感谢原文作者——原文链接
View的绘制流程主要是指measure，layout，draw这三步，即测量、绘制、布局
View的绘制流程从ViewRootImpl的performTraversals方法开始，在performTraversals方法中会调用performMeasure、performLayout、performDraw三个方法来遍历完成整棵视图树的绘制。
Measure
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
MeasureSpec——系统是通过View的MeasureSpec来确定View的测量宽高的。是一个32位的int值，高2位是specMode记录的是测量模式，低30位是specSize记录的是测量大小。
EXACTLY ： 精确值模式，表示父视图希望子视图的大小应该是由specSize的值来决定的，这个时候View的最终大小就是specSize所记录的大小。对应于LayoutParams中的 match_parent和具体数值的形式。比如 android:layout_width=”match_parent”，android:layout_width=”50dp”
AT_MOST ： 最大值模式，表示父容器指定了一个可用大小specSize，子视图最多只能是specSize中指定的大小，不能大于这个值。对应于LayoutParams中的 wrap_content的形式。
UNSPECIFIED ：父容器不对View有任何限制，View想多大就多大，一般不会用到
对于普通的View来说，其MeasureSpec由父容器的MeasureSpec和自身的LayoutParams共同确定。对于顶级View（DecorView），其MeasureSpec由窗口的尺寸和其自身的LayoutParams共同确定。
一个View是有两个MeasureSpec的，一个用于确定测量的宽，一个用于确定测量的高。回到performMeasure方法，来看看参数childWidthMeasureSpec和childHeightMeasureSpec，这两个MeasureSpec是顶级View的，它们由窗口的尺寸和其自身的LayoutParams共同确定。那它们又是怎么产生的？接下来就是揭晓谜底时候。
在ViewRootImpl的measureHierarchy中的getRootMeasureSpec中确定:
childWidthMeasureSpec = getRootMeasureSpec(desiredWindowWidth, lp.width);
childHeightMeasureSpec = getRootMeasureSpec(desiredWindowHeight, lp.height);
desiredWindowWidth和desiredWindowHeight是屏幕的尺寸。lp.width 和lp.height都是MATCH_PARENT。
private static int getRootMeasureSpec(int windowSize, int rootDimension) {
int measureSpec;
switch (rootDimension) {
case ViewGroup.LayoutParams.MATCH_PARENT:
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);
break;
case ViewGroup.LayoutParams.WRAP_CONTENT:
measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);
break;
default:
measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);
break;
}
return 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);
}
}
将SpecSize和SpecMode包装成32位的int值, 根View的MeasureSpec就诞生了。它将参与构成子元素的MeasureSpec。
现在转入ViewGroup的measureChild方法，这个方法是用来测量子View的。
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);
}
getChildMeasureSpec用于产生特定子view的MeasureSpec
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) {
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
有了MeasureSpec后，真正的测量过程，其实就是根据MeasureSpec的测量模式来确定测量大小。那么现在我们已经搞定了MeasureSpec，之后回到performMeasure方法，这里传入的是顶级View（DecorView）的MeasureSpec。那么我们就跟进看看到底View的测量过程是怎样的。
View的测量
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);
}
}
转入一个final修饰的measure方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
if (forceLayout || needsLayout) {
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL);
}
核心过程会转到onMeasure方法真正去记录，而这个onMeasure方法一般是需要重写，比如DecorView重写了View的onMeasure方法，TextView也是。且看View 的onMeasure()方法，不同的组件有不同的实现，对于Viewgroup，onMasure的核心就是通过子控件计算自己的宽高，然后通过setMeasuredDimention将值设置进去
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
在performMeasure之后，最后的核心是要将测量的高宽通过setMeasuredDimension传递进去。
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;
}
就是根据不同的MeasureSpecMode在getSuggestedMinimumWidth和之前计算好的MeasureSpec之中二选一
此外不同的ViewGroup以不同方式重写了onMeasure方法，ViewGroup除了完成自身的测量，还会遍历子元素，如此循环完成整棵视图树的测量过程。在ViewGroup中定义了一个measureChildren方法去遍历子元素，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);
}
}
}
会转到measureChild方法中去测量子元素。
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);
}
就这样，ViewGroup将measure过程传递到了子元素。如此反复完成整棵视图树的绘制。
以上就是ViewGroup的测量过程
核心过程就是从view顶层View开始，通过父容器的MeasureSpec和当前view的LP计算出当前view的MeasureSpec，然后通过onMeasure方法将确定的dimension传入当前view，onMeasure就是确定当前控件大小的。
Layout
layout方法来完成View布局过程。下层view通过onLayout方法完成布局。
public void layout(int l, int t, int r, int b) {
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
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);
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)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);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
---------------------
先通过setFrame方法确定view
四个顶点的位置，
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
//代码省略
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
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;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
//代码省略
return changed;
}
---------------------
经过了setFrame方法后，View在父容器中的位置也就确定了。
此外，还调用了onLayout方法，是用来确定子元素位置的，该方法中又会遍历子元素，然后调用子元素的layout来确定子元素在父容器中的位置。
和onMeasure方法相似，需要由子类去具体实现
比如FrameLayout的onLayout方法
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
下一步
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();
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;
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;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
最后会依次调用子元素的layout方法，子元素中又会调用setFrame方法确定自身位置并确定其子元素的位置，如此循环。
OnDraw
在performDraw方法中，会调用draw方法的重载，之后会转到draw(Canvas canvas)方法.
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
* 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);
// 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);
// we're done...
return;
}
//代码省略
}
onDraw(canvas)绘制自己，该方法同样在不同view中有不同的实现，dispatchDraw(canvas) 绘制子元素
protected void dispatchDraw(Canvas canvas) {
//代码省略
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
//代码省略
}
在dispatchDraw方法中会遍历子元素，转到drawChild方法，在drawChild方法中又会调用View的draw方法来完成子元素的绘制过程，如此循环完成整个视图树的绘制。
感谢原文作者——原文链接