Android 开发中Parcel存储类型和数据容器
jopen
13年前
<span style="font-family:'Times New Roman';font-size:10.5pt;">在<span style="font-family:Times New Roman;">android</span><span style="font-family:宋体;">中使用</span><span style="font-family:Times New Roman;">Parcel</span><span style="font-family:宋体;">（存储基本数据类型和引用数据类型的容器）通过</span><span style="font-family:Times New Roman;">IBinder</span><span style="font-family:宋体;">来绑定，这种方法的作用是让数据在进程间传递数据，</span><span style="font-family:Times New Roman;">Parcel</span><span style="font-family:宋体;">定位就是轻量级的高效的对象序列化和反序列化机制。</span></span> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">在<span style="font-family:Times New Roman;">Framework</span><span style="font-family:宋体;">中有</span><span style="font-family:Times New Roman;">parcel</span><span style="font-family:宋体;">类，源码路径是：</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">Frameworks/base/core/java/android/os/Parcel.java</span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">典型的源码片断如下：</span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <pre class="brush:java; toolbar: true; auto-links: false;">/ Write an integer value into the parcel at the current dataPosition(), growing dataCapacity() if needed. / public final native void writeInt(int val); / Write a long integer value into the parcel at the current dataPosition(), growing dataCapacity() if needed. / public final native void writeLong(long val); </pre> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">他的方法如下：</span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">Parcel parcel = Parcel.obtain();// </span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">获取一个<span style="font-family:Times New Roman;">Parcel </span><span style="font-family:宋体;">对象下面就可以对其进行方法进行操作了，</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">createXXX(),wirteXXX(),readXXX(),</span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">其中<span style="font-family:Times New Roman;">dataPosition(),</span><span style="font-family:宋体;">返回当前</span><span style="font-family:Times New Roman;">Parcel</span><span style="font-family:宋体;">当前对象存储数据的偏移量，而</span><span style="font-family:Times New Roman;">setDataPosition(),</span><span style="font-family:宋体;">设置当前</span><span style="font-family:Times New Roman;">Parcel</span><span style="font-family:宋体;">对象的偏移量，方便读取</span><span style="font-family:Times New Roman;">parcel</span><span style="font-family:宋体;">中的数据，可问题就出在我读取出来的数据要么是空（</span><span style="font-family:Times New Roman;">null</span><span style="font-family:宋体;">）</span><span style="font-family:Times New Roman;">,</span><span style="font-family:宋体;">要么永远是第一个偏移量处的值。</span><span style="font-family:Times New Roman;">Parcel</span><span style="font-family:宋体;">采用什么机制实现的，是以什么形式的存储的，然后我才能任意对其操作。</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">基本数据类型的取值范围，<span style="font-family:Times New Roman;">boolean 1bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">short 16bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">int 32bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">long 64bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">float 32bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">double 64bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">char 16bit</span><span style="font-family:宋体;">、</span><span style="font-family:Times New Roman;">byte 8bit</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">由此我可以猜想，<span style="font-family:Times New Roman;">Parcel 32bit </span><span style="font-family:宋体;">作为基本单位存储写入的变量</span><span style="font-family:Times New Roman;">,4byte*8=32bit,</span><span style="font-family:宋体;">在内存中的引用地址变量是采用</span><span style="font-family:Times New Roman;">16</span><span style="font-family:宋体;">进制进行编码，且作为偏移量，即偏移量是</span><span style="font-family:Times New Roman;">4</span><span style="font-family:宋体;">的倍数，</span><span style="font-family:Times New Roman;">0,4,8,12,16,20,24,28,32,36,40,44,48……4*N,f(x) = 4*y</span><span style="font-family:宋体;">｛</span><span style="font-family:Times New Roman;">y>=0&y</span><span style="font-family:宋体;">是自然数｝</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">我想绝对不会出现向偏移量是<span style="font-family:Times New Roman;">3</span><span style="font-family:宋体;">，</span><span style="font-family:Times New Roman;">6</span><span style="font-family:宋体;">，</span><span style="font-family:Times New Roman;">9</span><span style="font-family:宋体;">这样的数据。。。