(Go) Refresh GMail SMTP OAuth2 Access Token

Demonstrates how to refresh an access token for sending email via GMail (SMTP).

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

    // See Get GMail SMTP Access Token 
    // for sample code showing how to obtain a GMail SMTP access token for desktop apps.

    // It is assumed we previously obtained an OAuth2 access token for GMail (one with a scope for sending email).

    jsonToken := chilkat.NewJsonObject()
    success := jsonToken.LoadFile("qa_data/tokens/_gmailSmtp.json")
    if success != true {
        fmt.Println("Failed to load _gmailSmtp.json")
        jsonToken.DisposeJsonObject()
        return
    }

    // The access token JSON looks something like this:

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

    oauth2 := chilkat.NewOAuth2()

    oauth2.SetTokenEndpoint("https://www.googleapis.com/oauth2/v4/token")

    // Replace these with actual values.
    oauth2.SetClientId("GOOGLE-CLIENT-ID")
    oauth2.SetClientSecret("GOOGLE-CLIENT-SECRET")

    // Get the "refresh_token"
    oauth2.SetRefreshToken(jsonToken.StringOf("refresh_token"))

    // Send the HTTP POST to refresh the access token..
    success = oauth2.RefreshAccessToken()
    if success != true {
        fmt.Println(oauth2.LastErrorText())
        jsonToken.DisposeJsonObject()
        oauth2.DisposeOAuth2()
        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",oauth2.AccessToken())

    // Save the new JSON access token response to a file.
    sbJson := chilkat.NewStringBuilder()
    jsonToken.SetEmitCompact(false)
    jsonToken.EmitSb(sbJson)
    sbJson.WriteFile("qa_data/tokens/_gmailSmtp.json","utf-8",false)

    fmt.Println("OAuth2 authorization granted!")
    fmt.Println("New Access Token = ", oauth2.AccessToken())

    jsonToken.DisposeJsonObject()
    oauth2.DisposeOAuth2()
    sbJson.DisposeStringBuilder()