Change margins inside a custom view

Question

In my open source library FloatingActioButton I need to programmatically change view margins specified by a user. Margins must be reduced by a shadow size (so that margin is set to the button itself and not to the shadow).

I override the onLayout of the custom view class and use the following code:

@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
    super.onLayout(changed, left, top, right, bottom);
    if (!hasLollipopApi() && !mMarginsSet) {
        if (getLayoutParams() instanceof ViewGroup.MarginLayoutParams) {
            ViewGroup.MarginLayoutParams lp = (ViewGroup.MarginLayoutParams) getLayoutParams();
            int leftMargin = lp.leftMargin - mShadowSize;
            int topMargin = lp.topMargin - mShadowSize;
            int rightMargin = lp.rightMargin - mShadowSize;
            int bottomMargin = lp.bottomMargin - mShadowSize;
            lp.setMargins(leftMargin, topMargin, rightMargin, bottomMargin);

            setLayoutParams(lp);
            mMarginsSet = true;
        }
    }
}

But when I test it on the sample project I see that correct margins are applied only the first time on the first fragment (ListView). When I click on another tab, a new fragment is created (so complete layout is created) and the FAB is shifted (margins are not reduced by the shadow size):

Correct margins:

Wrong margins:

Even when I return to the first tab, the FAB stays with wrong margins. I tried to debug and saw that values calculated in onLayout are the same in all cases. Margins calculated correctly.

Maybe onLayout is not correct place to change margins?

Answer 1

I fixed it by moving a call to set margins to the onMeasure method:

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    int size = getDimension(
        mType == TYPE_NORMAL ? R.dimen.fab_size_normal : R.dimen.fab_size_mini);
    if (mShadow && !hasLollipopApi()) {
        size += mShadowSize * 2;
        setMarginsWithoutShadow();
    }
    setMeasuredDimension(size, size);
}

private void setMarginsWithoutShadow() {
    if (!mMarginsSet) {
        if (getLayoutParams() instanceof ViewGroup.MarginLayoutParams) {
            ViewGroup.MarginLayoutParams layoutParams = (ViewGroup.MarginLayoutParams) getLayoutParams();
            int leftMargin = layoutParams.leftMargin - mShadowSize;
            int topMargin = layoutParams.topMargin - mShadowSize;
            int rightMargin = layoutParams.rightMargin - mShadowSize;
            int bottomMargin = layoutParams.bottomMargin - mShadowSize;
            layoutParams.setMargins(leftMargin, topMargin, rightMargin, bottomMargin);

            requestLayout();
            mMarginsSet = true;
        }
    }
}