《Android 开发艺术探索》 12-Bitmap的加载和Cache

public class MyBitmapLoadUtil {

    /**
     * 对一个Resources的资源文件进行指定长宽来加载进内存, 并把这个bitmap对象返回
     *
     * @param res   资源文件对象
     * @param resId 要操作的图片id
     * @param reqWidth 最终想要得到bitmap的宽度
     * @param reqHeight 最终想要得到bitmap的高度
     * @return 返回采样之后的bitmap对象
     */
    public static Bitmap decodeFixedSizeForResource(Resources res, int resId, int reqWidth, int reqHeight){
        // 首先先指定加载的模式 为只是获取资源文件的大小
        BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);

        //Calculate Size  计算要设置的采样率 并把值设置到option上
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

        // 关闭只加载属性模式, 并重新加载的时候传入自定义的options对象
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);

    }

    /**
     *  一个计算工具类的方法, 传入图片的属性对象和 想要实现的目标大小. 通过计算得到采样值
     */
    private static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
        //Raw height and width of image
        //原始图片的宽高属性
        final int height = options.outHeight;
        final int width = options.outWidth;
        int inSampleSize = 1;

        // 如果想要实现的宽高比原始图片的宽高小那么就可以计算出采样率, 否则不需要改变采样率
        if (reqWidth < height || reqHeight < width){
            int halfWidth = width/2;
            int halfHeight = height/2;

            // 判断原始长宽的一半是否比目标大小小, 如果小那么增大采样率2倍, 直到出现修改后原始值会比目标值大的时候
            while((halfHeight/inSampleSize) >= reqHeight && (halfWidth/inSampleSize) >= reqWidth){
                inSampleSize *= 2;
            }
        }

        return inSampleSize;

    }
}

public static DiskLruCache open(File directory, int appVersion, int valueCount, long maxSize)

public class SquareImageView extends ImageView {
    public SquareImageView(Context context) {
        super(context);
    }

    public SquareImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public SquareImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

      @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        // 由于这里准备提供给GridView控件使用, 对于一个水平线有几个是未知的
        // 那么通过父控件的测量传到这里的宽度规格, 也当做高度即可
        super.onMeasure(widthMeasureSpec, widthMeasureSpec);
    }
}

 // 根据连接网络的情况判断是否加载图片
if (!NetWorkUtil.isWifi(getApplicationContext())) {
  AlertDialog.Builder builder = new AlertDialog.Builder(this);
  builder.setMessage("首次使用会从手机网络下载图片, 是否确认下载?")
          .setTitle("友情提示")
          .setPositiveButton("好的.", new DialogInterface.OnClickListener() {
              @Override
              public void onClick(DialogInterface dialog, int which) {
                  mCanLoadForPhoneNet = true;
                  imageAdapter.notifyDataSetChanged();
              }
          })
          .setNegativeButton("不行!", new DialogInterface.OnClickListener() {
              @Override
              public void onClick(DialogInterface dialog, int which) {
                  Toast.makeText(getApplicationContext(), "瞅你扣那样!!!", Toast.LENGTH_LONG).show();
              }
          }).show();
}else{
  mCanLoadForPhoneNet = true;
}

// NetWorkUtil类
public class NetWorkUtil {
    public static boolean isWifi(Context context) {
        ConnectivityManager connectivityManager = (ConnectivityManager) context
                .getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo activeNetInfo = connectivityManager.getActiveNetworkInfo();
        if (activeNetInfo != null
                && activeNetInfo.getType() == ConnectivityManager.TYPE_WIFI) {
            return true;
        }
        return false;
    }
}

GridView gv_main = (GridView) findViewById(R.id.gv_main);
// 监听GridView的滑动状态
  gv_main.setOnScrollListener(new AbsListView.OnScrollListener() {
      @Override
      public void onScrollStateChanged(AbsListView view, int scrollState) {
          if (scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE){
              mIsGridViewIdle = true;
              // 并触发更新adapter
              imageAdapter.notifyDataSetChanged();
          }else{
              mIsGridViewIdle = false;
          }
      }

      @Override
      public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {

      }
  });

@Override
public View getView(int position, View convertView, ViewGroup parent) {
  ViewHolder holder = null;
  if (convertView == null){
      convertView = View.inflate(mContext, R.layout.item_photo_wall, null);
      holder = new ViewHolder();
      holder.mImageView = (ImageView) convertView.findViewById(R.id.iv_square);
      convertView.setTag(holder);
  }else{
      holder = (ViewHolder) convertView.getTag();
  }

  // 设置默认图片
  ImageView mImageView = holder.mImageView;
  mImageView.setImageResource(android.R.drawable.screen_background_dark_transparent);


  // 检测是否wifi 和 是否是滑动状态
  // 优化重点
  if (mCanLoadForPhoneNet && mIsGridViewIdle){
      // 加载图片
      mImageLoader.setImageView(mImageView).url(mUrls.get(position));
  }


  return convertView;
}