（1）前言
Android的进阶之路上，总少不了使用自定义控件。自定义控件按照不同的分法，有不同的分类，这里主要分为四类并在后面跟上例子:
1 继承自view，重写 onDraw方法；比如系统的TextView，ImageView
2 继承自ViewGroup，实现自己的自定义控件；卡片布局CascadLayout
3 继承自特定的view（比如ImageView），
圆角图片CircleImageView，自带清除按钮的EditText
4 继承自特定的ViewGroup,（比如LinearLayout，ListView）自定义控件-下拉刷新和上拉加载的listView
这里按照2继承自ViewGroup，实现自己的自定义控件为切入点，从LinearLayout继承ViewGroup源码角度分析自定义控件。
（哈哈，这里偷换概念，把系统的LinearLayout当做自定义控件，主要是一个学习目的。）。自定义ViewGroup的一个简单项目,请参考我的文章,卡片布局CascadLayout
view的工作流程是measure，layout和draw三大流程，也就是测量，布局和绘制，通过这三大步骤来完成这个view的布局以及显示。下面也按照这个思路来一步步解析。
（2）源码解析之构造函数：
这也是我们自定义viewGroup的第一步，完成自定义属性，然后在自己的构造函数中获取属性并完成初始化。
注意：
1 在values目录下面创建attrs.xml,当然也可以选择attrs_custom_viewgroup.xml这种按照attr_开头的形式进行定义。
2 继承viewGroup至少需要覆写一个构造函数
3 源码解析之measure过程：
在自定义ViewGroup中我们会选择重写onMeasure来完成测量过程。来一步一步分析一下。
3.1View 类中的mesure流程
首先在View类中有measure 方法中调用了onMeasure 方法，将measureSpec传递给onMeasure 方法。并且调用了我们每次onMeasure覆写之后都必须调用的方法setMeasuredDimension。
接下来看看View中的onMeasure方法。调用了setMeasuredDimension来存储测量宽和测量高。
注意：
1 可以看到view中的onMesure在两种MeasureSpec分别为At_MOST（对应LayoutParams中的wrapcontent）和EXACTLY（对应layoutparams中的matchparent或者fillparent）的情况下是一样的在getDefaultSize 方法中返回了result=specSize，也就是说直接继承View的自定义控件无论他的子view，wrapcontent，还是matchparent返回的测量尺寸大小都是一样的。显然这是不符合逻辑的，所以子view必须要做出自己的onMeasure操作。
2 **MesureSpec用于父元素对子元素进行控制，LayoutParams用于子元素告诉父元素自己想被怎么控制。**MesureSpec包含UNSPECIFIED，AT_MOST，EXACTLY。LayoutParams请看下面有详细介绍。
来看看LinearLayout的onMeasure操作，
3.2 LinearLayout的onMeasure流程
可以看到根据LinearLayout设置的方向（orientation）不同，分为了measureVertical和measureHorizontal，原理一样，这里分析measureVertical方法。这里代码注释截取自本篇博客,感谢原作者做的精彩分析
void measureVertical(int widthMeasureSpec, int heightMeasureSpec) {
mTotalLength = 0;//子view的高度和
int maxWidth = 0;//最大宽度
int childState = 0;//child的状态
int alternativeMaxWidth = 0;//子视图的最大宽度(不包含layout_weight>0的子view)
int weightedMaxWidth = 0;//子视图的最大宽度(仅包含layout_weight>0的子view)
boolean allFillParent = true; //子视图的宽度是否全是fillParent的，用于后续判断是否需要重新计算
float totalWeight = 0; //所有子view的weight之和
final int count = getVirtualChildCount();//实际的子view个数
final int widthMode = MeasureSpec.getMode(widthMeasureSpec);//LinearLayout宽度模式
final int heightMode = MeasureSpec.getMode(heightMeasureSpec); //LinearLayout高度模式
boolean matchWidth = false;
//子view的宽度是否要由父确定。如父LinearLayout为layout_width=wrap_content， 子view为fill_parent则matchWidth =true
final int baselineChildIndex = mBaselineAlignedChildIndex; //以LinearLayout中第几个子view的baseLine作为LinearLayout的基准线
final boolean useLargestChild = mUseLargestChild;//使用高度的child
int largestChildHeight = Integer.MIN_VALUE;//设定高度最高的child的默认值
//获得子view的高度，并记下最大的高度
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);//默认返回0
continue;
}
if (child.getVisibility() == View.GONE) {
i += getChildrenSkipCount(child, i);//默认返回0
continue;
}
if (hasDividerBeforeChildAt(i)) {
mTotalLength += mDividerHeight;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
//获取LinearLayout定义的LayoutParams
totalWeight += lp.weight;//计算总共的权重weight
if (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0) {
// Optimization: don't bother measuring children who are going to use
// leftover space. These views will get measured again down below if
// there is any leftover space.
