ListView实现顶部和底部内容指示器的方法

package net.learningandroid.lib.view;
 
import net.learningandroid.lib.R;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Rect;
import android.graphics.drawable.BitmapDrawable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;
import android.widget.ListView;
 
/**
 * 支持上下箭头的ListView
 *
 * @author Mr. Lu
 */
public class ArrowListView extends ListView {
 
  private final float scale = getContext().getResources().getDisplayMetrics().density;
  private float topArrowPadding;
  private float bottomArrowPadding;
 
  private static float DEFAULT_TOP_PADDING_DP = 2.0f;
  private static float DEFAULT_BOTTOM_PADDING_DP = 2.0f;
 
  public ArrowListView(Context context, AttributeSet attrs) {
    super(context, attrs);
 
    String strTopArrowPadding = attrs.getAttributeValue(null,
        "topArrowPadding");
    String strBottomArrowPadding = attrs.getAttributeValue(null,
        "bottomArrowPadding");
 
    topArrowPadding = convertDisplayUom(strTopArrowPadding,
        DEFAULT_TOP_PADDING_DP);
    bottomArrowPadding = convertDisplayUom(strBottomArrowPadding,
        DEFAULT_BOTTOM_PADDING_DP);
 
    Log.v("ArrowListView", String.valueOf(topArrowPadding));
  }
 
  /**
   * 单位转换
   */
  private float convertDisplayUom(String sour, float defaultValue) {
    try {
      if (sour.toLowerCase().endsWith("px")) {
        return Float.parseFloat(sour.toLowerCase().replace("px", ""));
      } else if (sour.toLowerCase().endsWith("dp")) {
        return Integer.parseInt(sour.toLowerCase().replace("dp",
            ""))
            * scale + 0.5f;
      }
    } catch (Exception e) {
    }
 
    return (defaultValue * scale + 0.5f);
  }
 
  /**
   * onDraw方法，根据ListView滚动位置绘出箭头.
   */
  @Override
  protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    Paint paint = new Paint();
 
    // 取得箭头的图片，此处是固定图片，其实上可以做成配置方式
    Bitmap topPic = ((BitmapDrawable) getResources().getDrawable(
        R.drawable.arrow_up)).getBitmap();
    Bitmap bottomPic = ((BitmapDrawable) getResources().getDrawable(
        R.drawable.arrow_down)).getBitmap();
 
    // 取得ListView的绘制区域大小
    Rect r = new Rect();
    this.getDrawingRect(r);
 
    // 计算箭头的绘制位置
    float top = r.top + topArrowPadding;
    float bottom = r.bottom - bottomArrowPadding - bottomPic.getHeight();
    float left = r.left + (r.right - r.left - topPic.getWidth()) / 2;
 
    // 计算是否需要绘制
    boolean drawTop = false;
    boolean drawBottom = false;
 
    if (this.getChildCount() > 0) {
      Rect rTop = new Rect();
      this.getChildAt(0).getLocalVisibleRect(rTop);
      Rect rBottom = new Rect();
      View lastChild = this.getChildAt(this.getChildCount() - 1);
      lastChild.getLocalVisibleRect(rBottom);
 
      drawTop = (this.getFirstVisiblePosition() > 0 || this
              .getFirstVisiblePosition() == 0
              && rTop.top > 0);
      drawBottom = (this.getLastVisiblePosition() 

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