首先给大家介绍一个源码查看网址：http://www.grepcode.com/，不需要就可以查看源码。
首先View的工作过程包括：measure（测量），layout（布局），draw（绘制）三大流程。ViewGroup则增加了对子View的处理。可以通过查看LinearLayout的实现源码更直观了解到View的工作原理。
主要内容：
1. LinearLayut初始化
2. LinearLayout measure
3. LinearLayout layout
4. LinearLayout draw一，LinearLayout初始化
LinearLayout的类结构：
LinearLayout的第三个构造方法指通过xml定义的LinearLayout初始化各种参数。包括orientation,gravity,baselineAligned,weightsume等。没有什么逻辑处理部分。
二，LinearLayout measure
xml中定义的宽，高是layout_weight,layout_height,意思是指父视图给定 的宽高，而不是自己定义的视图宽高。这就是为什么前面会有layout_前缀的原因。
measure本质的过程就是讲wrap_content,match_parent的相对值转化为xxdp绝对值的过程。如果只是定义了具体的dp，measure也就没有什么意义了。
以LinearLayout的竖直布局为例：
View的measure会回调onMeasure方法，LinearLayout重载onMeasure方法。
这个是measure的主要流程：
1. 子View的第一测量：
f (heightMode == MeasureSpec.EXACTLY && lp.height == 0 && lp.weight > 0)
也就是说子view是 不是wrap_content，并且height=0，设置了weight，则第一次不测量。子View的onMeasure不执行。
条件不成立的话：
measureChildBeforeLayout(
child, i, widthMeasureSpec, 0, heightMeasureSpec,
totalWeight == 0 ? mTotalLength : 0);
mtotalLength 是指 被View占了的空间。如下，每次循环结束，
都会执行
mTotalLength = Math.max(totalLength, totalLength + childHeight + lp.topMargin +
lp.bottomMargin + getNextLocationOffset(child));2.第二次测量：
int delta = heightSize - mTotalLength;
delta是指剩余的大小，有可能小于0，也有可能大于0。
第二次测量只要就是讲剩余空间均分为weight的View。
当然，如果share <0，wrap_content的子View的高度就会是0， 200dp或者match_parent的View就会被设置成int childHeight = child.getMeasuredHeight() + share; 的高度。
总之，linearlayout是Android空间里最简单的一个，大家一定要去看看源码。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;三，LinearLayout layout过程
layout目的就是父视图按照子视图大小和布局参数，将子视图放到适合的位置上。主要是gravity参数。
剔除Gone的子视图。
主要代码：
layout过程比较简单。见代码
void layoutVertical(int left, int top, int right, int bottom) {
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);
}
}
}
private void setChildFrame(View child, int left, int top, int width, int height) {
child.layout(left, top, left + width, top + height);
}根据gravity的不同，分为左中右位置，分别layout到不同的位置，调用child.layout方法。
四，LinearLayout draw
draw过程就是将视图绘制到canvas上，如果是viewgroup，则递归调用子view的draw。
void drawDividersVertical(Canvas canvas) {
final int count = getVirtualChildCount();
for (int i = 0; i < count; i++) {
final View child = getVirtualChildAt(i);
if (child != null && child.getVisibility() != GONE) {
if (hasDividerBeforeChildAt(i)) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int top = child.getTop() - lp.topMargin - mDividerHeight;
drawHorizontalDivider(canvas, top);
}
}
}
if (hasDividerBeforeChildAt(count)) {
final View child = getVirtualChildAt(count - 1);
int bottom = 0;
if (child == null) {
bottom = getHeight() - getPaddingBottom() - mDividerHeight;
} else {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
bottom = child.getBottom() + lp.bottomMargin;
}
drawHorizontalDivider(canvas, bottom);
}
}可以看到LinearLayout的ondraw没有做过多处理。具体可以查看View的draw方法。因为draw方法没有需要特殊处理的地方。
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