在项目中需要监听ScrollView滑动到顶端和底端的时候以实现自己的ScrollView，那么怎样去监听呢？今天查看了一下ScrollView的源码，找到了一种方法。先看一下源码中的overScrollBy()方法：
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;
}
此方法有9个参数，参数太多有点让人模糊。这里我们不管他们是什么意思，直接看36-43行代码，很显然这是一个垂直方向的边界检查，而这也正是我们想要的。因此我们可以重写此方法，在此方法中实现监听，代码如下：
@Override
protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) { int newScrollY = scrollY + deltaY;
final int bottom = maxOverScrollY + scrollRangeY;
final int top = -maxOverScrollY;
if (newScrollY > bottom) {
System.out.println("滑动到底端");
}
else if (newScrollY < top) {
System.out.println("滑动到顶端");
}
return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);
}
是不是很简单呢？