(Swift 3,4,5...) Refresh GMail OAuth2 Access Token

Demonstrates how to refresh an expired or soon-to-expire GMail OAuth2 access token.

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func chilkatTest() {
    // 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 GMail OAuth2 Access Token

    let jsonToken = CkoJsonObject()!
    var success: Bool = jsonToken.loadFile("qa_data/tokens/gmail.json")
    if success != true {
        print("Failed to load googleSheets.json")
        return
    }

    // The access token JSON looks like this:

    // 	{ 
    // 	 "access_token": "abac.Ci-ZabcdF37-VUrmSzxbHxTsMbtlN-uPfabcdAQN_2ekzj0s5osCzRabcdTDMGaa2Q",
    // 	 "token_type": "Bearer",
    // 	 "expires_in": 3600,
    // 	 "refresh_token": "1/lEe5abcdNJ77eOSDBabcdkQGVmDalkwabcdR2Hatbo"
    // 	}
    // 

    let oauth2 = CkoOAuth2()!

    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.string(of: "refresh_token")

    // Send the HTTP POST to refresh the access token..
    success = oauth2.refreshAccessToken()
    if success != true {
        print("\(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.update("access_token", value: oauth2.accessToken)

    // Save the new JSON access token response to a file.
    let sbJson = CkoStringBuilder()!
    jsonToken.emitCompact = false
    jsonToken.emitSb(sbJson)
    sbJson.writeFile("qa_data/tokens/gmail.json", charset: "utf-8", emitBom: false)

    print("OAuth2 authorization granted!")
    print("New Access Token = \(oauth2.accessToken!)")

}