用于处理“居中”页的PageTransformer

Question

我有一个ViewPager，它显示了三个不同的页面。我想通过使用Google示例中的ZoomOutPageTransformer来动画寻呼机：

public class ZoomOutPageTransformer implements ViewPager.PageTransformer {
    private static float MIN_SCALE = 0.85f;
    private static float MIN_ALPHA = 0.5f;

    public void transformPage(View view, float position) {
        int pageWidth = view.getWidth();
        int pageHeight = view.getHeight();
        int pageOffset = 1; //one page offset

        if (position < -1) { // [-Infinity,-2)
            // This page is way off-screen to the left.
            view.setAlpha(0);

        } else if (position <= 1) { // [-1,1]
            // Modify the default slide transition to shrink the page as well
            float scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position));
            float vertMargin = pageHeight * (1 - scaleFactor) / 2;
            float horzMargin = pageWidth * (1 - scaleFactor) / 2;
            if (position < 0) {
                view.setTranslationX(horzMargin - vertMargin / 2);
            } else {
                view.setTranslationX(-horzMargin + vertMargin / 2);
            }

            // Scale the page down (between MIN_SCALE and 1)
            view.setScaleX(scaleFactor);
            view.setScaleY(scaleFactor);

            // Fade the page relative to its size.
            view.setAlpha(MIN_ALPHA +
                    (scaleFactor - MIN_SCALE) /
                    (1 - MIN_SCALE) * (1 - MIN_ALPHA));

        } else { // (1,+Infinity]
            // This page is way off-screen to the right.
            view.setAlpha(0);
        }
    }

我添加了变量"pageOffset“。我现在想要的不是第一页，而是第二页(居中)。所以我得把整个动画移到右边一步。这样，居中的页面就会比其他页面“更大”、更可见。

但是我不能这么做。我试图将职位转换为position+offset，但没有成功。在这个片段中我应该怎么做才能得到我想要的结果？

编辑:我认为这会奏效--但是，它没有：

public void transformPage(View view, float position) {
  int pageWidth = view.getWidth();
  int pageHeight = view.getHeight();
  int pageOffset = 1; //one page offset

if (position < -1 - pageOffset) { // [-Infinity,-2)
    // This page is way off-screen to the left.
    view.setAlpha(0);

} else if (position - pageOffset <= 1) { // [-1,1]
    // Modify the default slide transition to shrink the page as well
    float scaleFactor = Math.max(MIN_SCALE, 1 - Math.abs(position - pageOffset));
    float vertMargin = pageHeight * (1 - scaleFactor) / 2;
    float horzMargin = pageWidth * (1 - scaleFactor) / 2;
    if (position - pageOffset < 0) {
        view.setTranslationX(horzMargin - vertMargin / 2);
    } else {
        view.setTranslationX(-horzMargin + vertMargin / 2);
    }

    // Scale the page down (between MIN_SCALE and 1)
    view.setScaleX(scaleFactor);
    view.setScaleY(scaleFactor);

    // Fade the page relative to its size.
    view.setAlpha(MIN_ALPHA +
            (scaleFactor - MIN_SCALE) /
            (1 - MIN_SCALE) * (1 - MIN_ALPHA));

} else { // (1,+Infinity]
    // This page is way off-screen to the right.
    view.setAlpha(0);
}

}

Answer 1

我不完全确定您的意图，因为pageOffset设置为1，从未更新过。尽管如此，也许对我对PageTransformer的理解的一个解释将提供一些启示：

if (mPageTransformer != null) { final int scrollX = getScrollX(); final int childCount = getChildCount(); for (int i = 0; i < childCount; i++) { final View child = getChildAt(i); final LayoutParams lp = (LayoutParams) child.getLayoutParams(); if (lp.isDecor) continue; final float transformPos = (float) (child.getLeft() - scrollX) / getClientWidth(); mPageTransformer.transformPage(child, transformPos); } }

这是取自onPageScrolled方法ViewPager的片段。在transformPage方法中，您应该使用position参数来确定要应用什么动画，如所提供的ZoomOutPageTransformer中所示：

if (position < -1) // This view is leftward beyond visibility.
else if (position <= 1) // This is our center view being scrolled into visibility.
else // This view is rightward beyond visibility.

另一个例子是：

    @Override
    public void transformPage(View page, float position) {
        if (position >= 0) {
            page.setAlpha(Math.abs(Math.abs(position) - 1));
        }
    }

根据具体情况，该方法可能会被调用三次。让我们看看有三个视图并从最左边(0)滚动到中间(1)的情况。如果我们处于过渡过程的一半，那么对于两个视图，Math.abs(Math.abs(position) - 1)都将是0.5。这样，我们对两种观点都会采用50%的不透明度。