Android Adapter的几个常用方法

View obtainView(int position, boolean[] isScrap) {
    isScrap[0] = false;
    View scrapView;
    //从回收器中获取view
    scrapView = mRecycler.getScrapView(position);

    View child;
    if (scrapView != null) {
      ...
      //若不为空，则传入convertView，这样的话重用了view，同时更新了数据
      child = mAdapter.getView(position, scrapView, this);
      ...
    } else {
      //若为空，则在getView中重新创建HolderView，且填入数据
      child = mAdapter.getView(position, null, this);
      ...
    }
    return child;
  }

  /**
   * Gets the data associated with the specified position in the list.
   *
   * @param position Which data to get
   * @return The data associated with the specified position in the list
   */
  public Object getItemAtPosition(int position) {
    T adapter = getAdapter();
    return (adapter == null || position < 0) ? null : adapter.getItem(position);
  }
  
  /**
   * @return The data corresponding to the currently selected item, or
   * null if there is nothing selected.
   */
  public Object getSelectedItem() {
    T adapter = getAdapter();
    int selection = getSelectedItemPosition();
    if (adapter != null && adapter.getCount() > 0 && selection >= 0) {
      return adapter.getItem(selection);
    } else {
      return null;
    }
  }

public long getItemIdAtPosition(int position) {
    T adapter = getAdapter();
    return (adapter == null || position < 0) ? INVALID_ROW_ID : adapter.getItemId(position);
  }
  
  
  private void fireOnSelected() {
    if (mOnItemSelectedListener == null)
      return;

    int selection = this.getSelectedItemPosition();
    if (selection >= 0) {
      View v = getSelectedView();
      //这里调用的getItemId得到的返回值与selection都属于同一个item的特征，其意义也就在于在选择接口的onItemSelected方法中可
      //以直接拿到该item的id，而不需要通过获取adapter来间接实现
      mOnItemSelectedListener.onItemSelected(this, v, selection,
          getAdapter().getItemId(selection));
    } else {
      mOnItemSelectedListener.onNothingSelected(this);
    }
  }
  
  int findSyncPosition() {
    ...
      rowId = adapter.getItemId(seed);
      if (rowId == idToMatch) { //从这里来看，getItemId似乎应该对于不同的item返回不同的值，保持唯一性
        // Found it!
        return seed;
      }
    ...
  }