本文已参与「新人创作礼」活动，一起开启掘金创作之路。
策略模式就是用统一的方法接口分别对不同类型的数据进行访问。比如说，现在我们想用pc看一部电影，此时应该怎么做呢？看电影嘛，当然需要各种播放电影的方法。rmvb要rmvb格式的方法，avi要avi的方法,mpeg要mpeg的方法。可是事实上，我们完全可以不去管是什么文件格式。因为播放器对所有的操作进行了抽象，不同的文件会自动调用相应的访问方法。
typedef struct _MoviePlay`
{`
struct _CommMoviePlay* pCommMoviePlay;
}MoviePlay;
typedef struct _CommMoviePlay
{`
HANDLE hFile;
void (*play)(HANDLE hFile);
}CommMoviePlay;
这个时候呢，对于用户来说，统一的文件接口就是MoviePlay。接下来的一个工作，就是编写一个统一的访问接口。
void play_movie_file(struct MoviePlay* pMoviePlay)
{
CommMoviePlay* pCommMoviePlay;
assert(NULL != pMoviePlay);
pCommMoviePlay = pMoviePlay->pCommMoviePlay;
pCommMoviePlay->play(pCommMoviePlay->hFile);
}
最后的工作就是对不同的hFile进行play的实际操作，写简单一点就是，
void play_avi_file(HANDLE hFile)
{
printf("play avi file!\n");
}
void play_rmvb_file(HANDLE hFile)
{
printf("play rmvb file!\n");
}
void play_mpeg_file(HANDLE hFile)
{
printf("play mpeg file!\n");
}
完整代码，
#include <stdio.h>
#include <assert.h>
typedef unsigned int HANDLE;
typedef struct _MoviePlay
{
struct _CommMoviePlay* pCommMoviePlay;
}MoviePlay;
typedef struct _CommMoviePlay
{
HANDLE hFile;
void (*play)(HANDLE hFile);
}CommMoviePlay;
void play_movie_file(MoviePlay* pMoviePlay)
{
CommMoviePlay* pCommMoviePlay;
assert(NULL != pMoviePlay);
pCommMoviePlay = pMoviePlay->pCommMoviePlay;
pCommMoviePlay->play(pCommMoviePlay->hFile);
}
void play_avi_file(HANDLE hFile)
{
printf("play avi file: %d!\n", hFile);
}
void play_rmvb_file(HANDLE hFile)
{
printf("play rmvb file: %d!\n", hFile);
}
void play_mpeg_file(HANDLE hFile)
{
printf("play mpeg file: %d!\n", hFile);
}
int main(void)
{
MoviePlay m_MoviePlay;
CommMoviePlay m_CommMoviePlay;
m_MoviePlay.pCommMoviePlay = &m_CommMoviePlay;
m_CommMoviePlay.hFile = 1; //imagine m_CommMoviePlay could be changed by another file as plug-in
m_CommMoviePlay.play = &play_avi_file; //imagine m_CommMoviePlay could be changed by another file as plug-in
play_movie_file(&m_MoviePlay);
m_CommMoviePlay.hFile = 2; //imagine m_CommMoviePlay could be changed by another file as plug-in
m_CommMoviePlay.play = &play_rmvb_file; //imagine m_CommMoviePlay could be changed by another file as plug-in
play_movie_file(&m_MoviePlay);
m_CommMoviePlay.hFile = 3; //imagine m_CommMoviePlay could be changed by another file as plug-in
m_CommMoviePlay.play = &play_mpeg_file; //imagine m_CommMoviePlay could be changed by another file as plug-in
play_movie_file(&m_MoviePlay);
return 0;
}
输出如下，可以看到最后调用的接口都是，play_movie_file(&m_MoviePlay);，但是最终输出可以不一样，如果我们把m_CommMoviePlay中的成员开放出来给别的文件改写作为插件，那么就可以做到main文件不变，只修改插件代码来完成不同解码器的切换了。
play avi file: 1!
play rmvb file: 2!
play mpeg file: 3!
改写成如下格式，会更清楚一点，可以把avi.h、rmvb.h和mpeg.h想象成插件
strategy_pattern.h
typedef unsigned int HANDLE;
typedef struct _MoviePlay
{
struct _CommMoviePlay* pCommMoviePlay;
}MoviePlay;
typedef struct _CommMoviePlay
{
HANDLE hFile;
void (*play)(HANDLE hFile);
}CommMoviePlay;
avi.h
void play_avi_file(HANDLE hFile)
{
printf("play avi file: %d!\n", hFile);
}
void change_to_avi_codec(CommMoviePlay* cMP)
{
cMP->hFile = 1;
cMP->play = &play_avi_file;
}
rmvb.h
void play_rmvb_file(HANDLE hFile)
{
printf("play rmvb file: %d!\n", hFile);
}
void change_to_rmvb_codec(CommMoviePlay* cMP)
{
cMP->hFile = 2;
cMP->play = &play_rmvb_file;
}
mpeg.h
void play_mpeg_file(HANDLE hFile)
{
printf("play mpeg file: %d!\n", hFile);
}
void change_to_mpeg_codec(CommMoviePlay* cMP)
{
cMP->hFile = 3;
cMP->play = &play_mpeg_file;
}
strategy_pattern.c
#include <stdio.h>
#include <assert.h>
#include "strategy_pattern.h"
#include "avi.h"
#include "rmvb.h"
#include "mpeg.h"
void play_movie_file(MoviePlay* pMoviePlay)
{
CommMoviePlay* pCommMoviePlay;
assert(NULL != pMoviePlay);
pCommMoviePlay = pMoviePlay->pCommMoviePlay;
pCommMoviePlay->play(pCommMoviePlay->hFile);
}
int main(void)
{
MoviePlay m_MoviePlay;
CommMoviePlay m_CommMoviePlay;
m_MoviePlay.pCommMoviePlay = &m_CommMoviePlay;
change_to_avi_codec(&m_CommMoviePlay);
play_movie_file(&m_MoviePlay);
change_to_rmvb_codec(&m_CommMoviePlay);
play_movie_file(&m_MoviePlay);
change_to_mpeg_codec(&m_CommMoviePlay);
play_movie_file(&m_MoviePlay);
return 0;
}
