相信很多Android开发的同学都知道View绘制流程大致是先measure、layout、draw。但是你们知道measure、layout、draw分别做些什么事情吗？进入activity怎么开始绘制view的？
本篇文章主要围绕View的整体流程（包括从什么时候开始进行View的绘制、流程顺序是怎么样的）、measure、layout、draw分别做什么进行讲解。
一、View绘制入口
我们先看下View是从什么时候开始绘制的。是从Activity onCreate中setContentView开始吗？No。从Activity onResume之后，这个需要从ActivityThread中的handleResumeActivity方法来说：
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
//...省略
//调用Activity的onResume方法
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
if (r == null) {
//...省略
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
//...省略
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
//此处便是绘制的入口，调用WindowManager的addView方法
wm.addView(decor, l);
}
//...省略
}
//...省略
}
ActivityThread中的handleResumeActivity先调用Activity的onResume方法，接着通过windowmanager的addView方法开始绘制View。WindowManager只是个接口，它的实现类是WindowManagerImpl类，那我们接着看WindowManagerImpl的addView方法：
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
WindowManagerImpl有调用mGlobal的addView方法，mGloabal是WindowManagerGlobal的实例对象，我们解析看下mGlobal的addView方法：
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
//...省略
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
//...省略
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
//...省略
try {
root.setView(view, wparams, panelParentView);
}
//...省略
}
}
mGloabal的addView先创建ViewRootImpl实列对象root，再设置View的layoutParams，将view加入mViews列表中，将root加入mRoots列表中，接着调用ViewRootImpl的setView方法将view(此view是DecorView)、wparams(是window的layoutParams)、panelParentView（在哪个父级窗口的根view，如果是activity则为空，如果是dialog则是对应的activity）。
明确一点：WindowManager用于类似activity等组件与window的通信管理类；ViewRootImpl则是View和window的交互、通信的桥梁。
好的，我们继续看下ViewRootImpl的setView做些什么事情：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
//将DecorView赋值给成员变量mView
mView = view;
//...省略
requestLayout();
//...省略
}
}
}
requestLayout是不是很熟悉？不错开发中我们有时会重新调用requestLayout进行更新布局重绘。我们先看下ViewRootImpl中requestLayout做什么：
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
//校验是否是ui线程
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
//...省略
}
}
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
void doTraversal() {
//...省略
performTraversals();
//...省略
}
ViewRootImpl的requestLayout先调用scheduleTraversals，接着scheduleTraversals又将mTraversalRunnable加入mChoreographer的执行队列中，mTraversalRunnable在mChoreographer调度室调用doTraversal方法，接着doTraversal又调用performTraversals方法，ok，我们继续看performTraversals方法的实现：
private void performTraversals() {
//...省略
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//...省略
performLayout(lp, mWidth, mHeight);
//...省略
performDraw();
//...省略
}
到这里是不是很熟悉了？先measure再layout最后draw。
二、Measure
步骤一中，我们分析到ViewRootImpl的performTraversals方法中调用performMeasure，我们从这个performMeasure开始分析：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
上面分析是已经明确说明了mView就是DecorView，我们看下DecorView的measure方法，首先DecorView继承自FrameLayout，而FrameLayout有继承自ViewGroup，ViewGroup继承自View。我们发现DecorView并没有重写measure方法，FrameLayout也没有，ViewGroup也没有，那么我们看下View下的measure方法：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//判断是否阴影、光效等边界
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
//代表父控件有阴影、光效等边界而当前view没有
//或者当前view有阴影、光效等边界而父控件没有
//则当前view的大小需要减去阴影、光效等边界的大小
Insets insets = getOpticalInsets();
int oWidth = insets.left + insets.right;
int oHeight = insets.top + insets.bottom;
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
//已宽的大小为高32位，高的大小为低32为计算出测量结果缓存的key
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
//如果测量缓存为空，则创建一个测量缓存对象
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
//是否强制绘制
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
//大小是否有变化
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
//大小是否精确值？
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
//大小是否是match_parent
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
//是否需要测量大小
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
//强制绘制或者需要测量
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
//需要绘制或者测量
// measure ourselves, this should set the measured dimension flag back
//调用onMeasure方法进行测量
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
//...省略
}
///...省略
}
View最终会调用onMeasure进行测量，DecorView重写了FrameLayout的onMeasure方法，我们看下DecorView的onMeasure方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
//...省略
//获取测量策略
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
//...省略
if (widthMode == AT_MOST) {
//宽度测量策略是wrap_content
final TypedValue tvw = isPortrait ? mWindow.mFixedWidthMinor : mWindow.mFixedWidthMajor;
if (tvw != null && tvw.type != TypedValue.TYPE_NULL) {
//根据window的大小来计算DecorView的宽度
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);
//获取DecorView的宽度大小
final int widthSize = MeasureSpec.getSize(widthMeasureSpec);
if (w > 0) {
//取window允许宽度的最大值和当前DecorView宽度大小，两者的最小值作为DecorView的宽度
