- 在上一篇笔记中分析了UI的绘制流程，但是未深入测量、布局、绘制的具体流程，该篇笔记对这三部分做深入的分析，入口分别是ViewRootImpl类的performTraversals方法中的performMeasure、performLayout、performDraw方法；
- DecorView是最外层的控件，整个窗体的绘制，是从DecorView开始，然后一层一层的调用下去。
一.MeasureSpec
- 在分析performMeasure的时候，传递了两个参数，我们需要先对这两个参数相关的信息进行分析；
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
- 分析getRootMeasureSpec方法
//参数2：不一定是具体的数值
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
//这是一个int类型的参数，但是这不是一个普通的int类型参数，这个参数的二进制当中，包含了测量模式，以及测量的具体数值
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
//窗口无法调整大小。强制根视图为windowsize
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
//窗口无法调整大小。强制根视图为windowsize
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
//窗口无法调整大小。强制根视图为windowsize
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return measureSpec;
}
- MeasureSpec.makeMeasureSpec
//在分析该方法之前，我们需要先了解MeasureSpec（是View的内部类）
//内部的二进制表达需要弄明白
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
//0x3:在内存中的二进制表现形式为
//00000000 00000000 00000000 00000011
//左移动30位之后，11000000 00000000 00000000 00000000
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
//00 000000 00000000 00000000 00000000
//这里有个细节，表述的时候不能直接说成是0
public static final int UNSPECIFIED = 0 << MODE_SHIFT;
//精准尺寸，当我们将控件的layout_width或layout_height指定为具体数值时，或match_parent，则控件大小已经确定，都是精确模式
//01 000000 00000000 00000000 00000000
public static final int EXACTLY = 1 << MODE_SHIFT;
//10 000000 00000000 00000000 00000000
public static final int AT_MOST = 2 << MODE_SHIFT;
//...
//生成测量规则
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);
}
}
}
二.测量的具体流程
- 从performMeasure方法入手
//childWidthMeasureSpec和childHeightMeasureSpec是DecorView的
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
//1.分析
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
- 分析performMeasure
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
//2.mView是DecorView,这里的参数传递的是父类的大小。实际上是需要传递自己的测量规则，但是DecorView比较特殊，其测量规则和父类的一样
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
- 分析DecorView的measure方法（调用View的）
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
//不重要：判断view布局模式是否有一些特殊的边界
boolean optical = isLayoutModeOptical(this);
//不重要
//判断view和该view的夫ciew的布局模式情况，如果两者不同步，则进行子view的size大小的修改
//有两种情况会进入到该if条件，
//一是子view有特殊的光学边界，而父view没有，此时optical为true
//一是父view有一个特殊的光学边界，而子view没有
//这里的optical为false
if (optical != isLayoutModeOptical(mParent)) {
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);
}
//4.获取到测量缓存的key，通过key去缓存中拿，看是否已经有测量结果
//long在内存中占用8个字节（64位），前面32位放宽的测量模式，后面的32位放高的测量模式
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
//5.判断缓存对象是否为空，为空就创建
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
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.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
//6.如果缓存中没有，就调用onMeasure进行测量
if (cacheIndex < 0 || sIgnoreMeasureCache) {
//measure ourselves, this should set the measured dimension flag back
//7.分析该方法
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
//如果缓存中有，就直接从缓存中拿出来，然后通知测量完毕
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
- onMeasure
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
//8.设置的参数是默认值 非重点
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
//getSuggestedMinimumWidth
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
//getDefaultSize
//size:默认大小
//measureSpec：父容器传递过来的测量规则
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
//获取到父容器传进来的测量规则包含的测量模式以及大小
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;
}
//setMeasuredDimension
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
//边界的判断
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
//9.setMeasuredDimensionRaw
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
//setMeasuredDimensionRaw
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
- 分析FrameLayout的onMeasure的方法
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;
//遍历所有的子控件
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//如果子控件都是显示的
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//测量子控件 分析
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
//获取到子控件的LayoutParams
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());
//如果measureMatchParentChildren为true
if (measureMatchParentChildren) {
//如果子控件的长或宽有一个设置的是MATCH_PARENT，就添加到一个集合
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
//加上自己的padding值
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
//用maxHeight和背景的高度进行对比
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
//最大宽度和前景图的宽高比较再取最大值
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
//设置最终的值
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++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
if (lp.width == LayoutParams.MATCH_PARENT) {
final int width = Math.max(0, getMeasuredWidth()
- getPaddingLeftWithForeground() - getPaddingRightWithForeground()
- lp.leftMargin - lp.rightMargin);
childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(
width, MeasureSpec.EXACTLY);
} else {
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);
}
//在父view的测量规则限定下，生成子view的测量规则
//调用child的测量方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
}
- measureChildWithMargins
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//获取到child的测量规则：根据父类的测量规则和自己的大小来生成自己的测量规则
//参数1：父类的测量规则
//参数2：父类的padding + child的margin
//参数3：child的width
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);
}
- 分析getChildMeasureSpec
//注意：测量规则的值，不是最终的大小，而是参考大小
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//获取到父容器的测量模式以及测量值
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
//比对父容器的测量值是否大于0
int size = Math.max(0, specSize - padding);
//用来装子view的测量模式以及测量大小
int resultSize = 0;
int resultMode = 0;
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
//当前的子view设置的是否是不是具体的值
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
//将父大小的值减去padding得到的值赋值给子view
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.
