本文详细解析了Android UI的绘制流程，从Activity启动到View的绘制全过程，包括测量、布局和绘制三个关键步骤。深入探讨了MeasureSpec的概念及其作用，以及View和ViewGroup如何协同工作完成界面的构建。
Android的Activity作为应用程序的载体存在，它代表着整个用户界面，提供了一个窗口来绘制各种视图。Activity启动时调用setContentView方法来加载一个内容视图，这个内容视图就是用户看到的界面。Android中View存在两种形式：一种是单一的View控件，另一种是可以包含其他View的ViewGroup容器。前面所说的内容视图就是以ViewGroup的形式存在的。在讲解View的绘制流程之前，我们先来了解一下Android的UI管理系统的层次关系。
PhoneWindow是Android系统中最基本的窗口系统，每个Activity会创建一个。PhoneWindow是Activity和View系统交互的接口。DecorView本质上是一个FrameLayout，是Activity中所有View的祖先。
1.1绘制流程
当一个应用启动时，会启动一个主Activity，Android系统会根据Activity的布局来对它进行绘制。绘制会从根视图ViewRoot的 performTraversals（） 方法开始，遍历整个视图树，每个View控件负责绘制自己，而ViewGroup还需要负责通知子View进行绘制。视图绘制的过程包括三个步骤，测量(measure)、布局(layout)和绘制(draw)。
preformTraversals 方法核心代码如下：
private void preformTraversals() {
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
...
// 测量流程
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
// 执行布局流程
performLayout(lp, mWidth, mHeight);
...
// 执行绘制流程
performDraw();
...
}MeasureSpec 表示的是一个32位的整型值，它的高2位表示测量模式SpecMode，低30位表示某种测量模式下的规则大小SpecSize。MeasureSpec 是View类的一个静态内部类，用来说明应该如何测量这个View。其核心代码如下：
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
/** @hide */
@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
@Retention(RetentionPolicy.SOURCE)
public @interface MeasureSpecMode {}
// 不指定测量模式
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
// 精确测量模式
public static final int EXACTLY = 1 << MODE_SHIFT;
// 最大值测量模式
public static final int AT_MOST = 2 << MODE_SHIFT;
// 根据指定的大小和模式创建一个MeasureSpec
public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
@MeasureSpecMode int mode) {
if (sUseBrokenMakeMeasureSpec) {
return size + mode;
} else {
return (size & ~MODE_MASK) | (mode & MODE_MASK);
}
}
/**
* Like {@link #makeMeasureSpec(int, int)}, but any spec with a mode of UNSPECIFIED
* will automatically get a size of 0. Older apps expect this.
*
* @hide internal use only for compatibility with system widgets and older apps
*/
public static int makeSafeMeasureSpec(int size, int mode) {
if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) {
return 0;
}
return makeMeasureSpec(size, mode);
}
...
// 微调某个MeasureSpec的大小
static int adjust(int measureSpec, int delta) {
final int mode = getMode(measureSpec);
int size = getSize(measureSpec);
if (mode == UNSPECIFIED) {
// No need to adjust size for UNSPECIFIED mode.
return makeMeasureSpec(size, UNSPECIFIED);
}
size += delta;
if (size < 0) {
Log.e(VIEW_LOG_TAG, "MeasureSpec.adjust: new size would be negative! (" + size +
") spec: " + toString(measureSpec) + " delta: " + delta);
size = 0;
}
return makeMeasureSpec(size, mode);
}
...