//如果LinearLayout高度是已经确定的。并且这个子view的height=0，weight>0，
//则mTotalLength只需要加上margin即可，
//由于是weight>0；该view的具体宽度等会还要计算
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + lp.topMargin + lp.bottomMargin);
} else {
int oldHeight = Integer.MIN_VALUE;
if (lp.height == 0 && lp.weight > 0) {
// heightMode is either UNSPECIFIED or AT_MOST, and this
// child wanted to stretch to fill available space.
// Translate that to WRAP_CONTENT so that it does not end up
// with a height of 0
oldHeight = 0;
lp.height = LayoutParams.WRAP_CONTENT;
}
// Determine how big this child would like to be. If this or
// previous children have given a weight, then we allow it to
// use all available space (and we will shrink things later
// if needed).
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
if (oldHeight != Integer.MIN_VALUE) {
lp.height = oldHeight;
}
final int childHeight = child.getMeasuredHeight();
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));
if (useLargestChild) {
largestChildHeight = Math.max(childHeight, largestChildHeight);
}
}
/**
* If applicable, compute the additional offset to the child's baseline
* we'll need later when asked {@link #getBaseline}.
*/
if ((baselineChildIndex >= 0) && (baselineChildIndex == i + 1)) {
mBaselineChildTop = mTotalLength;
}
// if we are trying to use a child index for our baseline, the above
// book keeping only works if there are no children above it with
// weight. fail fast to aid the developer.
if (i < baselineChildIndex && lp.weight > 0) {
//为什么i < baselinechildindex && lp.weight > 0不行。
//假如行的话，如果LinearLayout与其他view视图对其的话，
//由于weight>0的作用，会影响其他所有的view位置
//应该是由于效率的原因才不允许这样。
throw new RuntimeException("A child of LinearLayout with index "
+ "less than mBaselineAlignedChildIndex has weight > 0, which "
+ "won't work. Either remove the weight, or don't set "
+ "mBaselineAlignedChildIndex.");
}
boolean matchWidthLocally = false;
if (widthMode != MeasureSpec.EXACTLY && lp.width == LayoutParams.MATCH_PARENT) {
// The width of the linear layout will scale, and at least one
// child said it wanted to match our width. Set a flag
// indicating that we need to remeasure at least that view when
// we know our width.
//如果LinearLayout宽度不是已确定的，如是wrap_content,而子view是FILL_PARENT，
//则做标记matchWidth=true； matchWidthLocally = true;
matchWidth = true;
matchWidthLocally = true;
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);//最大子view的宽度
childState = combineMeasuredStates(childState, child.getMeasuredState());
//子view宽度是否全是FILL_PARENT
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
if (lp.weight > 0) {
/*
* Widths of weighted Views are bogus if we end up
* remeasuring, so keep them separate.
*/
//如父width是wrap_content，子是fill_parent，则子的宽度需要在父确定后才能确定。这里并不是真实的宽度
weightedMaxWidth = Math.max(weightedMaxWidth,
matchWidthLocally ? margin : measuredWidth);
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
}
i += getChildrenSkipCount(child, i);
}
if (mTotalLength > 0 && hasDividerBeforeChildAt(count)) {
mTotalLength += mDividerHeight;
}
if (useLargestChild &&
(heightMode == MeasureSpec.AT_MOST || heightMode == MeasureSpec.UNSPECIFIED)) {
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null) {
mTotalLength += measureNullChild(i);
continue;
}
if (child.getVisibility() == GONE) {
i += getChildrenSkipCount(child, i);
continue;
}
final LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams)
child.getLayoutParams();
// Account for negative margins
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + largestChildHeight +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
int heightSize = mTotalLength;
// Check against our minimum height
heightSize = Math.max(heightSize, getSuggestedMinimumHeight());
// Reconcile our calculated size with the heightMeasureSpec
int heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);
heightSize = heightSizeAndState & MEASURED_SIZE_MASK;
// Either expand children with weight to take up available space or
// shrink them if they extend beyond our current bounds
int delta = heightSize - mTotalLength;
if (delta != 0 && totalWeight > 0.0f) {
float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child.getVisibility() == View.GONE) {
continue;
}
LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
// Child said it could absorb extra space -- give him his share
int share = (int) (childExtra * delta / weightSum);
weightSum -= childExtra;
delta -= share;
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight +
lp.leftMargin + lp.rightMargin, lp.width);
// TODO: Use a field like lp.isMeasured to figure out if this
// child has been previously measured
if ((lp.height != 0) || (heightMode != MeasureSpec.EXACTLY)) {
// child was measured once already above...
