引言
工厂设计模式在Android系统源码中也经常能见到，如BitmapFactory,传入不同的参数类型，都是得到一个Bitmap对象。还有MediaPlayerFactory，在MediaPlayerFactory的体现更加直观。
class StagefrightPlayerFactory :
public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp& client,
int fd,
int64_t offset,
int64_t length,
float curScore) {
char buf[20];
lseek(fd, offset, SEEK_SET);
read(fd, buf, sizeof(buf));
lseek(fd, offset, SEEK_SET);
long ident = *((long*)buf);
// Ogg vorbis?
if (ident == 0x5367674f) // 'OggS'
return 1.0;
return 0.0;
}
virtual sp createPlayer() {
ALOGV(" create StagefrightPlayer");
return new StagefrightPlayer();
}
};
class NuPlayerFactory : public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp& client,
const char* url,
float curScore) {
static const float kOurScore = 0.8;
if (kOurScore <= curScore)
return 0.0;
if (!strncasecmp("http://", url, 7)
|| !strncasecmp("https://", url, 8)) {
size_t len = strlen(url);
if (len >= 5 && !strcasecmp(".m3u8", &url[len - 5])) {
return kOurScore;
}
if (strstr(url,"m3u8")) {
return kOurScore;
}
}
if (!strncasecmp("rtsp://", url, 7)) {
return kOurScore;
}
return 0.0;
}
virtual float scoreFactory(const sp& client,
const sp &source,
float curScore) {
return 1.0;
}
virtual sp createPlayer() {
ALOGV(" create NuPlayer");
return new NuPlayerDriver;
}
};
class SonivoxPlayerFactory : public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp& client,
const char* url,
float curScore) {
static const float kOurScore = 0.4;
static const char* const FILE_EXTS[] = { ".mid",
".midi",
".smf",
".xmf",
".mxmf",
".imy",
".rtttl",
".rtx",
".ota" };
if (kOurScore <= curScore)
return 0.0;
// use MidiFile for MIDI extensions
int lenURL = strlen(url);
for (int i = 0; i < NELEM(FILE_EXTS); ++i) {
int len = strlen(FILE_EXTS[i]);
int start = lenURL - len;
if (start > 0) {
if (!strncasecmp(url + start, FILE_EXTS[i], len)) {
return kOurScore;
}
}
}
return 0.0;
}
virtual float scoreFactory(const sp& client,
int fd,
int64_t offset,
int64_t length,
float curScore) {
static const float kOurScore = 0.8;
if (kOurScore <= curScore)
return 0.0;
// Some kind of MIDI?
EAS_DATA_HANDLE easdata;
if (EAS_Init(&easdata) == EAS_SUCCESS) {
EAS_FILE locator;
locator.path = NULL;
locator.fd = fd;
locator.offset = offset;
locator.length = length;
EAS_HANDLE eashandle;
if (EAS_OpenFile(easdata, &locator, &eashandle) == EAS_SUCCESS) {
EAS_CloseFile(easdata, eashandle);
EAS_Shutdown(easdata);
return kOurScore;
}
EAS_Shutdown(easdata);
}
return 0.0;
}
virtual sp createPlayer() {
ALOGV(" create MidiFile");
return new MidiFile();
}
};
MediaPlayerFactory有一个抽象方法createPlayer()；
有四个具体实现此方法的子类继承他：
1.StagefrightPlayerFactory
2.MuPlayerFactory
3.SonlvoxPlayerFactory
4.TestPlayerFactory
这四种MediaPlayerFactory分别会生成不同的MediaPlayer基类：StagefrightPlayer、NuPlayerDriver、MidiFile、TestPlayerStub。
可见，工厂设计模式离我们并不遥远。设计模式是一种代码思想，而并非是一种具体的代码。它的代码是可以根据需要去变动的，只有将设计思想融入到实际的开发中，它才是有用的。
工厂设计模式的分类
目前工厂模式的常见的分类主要有3种，简单工厂模式、工厂方法模式，抽象工厂模式。
简单工厂模式
简单工厂模式
public class StorageFactory {
private static volatile StorageFactory mInstance;
private StorageFactory() {
}
public static StorageFactory getInstance() {
if (mInstance == null) {
synchronized (StorageFactory.class) {
if (mInstance == null) {
mInstance = new StorageFactory();
}
}
}
return mInstance;
}
public enum StorageType {
Memory, Disk
}
public IStorageHolder createStorageHolder(StorageType storageType) {
switch (storageType) {
case Memory:
return MemoryStorageHolder.getInstance();
case Disk:
return new DiskStorageHolder();
default:
