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1，NSData 与 NSString
　　NSData --> NSString
　　NSString *aString = [[NSString alloc] initWithData:adata encoding:NSUTF8StringEncoding];
　　NSString --> NSData
　　NSString *aString = @"1234";
　　NSData *aData = [aString dataUsingEncoding: NSUTF8StringEncoding];
   iOS NSString 和NSData 转换
   NSString 转换成NSData 对象 
   NSData* xmlData = [@"testdata" dataUsingEncoding:NSUTF8StringEncoding]; 
   NSData 转换成NSString对象 
   NSData * data; 
   NSString *result = [[NSString alloc] initWithData:data  encoding:NSUTF8StringEncoding];
2，NSData 与 Byte
　　NSData --> Byte
　　NSString *testString = @"1234567890";
　　NSData *testData = [testString dataUsingEncoding: NSUTF8StringEncoding];
　　Byte *testByte = (Byte *)[testData bytes];
　　Byte --> NSData
　　Byte byte[] = {0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23};
　　NSData *adata = [[NSData alloc] initWithBytes:byte length:24];
3，NSData 与 UIImage
　　NSData --> UIImage
　　UIImage *aimage = [UIImage imageWithData: imageData];
　　//例：从本地文件沙盒中取图片并转换为NSData
　　NSString *path = [[NSBundle mainBundle] bundlePath];
　　NSString *name = [NSString stringWithFormat:@"ceshi.png"];
　　NSString *finalPath = [path stringByAppendingPathComponent:name];
　　NSData *imageData = [NSData dataWithContentsOfFile: finalPath];
　　UIImage *aimage = [UIImage imageWithData: imageData];
　　UIImage－> NSData
　　NSData *imageData = UIImagePNGRepresentation(aimae);
4，NSData 与 NSMutableData
　　NSData --> MSMutableData
　　NSData *data=[[NSData alloc]init];
　　NSMutableData *mdata=[[NSMutableData alloc]init];   
　　mdata=[NSData dataWithData:data];
5，NSData合并为一个NSMutableData
- (NSString *)filePathWithName:(NSString *)filename
 {
         NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
         NSString *documentsDirectory = [paths objectAtIndex:0];
         return [documentsDirectory stringByAppendingPathComponent:filename];
 }
 
 - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {    
     //音频文件路径
         NSString *mp3Path1 = [[NSBundle mainBundle] pathForResource:@"1" ofType:@"mp3"];
         NSString *mp3Path2 = [[NSBundle mainBundle] pathForResource:@"2" ofType:@"mp3"];
         //音频数据
         NSData *sound1Data = [[NSData alloc] initWithContentsOfFile: mp3Path1];
         NSData *sound2Data = [[NSData alloc] initWithContentsOfFile: mp3Path2];
         //合并音频
         NSMutableData *sounds = [NSMutableData alloc];
         [sounds appendData:sound1Data];
         [sounds appendData:sound2Data];
         //保存音频
 
         NSLog(@"data length:%d", [sounds length]);
 
         [sounds writeToFile:[self filePathWithName:@"tmp.mp3"] atomically:YES];
         
         [window makeKeyAndVisible];
     
     return YES;
 }