// base new measurement on stored values
int childHeight = child.getMeasuredHeight() + share;
if (childHeight < 0) {
childHeight = 0;
}
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(childHeight, MeasureSpec.EXACTLY));
} else {
// child was skipped in the loop above.
// Measure for this first time here
child.measure(childWidthMeasureSpec,
MeasureSpec.makeMeasureSpec(share > 0 ? share : 0,
MeasureSpec.EXACTLY));
}
// Child may now not fit in vertical dimension.
childState = combineMeasuredStates(childState, child.getMeasuredState()
& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
}
final int margin = lp.leftMargin + lp.rightMargin;
final int measuredWidth = child.getMeasuredWidth() + margin;
maxWidth = Math.max(maxWidth, measuredWidth);
boolean matchWidthLocally = widthMode != MeasureSpec.EXACTLY &&
lp.width == LayoutParams.MATCH_PARENT;
alternativeMaxWidth = Math.max(alternativeMaxWidth,
matchWidthLocally ? margin : measuredWidth);
allFillParent = allFillParent && lp.width == LayoutParams.MATCH_PARENT;
final int totalLength = mTotalLength;
mTotalLength = Math.max(totalLength, totalLength + child.getMeasuredHeight() +
lp.topMargin + lp.bottomMargin + getNextLocationOffset(child));
}
// Add in our padding
mTotalLength += mPaddingTop + mPaddingBottom;
// TODO: Should we recompute the heightSpec based on the new total length?
} else {
alternativeMaxWidth = Math.max(alternativeMaxWidth,
weightedMaxWidth);
// We have no limit, so make all weighted views as tall as the largest child.
// Children will have already been measured once.
if (useLargestChild && heightMode != MeasureSpec.EXACTLY) {
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
float childExtra = lp.weight;
if (childExtra > 0) {
child.measure(
MeasureSpec.makeMeasureSpec(child.getMeasuredWidth(),
MeasureSpec.EXACTLY),
MeasureSpec.makeMeasureSpec(largestChildHeight,
MeasureSpec.EXACTLY));
}
}
}
}
if (!allFillParent && widthMode != MeasureSpec.EXACTLY) {
maxWidth = alternativeMaxWidth;
}
maxWidth += mPaddingLeft + mPaddingRight;
// Check against our minimum width
maxWidth = Math.max(maxWidth, getSuggestedMinimumWidth());
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
heightSizeAndState);//最后一定要调用setMeasureDimension
if (matchWidth) {
forceUniformWidth(count, heightMeasureSpec);
}
}这里对onMeasure中的一些片段进行分析：
如果LinearLayout高度是已经确定的。并且这个子view的height=0，weight>0， 则mTotalLength只需要加上margin即可，由于是weight>0；该view的具体高度等会还要计算 。
如果MesureSpec的宽度模式是UNSPECIFIED或者AT_MOST，也就是说子元素想自定义宽，但是lp .width是0，但是子元素weight不为零，所以说，这时候还不能确定它的宽度，但是不能把它宽度置为0.这也就是我们遇见的，如果利用了weight属性，会导致linearLayout测量两次。
measureChildBeforeLayout则是结合父元素传递MeasureSpec，以及padding和Margin来完成了child的初次测量。margin和padding也是我们覆写viewGroup需要注意的地方。 
这里将resolveSizeAndState传递给View，然后View会返回一个标志位boolean值。 
onMesure方法中比较重要的一个步骤，将MesureSpec传递给child，也就实现了测量向下传递。我们的高度已经通过mTotalLength确定。通过child.measure也可以测量宽度。 
最后也是最重要的setMeasuredDimension方法。
注意：
1 覆盖onMeasure方法时，必须调用 setMeasuredDimension(int, int)方法来保存评估结果的视图的宽度和高度.如果忘记将导致 measure(int, int)方法抛出IllegalStateException异常.
