Android: ScrollView監聽滑動到頂端和底端

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在項目中需要監聽ScrollView滑動到頂端和底端的時候以實現自己的ScrollView，那麼怎樣去監聽呢？今天查看了一下ScrollView的源碼，找到了一種方法。先看一下源碼中的overScrollBy()方法：

 1 protected boolean overScrollBy(int deltaX, int deltaY, 2             int scrollX, int scrollY, 3             int scrollRangeX, int scrollRangeY, 4             int maxOverScrollX, int maxOverScrollY, 5             boolean isTouchEvent) { 6         final int overScrollMode = mOverScrollMode; 7         final boolean canScrollHorizontal = 8                 computeHorizontalScrollRange() > computeHorizontalScrollExtent(); 9         final boolean canScrollVertical =10                 computeVerticalScrollRange() > computeVerticalScrollExtent();11         final boolean overScrollHorizontal = overScrollMode == OVER_SCROLL_ALWAYS ||12                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollHorizontal);13         final boolean overScrollVertical = overScrollMode == OVER_SCROLL_ALWAYS ||14                 (overScrollMode == OVER_SCROLL_IF_CONTENT_SCROLLS && canScrollVertical);15         int newScrollX = scrollX + deltaX;16         if (!overScrollHorizontal) {17             maxOverScrollX = 0;18         }19         int newScrollY = scrollY + deltaY;20         if (!overScrollVertical) {21             maxOverScrollY = 0;22         }23         // Clamp values if at the limits and record24         final int left = -maxOverScrollX;25         final int right = maxOverScrollX + scrollRangeX;26         final int top = -maxOverScrollY;27         final int bottom = maxOverScrollY + scrollRangeY;28         boolean clampedX = false;29         if (newScrollX > right) {30             newScrollX = right;31             clampedX = true;32         } else if (newScrollX < left) {33             newScrollX = left;34             clampedX = true;35         }36         boolean clampedY = false;37         if (newScrollY > bottom) {38             newScrollY = bottom;39             clampedY = true;40         } else if (newScrollY < top) {41             newScrollY = top;42             clampedY = true;43         }44         onOverScrolled(newScrollX, newScrollY, clampedX, clampedY);45         return clampedX || clampedY;46     }

此方法有9個參數，參數太多有點讓人模糊。這裡我們不管他們是什麼意思，直接看36-43行代碼，很顯然這是一個垂直方向的邊界檢查，而這也正是我們想要的。因此我們可以重寫此方法，在此方法中實現監聽，代碼如下：

 1  @Override 2     protected boolean overScrollBy(int deltaX, int deltaY, int scrollX, int scrollY, int scrollRangeX, int scrollRangeY, int maxOverScrollX, int maxOverScrollY, boolean isTouchEvent) { 3  4         int newScrollY = scrollY + deltaY; 5         final int bottom = maxOverScrollY + scrollRangeY; 6         final int top = -maxOverScrollY; 7         if (newScrollY > bottom) { 8             System.out.println("滑動到底端"); 9         }10         else if (newScrollY < top) {11             System.out.println("滑動到頂端");12         }13         return super.overScrollBy(deltaX, deltaY, scrollX, scrollY, scrollRangeX, scrollRangeY, maxOverScrollX, maxOverScrollY, isTouchEvent);14     }

是不是很簡單呢？