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) {
//高度测量策略是wrap_content
final TypedValue tvh = isPortrait ? mWindow.mFixedHeightMajor
: mWindow.mFixedHeightMinor;
if (tvh != null && tvh.type != TypedValue.TYPE_NULL) {
//根据window的大小来计算DecorView的高度
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);
//获取DecorView的高度大小
final int heightSize = MeasureSpec.getSize(heightMeasureSpec);
if (h > 0) {
//取window允许高度的最大值和当前DecorView高度，两者的最小值作为DecorView的高度
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);
}
}
//调用父类的onMeasure方法进行测量
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int width = getMeasuredWidth();
boolean measure = false;
widthMeasureSpec = MeasureSpec.makeMeasureSpec(width, EXACTLY);
if (!fixedWidth && widthMode == AT_MOST) {
//如果不是Window的大小不确定并且DecorView的测量策略是wrap_content
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) {
//宽度小于最小值
widthMeasureSpec = MeasureSpec.makeMeasureSpec(min, EXACTLY);
measure = true;
}
}
}
// TODO: Support height?
if (measure) {
//需要二次测量
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
}
首先需要说明下三种测量策略的意义：
- UNSPECIFIED： 不指定测量模式，一般开发中没用到此测量策略，此测量策略说的是父控件并没有对子控件的大小进行约束，子控件想要多大的大小都可以
- EXACTLY：准确测量模式，对应的是我们开发中用到的match_parent或者准确的值，控件最终的大小由父控件确定
- AT_MOST：最大测量模式，对应的是我们开发中用到wrap_content，控件的大小可以任何不超过父控件最大的尺寸。
根据上面DecorView onMeasure代码，可以知道DecorView的大小受window的大小和自身LayoutParams的影响。
DecorView 完成自身初步测量之后，调用父类即FrameLayout的onMeasure方法，我们接着往下看FrameLayout的onMeasure方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//获取子控件个数
int count = getChildCount();
//是否是match_parent
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++) {
//遍历子控件
//根据下标取子控件
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//需要测量所以子控件或者子控件可见状态时Visible或者InVisible
//对子控件进行测量
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
//获取子控件的layoutParams
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//根据子控件的大小计算最大值，用来作为当前控件的最大值
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
//子控件的测量状态
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
//记录match_parent的子控件
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
//最大值加上内边距的值
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
//重新计算最大值
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) {
//依据Drawable的大小来重新计算最大值
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//设置测量结果的值
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++) {
//遍历设置宽度或者高度为match_parent的子控件
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
//宽度为match_parent，取当前控件的宽度值减去内边距、外边距作为子控件的宽度
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
//宽度不为match_parent，已经当前控件的大小、内边距、外边距重新计算子控件的宽度
childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
getPaddingLeftWithForeground() + getPaddingRightWithForeground() +
lp.leftMargin + lp.rightMargin,
lp.width);
}
final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
//高度为match_parent，取当前控件的高度值减去内边距、外边距作为子控件的高度
final int height = Math.max(0, getMeasuredHeight()
- getPaddingTopWithForeground() - getPaddingBottomWithForeground()
- lp.topMargin - lp.bottomMargin);
childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
height, MeasureSpec.EXACTLY);
} else {
//高度不为match_parent，已经当前控件的高度、内边距、外边距重新计算子控件的高度
childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
lp.topMargin + lp.bottomMargin,
lp.height);
}
//重新对子控件进行测量
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
//依据父控件的大小和内边距、子控件的边界测量子控件的大小
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
获取子控件的layoutParams
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//计算子控件宽度的测量值
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
//计算子控件高度的测量值
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
//调用子控件的measure方法对子控件进行测量
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
//根据父控件的测量值、父控件的内边距、子控件的大小计算子控件的测量值
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//获取父控件测量模式
int specMode = MeasureSpec.getMode(spec);
//获取父控件大小
int specSize = MeasureSpec.getSize(spec);
//计算子控件的最大值
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:
//父控件的大小是准确值模式
if (childDimension >= 0) {
//子控件的大小是准确值
//使用子控件的准确值作为子控件的大小
resultSize = childDimension;
//设置控件的测量模式为准确模式
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
//子控件的大小是match_parent
//使用父控件允许的最大值作为子控件的大小
resultSize = size;
//设置控件的测量模式为准确模式
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
//子控件的大小是wrap_content
//使用父控件允许的最大值作为子控件的大小
resultSize = size;
//设置控件的测量模式为最大模式
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
//父控件的大小是最大值模式
if (childDimension >= 0) {
// Child wants a specific size... so be it
//子控件的大小是准确值
//使用子控件的准确值作为子控件的大小
resultSize = childDimension;
//设置控件的测量模式为准确模式
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.
