【朝花夕拾】Android自定义View篇之（一）View绘制流程

//===========ActivityThread.java==========
final void handleResumeActivity(...) {
......
//跟踪代码后发现其初始赋值为mWindow = new PhoneWindow(this, window, activityConfigCallback);
r.window = r.activity.getWindow();
//从PhoneWindow实例中获取DecorView
View decor = r.window.getDecorView();
......
//跟踪代码后发现，vm值为上述PhoneWindow实例中获取的WindowManager。
ViewManager wm = a.getWindowManager();
......
//当前window的属性，从代码跟踪来看是PhoneWindow窗口的属性
WindowManager.LayoutParams l = r.window.getAttributes();
......
wm.addView(decor, l);
......
}

//===============Activity.java=============
private Window mWindow;
public Window getWindow() {
return mWindow;
} final void attach(...){
......
mWindow = new PhoneWindow(this, window, activityConfigCallback);
......
}

//==========PhoneWindow.java===========
// This is the top-level view of the window, containing the window decor.
private DecorView mDecor;
......
public PhoneWindow(...){
......
mDecor = (DecorView) preservedWindow.getDecorView();
......
} @Override
public final View getDecorView() {
......
return mDecor;
}

//===================Window.java===================
//The current window attributes.
private final WindowManager.LayoutParams mWindowAttributes =
new WindowManager.LayoutParams();
......
public final WindowManager.LayoutParams getAttributes() {
return mWindowAttributes;
}
...... //==========WindowManager.java的内部类LayoutParams extends ViewGroup.LayoutParams=============
public LayoutParams() {
super(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT);
......
} //==============ViewGroup.java内部类LayoutParams====================
public LayoutParams(int width, int height) {
this.width = width;
this.height = height;
}

// =====================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);
......
performLayout(lp, mWidth, mHeight);
......
performDraw();
}

/**
* A MeasureSpec encapsulates the layout requirements passed from parent to child.
* Each MeasureSpec represents a requirement for either the width or the height.
* A MeasureSpec is comprised of a size and a mode. There are three possible
* modes:
* <dl>
* <dt>UNSPECIFIED</dt>
* <dd>
* The parent has not imposed any constraint on the child. It can be whatever size
* it wants.
* </dd>
*
* <dt>EXACTLY</dt>
* <dd>
* The parent has determined an exact size for the child. The child is going to be
* given those bounds regardless of how big it wants to be.
* </dd>
*
* <dt>AT_MOST</dt>
* <dd>
* The child can be as large as it wants up to the specified size.
* </dd>
* </dl>
*
* MeasureSpecs are implemented as ints to reduce object allocation. This class
* is provided to pack and unpack the &lt;size, mode&gt; tuple into the int.
*/
public static class MeasureSpec {
private static final int MODE_SHIFT = 30;
private static final int MODE_MASK = 0x3 << MODE_SHIFT;
......
/**
* Measure specification mode: The parent has not imposed any constraint
* on the child. It can be whatever size it wants.
*/
public static final int UNSPECIFIED = 0 << MODE_SHIFT; /**
* Measure specification mode: The parent has determined an exact size
* for the child. The child is going to be given those bounds regardless
* of how big it wants to be.
*/
public static final int EXACTLY = 1 << MODE_SHIFT; /**
* Measure specification mode: The child can be as large as it wants up
* to the specified size.
*/
public static final int AT_MOST = 2 << MODE_SHIFT;
......
/**
* Creates a measure specification based on the supplied size and mode.
*......
*@return the measure specification based on size and mode
*/
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);
}
...... }
......
/**
* Extracts the mode from the supplied measure specification.
*......
*/
@MeasureSpecMode
public static int getMode(int measureSpec) {
//noinspection ResourceType
return (measureSpec & MODE_MASK);
} /**
* Extracts the size from the supplied measure specification.
*......
* @return the size in pixels defined in the supplied measure specification
*/
public static int getSize(int measureSpec) {
return (measureSpec & ~MODE_MASK);
}
......
}