return null;
}
}
}
在上方代码上可以看到，StorageFactory掌握着IStorageHolder实现类的初始化。如果不是经常去增加IStorageHolder实现类的类型，应该简单工厂模式就可以满足解耦的需要。但是如果去新增IStorageHolder实现类的类型，要对原代码改动较大，要去新增type类型，多增加一个case判断，在使用的位置也需要去改动。不符合开闭原则，于是就衍生了工厂方法模式。
工厂方法模式
工厂方法模式
public class StorageFactory {
private static volatile StorageFactory mInstance;
private StorageFactory() {
}
public static StorageFactory getInstance() {
if (mInstance == null) {
synchronized (StorageFactory.class) {
if (mInstance == null) {
mInstance = new StorageFactory();
}
}
}
return mInstance;
}
public IStorageHolder createStorageHolder(IStorageFactory iStorageFactory) {
return iStorageFactory.createStorageFactory();
}
}
public class DiskStorageFactory implements IStorageFactory{
@Override
public IStorageHolder createStorageFactory() {
return new DiskStorageHolder();
}
}
public class DiskStorageHolder implements IStorageHolder {
private DiskLruCacheHelper mDiskLruCacheHelper;
public DiskStorageHolder(Context context, String uniqueName) {
if (mDiskLruCacheHelper == null){
try {
mDiskLruCacheHelper = new DiskLruCacheHelper(context,uniqueName);
} catch (IOException e) {
e.printStackTrace();
}
}
}
@Override
public IStorageHolder save(String key, String value) {
mDiskLruCacheHelper.put(key,value);
return this;
}
@Override
public IStorageHolder save(String key, int value) {
save(key,String.valueOf(value));
return this;
}
@Override
public IStorageHolder save(String key, Serializable value) {
mDiskLruCacheHelper.put(key,value);
return this;
}
@Override
public String getString(String key) {
return mDiskLruCacheHelper.getAsString(key);
}
@Override
public int getInt(String key) {
return Integer.parseInt(mDiskLruCacheHelper.getAsString(key));
}
@Override
public Object getSerializable(String key) {
return mDiskLruCacheHelper.getAsSerializable(key);
}
}
从上方代码可以看到，StorageFactory只有一个创建IStorageHolder的方法createStorageHolder()，这个方法传入的是IStorageFactory，也就是说传入的是一个对象的工厂，再通过这个特定类的对象工厂去创建IStorageHolder。这种模式很好的解决了新增类型需要改动原代码较多的问题，新增类型需要去新增工厂和IStorageHolder的实现类就行。解决了不符合开闭原则的问题。但是目前还有一个问题，如果新增的类型很多，那么要增加的类的数量就会暴涨，且每个类的逻辑都差不多，显然目前的这种方式还是存在问题的。于是就衍生了抽象工厂模式。
抽象工厂模式
抽象工厂模式
public class StorageFactory implements IStorageFactory {
private static volatile StorageFactory mInstance;
private IStorageHolder mMemoryHolder, mDiskStorageHolder, mPreferenceStorageHolder;
private StorageFactory() {
}
public static StorageFactory getInstance() {
if (mInstance == null) {
synchronized (StorageFactory.class) {
if (mInstance == null) {
mInstance = new StorageFactory();
}
}
}
return mInstance;
}
@Override
public IStorageHolder createStorageHolder(Class cla) {
try {
return cla.newInstance();
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
public IStorageHolder createMemoryHolder() {
if (mMemoryHolder == null) {
mMemoryHolder = createStorageHolder(MemoryStorageHolder.class);
}
return mMemoryHolder;
}
public IStorageHolder createDiskStorageHolder(Context context, String uniqueName) {
if (mDiskStorageHolder == null) {
mDiskStorageHolder = new DiskStorageHolder(context.getApplicationContext(), uniqueName);
}
return mDiskStorageHolder;
}
public IStorageHolder createPreferenceStorageHolder(Context context) {
if (mPreferenceStorageHolder == null) {
mPreferenceStorageHolder = new PreferenceStorageHolder(context);
}
return mPreferenceStorageHolder;
}
}
在上方的代码中可以看到，如果新增类型，需要在StorageFactory中去新增一个获取IStorageHolder的方法，新增类型对原代码只是新增，改动量小。能很好的解决前两种工厂模式中出现的问题，也符合了开闭原则。
总结
任何一种设计模式，他的代码都可以是多变的，只有把设计模式用到对的地方，那么它带来的可扩展性，和代码的可维护性，会是一种比较大的飞跃。