1，ViewRoot和DecorView
ViewRoot对应于ViewRootImpl类，它是连接WindowManager和DecorView的纽带，View的三大流程（measure、layout、draw）都是通过它完成的。在ActivityThread中，当Activity对象被创建后，会将DecorView添加到Window中，同时创建ViewRootImpl对象，并将ViewRootImpl对象和DecorView建立关联。
View的绘制流程是从ViewRoot的performTraversals方法开始的，它经过measure、layout、draw三个过程将View绘制出来。
measure过程决定了View的宽高，Measure完成以后，可以通过getMeasureWidth/getMeasureHeight获取View的宽高；
Layout过程决定了View的四个顶点的坐标和实际View的宽高，完成以后，可以通过getTop、getBottom、getLeft、getRight获取view的四个顶点的坐标，也可以通过getWidth、getHeight获取view的最终宽高；
Draw过程决定了View的显示，只有draw完成后，view的内容才能显示到屏幕上。
2，MeasureSpec
2.1，MeasureSpec基础
MessureSpec决定一个view的尺寸规格，测量过程中会受到父容器的影响。测量过程中，系统会将View的LayoutParams根据父容器所施加的规则转换成对应的MeasureSpec，然后根据这个MeasureSpec来测量view的宽高。
MeasureSpec是一个32位的int值，高2位表示测量模式（SpecMode），低30位代表测试规格（SpecSize）。下面是3种测量模式的介绍：
UNSPECIFIED：父容器不对View有任何限制，要多大就有多大，这种情况一般用于系统内部，表示一种测量状态。
EXACTLY：父容器已经检测出View的精确大小，这时候View的最终大小由SpecSize多指定的值，它对应于LayoutParams中的match_parent和具体的数值这2种模式（layout_width/layout_heigth设置match_parent或者具体的单位，如20dp等）。
AT_MOST：父容器指定了一个可用大小即SpecSize，View的大小不能大于这个值，具体是什么值需要看View的具体实现，它对应于LayoutParams的wrap_content（即layout_width/layout_heigth设置wrap_content）。
2.2，MeasureSpec和LayoutParams的对应关系
在View测量的时候，系统会将LayoutParams在父容器的约束下转换成对象的MeasureSpec，然后再根据这个MeasureSpec来确定View测量后的宽高。MeasureSpec并不是唯一由LayoutParams决定的，LayoutParams需要和父容器一起才能决定View的MeasureSpec。
对于普通的View来说V，View的measure过程是通过ViewGroup传递来的，下面是ViewGroup的measureChildWithMargins方法：
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);
}
从上述代码中可以看出，对子元素的measure之前，会调用getChildMeasureSpec方法得到子元素的MeasureSpec，并且子元素的MeasureSpec跟父容器的MeasureSpec和子元素的LayoutParams有关，此外还跟子元素的margin和padding也有关，下面是getChildMeasureSpec方法的源码：
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 = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
以上代码可以清楚的了解父容器的MeasureSpec和子元素的MeasureSpec的关系：
3，View的工作流程
3.1，measure过程
一个原始View通过measure过程，就完成了其测量宽高；而一个ViewGroup类型的组件，除了完成自己的测量外，还会遍历其子元素的measure方法，各个子元素又会递归去执行这个流程。
3.1.1，View的measure过程
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
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;
}
...