由此我们可以推断出，无论他存储的是基本数据类型或引用数据类型的变量，都是以</span><span style="font-family:Times New Roman;">32bit</span><span style="font-family:宋体;">基本单位作为偏移量。</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">parcel.writeInt(1);parcel.writeInt(2);parcel.writeInt(3);parcel.writeInt(4);parcel.writeInt(5);parcel.writeInt(6);parcel.writeInt(7);parcel.writeInt(81011111);parcel.writeFloat(1f);parcel.writeFloat(1000000000000000000000000000000000000f);parcel.writeXXX(),<span style="font-family:宋体;">每写一次数据，在</span><span style="font-family:Times New Roman;">32bit</span><span style="font-family:宋体;">的空间里能够存储要放入的变量，怎只占一个偏移量，也就之一动</span><span style="font-family:Times New Roman;">4</span><span style="font-family:宋体;">个位置。</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">而当存储的数据如：<span style="font-family:Times New Roman;">parcel.writeFloat(1000000000000000000000000000000000000f);</span><span style="font-family:宋体;">他就自动往后移动，</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">parcel.writeString(“a”);parcel.writeString(“b”);parcel.writeString(“d”);parcel.writeString(“c”);<span style="font-family:宋体;">和</span><span style="font-family:Times New Roman;">parcel.writeString(“abcd”); </span><span style="font-family:宋体;">的区别。</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">有此可见，他的内存的分配原来是这样的。那我怎样才能把我存进去的数据依次的去出来呢？<span style="font-family:Times New Roman;">setDataPosition(),</span><span style="font-family:宋体;">设置</span><span style="font-family:Times New Roman;">parcel</span><span style="font-family:宋体;">的偏移量，在</span><span style="font-family:Times New Roman;">readXXX(),</span><span style="font-family:宋体;">读取数据</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <pre class="brush:java; toolbar: true; auto-links: false;">int size = parcel.dataSize(); int i = 0; while (i <= size ) { parcel.setDataPosition(i); int curr_int = parcel.readInt(); i+=4; int j = 0; j++; }</pre> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"><span style="font-family:'Times New Roman';font-size:10.5pt;">由此可见<span style="font-family:Times New Roman;">parcel</span><span style="font-family:宋体;">写入数据是按照</span><span style="font-family:Times New Roman;">32bit </span><span style="font-family:宋体;">为基本的容器，依次存储写入的数据，基本和引用（其实引用的也是有多个基本数据类型组合而成</span><span style="font-family:Times New Roman;">OBJECTS</span><span style="font-family:宋体;">－属性｜方法），读取的时候我们就可以按照这种规律根据目标数据的偏移量的位置（</span><span style="font-family:Times New Roman;">curr_position</span><span style="font-family:宋体;">），以及偏移量的大小</span><span style="font-family:Times New Roman;">(size)</span><span style="font-family:宋体;">，取出已经存进去的数据了。</span></span></p> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <pre class="brush:java; toolbar: true; auto-links: false;">int i ＝ curr_position； while (i <= size ) { parcel.setDataPosition(i); int curr_int = parcel.readXXXt(); i+=4; int j = 0; j++; }</pre> <p style="margin-top:0pt;margin-bottom:0pt;" class="p0"></p> <span style="font-family:'宋体';font-size:10.5pt;">最后提醒一句</span> <span style="font-family:'Times New Roman';font-size:10.5pt;">，</span> <span style="font-family:'宋体';font-size:10.5pt;">在</span> <span style="font-family:'Times New Roman';font-size:10.5pt;">java<span style="font-family:宋体;">中基本数据类型的取值范围，引用类型的数据，相当于</span><span style="font-family:Times New Roman;">c</span><span style="font-family:宋体;">中的指针，以及各进制之间的相互转换和灵活的引用，以及定制自己想要的任意进制数据类型。</span></span> <span style="font-family:'Times New Roman';font-size:10.5pt;">译成串行化、序列化<span style="font-family:Times New Roman;">……</span><span style="font-family:宋体;">，其作用是能将数据对象存入字节流当中，在需要时重新生成对象。主要应用是利用外部存储设备保存对象状态，以及通过网络传输对象等。</span></span> <span style="font-family:'Times New Roman';font-size:10.5pt;"> </span>