onMeasure 流程
分VERTICAL和HORIZONTAL两种情况，以VERTICAL为例说明，即measureVertical方法。【在有weight的情况也需要测量两次】
mTotalLength：记录子view的总高度
totalWeight：记录权重总和
线性布局可以设置android:divider分割线。
android:measureWithLargestChild="true"，所有带权重的子元素都会具有最大子元素的尺寸，且只有当父view布局方向上的宽度或高度为wrap_content才有效。
第一次measure：
// See how tall everyone is. Also remember max width.
// 宽度为最大子view的宽度
for (int i = 0; i < count; ++i) {
...
final boolean useExcessSpace = lp.height == 0 && lp.weight > 0;
if (heightMode == MeasureSpec.EXACTLY && useExcessSpace) {
//当前的LinearLayout是EXACTLY模式，且子view的高度为0，且权重大于0，
//这个子view只有在LinearLayout高度有剩余的时候，才会根据权重的
//占比去平 分剩余空间，这是二次测量的这部分。
skippedMeasure = true;
//Optimization: don't bother measuring children who are only
// laid out using excess space. These views will get measured
// later if we have space to distribute.
} else {
//如果当前的LinearLayout不是EXACTLY模式，且子View的weight大于0，
//优先会把当前LinearLayout的全部可用高度用于子View测量
final int usedHeight = totalWeight == 0 ? mTotalLength : 0;
measureChildBeforeLayout(child, i, widthMeasureSpec, 0,
heightMeasureSpec, usedHeight); //会调用到child.measure()
}
}
第二次measure：【只有在子view设置了layout_weight时才会进行】
int remainingExcess = heightSize - mTotalLength
+ (mAllowInconsistentMeasurement ? 0 : consumedExcessSpace);
if (skippedMeasure || remainingExcess != 0 && totalWeight > 0.0f) {
float remainingWeightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;
mTotalLength = 0;
for (int i = 0; i < count; ++i) {
final View child = getVirtualChildAt(i);
if (child == null || child.getVisibility() == View.GONE) {
continue;
}
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final float childWeight = lp.weight;
if (childWeight > 0) {
//权重的分配是一个一个分的，而不是一次分配完成的，如weight分别为：1，2，1。
//则第一个view为剩余的1/4；第二个view为剩余的1/3；第三个view为剩余的全部。
final int share = (int) (childWeight * remainingExcess / remainingWeightSum);
remainingExcess -= share;
remainingWeightSum -= childWeight;
final int childHeight;
if (mUseLargestChild && heightMode != MeasureSpec.EXACTLY) {
childHeight = largestChildHeight;
} else if (lp.height == 0 && (!mAllowInconsistentMeasurement
|| heightMode == MeasureSpec.EXACTLY)) {
// This child needs to be laid out from scratch using
// only its share of excess space.
childHeight = share;
} else {
// This child had some intrinsic height to which we
// need to add its share of excess space.
childHeight = child.getMeasuredHeight() + share;
}
final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
Math.max(0, childHeight), MeasureSpec.EXACTLY);
final int childWidthMeasureSpec = getChildMeasureSpec(widthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin +
lp.rightMargin, lp.width);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
...
}
}
}
如果是EXACTLY模式下的weight不为0，且高度设置为0的子View优先级是最低的。如果LinearLayout剩余空间不足，就会不显示 。【尽管该子view排序在前】
onLayout 流程
也分VERTICAL和HORIZONTAL两种情况，以VERTICAL为例子。
- 首先根据LinearLayout设置的android:gravity确定childTop的值。
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;
}
- 遍历子view根据android:layout_gravity的设置确定childLeft的值。
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;
child.layout(childLeft, childTop + getLocationOffset(child),
childWidth, childHeight);
//childTop累加
childTop += childHeight + lp.bottomMargin + getNextLocationOffset(child);
i += getChildrenSkipCount(child, i);
}
}
