ios - How can I call a category method from my UIViewcontroller class? -

error

hi have category base64 encoding, need call thing main class method. following category

code

#import <foundation/foundation.h>  @interface nsstring (addition) - (nsstring *) base64stringfromdata:(nsdata *)data length:(int)length;  @end 

in .m file

#import "nsstring+addition.h" static char base64encodingtable[64] = {     'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',     'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'a', 'b', 'c', 'd', 'e', 'f',     'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v',     'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/' };  @implementation nsstring (addition)  - (nsstring *) base64stringfromdata: (nsdata *)data length: (int)length {     unsigned long ixtext, lentext;     long ctremaining;     unsigned char input[3], output[4];     short i, charsonline = 0, ctcopy;     const unsigned char *raw;     nsmutablestring *result;      lentext = [data length];     if (lentext < 1)         return @"";     result = [nsmutablestring stringwithcapacity: lentext];     raw = [data bytes];     ixtext = 0;      while (true) {         ctremaining = lentext - ixtext;         if (ctremaining <= 0)             break;         (i = 0; < 3; i++) {             unsigned long ix = ixtext + i;             if (ix < lentext)                 input[i] = raw[ix];             else                 input[i] = 0;         }         output[0] = (input[0] & 0xfc) >> 2;         output[1] = ((input[0] & 0x03) << 4) | ((input[1] & 0xf0) >> 4);         output[2] = ((input[1] & 0x0f) << 2) | ((input[2] & 0xc0) >> 6);         output[3] = input[2] & 0x3f;         ctcopy = 4;         switch (ctremaining) {             case 1:                 ctcopy = 2;                 break;             case 2:                 ctcopy = 3;                 break;         }          (i = 0; < ctcopy; i++)             [result appendstring: [nsstring stringwithformat: @"%c", base64encodingtable[output[i]]]];          (i = ctcopy; < 4; i++)             [result appendstring: @"="];          ixtext += 3;         charsonline += 4;          if ((length > 0) && (charsonline >= length))             charsonline = 0;     }          return result; }  @end 

my uiviewcontroller methode code

i need call ategory method method

- (void)completetransaction:(skpaymenttransaction *)transaction {     nslog(@"completetransaction...");      uialertview *alert=[[uialertview alloc]initwithtitle:@"alert" message:@"transaction completed." delegate:nil cancelbuttontitle:@"ok" otherbuttontitles:nil, nil];     [alert show];     [self providecontentforproductidentifier:transaction.payment.productidentifier];     nslog(@"naveen%@",transaction.transactionreceipt);     //call category method here      [[skpaymentqueue defaultqueue] finishtransaction:transaction]; } 

 nsstring *tsr=[nsstring base64stringfromdata:transaction.transactionreceipt length:[transaction.transactionreceipt length]];     nslog(@"%@",tsr);