widthMeasureSpec is 0 when in HorizontalScrollView

问题

I'm writing a custom view in Android. Since I need precise size management, I've overridden onMeasure. Its documentation says, that View.MeasureSpec.getMode returns one of three values, defined as constants. But when my view was placed inside HorizontalScrollView, widthMeasureSpec was 0 - I was unable to get both mode and size from it.

What is even weirder is that this parameter was 0 even if I explicitly defined width for my view.

Why does it happen, how should I interpret the 0 value and what should I do in this particular case?

回答1:

0 means the mode is UNSPECIFIED. This means you can be as big as you want, which makes sense since it is a ScrollView...intended for a View bigger than your actual screen.

Being unspecified, you don't have to care about the size, this is why it is 0.

If you look at the source of HorizontalScrollView you can also see that it just passes width: 0, UNSPECIFIED to the child:

@Override
protected void measureChild(View child, int parentWidthMeasureSpec, int parentHeightMeasureSpec) {
    ViewGroup.LayoutParams lp = child.getLayoutParams();

    int childWidthMeasureSpec;
    int childHeightMeasureSpec;

    childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec, mPaddingTop
            + mPaddingBottom, lp.height);

    childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

---

To get the child view as big as the parent you can use fillViewPort from xml or java which will lead to a call with mode EXACTLY:

if (!mFillViewport) {
    return;
}
// ...
int childWidthMeasureSpec = MeasureSpec.makeMeasureSpec(width, MeasureSpec.EXACTLY);

This is one way to handle your child view being as big as the ScrollView.

来源：https://stackoverflow.com/questions/35589320/widthmeasurespec-is-0-when-in-horizontalscrollview