对于我们每一个android工程师来说，自定义view对我们来说都是开发中不得不面对的问题，而且面对现在五花八门的应用市场，而作为进阶高级工程师的我们，自定义view将成为我们不得不掌握的一大知识。而自定义view的整个流程又可以分为measure,layout与draw三个阶段。本文将从源码与实际应用的角度去解读自定义view的整个流程以及实现过程。
而view的绘制主要是从viewRootImpl的performTraversals()开始的，而viewRootImpl的定义是从view被加入到window开始的。首先让我们从源码角度来分析下：
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
} else {
// If there's no parent, then hardware acceleration for this view is
// set from the application's hardware acceleration setting.
final Context context = view.getContext();
if (context != null
&& (context.getApplicationInfo().flags
& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// Start watching for system property changes.
if (mSystemPropertyUpdater == null) {
mSystemPropertyUpdater = new Runnable() {
@Override public void run() {
synchronized (mLock) {
for (int i = mRoots.size() - 1; i >= 0; --i) {
mRoots.get(i).loadSystemProperties();
}
}
}
};
SystemProperties.addChangeCallback(mSystemPropertyUpdater);
}
int index = findViewLocked(view, false);
if (index >= 0) {
if (mDyingViews.contains(view)) {
// Don't wait for MSG_DIE to make it's way through root's queue.
mRoots.get(index).doDie();
} else {
throw new IllegalStateException("View " + view
+ " has already been added to the window manager.");
}
// The previous removeView() had not completed executing. Now it has.
}
// If this is a panel window, then find the window it is being
// attached to for future reference.
if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
final int count = mViews.size();
for (int i = 0; i < count; i++) {
if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
panelParentView = mViews.get(i);
}
}
}
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
}
// do this last because it fires off messages to start doing things
try {
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
我们会在这里去创建我们的ViewRootImpl对象，并将传进来的view对象交给传给ViewRootImpl对象，即：
root = new ViewRootImpl(view.getContext(), display);
root.setView(view, wparams, panelParentView);
而在setView方法中，我们很快找到了熟悉的标记，requestLayout()方法，这个方法主要用于view的重新布局与重绘：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
...
requestLayout();
...
}
我们继续跟踪：
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
首先会进行线程判断checkThread():
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
这里我们可以看到，通过checkThread可以检测到，只有创建view的地方才可以操作view相关的内容，而我们的view是从window中添加进来的，所以这里的thread就是我们的主线程了，这就是为什么我们不能通过子线程更新UI的缘故。
接下来我们继续向下看scheduleTraversals()的实现：
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
我们主要看一下下面的方法实现，因为他跟我们接下来的绘制息息相关：
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
而这里的mTraversalRunnable最终会被执行，mTraversalRunnable是一个Runnable对象：
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
我们接下来看doTraversal()中的实现：
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
到这里我们清楚的看到，最终调用到了performTraversals()方法，我们挑重点来看：
private void performTraversals() {
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, mWidth, mHeight);
...
performDraw();
...
}
我们接着来看performMeasure()中的实现：
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);
}
}
这个时候我们看到viewRootImpl将对控件的测量传递到了view中，而mView正是我们之前看到的setView()传过来的View对象。接下来我们继续跟踪measure()中的实现：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
到这里我们应该就比较熟悉了，我们在自定义view处理时经常会重写onMeasure()方法，我们来看看onMeasure()中的实现：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
只有一行代码，根据方法名我们也可以看出，这里会直接将我们测量得到的结果设置给变量。而参数是一个方法名，那我们很有必要看一下它的方法实现：
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;
}
到这里我们看到，通过specMode会为我们的result重新赋值。这里简单说一下measureSpec：
measureSpec是一个32位表示的整数，前两位代表了specMode的模式，通过MeasureSpec.getMode(measureSpec)可获取到；后30为表示它的测量大小，通过MeasureSpec.getSize(measureSpec)可得到。measureSpec一共有3种模式：
MeasureSpec.UNSPECIFIED：表示一种不确定状态，一般用于系统内部的测量过程，这种情况下的大小为getSuggestedMinimumWidth()或getSuggestedMinimumHeight()的值，这两个方法后面再说；
MeasureSpec.AT_MOST：这种状态下表示view会充满它的父控件的剩余空间；
MeasureSpec.EXACTLY: 这种状态表示一种确定的状态，即view的宽度或高度是一个确定值，或者充满其父控件的剩余空间。
接下来我们看一下getSuggestedMinimumWidth()，由于getSuggestedMinimumHeight()实现方式一样，所以这里只看getSuggestedMinimumWidth()中的实现：
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
首先会判断view是否设置有mBackground属性，如果没有设置就会返回mMinWidth的值，而mMinWidth对应minWidth属性，默认为0；如果设置有mBackground属性，那么会获取到minWidth与mBackground的宽度的最大值，我们看一下mBackground.getMinimumWidth()方法：
public int getMinimumWidth() {
final int intrinsicWidth = getIntrinsicWidth();
return intrinsicWidth > 0 ? intrinsicWidth : 0;
}
这里实现的是Drawable的getIntrinsicWidth()方法，表示Drawable的内在宽度，而Drawable是一个抽象类，它的实现主要依赖于其子类，比如默认情况下，ShapeDrawable,ColorDrawable是没有宽高的，而BitmapDrawable是有宽高的，其宽高是图片的宽高。
那么现在我们知道了，当specMode为UNSPECIFIED时，默认设置的是mMinWidth与mBackground的最大值。
这里是我们整个view的measure过程。
我们接下来看view的layout过程:
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
...