//============================ViewGroup.java===============================
/**
* LayoutParams are used by views to tell their parents how they want to be
* laid out.
*......
* <p>
* The base LayoutParams class just describes how big the view wants to be
* for both width and height. For each dimension, it can specify one of:
* <ul>
* <li>FILL_PARENT (renamed MATCH_PARENT in API Level 8 and higher), which
* means that the view wants to be as big as its parent (minus padding)
* <li> WRAP_CONTENT, which means that the view wants to be just big enough
* to enclose its content (plus padding)
* <li> an exact number
* </ul>
* There are subclasses of LayoutParams for different subclasses of
* ViewGroup. For example, AbsoluteLayout has its own subclass of
* LayoutParams which adds an X and Y value.</p>
* ......
* @attr ref android.R.styleable#ViewGroup_Layout_layout_height
* @attr ref android.R.styleable#ViewGroup_Layout_layout_width
*/
public static class LayoutParams {
...... /**
* Special value for the height or width requested by a View.
* MATCH_PARENT means that the view wants to be as big as its parent,
* minus the parent's padding, if any. Introduced in API Level 8.
*/
public static final int MATCH_PARENT = -1; /**
* Special value for the height or width requested by a View.
* WRAP_CONTENT means that the view wants to be just large enough to fit
* its own internal content, taking its own padding into account.
*/
public static final int WRAP_CONTENT = -2; /**
* Information about how wide the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT
* in API Level 8) or WRAP_CONTENT, or an exact size.
*/
public int width; /**
* Information about how tall the view wants to be. Can be one of the
* constants FILL_PARENT (replaced by MATCH_PARENT
* in API Level 8) or WRAP_CONTENT, or an exact size.
*/
public int height;
......
}

//=============ViewRootImpl.java==============
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
......
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
......
}

//========================ViewRootImpl.java======================
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
......
mView = view;
...... mWindowAttributes.copyFrom(attrs);
......
}

//===========================View.java===============================
/**
* <p>
* This is called to find out how big a view should be. The parent
* supplies constraint information in the width and height parameters.
* </p>
*
* <p>
* The actual measurement work of a view is performed in
* {@link #onMeasure(int, int)}, called by this method. Therefore, only
* {@link #onMeasure(int, int)} can and must be overridden by subclasses.
* </p>
*
*
* @param widthMeasureSpec Horizontal space requirements as imposed by the
* parent
* @param heightMeasureSpec Vertical space requirements as imposed by the
* parent
*
* @see #onMeasure(int, int)
*/
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
......
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
......
}

//===========================View.java===============================
/**
* <p>
* Measure the view and its content to determine the measured width and the
* measured height. This method is invoked by {@link #measure(int, int)} and
* should be overridden by subclasses to provide accurate and efficient
* measurement of their contents.
* </p>
*
* <p>
* <strong>CONTRACT:</strong> When overriding this method, you
* <em>must</em> call {@link #setMeasuredDimension(int, int)} to store the
* measured width and height of this view. Failure to do so will trigger an
* <code>IllegalStateException</code>, thrown by
* {@link #measure(int, int)}. Calling the superclass'
* {@link #onMeasure(int, int)} is a valid use.
* </p>
*
* <p>
* The base class implementation of measure defaults to the background size,
* unless a larger size is allowed by the MeasureSpec. Subclasses should
* override {@link #onMeasure(int, int)} to provide better measurements of
* their content.
* </p>
*
* <p>
* If this method is overridden, it is the subclass's responsibility to make
* sure the measured height and width are at least the view's minimum height
* and width ({@link #getSuggestedMinimumHeight()} and
* {@link #getSuggestedMinimumWidth()}).
* </p>
*
* @param widthMeasureSpec horizontal space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
* @param heightMeasureSpec vertical space requirements as imposed by the parent.
* The requirements are encoded with
* {@link android.view.View.MeasureSpec}.
*
* @see #getMeasuredWidth()
* @see #getMeasuredHeight()
* @see #setMeasuredDimension(int, int)
* @see #getSuggestedMinimumHeight()
* @see #getSuggestedMinimumWidth()
* @see android.view.View.MeasureSpec#getMode(int)
* @see android.view.View.MeasureSpec#getSize(int)
*/
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