//子控件的大小是match_parent
//使用父控件允许的最大值作为子控件的大小
resultSize = size;
//设置控件的测量模式为最大值模式
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
//子控件的大小是wrap_content
//使用父控件允许的最大值作为子控件的大小
resultSize = size;
//设置控件的测量模式为最大模式
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
//父控件的大小是不测量模式
if (childDimension >= 0) {
// Child wants a specific size... let him have it
//子控件的大小是准确值
//使用子控件的准确值作为子控件的大小
resultSize = childDimension;
//设置控件的测量模式为准确模式
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
//子控件的大小是match_parent
//使用父控件允许的最大值作为子控件的大小
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
//设置控件的测量模式为不测量模式
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
//子控件的大小是wrap_content
//使用父控件允许的最大值作为子控件的大小
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
//设置控件的测量模式为不测量模式
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
//根据测量大小、测量模式生成测量结果MeasureSpec
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
FrameLayout的onMeasure方法大致的流程是根据FrameLayout的测量值、内边距，遍历其子控件，对子控件进行测量（依据的是FramentLayout的测量值和内部距以及子控件的外边距）。然后将测量结果保存到mMeasuredWidth和mMeasuredHeight成员变量中，作为空间的大小。
measure主要做的是对控件及其子孙控件进行大小的测量，得到最终的控件大小。
DecorView的大小受到Window和自身LayoutParams的值影响，其他控件受到其父控件及其自身控件LayoutParams的影响。
三、Layout
Layout的过程是用来确定View在父容器的布局位置。根据上面的分析Layout的起始位置在ViewRootImpl的performLayout方法，我们看下其实现：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
//...省略
final View host = mView;
if (host == null) {
return;
}
//...省略
try {
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
}
//...省略
}
我们已经知道ViewRootImpl的mViews是DecorView，DecorView继承自FrameLayout,FrameLayout继承自ViewGroup,ViewGroup继承自View，DecorView、FrameLayout、ViewGroup都没有重写layout方法，所以我们先进入View的layout方法看下其实现：
public void layout(int l, int t, int r, int b) {
//...省略
//如果有阴影、光效等边界效果，则需要加上父控件的效果边界减去自控件的效果边
界
//如果没有效果边界，则直接依据l、t、r、b设置其位置信息
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//位置信息有变化或者需要重新计算布局位置信息，则调用onLayout方法
onLayout(changed, l, t, r, b);
//...省略
}
DecorView重写了onLayout的方法，我们看下其实现：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
//调用父类的onLayout方法进行计算布局位置信息
super.onLayout(changed, left, top, right, bottom);
//获取外边界信息
getOutsets(mOutsets);
//根据外边界重新计算位置信息
if (mOutsets.left > 0) {
offsetLeftAndRight(-mOutsets.left);
}
if (mOutsets.top > 0) {
offsetTopAndBottom(-mOutsets.top);
}
//如果使用了浮动效果，则需要重新计算位置
if (mApplyFloatingVerticalInsets) {
offsetTopAndBottom(mFloatingInsets.top);
}
if (mApplyFloatingHorizontalInsets) {
offsetLeftAndRight(mFloatingInsets.left);
}
//...省略
}
DecorView的onLayout方法先调用其父控件的onLayout方法，那么我先看下FrameLayout的onLayout方法的实现：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
//调用layoutChildren方法，计算子控件的位置信息
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom, boolean forceLeftGravity) {
//获取子控件的个数
final int count = getChildCount();
//获取水平内边距
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
//获取垂直内边距
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
//遍历子控件
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
//如果子控件的可见性是Visible或者inVisible
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获取子控件的大小
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
//获取子控件相对父控件的对齐方式
int gravity = lp.gravity;
if (gravity == -1) {
//对齐方式默认是左上
gravity = DEFAULT_CHILD_GRAVITY;
}
//获取布局方向、绝对对齐方式、垂直对齐方式
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:
//水平居中，则起始左位置=左内边距+（父控件大小-子控件大小）/ 2 + 左外边距 - 右外边距
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
//右对齐，则起始左位置=右内边距 - 子控件大小 - 右外边距
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
//左对齐，则起始左位置=左内边距 + 左外边距
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
//顶部对齐，则起始顶部位置=顶部内边距+顶部外边距
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
