新浪乐居有道面试题"如果父视图的高设置为wrap_content，它的子View高也是wrap_content，那么子View的高度如何显示？"这种问题就是考察你对源码的熟悉度。
我们直接查看View工作原理源码，梳理出三个方法如下，都是View类中的方法。
onMeasure()方法
getSuggestMinimumWidth()方法
getDefaultSize()方法
onMeasure()方法
测量方法onMeasure()接受父视图施加的宽高"测量规格"。
//widthMeasureSpec父类的宽度测量准则
//heightMeasureSpec父类的高度测量准则
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int widthSize = getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec);
int heightSize = getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec);
setMeasureDimension(widthSize, heightSize);
}
getSuggestedMinimumWidth()方法
该方法在父容器对子View宽度没有任何限制的情况下才会有作用，一般用于系统的内部测量，在此我们不做研究。
getDefaultSize()方法
这里就是考察的重点，一般情况下子view的高度由父视图的"测量规格来决定"。
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.exactly:
case MeasureSpec.at_most: {
result = specSize;
}
break;
}
return result;
}
得出结论：无论子View高度为match_parent、wrap_content或者具体的高度值(如100px)测量的高度都是父容器的高度。
直接继承View的自定义控件需要重写onMeasure()方法并设置wrap_content时的自身大小，否则在布局中使用wrap_content就相当于使用match_parent。指定的默认的内部宽/高（mWidth和mHeight），这个没有固定的依据，根据需要灵活指定即可。如果查看TextView、ImageView等的源码就可以知道，针对wrap_content情形，它们的onMeasure()方法均做了特殊处理。
以上就是本篇文章所要讲述的问题了，以下的源码仅供喜欢钻研的读者阅读。
以下为TextView的onMeasure()方法源码
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int widthMode = MeasureSpec.getMode(widthMeasureSpec);
int heightMode = MeasureSpec.getMode(heightMeasureSpec);
int widthSize = MeasureSpec.getSize(widthMeasureSpec);
int heightSize = MeasureSpec.getSize(heightMeasureSpec);
int width;
int height;
BoringLayout.Metrics boring = UNKNOWN_BORING;
BoringLayout.Metrics hintBoring = UNKNOWN_BORING;
if (mTextDir == null) {
mTextDir = getTextDirectionHeuristic();
}
int des = -1;
boolean fromexisting = false;
final float widthLimit = (widthMode == MeasureSpec.AT_MOST)
? (float) widthSize : Float.MAX_VALUE;
if (widthMode == MeasureSpec.EXACTLY) {
// Parent has told us how big to be. So be it.
width = widthSize;
} else {
if (mLayout != null && mEllipsize == null) {
des = desired(mLayout);
}
if (des < 0) {
boring = BoringLayout.isBoring(mTransformed, mTextPaint, mTextDir, mBoring);
if (boring != null) {
mBoring = boring;
}
} else {
fromexisting = true;
}
if (boring == null || boring == UNKNOWN_BORING) {
if (des < 0) {
des = (int) Math.ceil(Layout.getDesiredWidthWithLimit(mTransformed, 0,
mTransformed.length(), mTextPaint, mTextDir, widthLimit));
}
width = des;
} else {
width = boring.width;
}
final Drawables dr = mDrawables;
if (dr != null) {
width = Math.max(width, dr.mDrawableWidthTop);
width = Math.max(width, dr.mDrawableWidthBottom);
}
if (mHint != null) {
int hintDes = -1;
int hintWidth;
if (mHintLayout != null && mEllipsize == null) {
hintDes = desired(mHintLayout);
}
if (hintDes < 0) {
hintBoring = BoringLayout.isBoring(mHint, mTextPaint, mTextDir, mHintBoring);
if (hintBoring != null) {
mHintBoring = hintBoring;
}
}
if (hintBoring == null || hintBoring == UNKNOWN_BORING) {
if (hintDes < 0) {
hintDes = (int) Math.ceil(Layout.getDesiredWidthWithLimit(mHint, 0,
mHint.length(), mTextPaint, mTextDir, widthLimit));
}
hintWidth = hintDes;
} else {
hintWidth = hintBoring.width;
}
if (hintWidth > width) {
width = hintWidth;
}
}
width += getCompoundPaddingLeft() + getCompoundPaddingRight();
if (mMaxWidthMode == EMS) {
width = Math.min(width, mMaxWidth * getLineHeight());
} else {
width = Math.min(width, mMaxWidth);
}
if (mMinWidthMode == EMS) {
width = Math.max(width, mMinWidth * getLineHeight());
} else {
width = Math.max(width, mMinWidth);
}
// Check against our minimum width
width = Math.max(width, getSuggestedMinimumWidth());
if (widthMode == MeasureSpec.AT_MOST) {
width = Math.min(widthSize, width);
}
}
int want = width - getCompoundPaddingLeft() - getCompoundPaddingRight();
int unpaddedWidth = want;
if (mHorizontallyScrolling) want = VERY_WIDE;
int hintWant = want;
int hintWidth = (mHintLayout == null) ? hintWant : mHintLayout.getWidth();
if (mLayout == null) {
makeNewLayout(want, hintWant, boring, hintBoring,
width - getCompoundPaddingLeft() - getCompoundPaddingRight(), false);
} else {
final boolean layoutChanged = (mLayout.getWidth() != want) || (hintWidth != hintWant)
|| (mLayout.getEllipsizedWidth()
!= width - getCompoundPaddingLeft() - getCompoundPaddingRight());
final boolean widthChanged = (mHint == null) && (mEllipsize == null)
&& (want > mLayout.getWidth())
&& (mLayout instanceof BoringLayout
|| (fromexisting && des >= 0 && des <= want));
final boolean maximumChanged = (mMaxMode != mOldMaxMode) || (mMaximum != mOldMaximum);
if (layoutChanged || maximumChanged) {
if (!maximumChanged && widthChanged) {
mLayout.increaseWidthTo(want);
} else {
makeNewLayout(want, hintWant, boring, hintBoring,
width - getCompoundPaddingLeft() - getCompoundPaddingRight(), false);
}
} else {
// Nothing has changed
}
}
if (heightMode == MeasureSpec.EXACTLY) {
// Parent has told us how big to be. So be it.
height = heightSize;
mDesiredHeightAtMeasure = -1;
} else {
int desired = getDesiredHeight();
height = desired;
mDesiredHeightAtMeasure = desired;
if (heightMode == MeasureSpec.AT_MOST) {
height = Math.min(desired, heightSize);
}
}
int unpaddedHeight = height - getCompoundPaddingTop() - getCompoundPaddingBottom();
if (mMaxMode == LINES && mLayout.getLineCount() > mMaximum) {
unpaddedHeight = Math.min(unpaddedHeight, mLayout.getLineTop(mMaximum));
}
/*
* We didn't let makeNewLayout() register to bring the cursor into view,
* so do it here if there is any possibility that it is needed.
*/
if (mMovement != null
|| mLayout.getWidth() > unpaddedWidth
|| mLayout.getHeight() > unpaddedHeight) {
registerForPreDraw();
} else {
scrollTo(0, 0);
}
setMeasuredDimension(width, height);
}
