Android ListView 弹性滚动简单实现

通常情况下，Android默认的ListView 在滚动到顶端或底端的时候，并没有很好的提示。在IOS系统的交互中，列表是有弹性的，当列表滑动到顶部或者底部时会继续往上或者往下滑动一段距离。但是在Android系统中，Google并没有赋予listview这种良好的交互效果，仅在Android 5.X版本的系统中，给这种行为添加了一个半月型的波纹阴影。

搜了一下网上的解决方案，基本上都是通过重写ListView来实现弹性效果的。比如增加HeaderView 或者嵌套scrollView,方法很多，这里不再赘述。不过这篇博文主要给大家介绍一个非常简单的方法来实现这种效果。

查看ListView的源代码时候可以发现，其中一个方法，可以控制ListView滑动到边缘的处理方法，源码如下：

/**
 * Scroll the view with standard behavior for scrolling beyond the normal
 * content boundaries. Views that call this method should override
 * {@link #onOverScrolled(int, int, boolean, boolean)} to respond to the
 * results of an over-scroll operation.
 *
 * Views can use this method to handle any touch or fling-based scrolling.
 *
 * @param deltaX Change in X in pixels
 * @param deltaY Change in Y in pixels
 * @param scrollX Current X scroll value in pixels before applying deltaX
 * @param scrollY Current Y scroll value in pixels before applying deltaY
 * @param scrollRangeX Maximum content scroll range along the X axis
 * @param scrollRangeY Maximum content scroll range along the Y axis
 * @param maxOverScrollX Number of pixels to overscroll by in either direction
 * along the X axis.
 * @param maxOverScrollY Number of pixels to overscroll by in either direction
 * along the Y axis.
 * @param isTouchEvent true if this scroll operation is the result of a touch event.
 * @return true if scrolling was clamped to an over-scroll boundary along either
 * axis, false otherwise.
 */
@SuppressWarnings({"UnusedParameters"})
protected boolean overScrollBy(int deltaX, int deltaY,
int scrollX, int scrollY,
int scrollRangeX, int scrollRangeY,
int maxOverScrollX, int maxOverScrollY,
boolean isTouchEvent) {
final int overScrollMode = mOverScrollMode;
final boolean canScrollHorizontal =
                computeHorizontalScrollRange() > computeHorizontalScrollExtent();
final boolean canScrollVertical =
                computeVerticalScrollRange() > computeVerticalScrollExtent();
final boolean overScrollHorizontal = overScrollMode == OVER_SCROLL_ALWAYS ||
                (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollHorizontal);
final boolean overScrollVertical = overScrollMode == OVER_SCROLL_ALWAYS ||
                (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollVertical);

int newScrollX = scrollX + deltaX;
if (!overScrollHorizontal) {
            maxOverScrollX = 0;
        }

int newScrollY = scrollY + deltaY;
if (!overScrollVertical) {
            maxOverScrollY = 0;
        }

// Clamp values if at the limits and record
final int left = -maxOverScrollX;
final int right = maxOverScrollX + scrollRangeX;
final int top = -maxOverScrollY;
final int bottom = maxOverScrollY + scrollRangeY;

boolean clampedX = false;
if (newScrollX > right) {
            newScrollX = right;
            clampedX = true;
        } else if (newScrollX < left) {
            newScrollX = left;
            clampedX = true;
        }

boolean clampedY = false;
if (newScrollY > bottom) {
            newScrollY = bottom;
            clampedY = true;
        } else if (newScrollY < top) {
            newScrollY = top;
            clampedY = true;
        }

        onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);

return clampedX || clampedY;
    }

从注释当中可以看到这样一个参数

maxOverScrollY
Number of pixels to overscroll by in either >direction along the Y axis.

这个参数标识了ListView 沿Y轴方向超出范围滚动的像素数，我们可以修改这个值，就可以让ListView具有弹性。只要重写ListView中的这个方法，设置maxOverScrollY 为我们自己定义的偏移值maxOverOffset.代码如下：

public class FlexibleListView extends ListView {

private static int maxOverOffset = 50;
private Context mContext;

public FlexibleListView(Context context, AttributeSet attrs,
int defStyleAttr) {
super(context, attrs, defStyleAttr);
this.mContext = context;
        initView();
    }

public FlexibleListView(Context context, AttributeSet attrs) {
super(context, attrs);
this.mContext = context;
        initView();
    }

public FlexibleListView(Context context) {
super(context);
this.mContext = context;
        initView();
    }

private void initView() {
        DisplayMetrics metrics = mContext.getResources().getDisplayMetrics();
float density = metrics.density;
//Log.d("TAG", "屏幕密度比例因子为" + density);
        maxOverOffset = (int) (density * maxOverOffset);
    }

@Override
protected boolean overScrollBy(int deltaX, int deltaY,
int scrollX, int scrollY,
int scrollRangeX, int scrollRangeY,
int maxOverScrollX, int maxOverScrollY,
boolean isTouchEvent) {
return super.overScrollBy(deltaX, deltaY,
                scrollX, scrollY,
                scrollRangeX, scrollRangeY,
                maxOverScrollX, maxOverOffset,
                isTouchEvent);

    }
}

注意，为了保持不同分辨率屏幕下弹性距离保持基本保持一致，偏移值 maxOverOffset 根据屏幕密度比例进行了重新计算。

效果如下图所示：

完整demo读者自己写下，比较简单，这里不再给出，这个例子提醒我们，平常在开发过程中注意阅读源码，往往能得到意想不到的结果。