} 重点关注三种测量模式，在后面的Measure 阶段用到
·UNSPECIFIED ： 不指定测量模式，父视图没有限制子视图的大小，子视图可以是想要的任何尺寸，通常用于系统内部，应用开发中很少使用到。
·EXACTLY ： 精确测量模式，当视图的layout_width 或者layout_height 指定为具体数值或者match_parent 时生效，表示父视图已经决定了子视图的精确大小，这种模式下的测量值就是SpecSize 的值
·AT_MOST ： 最大值模式，当该视图的layout_width或layout_height 是 wrap_content 的模式时生效，此时子视图的只存可以是不超过父视图允许的最大尺寸的任何尺寸。
对DecorView 而言，它的MeasureSpec是由窗口尺寸 和自身的LayoutParams 共同决定，对于普通的View，它的MeasureSpec 由父视图的MeasureSpec 和自身的LayoutParams 共同决定的。
1.3测量(Measure)
Measure操作用来计算View的实际大小，由前面的分析可知，页面的测量流程是从performMeasure开始的，代码实现如下：
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);
}
}从上面可以看到，具体的测量操作是分发给ViewGroup的，由ViewGroup在它的measureChild 方法中传递给子View。ViewGroup遍历所有子View，并逐个调用子View的measure方法实现测量操作。代码如下：
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
···
onMeasure（widthMeasureSpec，heighMeasureSpec）
···
}protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec){
//setMeasureDimension方法用于设置View的测量宽高
setMeasureDimension(getDefaultSize(getSuggestedMinimumWidth(),widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(),heightMeasureSpec));
}如果没有的重写onMeasure方法，则默认会直接调用gerDefaultSize来获取View 的宽高，其代码实现如下：
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;
}布局过程跟测量过程类似，Layout过程是用来确定View在父容器中的布局位置。它是由父容器获取子View的位置参数后，调用子View的layout方法并将位置参数传入实现的。ViewRootImpl中的performLayout代码如下：
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (DEBUG_ORIENTATION || DEBUG_LAYOUT) {
Log.v(mTag, "Laying out " + host + " to (" +
host.getMeasuredWidth() + ", " + host.getMeasuredHeight() + ")");
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "layout");
try {
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.
ArrayList<View> 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");
view.requestLayout();
}
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
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<View> finalRequesters = validLayoutRequesters;
// Post second-pass requests to the next frame
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;
}1.5绘制(draw)
绘制操作用来将控件绘制出来，绘制的流程从performDraw开始。最终会调用每个View的draw方法绘制每个具体的View，绘制基本上可以分为六个步骤。performDraw的代码如下：
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
// For whatever reason we didn't create a HardwareRenderer, end any
// hardware animations that are now dangling
if (mAttachInfo.mPendingAnimatingRenderNodes != null) {
final int count = mAttachInfo.mPendingAnimatingRenderNodes.size();
for (int i = 0; i < count; i++) {
mAttachInfo.mPendingAnimatingRenderNodes.get(i).endAllAnimators();
}
mAttachInfo.mPendingAnimatingRenderNodes.clear();
}
if (mReportNextDraw) {
mReportNextDraw = false;
// if we're using multi-thread renderer, wait for the window frame draws
if (mWindowDrawCountDown != null) {
try {
mWindowDrawCountDown.await();
} catch (InterruptedException e) {
Log.e(mTag, "Window redraw count down interruped!");
}
mWindowDrawCountDown = null;
}
if (mAttachInfo.mHardwareRenderer != null) {
mAttachInfo.mHardwareRenderer.fence();
mAttachInfo.mHardwareRenderer.setStopped(mStopped);
}
if (LOCAL_LOGV) {
Log.v(mTag, "FINISHED DRAWING: " + mWindowAttributes.getTitle());
}
if (mSurfaceHolder != null && mSurface.isValid()) {
mSurfaceHolderCallback.surfaceRedrawNeeded(mSurfaceHolder);
SurfaceHolder.Callback callbacks[] = mSurfaceHolder.getCallbacks();
if (callbacks != null) {
for (SurfaceHolder.Callback c : callbacks) {
if (c instanceof SurfaceHolder.Callback2) {
((SurfaceHolder.Callback2)c).surfaceRedrawNeeded(mSurfaceHolder);
}
}
}
}
try {
mWindowSession.finishDrawing(mWindow);
} catch (RemoteException e) {
}
}
}
private void draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
if (!surface.isValid()) {
return;
}
if (DEBUG_FPS) {
trackFPS();
}
if (!sFirstDrawComplete) {
synchronized (sFirstDrawHandlers) {
sFirstDrawComplete = true;
final int count = sFirstDrawHandlers.size();
for (int i = 0; i< count; i++) {
mHandler.post(sFirstDrawHandlers.get(i));
}
}
}
scrollToRectOrFocus(null, false);
if (mAttachInfo.mViewScrollChanged) {
mAttachInfo.mViewScrollChanged = false;
mAttachInfo.mTreeObserver.dispatchOnScrollChanged();
}
boolean animating = mScroller != null && mScroller.computeScrollOffset();
final int curScrollY;
if (animating) {
curScrollY = mScroller.getCurrY();
} else {
curScrollY = mScrollY;
}
if (mCurScrollY != curScrollY) {
mCurScrollY = curScrollY;
fullRedrawNeeded = true;
if (mView instanceof RootViewSurfaceTaker) {
((RootViewSurfaceTaker) mView).onRootViewScrollYChanged(mCurScrollY);
}
}
final float appScale = mAttachInfo.mApplicationScale;
final boolean scalingRequired = mAttachInfo.mScalingRequired;
int resizeAlpha = 0;
final Rect dirty = mDirty;
if (mSurfaceHolder != null) {
// The app owns the surface, we won't draw.
