private Bitmap getBitmapFromSharedPreferences(){
        SharedPreferences sharedPreferences=getSharedPreferences("testSP", Context.MODE_PRIVATE);
        //第一步:取出字符串形式的Bitmap
        String imageString=sharedPreferences.getString("image", "");
        //第二步:利用Base64将字符串转换为ByteArrayInputStream
        byte[] byteArray= Base64.decode(imageString, Base64.DEFAULT);
        ByteArrayInputStream byteArrayInputStream=new ByteArrayInputStream(byteArray);
        //第三步:利用ByteArrayInputStream生成Bitmap
        Bitmap bitmap=BitmapFactory.decodeStream(byteArrayInputStream);
        return bitmap;
    }

  private void saveBitmapToSharedPreferences(Bitmap bitmap){
//      Bitmap bitmap=BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
        //第一步:将Bitmap压缩至字节数组输出流ByteArrayOutputStream
        ByteArrayOutputStream byteArrayOutputStream=new ByteArrayOutputStream();
        bitmap.compress(Bitmap.CompressFormat.PNG, 80, byteArrayOutputStream);
        //第二步:利用Base64将字节数组输出流中的数据转换成字符串String
        byte[] byteArray=byteArrayOutputStream.toByteArray();
        String imageString=new String(Base64.encodeToString(byteArray, Base64.DEFAULT));
        //第三步:将String保持至SharedPreferences
        SharedPreferences sharedPreferences=getActivity().getSharedPreferences("testSP", Context.MODE_PRIVATE);
        SharedPreferences.Editor editor=sharedPreferences.edit();
        editor.putString("image", imageString);
        editor.commit();
    }