x-surfer

 一种方式是把Bitmap 缓存到内存，但是我们需要使用弱引用来缓存这个Bitmap： Memory Cache

另一种方式是我们的内存即使弱引用了这个还是不够用，或者频繁访问网络造成流量消耗太多我们可以把这个bitmap先保存在本地sd卡中，避免过多的流量消耗。File Cache

1.   Memory Cache

1. publicclassMemoryCache{

2. privatestaticfinalString TAG ="MemoryCache";

3. //WeakReference Map: key=string, value=Bitmap

4. privateWeakHashMap<String,Bitmap> cache =newWeakHashMap<String,Bitmap>();

5. /**

6.      * Search the memory cache by a unique key. 

7.      * @param key Should be unique. 

8.      * @return The Bitmap object in memory cache corresponding to specific key.

9.      * */

10. publicBitmap get(String key){

11. if(key !=null)

12. return cache.get(key);

13. returnnull;

14. }

15. /**

16.      * Put a bitmap into cache with a unique key.

17.      * @param key Should be unique.

18.      * @param value A bitmap.

19.      * */

20. publicvoid put(String key,Bitmap value){

21. if(key !=null&&!"".equals(key)&& value !=null){

22.             cache.put(key, value);

23. //Log.i(TAG, "cache bitmap: " + key);

24. Log.d(TAG,"size of memory cache: "+ cache.size());

25. }

26. }

27. /**

28.      * clear the memory cache.

29.      * */

30. publicvoid clear(){

31.         cache.clear();

32. }

33. }

2.    File Cache

1. publicclassFileCache{

2. privatestaticfinalString TAG ="MemoryCache";

3. privateFile cacheDir;//the directory to save images

4. /**

5.      * Constructor

6.      * @param context The context related to this cache.

7.      * */

8. publicFileCache(Context context){

9. // Find the directory to save cached images

10. if(android.os.Environment.getExternalStorageState()

11. .equals(android.os.Environment.MEDIA_MOUNTED))

12.             cacheDir =newFile(

13.                     android.os.Environment.getExternalStorageDirectory(),

14. Config.CACHE_DIR);

15. else

16.             cacheDir = context.getCacheDir();

17. if(!cacheDir.exists())

18.             cacheDir.mkdirs();

19. Log.d(TAG,"cache dir: "+ cacheDir.getAbsolutePath());

20. }

21. /**

22.      * Search the specific image file with a unique key.

23.      * @param key Should be unique.

24.      * @return Returns the image file corresponding to the key.

25.      * */

26. publicFile getFile(String key){

27. File f =newFile(cacheDir, key);

28. if(f.exists()){

29. Log.i(TAG,"the file you wanted exists "+ f.getAbsolutePath());

30. return f;

31. }else{

32. Log.w(TAG,"the file you wanted does not exists: "+ f.getAbsolutePath());

33. }

34. returnnull;

35. }

36. /**

37.      * Put a bitmap into cache with a unique key.

38.      * @param key Should be unique.

39.      * @param value A bitmap.

40.      * */

41. publicvoid put(String key,Bitmap value){

42. File f =newFile(cacheDir, key);

43. if(!f.exists())

44. try{

45.                 f.createNewFile();

46. }catch(IOException e){

47.                 e.printStackTrace();

48. }

49. //Use the util's function to save the bitmap.

50. if(BitmapHelper.saveBitmap(f, value))

51. Log.d(TAG,"Save file to sdcard successfully!");

52. else

53. Log.w(TAG,"Save file to sdcard failed!");

54. }

55. /**

56.      * Clear the cache directory on sdcard.

57.      * */

58. publicvoid clear(){

59. File[] files = cacheDir.listFiles();

60. for(File f : files)

61.             f.delete();

62. }

63. }

3.    关于上面的如何把BitMap 保存到文件中，这里有个工具方法：BitmapHelper 这个类中我们写保存bitmap的代码

1. publicstaticboolean saveBitmap(File file,Bitmap bitmap){

2. if(file ==null|| bitmap ==null)

3. returnfalse;

4. try{

5. BufferedOutputStream out =newBufferedOutputStream(newFileOutputStream(file));

6. return bitmap.compress(CompressFormat.JPEG,100, out);

7. }catch(FileNotFoundException e){

8.         e.printStackTrace();

9. returnfalse;

10. }

11. }

12.