/**
* <p>This method must be called by {@link #onMeasure(int, int)} to store the
* measured width and measured height. Failing to do so will trigger an
* exception at measurement time.</p>
*
* @param measuredWidth The measured width of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
* @param measuredHeight The measured height of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
*/
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
......
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

/**
* Returns the suggested minimum width that the view should use. This
* returns the maximum of the view's minimum width
* and the background's minimum width
* ({@link android.graphics.drawable.Drawable#getMinimumWidth()}).
* <p>
* When being used in {@link #onMeasure(int, int)}, the caller should still
* ensure the returned width is within the requirements of the parent.
*
* @return The suggested minimum width of the view.
*/
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}

/**
* Utility to return a default size. Uses the supplied size if the
* MeasureSpec imposed no constraints. Will get larger if allowed
* by the MeasureSpec.
*
* @param size Default size for this view
* @param measureSpec Constraints imposed by the parent
* @return The size this view should be.
*/
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;
}

/**
* ......
* @param measuredWidth The measured width of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
* @param measuredHeight The measured height of this view. May be a complex
* bit mask as defined by {@link #MEASURED_SIZE_MASK} and
* {@link #MEASURED_STATE_TOO_SMALL}.
*/
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
......
}

//==================View.java==============
public static final int MEASURED_SIZE_MASK = 0x00ffffff;
/**
* ......
* @return The raw measured width of this view.
*/
public final int getMeasuredWidth() {
return mMeasuredWidth & MEASURED_SIZE_MASK;
}

//==================View.java==============
/**
* ......
* @return The raw measured height of this view.
*/
public final int getMeasuredHeight() {
return mMeasuredHeight & MEASURED_SIZE_MASK;
}

int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);

/**
* Figures out the measure spec for the root view in a window based on it's
* layout params.
*
* @param windowSize
* The available width or height of the window
*
* @param rootDimension
* The layout params for one dimension (width or height) of the
* window.
*
* @return The measure spec to use to measure the root view.
*/
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;
}

//===========================ViewRootImpl.java=======================
......
final Rect mWinFrame; // frame given by window manager.
......
private void performTraversals() {
......
Rect frame = mWinFrame;
......
mWidth = frame.width();
......
}

//=================================ViewRootImpl.java================================
......
final WindowManager.LayoutParams mWindowAttributes = new WindowManager.LayoutParams();
......
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
......
mWindowAttributes.copyFrom(attrs);
......
}
private void performTraversals() {
......
WindowManager.LayoutParams lp = mWindowAttributes;
......
}

//================ViewGroup.java===============
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding.
* The heavy lifting is done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param parentHeightMeasureSpec The height requirements for this view
*/
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);
}

//===================ViewGroup.java===================
/**
* Ask one of the children of this view to measure itself, taking into
* account both the MeasureSpec requirements for this view and its padding
* and margins. The child must have MarginLayoutParams The heavy lifting is
* done in getChildMeasureSpec.
*
* @param child The child to measure
* @param parentWidthMeasureSpec The width requirements for this view
* @param widthUsed Extra space that has been used up by the parent
* horizontally (possibly by other children of the parent)
* @param parentHeightMeasureSpec The height requirements for this view
* @param heightUsed Extra space that has been used up by the parent
* vertically (possibly by other children of the parent)
*/
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);
}

//==================ViewGroup.java====================
/**
* Does the hard part of measureChildren: figuring out the MeasureSpec to
* pass to a particular child. This method figures out the right MeasureSpec
* for one dimension (height or width) of one child view.
*
* The goal is to combine information from our MeasureSpec with the
* LayoutParams of the child to get the best possible results. For example,
* if the this view knows its size (because its MeasureSpec has a mode of
* EXACTLY), and the child has indicated in its LayoutParams that it wants
* to be the same size as the parent, the parent should ask the child to
* layout given an exact size.
*
* @param spec The requirements for this view
* @param padding The padding of this view for the current dimension and
* margins, if applicable
* @param childDimension How big the child wants to be in the current
* dimension
* @return a MeasureSpec integer for the child
*/
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);
}

//=============DecorView.java=============
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
......
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
......
} //=============FrameLayout.java=============
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
......
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
......
}
......
setMeasuredDimension(......)
...... }

