引言
工廠設(shè)計(jì)模式在Android系統(tǒng)源碼中也經(jīng)常能見到，如BitmapFactory,傳入不同的參數(shù)類型，都是得到一個(gè)Bitmap對(duì)象。還有MediaPlayerFactory，在MediaPlayerFactory的體現(xiàn)更加直觀。
class StagefrightPlayerFactory :
public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp<IMediaPlayer>& 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<MediaPlayerBase> createPlayer() {
ALOGV(" create StagefrightPlayer");
return new StagefrightPlayer();
}
};
class NuPlayerFactory : public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp<IMediaPlayer>& 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<IMediaPlayer>& client,
const sp<IStreamSource> &source,
float curScore) {
return 1.0;
}
virtual sp<MediaPlayerBase> createPlayer() {
ALOGV(" create NuPlayer");
return new NuPlayerDriver;
}
};
class SonivoxPlayerFactory : public MediaPlayerFactory::IFactory {
public:
virtual float scoreFactory(const sp<IMediaPlayer>& 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<IMediaPlayer>& 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<MediaPlayerBase> createPlayer() {
ALOGV(" create MidiFile");
return new MidiFile();
}
};
MediaPlayerFactory有一個(gè)抽象方法createPlayer()；
有四個(gè)具體實(shí)現(xiàn)此方法的子類繼承他：
1.StagefrightPlayerFactory
2.MuPlayerFactory
3.SonlvoxPlayerFactory
4.TestPlayerFactory
這四種MediaPlayerFactory分別會(huì)生成不同的MediaPlayer基類：StagefrightPlayer、NuPlayerDriver、MidiFile、TestPlayerStub。
可見，工廠設(shè)計(jì)模式離我們并不遙遠(yuǎn)。設(shè)計(jì)模式是一種代碼思想，而并非是一種具體的代碼。它的代碼是可以根據(jù)需要去變動(dòng)的，只有將設(shè)計(jì)思想融入到實(shí)際的開發(fā)中，它才是有用的。
工廠設(shè)計(jì)模式的分類
目前工廠模式的常見的分類主要有3種，簡(jiǎn)單工廠模式、工廠方法模式，抽象工廠模式。
簡(jiǎn)單工廠模式
簡(jiǎn)單工廠模式
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;
}
}
}
在上方代碼上可以看到，StorageFactory掌握著IStorageHolder實(shí)現(xiàn)類的初始化。如果不是經(jīng)常去增加IStorageHolder實(shí)現(xiàn)類的類型，應(yīng)該簡(jiǎn)單工廠模式就可以滿足解耦的需要。但是如果去新增IStorageHolder實(shí)現(xiàn)類的類型，要對(duì)原代碼改動(dòng)較大，要去新增type類型，多增加一個(gè)case判斷，在使用的位置也需要去改動(dòng)。不符合開閉原則，于是就衍生了工廠方法模式。
工廠方法模式
工廠方法模式
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);
}
}
從上方代碼可以看到，StorageFactory只有一個(gè)創(chuàng)建IStorageHolder的方法createStorageHolder()，這個(gè)方法傳入的是IStorageFactory，也就是說傳入的是一個(gè)對(duì)象的工廠，再通過這個(gè)特定類的對(duì)象工廠去創(chuàng)建IStorageHolder。這種模式很好的解決了新增類型需要改動(dòng)原代碼較多的問題，新增類型需要去新增工廠和IStorageHolder的實(shí)現(xiàn)類就行。解決了不符合開閉原則的問題。但是目前還有一個(gè)問題，如果新增的類型很多，那么要增加的類的數(shù)量就會(huì)暴漲，且每個(gè)類的邏輯都差不多，顯然目前的這種方式還是存在問題的。于是就衍生了抽象工廠模式。
抽象工廠模式
抽象工廠模式
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<? extends IStorageHolder> 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;
}
}
在上方的代碼中可以看到，如果新增類型，需要在StorageFactory中去新增一個(gè)獲取IStorageHolder的方法，新增類型對(duì)原代碼只是新增，改動(dòng)量小。能很好的解決前兩種工廠模式中出現(xiàn)的問題，也符合了開閉原則。
總結(jié)
任何一種設(shè)計(jì)模式，他的代碼都可以是多變的，只有把設(shè)計(jì)模式用到對(duì)的地方，那么它帶來的可擴(kuò)展性，和代碼的可維護(hù)性，會(huì)是一種比較大的飛躍。