dirty.setEmpty();
if (animating && mScroller != null) {
mScroller.abortAnimation();
}
return;
}
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
}
if (DEBUG_ORIENTATION || DEBUG_DRAW) {
Log.v(mTag, "Draw " + mView + "/"
+ mWindowAttributes.getTitle()
+ ": dirty={" + dirty.left + "," + dirty.top
+ "," + dirty.right + "," + dirty.bottom + "} surface="
+ surface + " surface.isValid()=" + surface.isValid() + ", appScale:" +
appScale + ", width=" + mWidth + ", height=" + mHeight);
}
mAttachInfo.mTreeObserver.dispatchOnDraw();
int xOffset = -mCanvasOffsetX;
int yOffset = -mCanvasOffsetY + curScrollY;
final WindowManager.LayoutParams params = mWindowAttributes;
final Rect surfaceInsets = params != null ? params.surfaceInsets : null;
if (surfaceInsets != null) {
xOffset -= surfaceInsets.left;
yOffset -= surfaceInsets.top;
// Offset dirty rect for surface insets.
dirty.offset(surfaceInsets.left, surfaceInsets.right);
}
boolean accessibilityFocusDirty = false;
final Drawable drawable = mAttachInfo.mAccessibilityFocusDrawable;
if (drawable != null) {
final Rect bounds = mAttachInfo.mTmpInvalRect;
final boolean hasFocus = getAccessibilityFocusedRect(bounds);
if (!hasFocus) {
bounds.setEmpty();
}
if (!bounds.equals(drawable.getBounds())) {
accessibilityFocusDirty = true;
}
}
mAttachInfo.mDrawingTime =
mChoreographer.getFrameTimeNanos() / TimeUtils.NANOS_PER_MS;
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (mAttachInfo.mHardwareRenderer != null && mAttachInfo.mHardwareRenderer.isEnabled()) {
// If accessibility focus moved, always invalidate the root.
boolean invalidateRoot = accessibilityFocusDirty || mInvalidateRootRequested;
mInvalidateRootRequested = false;
// Draw with hardware renderer.
mIsAnimating = false;
if (mHardwareYOffset != yOffset || mHardwareXOffset != xOffset) {
mHardwareYOffset = yOffset;
mHardwareXOffset = xOffset;
invalidateRoot = true;
}
if (invalidateRoot) {
mAttachInfo.mHardwareRenderer.invalidateRoot();
}
dirty.setEmpty();
// Stage the content drawn size now. It will be transferred to the renderer
// shortly before the draw commands get send to the renderer.
final boolean updated = updateContentDrawBounds();
if (mReportNextDraw) {
// report next draw overrides setStopped()
// This value is re-sync'd to the value of mStopped
// in the handling of mReportNextDraw post-draw.
mAttachInfo.mHardwareRenderer.setStopped(false);
}
if (updated) {
requestDrawWindow();
}
mAttachInfo.mHardwareRenderer.draw(mView, mAttachInfo, this);
} else {
// If we get here with a disabled & requested hardware renderer, something went
// wrong (an invalidate posted right before we destroyed the hardware surface
// for instance) so we should just bail out. Locking the surface with software
// rendering at this point would lock it forever and prevent hardware renderer
// from doing its job when it comes back.
