18.设计模式-桥接模式

桥接模式（Bridge）:将抽象部分与它的实现部分分离，使它们都可以独立地变化

合成／聚合复用原则（CARP），尽量使用合成／聚合，尽量不要使用类继承。

由于实现的方式有多种，桥接模式的核心意图就是把这些实现独立出来，让它们各自地变化。这就使得每种实现的变化不会影响其他实现，从而达到应对变化的目的。

代码

本例中就是让'手机'既可以按照品牌来分类，也可以按照功能来分类。当品牌需要实现功能时，通过调用功能类实现。

业务类

c 复制代码

#include <stdio.h>
#include <stdlib.h>

typedef struct HandsetSoft {
    void (*run)();
} HandsetSoft;
typedef struct HandsetGame {
    HandsetSoft base;
} HandsetGame;

void HandsetGameRun() {
    printf("运行手机游戏\n");
}

HandsetSoft *InitHandsetGame() {
    HandsetGame *obj = (HandsetGame *)malloc(sizeof(HandsetGame));
    obj->base.run = HandsetGameRun;
    return (HandsetSoft *)obj;
}

typedef struct HandsetAddressList {
    HandsetSoft base;
} HandsetAddressList;
void HandsetAddressListRun() {
    printf("运行手机通讯录\n");
}

HandsetSoft *InitHandsetAddressList() {
    HandsetAddressList *obj = (HandsetAddressList *)malloc(sizeof(HandsetAddressList));
    obj->base.run = HandsetAddressListRun;
    return (HandsetSoft *)obj;
}
typedef struct HandsetBrand {
    HandsetSoft *soft;
    void (*run)(struct HandsetBrand *);
    void (*setSoft)(struct HandsetBrand *, struct HandsetSoft *);
} HandsetBrand;

typedef struct HandsetBrandN {
    HandsetBrand base;
} HandsetBrandN;

void HandsetBrandNSetSoft(HandsetBrand *obj, HandsetSoft *soft) {
    obj->soft = soft;
}

void HandsetBrandNRun(HandsetBrand *obj) {
    printf("手机品牌N ");
    obj->soft->run();
}

HandsetBrand *InitHandsetBrandN() {
    HandsetBrandN *obj = (HandsetBrandN *)malloc(sizeof(HandsetBrandN));
    obj->base.setSoft = HandsetBrandNSetSoft;
    obj->base.run = HandsetBrandNRun;
    return (HandsetBrand *)obj;
}

typedef struct HandsetBrandM {
    HandsetBrand base;
} HandsetBrandM;

void HandsetBrandMSetSoft(HandsetBrand *obj, HandsetSoft *soft) {
    if(obj->soft != NULL) {
        free(obj->soft);
    }
    obj->soft = soft;
}

void HandsetBrandMRun(HandsetBrand *obj) {
    printf("手机品牌M ");
    obj->soft->run();
}

HandsetBrand *InitHandsetBrandM() {
    HandsetBrandM *obj = (HandsetBrandM *)malloc(sizeof(HandsetBrandM));
    obj->base.setSoft = HandsetBrandMSetSoft;
    obj->base.run = HandsetBrandMRun;
    obj->base.soft = NULL;
    return (HandsetBrand *)obj;
}

客户端

c 复制代码

int main() {
    HandsetBrand *ab = InitHandsetBrandN();
    ab->setSoft(ab, InitHandsetGame());
    ab->run(ab);
    ab->setSoft(ab,InitHandsetAddressList());
    ab->run(ab);
    free(ab->soft);
    free(ab);
    ab = InitHandsetBrandM();
    ab->setSoft(ab, InitHandsetGame());
    ab->run(ab);
    ab->setSoft(ab, InitHandsetAddressList());
    ab->run(ab);
    free(ab->soft);
    free(ab);
    return 0;
}

UML图

总结

合成／聚合复用原则的好处：优先使用对象的合成/聚合将有助于你保持每个类被封装，并被集中在单个任务上。这样类和类继承层次会保持较小规模，并且不太可能增长为不可控制的庞然大物。