来生成画布Canvas，相关操作在Native层进行，我们以后再分析。然后调用View#draw方法在画布上进行绘制，接下来我们看下DecorView的draw方法做了什么，代码如下：
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
可以看到这里没有做什么特别的事情，主要的工作还是在View中完成的，代码如下：
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// we're done...
return;
}
...
}
注释里写的很清楚，要依次执行6个步骤，不过步骤2和5是可以跳过的，这里我们主要看步骤3和步骤4。步骤3会调用View本身的onDraw方法，这个方法是空实现，如果自定义View一般需要自行实现。dispatchDraw在View中也是空实现，这个方法主要是针对ViewGroup的，所以我们看下ViewGroup中相应的实现，代码如下：
protected void dispatchDraw(Canvas canvas) {
...
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
最终依然是由View来处理，代码如下：
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
...
if (!drawingWithRenderNode) {
// apply clips directly, since RenderNode won't do it for this draw
if ((parentFlags & ViewGroup.FLAG_CLIP_CHILDREN) != 0 && cache == null) {
if (offsetForScroll) {
canvas.clipRect(sx, sy, sx + getWidth(), sy + getHeight());
} else {
if (!scalingRequired || cache == null) {
canvas.clipRect(0, 0, getWidth(), getHeight());
} else {
canvas.clipRect(0, 0, cache.getWidth(), cache.getHeight());
}
}
}
if (mClipBounds != null) {
// clip bounds ignore scroll
canvas.clipRect(mClipBounds);
}
}
if (!drawingWithDrawingCache) {
if (drawingWithRenderNode) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
((DisplayListCanvas) canvas).drawRenderNode(renderNode);
} else {
// Fast path for layouts with no backgrounds
if ((mPrivateFlags & PFLAG_SKIP_DRAW) == PFLAG_SKIP_DRAW) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
dispatchDraw(canvas);
} else {
draw(canvas);
}
}
} else if (cache != null) {
mPrivateFlags &= ~PFLAG_DIRTY_MASK;
if (layerType == LAYER_TYPE_NONE || mLayerPaint == null) {
// no layer paint, use temporary paint to draw bitmap
Paint cachePaint = parent.mCachePaint;
if (cachePaint == null) {
cachePaint = new Paint();
cachePaint.setDither(false);
parent.mCachePaint = cachePaint;
}
cachePaint.setAlpha((int) (alpha * 255));
canvas.drawBitmap(cache, 0.0f, 0.0f, cachePaint);
} else {
// use layer paint to draw the bitmap, merging the two alphas, but also restore
int layerPaintAlpha = mLayerPaint.getAlpha();
if (alpha < 1) {
mLayerPaint.setAlpha((int) (alpha * layerPaintAlpha));
}
canvas.drawBitmap(cache, 0.0f, 0.0f, mLayerPaint);
if (alpha < 1) {
mLayerPaint.setAlpha(layerPaintAlpha);
}
}
}
...
}
这里主要是使用了缓存，如果没有缓存，就会调用View#draw方法进行绘制，或者是通知子View进行绘制。
至此，View的绘制流程我们就分析完毕了。
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