檢索Documents目錄的完整路徑的兩種方法：

NSString *documentsDirectory = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"];

NSArray *paths=NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);    
NSString *documentsDirectory = [paths objectAtIndex:0];

判斷Documents下的某個資源文件是否存在

    NSString  *filename=@"123.mp3";
    NSString *documentsDirectory = [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"];
    NSFileManager *fileManager = [NSFileManager defaultManager];
    NSString *filePath = [documentsDirectory stringByAppendingPathComponent:filename];
    if(![fileManager fileExistsAtPath:filePath])
    {
        NSLog(@"文件不存在");
    }else{
        NSLog(@"文件存在");
    }