final View host = mView;
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
其实最终还是调用到了mView的layout方法，这里传进来的就是我们之前测量好的width和height了(并非最终的width和height)，我们来看一下它的layout中的实现：
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);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
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);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
}
这段代码就比较清晰了，其实通过view的四个边缘的坐标确定它的位置，而这里的实现主要是通过setFrame()方法完成的：
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
if (DBG) {
Log.d("View", this + " View.setFrame(" + left + "," + top + ","
+ right + "," + bottom + ")");
}
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);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
if ((mViewFlags & VISIBILITY_MASK) == VISIBLE || mGhostView != null) {
// If we are visible, force the DRAWN bit to on so that
// this invalidate will go through (at least to our parent).
// This is because someone may have invalidated this view
// before this call to setFrame came in, thereby clearing
// the DRAWN bit.
mPrivateFlags |= PFLAG_DRAWN;
invalidate(sizeChanged);
// parent display list may need to be recreated based on a change in the bounds
// of any child
invalidateParentCaches();
}
// Reset drawn bit to original value (invalidate turns it off)
mPrivateFlags |= drawn;
mBackgroundSizeChanged = true;
if (mForegroundInfo != null) {
mForegroundInfo.mBoundsChanged = true;
}
notifySubtreeAccessibilityStateChangedIfNeeded();
}
return changed;
}
这里会为view的mLeft，mTop，mRight，mBottom重新赋值为我们传进来的参数，而这里的赋值我们看到mLeft和mTop都为0，而mRight和mBottom是我们之前传进来的getMeasuredWidth()和getMeasuredHeight()，就是接下来会调用到onLayout中，然而onLayout并没有我们想象中那样有一堆实现，它是一个空方法，是系统提供给我们用来实现我们自定义view的一个接口。
通过以上的分析流程，我们就可以比较明确的知道，如果要获取一个view的宽度(或高度) ，通过getMeasuredWidth()和getWidth()获得的值是一样的：
public final int getWidth() {
return mRight - mLeft;
}
那么为什么还要区分getMeasuredWidth()和getWidth()这两个方法呢？
首先，当view的measure过程完成时，那么就可以获取到它的getMeasuredWidth()了，但只有layout完成时(具体说是它的setFrame()完成时)，才能通过getWidth()获取到它的值。
第二点就是由于在layout阶段系统为我们提供了onLayout回调，并传递了它的四个边界参数给我们，那么当我们通过在回调中调用onLayout(l, t, 1+50, t+50)，那么我们就会为view的实际宽高增加了50px，这个时候它的实际宽高就会与测量值不一致。
这是我们view的layout流程。
我们接下来分析它的draw过程：
draw过程其实与measure,layout过程一样，开始于performTraversals的performDraw，我们接下来看performDraw的过程：
private void performDraw() {
...
draw(fullRedrawNeeded);
...
}
这里我们只看和绘制相关的流程，在performDraw()中调用了draw()方法，我们继续跟踪：
private void draw(boolean fullRedrawNeeded) {
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
...
}
这里会调用到它的drawSoftware()方法，而view的绘制也是从这里开始的：
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
...
mView.draw(canvas);
...