// Before we request a new frame we must however attempt to reinitiliaze the
// hardware renderer if it's in requested state. This would happen after an
// eglTerminate() for instance.
if (mAttachInfo.mHardwareRenderer != null &&
!mAttachInfo.mHardwareRenderer.isEnabled() &&
mAttachInfo.mHardwareRenderer.isRequested()) {
try {
mAttachInfo.mHardwareRenderer.initializeIfNeeded(
mWidth, mHeight, mAttachInfo, mSurface, surfaceInsets);
} catch (OutOfResourcesException e) {
handleOutOfResourcesException(e);
return;
}
mFullRedrawNeeded = true;
scheduleTraversals();
return;
}
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}
/**
* @return true if drawing was successful, false if an error occurred
*/
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
try {
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
canvas = mSurface.lockCanvas(dirty);
// The dirty rectangle can be modified by Surface.lockCanvas()
//noinspection ConstantConditions
if (left != dirty.left || top != dirty.top || right != dirty.right
|| bottom != dirty.bottom) {
attachInfo.mIgnoreDirtyState = true;
}
// TODO: Do this in native
canvas.setDensity(mDensity);
} catch (Surface.OutOfResourcesException e) {
handleOutOfResourcesException(e);
return false;
} catch (IllegalArgumentException e) {
Log.e(mTag, "Could not lock surface", e);
// Don't assume this is due to out of memory, it could be
// something else, and if it is something else then we could
// kill stuff (or ourself) for no reason.
mLayoutRequested = true; // ask wm for a new surface next time.
return false;
}
try {
if (DEBUG_ORIENTATION || DEBUG_DRAW) {
Log.v(mTag, "Surface " + surface + " drawing to bitmap w="
+ canvas.getWidth() + ", h=" + canvas.getHeight());
//canvas.drawARGB(255, 255, 0, 0);
}
// If this bitmap's format includes an alpha channel, we
// need to clear it before drawing so that the child will
// properly re-composite its drawing on a transparent
// background. This automatically respects the clip/dirty region
// or
// If we are applying an offset, we need to clear the area
// where the offset doesn't appear to avoid having garbage
// left in the blank areas.
if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
canvas.drawColor(0, PorterDuff.Mode.CLEAR);
}
dirty.setEmpty();
mIsAnimating = false;
mView.mPrivateFlags |= View.PFLAG_DRAWN;
if (DEBUG_DRAW) {
Context cxt = mView.getContext();
Log.i(mTag, "Drawing: package:" + cxt.getPackageName() +
", metrics=" + cxt.getResources().getDisplayMetrics() +
", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());
}
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) {
Log.e(mTag, "Could not unlock surface", e);
mLayoutRequested = true; // ask wm for a new surface next time.
//noinspection ReturnInsideFinallyBlock
return false;
}
if (LOCAL_LOGV) {
Log.v(mTag, "Surface " + surface + " unlockCanvasAndPost");
}
}
return true;
}
可以看到最终调用到每个View的draw方法来绘制每个具体的View，绘制基本可以分为六个步骤，如下：
public void draw(Canvas canvas){
//1、绘制View 的背景
drawBackground(canvas);
//2、如果需要，保存canvas的图层，为fading做准备
saveCount = canvas.getSaveCount();
...
canvas.saveLayer(left, top, right, top + length, null, flags);
...
//3、绘制view的内容
onDraw(canvas);
//4、绘制View 的子View
dispatchDraw(canvas);
//5、如果需要，绘制View的fading图层
canvas.drawRect(left, top, right, top + length, p);
...
canvas.restoreToCount(saveCount);
//6、绘制View 的装饰内容，比如滚动条等
onDrawScrollBars(canvas);
}
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