android中ListView的定位:使用setSelectionFromTop

如果一个ListView太长,有时我们希望ListView在从其他界面返回的时候能够恢复上次查看的位置,这就涉及到ListView的定位问题:

解决的办法如下:

// 保存当前第一个可见的item的索引和偏移量
int index = mList.getFirstVisiblePosition();
View v = mList.getChildAt(0);
int top = (v == null) ? 0 : v.getTop();
// ...
//根据上次保存的index和偏移量恢复上次的位置
mList.setSelectionFromTop(index, top);

这里使用了setSelectionFromTop来定位ListView。其实还可以使用setSelection也可以定位,只是setSelectionFromTop要比setSelection更精准。因为通过getFirstVisiblePosition得到的第一个item可能已经有一部分是不可见的了,如果用setSelection无法反映出这不可见的部分。

为了说明setSelectionFromTop的参数值的意义,以及与setSelection的区别,下面从源码上来分析:

看一下setSelectionFromTop()的具体实现,代码如下:

/**
  * Sets the selected item and positions the selection y pixels from the top edge
  * of the ListView. (If in touch mode, the item will not be selected but it will
  * still be positioned appropriately.)
  *
  * @param position Index (starting at 0) of the data item to be selected.
  * @param y The distance from the top edge of the ListView (plus padding) that the
  *        item will be positioned.
  */
 public void setSelectionFromTop(int position, int y) {
     if (mAdapter == null) {
         return;
     }
     if (!isInTouchMode()) {
         position = lookForSelectablePosition(position, true);
         if (position >= 0) {
             setNextSelectedPositionInt(position);
         }
     } else {
         mResurrectToPosition = position;
     }
     if (position >= 0) {
         mLayoutMode = LAYOUT_SPECIFIC;
         mSpecificTop = mListPadding.top + y;
         if (mNeedSync) {
             mSyncPosition = position;
             mSyncRowId = mAdapter.getItemId(position);
         }
         requestLayout();
     }
 }

从上面的代码可以得知,setSelectionFromTop()的作用是设置ListView选中的位置,同时在Y轴设置一个偏移量。

而setSelection()方法,传入一个index整型数值,就可以让ListView定位到指定Item的位置。

这两个方法有什么区别呢?看一下setSelection()的具体实现,代码如下:

/**
  * Sets the currently selected item. If in touch mode, the item will not be selected
  * but it will still be positioned appropriately. If the specified selection position
  * is less than 0, then the item at position 0 will be selected.
  *
  * @param position Index (starting at 0) of the data item to be selected.
  */
 @Override
 public void setSelection(int position) {
     setSelectionFromTop(position, 0);
 }

原来,setSelection()内部就是调用了setSelectionFromTop(),只不过是Y轴的偏移量是0而已。现在应该对setSelection()和setSelectionFromTop()有了更深刻的认识了。