}
到这里我们就可以比较清楚的看到，最终还是调用了mView的draw()进行处理，而mView我们之前已经分析过了，它是通过WM通过addWindow()加载进来的view，最后通过ViewRootImpl的setView()设置给了mView.那我们继续来看view的draw()方法的实现：
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;
}
/*
* 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) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} 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);
// 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);
}
其实从整个view的draw()过程中，我们可以看到view的绘制顺序：
1.首先会进行背景的绘制，即drawBackground()；
2.绘制内容，即onDraw();
3.绘制子类（如果存在的话），dispatchDraw();
4.绘制进度条，通过canvas.drawRect()实现；
5.绘制前景，即onDrawForeground()。
往往后绘制的内容会覆盖先绘制的内容，所以我们可以根据绘制顺序完成我们对目标view的绘制，这里面除了背景的绘制是私有方法我们不能重写外，其它的我们都可以自己实现。
另外，我们可以通过通过设置view不具备自身的绘制功能，这样可以大大提高我们的运行效率：
setWillNotDraw(true);
如果我们在自定义view中这样设置，系统会进行自我优化，不会对自身进行绘制。默认情况下，view是没有启用这个标志的，而viewGroup默认会启用。
关于自定义view的源码部分我们已经了解的差不多了，但很多时候并不是一个单单的自定义view就能搞定我们的需求，我们还有必要看一下自定义ViewGroup中的一些知识：
由于ViewGroup的实现基本都是由具体的子类实现，而针对于不同的布局其实现流程又差别太大，所以我们这里只讨论ViewGroup的一个实现过程，而对于ViewGroup的measure过程，其实就是对子view的一个遍历measure过程，这一点我们可以从下面的代码中看到：
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);
}
我们就以width为例，继续来看childWidthMeasureSpec是如何获取的：
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);
}
从参数我们就可以知道：
spec表示父view的容器大小，即parentWidthMeasureSpec；
padding表示父view的内间距与已使用空间与子view的margin之和；
childDimension表示子view在布局中设置的宽度。
从代码中我们可以看到，子view的measureSpec实际上由父view的MeasureSpec与子view的LayoutParams共同决定。
当父view为EXACTLY模式时，如果子view的size不小于0（具体数值），即childDimension不小于0，那么子view的模式也是EXACTLY，大小为childDimension；如果childDimension为LayoutParams.MATCH_PARENT，那么子view的模式为EXACTLY，大小为填满父view的剩余空间；如果childDimension为LayoutParams.WRAP_CONTENT，那么子view的模式为AT_MOST，大小为填满父view的剩余空间。
当父view为AT_MOST模式时，如果子view的size不小于0（具体数值），即childDimension不小于0，那么子view的模式是EXACTLY，大小为childDimension；如果childDimension为LayoutParams.MATCH_PARENT，那么子view的模式为AT_MOST，大小为填满父view的剩余空间；如果childDimension为LayoutParams.WRAP_CONTENT，那么子view的模式为AT_MOST，大小为填满父view的剩余空间。
当父view为UNSPECIFIED模式时，如果子view的size不小于0（具体数值），即childDimension不小于0，那么子view的模式是EXACTLY，大小为childDimension；如果childDimension为LayoutParams.MATCH_PARENT，那么子view的模式为UNSPECIFIED，大小取决于sUseZeroUnspecifiedMeasureSpec属性，默认为false，当targetSdkVersion < 23为true，此时大小为0，否则填满父view的剩余空间；如果childDimension为LayoutParams.WRAP_CONTENT，那么子view的模式为UNSPECIFIED，大小取决于sUseZeroUnspecifiedMeasureSpec属性，默认为false，当targetSdkVersion < 23为true，此时大小为0，否则填满父view的剩余空间。
上面三条便是view的MeasureSpec的创建规则，这个时候我们要注意，如果我们的自定义view在布局中设置为wrap_content时，并不能达到我们想要的效果，不管父view是什么模式，基本上都会充满父view的剩余空间，所以针对这种情况我们要做特殊处理。
当完成对子view的measureSpec的构造之后，最终会调用到子view的measure()，完成子view的一个measure过程。而要完成所有子view的measure过程，需要在viewGroup的子类中进行循环遍历，如果child不为gone，那么就会进行这样一个measure的过程。
ViewGroup的layout过程也是由不同的子类实现，除了完成自身的一个layout,还要完成所有子 view的一个layout过程。另外ViewGroup的onLayout()是一个抽象方法，我们如果需要自定义ViewGroup时，就必须实现它的onLayout()方法。
另外ViewGroup是没有自己的draw()实现的，它的draw过程主要是对子view的一个draw过程，类似于measure与layout的过程，ViewGroup主要实现了dispatchDraw()方法，来完成对所有子view的一个draw()过程。
ViewGroup的一个自定义过程到这里就结束了，有兴趣的同学可以针对具体的ViewGroup做具体分析。
感谢大家的阅读！
