// DecorView.java
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
// 对 AT_MOST 进行处理
if (widthMode == AT_MOST) {
}
if (heightMode == AT_MOST) {
}
// 父类的测量，重点
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}
// FrameLayout.java
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
// 子 view 数量
int count = getChildCount();
// viewGroup 的宽高
int maxHeight = 0;
int maxWidth = 0;
int childState = 0;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
// 测量子 View
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
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());
// 需要重新测量 MATCH_PARENT 子 view 的大小
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
// Account for padding too
// 加上 padding
maxWidth += getPaddingLeftWithForeground() + getPaddingRightWithForeground();
maxHeight += getPaddingTopWithForeground() + getPaddingBottomWithForeground();
// Check against our minimum height and width
// 与背景比较
maxHeight = Math.max(maxHeight, getSuggestedMinimumHeight());
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
// Check against our foreground's minimum height and width
// 与前景比较
final Drawable drawable = getForeground();
if (drawable != null) {
maxHeight = Math.max(maxHeight, drawable.getMinimumHeight());
maxWidth = Math.max(maxWidth, drawable.getMinimumWidth());
}
// 保存结果
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
count = mMatchParentChildren.size();
if (count > 1) {
for (int i = 0; i < count; i++) {
final View child = mMatchParentChildren.get(i);
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
final int childWidthMeasureSpec;
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);
}
// 子 view 测量
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
}
// 测量指定的子 View，包括 padding 和 margins。
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
// 确定子 View 的测量参数，见下
// 和后边处理MATCH_PARENT模式的child唯一不同之处在于padding值的大小，也就是将Foreground的padding值加了进来。
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);
// 测量子 view
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
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
// 父布局给了确定的尺寸
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.
// 与父布局一样大
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.
// 子布局自己决定，不能超过父布局
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
// 父布局有一个确定的最大尺寸
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);
}
以上就是整个 View 和 ViewGroup 的整个 Measure 过程。
Layout
回到 ViewRootImpl 类中，执行 performLayout（）过程。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
try {
// 进行放置
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
int numViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
// 放置子 View
int numValidRequests = validLayoutRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = validLayoutRequesters.get(i);
//
view.requestLayout();
}
// 再次测量
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
// 重新放置
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
// 再次检查是否仍有需要layout的View，如果有，就到下一帧再继续
getRunQueue().post(new Runnable() {
@Override
public void run() {
int numValidRequests = finalRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = finalRequesters.get(i);
view.requestLayout();
}
mInLayout = false;
上述过程是先让 DecorView 进行放置，然后有需要就放置所有的子 View，调用 requestLayout 方法。然后再对整个 View Tree 进行测量，进行二次放置。
// View.java
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
// 重新测量
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// 放置，在 view 中空实现，在 viewGroup 中放置子 view
onLayout(changed, l, t, r, b);
// layout 有变化
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList<OnLayoutChangeListener> listenersCopy =
(ArrayList<OnLayoutChangeListener>)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
// 自定义 viewGroup 需要实现
// DecorView.java
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
getOutsets(mOutsets);
if (mOutsets.left > 0) {
offsetLeftAndRight(-mOutsets.left);
}
if (mOutsets.top > 0) {
offsetTopAndBottom(-mOutsets.top);
}
if (mApplyFloatingVerticalInsets) {
offsetTopAndBottom(mFloatingInsets.top);
}
if (mApplyFloatingHorizontalInsets) {
offsetLeftAndRight(mFloatingInsets.left);
}
// If the application changed its SystemUI metrics, we might also have to adapt
// our shadow elevation.
updateElevation();
mAllowUpdateElevation = true;
if (changed && mResizeMode == RESIZE_MODE_DOCKED_DIVIDER) {
getViewRootImpl().requestInvalidateRootRenderNode();
}
}
// FrameLayout.java
// 放置子 View
@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) {
// 子 View 的数量
final int count = getChildCount();
// 与父布局 Framelayout 的距离
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
// 默认左上角
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) {
// 水平居中
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
private void performDraw() {
// 主要是调用 draw 方法
try {
boolean canUseAsync = draw(fullRedrawNeeded);
}
}
private boolean draw(boolean fullRedrawNeeded) {
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
/**
* @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, Rect surfaceInsets) {
// Draw with software renderer.
final Canvas canvas;
// 获取画布
canvas = mSurface.lockCanvas(dirty);
// 一系列设置之后
mView.draw(canvas);
}
// DecorView.java
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
// 还是调用 View 的 draw 方法
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)
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;
}
}
在 draw 过程中，一般有 6 个步骤，2 - 5 可以省略。这里主要看 3 和 4 步。
/**
* Implement this to do your drawing.
*
* @param canvas the canvas on which the background will be drawn
*/
// 自定义 View 实现， 3
protected void onDraw(Canvas canvas) {
}
/**
* 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
*/
// 自定义 ViewGroup 实现
protected void dispatchDraw(Canvas canvas) {
}
View: working process
[TOC]
在上一篇文章Activity： Lifecycle and ActivityThread 分析了 Activity 启动后执行生命周期方法的基础上，走到了 performTraversals() 方法，这篇文章由此开始，讲解 View 的工作流程。
上面是该函数的三个主要过程，下面逐步去看一下每个过程分别做了什么。
View的基本介绍
ViewRoot 和 DecorView
ViewRoot 对应与 ViewRootImpl， 是连接 WindowManager 和 DecorView 的纽带，
View 的三大流程同时通过 ViewRoot 来完成。
在 ActivityThread 中，当 Activity 对象被创建完毕，会将 DecorView 添加到 Window 中，同时会创建 ViewRootImpl 对象，同 DecorView 建立关联。
Measure
这里的 measureSpec 应该很熟悉了，是一个32为的 int 型整数。高两位表示测量模式，低30位表示具体大小。
其中测量模式有三种：
UNSPECIFIED：父容器不作限制，子View想多大就多大，一般用于系统内部。
EXACTLY：精确模式，大小为SpecSize，父容器完全决定子View的大小，对应LayoutParams中的match_parent和具体数值。
AT_MOST：最大模式，大小不能大于SpecSize，也就是子View的大小有上限，对应于LayoutParams中的warp_content。
获得了 view 的测量数据后，
上述就是 View 的测量过程，ViewGroup 的测量过程类似，只不过还要测量子 View 的大小来确定自己的大小。
以 DecorView 为例：
以上就是整个 View 和 ViewGroup 的整个 Measure 过程。
Layout
回到 ViewRootImpl 类中，执行 performLayout（）过程。
上述过程是先让 DecorView 进行放置，然后有需要就放置所有的子 View，调用 requestLayout 方法。然后再对整个 View Tree 进行测量，进行二次放置。
因此这里知道， FrameLayout 放置子 View 就是根据其 Gravity 放置在对应的位置，然后根据顺序进行叠加放置。
Draw
在 draw 过程中，一般有 6 个步骤，2 - 5 可以省略。这里主要看 3 和 4 步。
看一下在 FrameLayout 的是如何处理的。
到此为止，整个 View 的绘制流程就分析完毕了。
自定义 View
总结
在 ViewRootImpl 的方法 performTraversals 中，会实现 view 的整个绘制过程。
测量：View 测量自身，ViewGroup 除了测量自己，还需测量子 View 来确定自己的大小
放置：ViewGroup 放置所有的 子View
绘制：使用 Canvas 进行绘制。