}
上面的代码是View的measure方法，可以发现该方法是final类型，表示measure是不能被子类重写的，View的measure方法内部又会调用onMeasure方法：
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
在onMeasure方法中，又调用了setMeasuredDimension方法，setMeasuredDimension方法会设置View的宽高测量值，看其参数有个方法getDefaultSize如下：
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;
}
getDefaultSize方法功能很单调，首先获取MeasureSpec中的specMode，根据specMode设置对应的specSize的值，并且返回，specSize就是View的测量时的宽高。
注意：此时measure方法测量的宽高叫做测量后的宽高，并不一定代表就是View的最终宽高，因为View的最终宽高是在layout过程确定下来的，但是大多数时候测量时宽高和最终宽高是一样的。
setMeasuredDimension就是通过getDefaultSize获取specSize传入当做参数，从而测量出View的宽高。所以View的测量宽高其实就是由specSize决定的，那么有个问题就是，直接继承View的自定义View需要重写onMeasure方法并且设置wrap_content时的自身大小，否则自定义View的使用wrap_content和使用match_parent的效果是一样的。这个原因可以通过分析上述的代码和前面的图2来理解，首先若自定义View设置了wrap_content时，那么在getDefaultSize方法中其对应的specMode是AT_MOST，此时它的宽高取的是specSize，在图2中可以知道，这种情况下specSize是parentSize，parentSize是父容器目前的大小，此时设置的wrap_content和match_parent效果是一样的。但是通过下面的处理可以解决这个问题：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
int widthSpecMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSpecSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSpecMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSpecSize = MeasureSpec.getSize(heightMeasureSpec);
if (widthSpecMode == MeasureSpec.AT_MOST && heightSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, mHeight);
} else if (widthSpecMode == MeasureSpec.AT_MOST) {
setMeasuredDimension(mWidth, heightSpecSize);
} else if (heightMeasureSpec == MeasureSpec.AT_MOST) {
setMeasuredDimension(widthSpecSize, mHeight);
}
}
3.1.2，ViewGroup的measure过程
对于ViewGroup的measure过程来说，它除了完成对自身的测量外，还会递归measure其子元素宽高，在ViewGroup中measureChildren方法就是完成对子元素的遍历measure的开端：
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);
}
}
}
在上面的源码中可以发现ViewGroup是首先遍历了子元素，并且调用了measureChild方法完成对子元素的measure，下面是measureChild方法的代码：
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);
}
通过上面的代码可以明显发现，measureChild方法内部首先会获取子元素的LayoutParams，然后通过getChildMeasureSpec方法获取子元素的MeasureSpec，最终调用View的measure方法完成子元素的测量过程。关于getChildMeasureSpec方法，本篇笔记上面有介绍。
measure完成以后，通过getMeasuredWidth和getMeasuredHeight就可以获取View的测量宽高了，但是在某些极端情况下，系统需要多次measure后才能确定View的宽高，这时候我们通过getMeasuredWidth和getMeasuredHeight获取的宽高就不准确了，所以最好的办法是在onLayout方法中获取View的测量宽高或者是最终宽高。
3.2，layout过程
layout过程是确定子元素位置，当ViewGroup的位置被确定后，它会在onLayout中遍历所有的子元素并调用其layout方法，在layout方法中又调用onLayout方法。layout方法确定View自身的位置，onLayout是确定所有子元素的位置。下面是View的layout方法代码：
@SuppressWarnings({"unchecked"})
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;
}
上面的代码中首先是调用了setFrame方法，setFrame方法是用来设置View的四个顶点的位置的，即初始化mLeft、mTop、mRight和mBottom这四个值。View的顶点一旦确定，那么View在父容器的位置就确定了。接着就会调用onLayout方法，这个方法的用途就是父容器确定子元素的位置，onLayout并没有具体的实现，需要针对不同的场景重写这个方法。下面是setFrame方法：
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;
// 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);
}
...
return changed;
}
通过setFrame方法，mLeft = left;mTop = top;mRight = right;mBottom = bottom; newWidth = right - left;newHeight = bottom - top;这些计算，View的位置属性被确定下来了。
3.3，draw过程
draw的过程遵循以下几个步骤：
（1）绘制背景background.draw(canvas)。
（2）绘制自己onDraw。
（3）绘制children（dispatchDraw）。
（4）绘制装饰（onDrawScrollBars）。
下面是draw方法的源码：
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);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(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);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
}
我们发现在draw方法的第二步onDraw绘制自身内容和第三步绘制children的dispatchDraw方法都是空实现：
protected void onDraw(Canvas canvas) {
}
protected void dispatchDraw(Canvas canvas) {
}
那么这2个方法，尤其是onDraw就需要控件根据自身的需要，自己实现如何绘制了。
ps：感谢任玉刚的《Android开发艺术探索》，这篇笔记只是对相关内容的简单总结，方便自己以后查看复习，如果大家觉得还可以，请购买正版书籍研读，非常不错！