View绘制的流程
View的绘制是从上往下一层层迭代下来的。
DecorView -> ViewGroup (-->ViewGroup) ->View。还记得我是事件分发的总结中说过，布局更像一个盒子结构设计，所以外层盒子都没绘制，怎么放里面的东西呢
Measure流程
如果有了widthMeasureSpec, heightMeasureSpec，通过一定的处理(可以重写，自定义处理步骤)，从中获取View的宽/高，调用setMeasuredDimension()方法，指定View的宽高，完成测量工作
1、日常使用代码示例：
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
// 获取宽-测量规则的模式和大小
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
// 获取高-测量规则的模式和大小
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
// 设置wrap_content的默认宽 / 高值
// 默认宽/高的设定并无固定依据,根据需要灵活设置
// 类似TextView,ImageView等针对wrap_content均在onMeasure()对设置默认宽 / 高值有特殊处理,具体读者可以自行查看
int mWidth = 400;
int mHeight = 400;
// 当布局参数设置为wrap_content时，设置默认值
if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT && getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
setMeasuredDimension(mWidth, mHeight);
// 宽 / 高任意一个布局参数为= wrap_content时，都设置默认值
} else if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT) {
setMeasuredDimension(mWidth, heightSize);
} else if (getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
setMeasuredDimension(widthSize, mHeight);
}
2、MeasureSpec是如何确定的
- 对于DecorView,其确定是通过屏幕的大小，和自身的布局参数LayoutParams
- 对于其他View(包括ViewGroup),其确定是通过父布局的MeasureSpec和自身的布局参数LayoutParams（在这你就可以想想布局中的match
_parent和wrap_content设置了，是不是你想要的效果）
Layout流程
测量完View大小后，就需要将View布局在Window中，View的布局主要通过确定上下左右四个点来确定的。
其中布局也是自上而下，不同的是ViewGroup先在layout()中确定自己的布局，然后在onLayout()方法中再调用子View的layout()方法，让子View布局。在Measure过程中，ViewGroup一般是先测量子View的大小，然后再确定自身的大小。
ViewGroup中有用，View中不用复写
日常示例使用代码：
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
int mPaddingLeft = getPaddingLeft();
int mPaddingRight = getPaddingRight();
int mPaddingTop = getPaddingTop();
int lineX = mPaddingLeft;
int lineY = mPaddingTop;
int lineWidth = r - l;
usefulWidth = lineWidth - mPaddingLeft - mPaddingRight;
int lineUsed = mPaddingLeft + mPaddingRight;
int lineHeight = 0;
int lineNum = 0;
lineNumList.clear();
for (int i = 0; i < this.getChildCount(); i++) {
View child = this.getChildAt(i);
if (child.getVisibility() == GONE) {
continue;
}
int spaceWidth = 0;
int spaceHeight = 0;
int left = 0;
int top = 0;
int right = 0;
int bottom = 0;
int childWidth = child.getMeasuredWidth();
int childHeight = child.getMeasuredHeight();
LayoutParams childLp = child.getLayoutParams();
if (childLp instanceof MarginLayoutParams) {
MarginLayoutParams mlp = (MarginLayoutParams) childLp;
spaceWidth = mlp.leftMargin + mlp.rightMargin;
spaceHeight = mlp.topMargin + mlp.bottomMargin;
left = lineX + mlp.leftMargin;
top = lineY + mlp.topMargin;
right = lineX + mlp.leftMargin + childWidth;
bottom = lineY + mlp.topMargin + childHeight;
} else {
left = lineX;
top = lineY;
right = lineX + childWidth;
bottom = lineY + childHeight;
}
spaceWidth += childWidth;
spaceHeight += childHeight;
if (lineUsed + spaceWidth > lineWidth) {
//approach the limit of width and move to next line
lineNumList.add(lineNum);
lineY += lineHeight + lineSpacing;
lineUsed = mPaddingLeft + mPaddingRight;
lineX = mPaddingLeft;
lineHeight = 0;
lineNum = 0;
if (childLp instanceof MarginLayoutParams) {
MarginLayoutParams mlp = (MarginLayoutParams) childLp;
left = lineX + mlp.leftMargin;
top = lineY + mlp.topMargin;
right = lineX + mlp.leftMargin + childWidth;
bottom = lineY + mlp.topMargin + childHeight;
} else {
left = lineX;
top = lineY;
right = lineX + childWidth;
bottom = lineY + childHeight;
}
}
child.layout(left, top, right, bottom);
lineNum ++;
if (spaceHeight > lineHeight) {
lineHeight = spaceHeight;
}
lineUsed += spaceWidth;
lineX += spaceWidth;
}
// add the num of last line
lineNumList.add(lineNum);
}
可能这个示例代码不是很具有代表性，毕竟需要重写onLayout的操作还是少，关键是遍历View child = this.getChildAt(i) ，child.layout(left, top, right, bottom);
Draw过程
无论单一View,或者ViewGroup都需要实现该方法，各凭本事了