2 覆写onMesure需要让遍历所有子view，并让它们测量自身，才能实现整个measure测量的向下传递。
3 margin和padding也是我们覆写viewGroup需要注意的地方。
4 源代码解析之定义LayoutParams
想必你肯定在使用RelativeLayout或者LinarLayout的时候使用过各种LayoutParams，比如android:layout_height，android:layout_weight属性。LayoutParams是子元素用来用于告诉父元素它（子元素）想怎么摆放。基础的LayoutParams描述了view的宽高，对于宽和高我们可以指定为定义xml布局的时候进行的操作，wrap_content, match_parent,fill_parent 或者是一个具体的数值， 这也就是我们使用布局文件的时候进行的那些操作。
1 ViewGroup的子类都会有自己的LayoutParams来定义自己的x，y以及它所特有的属性。
2 View 的绘制流程是从ViewRoot的PerformTraversal开始依次往下层的View传递的，这也就是我们需要通过LayoutParams来告诉父元素（ViewGroup）我们子元素（ViewGroup下面的view或者是ViewGroup）想怎么摆放的原因，不然你老爸怎么知道该在东边还是西边把你生出来？就是是说覆写ViewGroup必须有自己的LayoutParams。
3 可以这样去形容LayoutParams，在象棋的棋盘上，每个棋子都占据一个位置，也就是每个棋子都有一个位置的信息，如这个棋子在4行4列，这里的“4行4列”就是棋子的LayoutParams。
5 源码解析之layout过程
建立在view的measure之上，并且利用measure测量的参数作为依据。
5.1View类的measure流程
通过下图，我们可以看到view的layout方法调用了onLayout()，onLayoutChange()。
5.2LinearLayout的onMeasure流程
依然可以看到是分为两个方向进行layout。分析layoutVertical
void layoutVertical(int left, int top, int right, int bottom) {
//layout四个参数分别为左上右下
final int paddingLeft = mPaddingLeft;
int childTop;
int childLeft;
// Where right end of child should go
final int width = right - left;
int childRight = width - mPaddingRight;
// Space available for child
int childSpace = width - paddingLeft - mPaddingRight;
final int count = getVirtualChildCount();
final int majorGravity = mGravity & Gravity.VERTICAL_GRAVITY_MASK;
final int minorGravity = mGravity & Gravity.RELATIVE_HORIZONTAL_GRAVITY_MASK;
switch (majorGravity) {
case Gravity.BOTTOM:
// mTotalLength contains the padding already
childTop = mPaddingTop + bottom - top - mTotalLength;
break;
// mTotalLength contains the padding already
case Gravity.CENTER_VERTICAL:
childTop = mPaddingTop + (bottom - top - mTotalLength) / 2;
break;
case Gravity.TOP:
default:
childTop = mPaddingTop;
break;
}
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child == null) {
childTop += measureNullChild(i);
} else if (child.getVisibility() != GONE) {
final int childWidth = child.getMeasuredWidth();
final int childHeight = child.getMeasuredHeight();
final LinearLayout.LayoutParams lp =
(LinearLayout.LayoutParams) child.getLayoutParams();
int gravity = lp.gravity;
if (gravity < 0) {
gravity = minorGravity;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = paddingLeft + ((childSpace - childWidth) / 2)
+ lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
childLeft = childRight - childWidth - lp.rightMargin;
break;
case Gravity.LEFT:
default:
childLeft = paddingLeft + lp.leftMargin;
break;
}
if (hasDividerBeforeChildAt(i)) {
childTop += mDividerHeight;
}
childTop += lp.topMargin;
setChildFrame(child, childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
}layout相比measure过程就简单很多，主要是根据不同的gravity属性来确定子元素的child的位置，所以最后通过setChildFrame也就完成了整个layout过程
1 无论是onLayout还是onMesure，都会用到上面提到的LayoutParams。
2 onLayout最后通过child.layout方法来确定子view的位置。
3 每一个父控件都调用layout方法去放置或者布局自己的子控件，子控件通过在自己的onLayout方法中回调layout方法实现了循环遍历所有子孙控件的位置摆放。
4layout过程中还是要注意的是margin值和padding值。
6 源码解析之onDraw过程
在调用View类的draw流程的时候，一定要确保layout已经完成。不过这个不用我们去关心。只用去覆写onDraw就行了。
对于继承ViewGroup，其实一般情况下是不需要重写onDraw方法的，让子元素自己去绘制自身就行。但是背景还有这里的divider就需要自行绘制了。
好了。写了整整一天的一篇文章，反复整理了几次，还是发现很多逻辑感觉不清晰，感觉源代码分析，轻重还是掌握不好。但是希望对看到的人有帮助。再接再厉，希望有问题的地方，能得到大家的指点迷津。最后，再来看看我这个简单的demo卡片布局CascadLayout