//垂直居中，则起始顶部位置=顶部内边距+（父控件大小 - 子控件大小）/ 2 + 顶部外边距 - 底部外边距
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
//底部对齐，则起始顶部位置=底部内边距 - 子控件大小 - 底部外边距
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
//顶部对齐，则起始顶部位置=顶部内边距+顶部外边距
childTop = parentTop + lp.topMargin;
}
//子控件调用layout进行计算布局位置信息
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
layout的操作主要是计算出控件及其子孙控件的布局位置信息，已经的是measure测量的大小、内边距、外边距的值以及对齐方式。而如果是容器控件（ViewGroup）则会遍历其子孙控件计算它们的布局位置信息。
四、Draw
上面经过了测量大小、计算位置信息之后，接下来就是绘制的操作了，根据控件的大小、位置信息、控件的属性进行绘制。我们从绘制的起始位置ViewRootImpl的performDraw方法开始看：
private void performDraw() {
//...省略
try {
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
}
//...省略
}
private boolean draw(boolean fullRedrawNeeded) {
//...省略
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
//...省略
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
final Canvas canvas;
//...省略
canvas = mSurface.lockCanvas(dirty);
//...省略
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
}
//...省略
}
ViewRootImpl的performDraw调用其内部draw方法，draw方法有调用drawSoftware方法，drawSoftware中调用DecorView的draw方法。我们看下DecorView的draw方法：
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
DecorView的draw方法先调用父类的draw方法，再绘制menu的背景。DecorView的父类FrameLayout没有重写draw方法，FrameLayout的父类ViewGroup也没有重写draw方法，那么我们直接看ViewGroup的父类View的draw方法：
public void draw(Canvas canvas) {
//...省略
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background 绘制背景
* 2. If necessary, save the canvas' layers to prepare for fading 保存画布
* 3. Draw view's content 绘制内容
* 4. Draw children 绘制子控件
* 5. If necessary, draw the fading edges and restore layers 绘制渐变并恢复画布
* 6. Draw decorations (scrollbars for instance) 绘制一些装饰，比如滚动条
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
//第一步、绘制背景
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) {
//没有边界效果：如渐变等
// Step 3, draw the content
//第三步、如果没有边界信息，直接绘制内容
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
//第四步、绘制子控件
dispatchDraw(canvas);
//绘制一些高亮效果
drawAutofilledHighlight(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)
//第六步、绘制前景
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
//绘制焦点高亮效果
drawDefaultFocusHighlight(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
//...省略
// Step 2, save the canvas' layers
//第二步、保存当前画布
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
if (drawTop) {
canvas.saveUnclippedLayer(left, top, right, top + length);
}
if (drawBottom) {
canvas.saveUnclippedLayer(left, bottom - length, right, bottom);
}
if (drawLeft) {
canvas.saveUnclippedLayer(left, top, left + length, bottom);
}
if (drawRight) {
canvas.saveUnclippedLayer(right - length, top, right, bottom);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// Step 3, draw the content
//第三步、绘制内容
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
//第四步、绘制子控件
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
//第五步、绘制渐变等边界并恢复画布
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
//恢复画布
canvas.restoreToCount(saveCount);
//绘制高亮效果
drawAutofilledHighlight(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)
//第六步、绘制前景
onDrawForeground(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
}
View的绘制主要分为6个步骤：
1、绘制背景
2、保存画布
3、绘制内容
4、绘制子控件
5、绘制渐变等边界效果并恢复画布
6、绘制前景，比如滚动条等
其中第2步和第5步是可以省略的
五、结语
综合上面的流程、源码分析，我们可以知道：
- 绘制是在Activity onResume之后执行的
- 绘制的流程主要有：
a、measure->onMeasure
b、layout->onLayout
c、draw->onDraw
其中 - measure主要是测量控件的大小，对子控件一级一级往下测量是在onMeasure方法
- layout主要技术控件的位置新，对子控件一级一级往下技术位置信息是在onLayout方法中
- draw主要是绘制控件，onDraw是绘制控件的内容，对子控件一级一级往下绘制是在dispatchDraw方法中
那么我们来思考一个问题：为什么要先measure再layout最后才是draw呢？其实根据前面的分析和常理想想，也不难能得出答案。要先测量出控件的大小、在根据大小计算出控件在父容器中的位置，有了控件的大小、位置信息，就可以根据每个控件的不同特性绘制出其想要的效果。