(Objective-C) Google Sheets Refresh an Expired Access Token

Demonstrates how to refresh an expired access token. If a Google Sheets REST API call results in a 401 "unauthorized" response, then it is likely the access token expired. An application should be able to automatically refresh the access token with no user interaction and then re-try the request with the newly obtained access token.

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#import <CkoJsonObject.h>
#import <CkoOAuth2.h>
#import <CkoStringBuilder.h>

// This example requires the Chilkat API to have been previously unlocked.
// See Global Unlock Sample for sample code.

// It is assumed we previously obtained an OAuth2 access token.
// This example loads the JSON access token file 
// saved by this example: Get Google Sheets OAuth2 Access Token

CkoJsonObject *jsonToken = [[CkoJsonObject alloc] init];
BOOL success = [jsonToken LoadFile: @"qa_data/tokens/googleSheets.json"];
if (success != YES) {
    NSLog(@"%@",@"Failed to load googleSheets.json");
    return;
}

// The access token JSON looks like this:

// 	{ 
// 	 "access_token": "abac.Ci-ZabcdF37-NUrmSzxbHxTsMbtlN-uPfabcdAQN_2ekzj0s5ofCzRabcdTDMGaa2Q",
// 	 "token_type": "Bearer",
// 	 "expires_in": 3600,
// 	 "refresh_token": "1/lEe5abcdNJ77eOSDBabcdkQGVmDalkwabcdR2Hatbo"
// 	}
// 

CkoOAuth2 *oauth2 = [[CkoOAuth2 alloc] init];

oauth2.TokenEndpoint = @"https://www.googleapis.com/oauth2/v4/token";

// Replace these with actual values.
oauth2.ClientId = @"GOOGLE-CLIENT-ID";
oauth2.ClientSecret = @"GOOGLE-CLIENT-SECRET";

// Get the "refresh_token"
oauth2.RefreshToken = [jsonToken StringOf: @"refresh_token"];

// Send the HTTP POST to refresh the access token..
success = [oauth2 RefreshAccessToken];
if (success != YES) {
    NSLog(@"%@",oauth2.LastErrorText);
    return;
}

// The response contains a new access token, but we must keep
// our existing refresh token for when we need to refresh again in the future.
[jsonToken UpdateString: @"access_token" value: oauth2.AccessToken];

// Save the new JSON access token response to a file.
CkoStringBuilder *sbJson = [[CkoStringBuilder alloc] init];
jsonToken.EmitCompact = NO;
[jsonToken EmitSb: sbJson];
[sbJson WriteFile: @"qa_data/tokens/googleSheets.json" charset: @"utf-8" emitBom: NO];

NSLog(@"%@",@"OAuth2 authorization granted!");
NSLog(@"%@%@",@"New Access Token = ",oauth2.AccessToken);