//=====================ViewRootImpl.java=================
private void performTraversals() {
......
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
......
}

performLayout(lp, mWidth, mHeight);

//=====================ViewRootImpl.java===================
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
......
final View host = mView;
......
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
......
}

//==================ViewGroup.java================
@Override
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;
}
}

//=================View.java================
/**
* Assign a size and position to a view and all of its
* descendants
*
* <p>This is the second phase of the layout mechanism.
* (The first is measuring). In this phase, each parent calls
* layout on all of its children to position them.
* This is typically done using the child measurements
* that were stored in the measure pass().</p>
*
* <p>Derived classes should not override this method.
* Derived classes with children should override
* onLayout. In that method, they should
* call layout on each of their children.</p>
*
* @param l Left position, relative to parent
* @param t Top position, relative to parent
* @param r Right position, relative to parent
* @param b Bottom position, relative to parent
*/
@SuppressWarnings({"unchecked"})
public void layout(int l, int t, int r, int b) {
......
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
......
}
......
}

//=================View.java================
/**
* Assign a size and position to this view.
*
* This is called from layout.
*
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
* @return true if the new size and position are different than the
* previous ones
* {@hide}
*/
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
......
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
......
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight); // Invalidate our old position
invalidate(sizeChanged); mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
......
}
return changed;
}

//============View.java============
/**
* Called from layout when this view should
* assign a size and position to each of its children.
*
* Derived classes with children should override
* this method and call layout on each of
* their children.
* @param changed This is a new size or position for this view
* @param left Left position, relative to parent
* @param top Top position, relative to parent
* @param right Right position, relative to parent
* @param bottom Bottom position, relative to parent
*/
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}

//=============ViewGroup.java===========
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);

//==============DecorView.java================
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
......
}

//================FrameLayout.java==============
@Override
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();
......
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();
......
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}

//==================ViewRootImpl.java=================
private void performDraw() {
......
boolean canUseAsync = draw(fullRedrawNeeded);
......
} private boolean draw(boolean fullRedrawNeeded) {
......
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
......
} private boolean drawSoftware(......){
......
mView.draw(canvas);
......
}

//================DecorView.java==============
@Override
public void draw(Canvas canvas) {
super.draw(canvas); if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}

//====================View.java=====================
/**
* Manually render this view (and all of its children) to the given Canvas.
* The view must have already done a full layout before this function is
* called. When implementing a view, implement
* {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
* If you do need to override this method, call the superclass version.
*
* @param canvas The Canvas to which the View is rendered.
*/
@CallSuper
public void draw(Canvas canvas) {
......
/*
* 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)
......
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas); // Step 4, draw the children
dispatchDraw(canvas);
......
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);45 ......
}

//=================View.java===============
/**
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
protected void onDraw(Canvas canvas) {
}

//===============DocerView.java==============
@Override
public void onDraw(Canvas c) {
super.onDraw(c);
mBackgroundFallback.draw(this, mContentRoot, c, mWindow.mContentParent,
mStatusColorViewState.view, mNavigationColorViewState.view);
}

//================View.java===============
/**
* Called by draw to draw the child views. This may be overridden
* by derived classes to gain control just before its children are drawn
* (but after its own view has been drawn).
* @param canvas the canvas on which to draw the view
*/
protected void dispatchDraw(Canvas canvas) { }

//=============ViewGroup.java============
@Override
protected void dispatchDraw(Canvas canvas) {
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
......
for (int i = 0; i < childrenCount; i++) {
more |= drawChild(canvas, child, drawingTime);
......
}
......
}

/**
* Draw one child of this View Group. This method is responsible for getting
* the canvas in the right state. This includes clipping, translating so
* that the child's scrolled origin is at 0, 0, and applying any animation
* transformations.
*
* @param canvas The canvas on which to draw the child
* @param child Who to draw
* @param drawingTime The time at which draw is occurring
* @return True if an invalidate() was issued
*/
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}

//============View.java===========
/**
* This method is called by ViewGroup.drawChild() to have each child view draw itself.
*
* This is where the View specializes rendering behavior based on layer type,
* and hardware acceleration.
*/
boolean draw(Canvas canvas, ViewGroup parent, long drawingTime) {
......
draw(canvas);
......
}