解决ViewPager.setCurrentItem不能实现平滑移动

系统代码的duration被设置为0，所以不能实现平滑移动

void smoothScrollTo(int x, int y, int velocity) {
        if (getChildCount() == 0) {
            // Nothing to do.
            setScrollingCacheEnabled(false);
            return;
        }
        int sx = getScrollX();
        int sy = getScrollY();
        int dx = x - sx;
        int dy = y - sy;
        if (dx == 0 && dy == 0) {
            completeScroll(false);
            populate();
            setScrollState(SCROLL_STATE_IDLE);
            return;
        }

        setScrollingCacheEnabled(true);
        setScrollState(SCROLL_STATE_SETTLING);

        final int width = getClientWidth();
        final int halfWidth = width / 2;
        final float distanceRatio = Math.min(1f, 1.0f * Math.abs(dx) / width);
        final float distance = halfWidth + halfWidth *
                distanceInfluenceForSnapDuration(distanceRatio);

        int duration = 0;
        velocity = Math.abs(velocity);
        if (velocity > 0) {
            duration = 4 * Math.round(1000 * Math.abs(distance / velocity));
        } else {
            final float pageWidth = width * mAdapter.getPageWidth(mCurItem);
            final float pageDelta = (float) Math.abs(dx) / (pageWidth + mPageMargin);
            duration = (int) ((pageDelta + 1) * 100);
        }
        duration = Math.min(duration, MAX_SETTLE_DURATION);

        mScroller.startScroll(sx, sy, dx, dy, duration);
        ViewCompat.postInvalidateOnAnimation(this);
    }

由于smoothScrollTo方法不能被重写，切方法过于复杂，只能考虑重写mScroller的startScroll方法。
ViewPager对外不提供设置Scroller的方法，只能通过反射设置改对象

public class MyViewPager extends ViewPager {
    public MyViewPager(Context context, AttributeSet attrs) {
        super(context, attrs);
        setMyScroller();
    }

    private void setMyScroller() {
        try {
            Class viewpager = ViewPager.class;
            Field scroller = viewpager.getDeclaredField("mScroller");
            scroller.setAccessible(true);
            scroller.set(this, new MyScroller(getContext()));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public class MyScroller extends Scroller {
        public MyScroller(Context context) {
            super(context, new DecelerateInterpolator());
        }

        @Override
        public void startScroll(int startX, int startY, int dx, int dy, int duration) {
            super.startScroll(startX, startY, dx, dy, 1000);
        }
    }


}