//依旧将父类的值赋值给子view
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
//生成子view的测量规则
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
- LinearLayout的性能比不上RelativeLayout，就测量而言，RelativeLayout不需要计算子view的累加；越简单的东西运行起来性能越佳；
三.布局的具体流程
-
分析performLayout
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { //... final View host = mView; //... //1.分析 host 是DecorView //这是根布局 //左上都是0，右下为其宽和高 host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); //... }
public void layout(int l, int t, int r, int b) {
//...
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//2.分析
//参数1：一开始是false
onLayout(changed, l, t, r, b);
//...
}
//...
}
- 分析framelayout的onLayout方法
//现在分析的是DecoreView（继承子framelayout）
//view的onLayout方法空实现
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
//摆放子控件
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
//framelayout的onLayout方法
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();
//布局子控件
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
//获取子view测量的宽高
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;
}
//调用child的layout方法
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
四.绘制的具体流程
- 分析performDraw
private void performDraw() {
//...
try {
//1.分析draw(fullRedrawNeeded)
boolean canUseAsync = draw(fullRedrawNeeded);
if (usingAsyncReport && !canUseAsync) {
mAttachInfo.mThreadedRenderer.setFrameCompleteCallback(null);
usingAsyncReport = false;
}
}
//...
}
- draw(fullRedrawNeeded)
private boolean draw(boolean fullRedrawNeeded) {
//获取到绘制的画板
Surface surface = mSurface;
//...
//确认绘制区域
final Rect dirty = mDirty;
//...
//根据window的信息进行绘制区域的初始化
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
}
//...
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (mAttachInfo.mThreadedRenderer != null && mAttachInfo.mThreadedRenderer.isEnabled()) {
//...
} else {
//...
//2.绘制流程 传入画板 传入位置信息
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
return useAsyncReport;
}
- drawSoftware
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
//用软件渲染器绘制 ---> 自定义canvas画布对象
final Canvas canvas;
// We already have the offset of surfaceInsets in xoff, yoff and dirty region,
// therefore we need to add it back when moving the dirty region.
//当移动画布区域时，我们需要将其添加回
int dirtyXOffset = xoff;
int dirtyYOffset = yoff;
if (surfaceInsets != null) {
dirtyXOffset += surfaceInsets.left;
dirtyYOffset += surfaceInsets.top;
}
try {
dirty.offset(-dirtyXOffset, -dirtyYOffset);
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
//由画布对象去初始化canvas画布，在这一步canvas拥有了最开始的绘制坐标
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);
}
//...
try {
//...
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
//调用view绘制的方法，将画布传入进去，画布实际上是surface里面获取的 分析
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;
}
}
}
//...
return true;
}
- mView.draw(canvas)
//绘制的操作都是由底层去做的（我们只需要调用api即可）
//绘制是每个控件都有的功能，只是父控件调用子控件实现绘制
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 绘制背景
* 2. If necessary, save the canvas' layers to prepare for fading 如果需要，保存图层
* 3. Draw view's content 绘制view内容
* 4. Draw children 绘制子view
* 5. If necessary, draw the fading edges and restore layers 如果需要，恢复保存的图层
* 6. Draw decorations (scrollbars for instance) 画装饰 滚动条
*/
// 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);
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)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
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)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
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
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
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);
}
canvas.restoreToCount(saveCount);
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)
onDrawForeground(canvas);
if (debugDraw()) {
debugDrawFocus(canvas);
}
}
- FrameLayout没有onDraw方法：其原因是，容器不会去绘制。容器只会绘制背景和用来装控件，控件才会由onDraw方法；
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