how to get a token in oauth 2, with swift 3 ios, this code is with the twitter api

We go in parts: Read well the official documentation of any API, library, there you will find everything you need.

  

To generate the token you need to create the string in Base 64 of   consumerKey + consumerSecret, formatted as follows consumerKey: consumerSecret.

So, we have:

consumerKey:    "VTkUsZX4Yqzlzlu6MBLdcIJoP",
consumerSecret: "1TdkkuboBIJjEOZsOk7NIiFlQbLtfAf1Lbvgqp72qLfGLXDACf",

This is transformed under the format of consumerKey:consumerSecret

var preparandoCadena = "VTkUsZX4Yqzlzlu6MBLdcIJoP:1TdkkuboBIJjEOZsOk7NIiFlQbLtfAf1Lbvgqp72qLfGLXDACf";

You apply some swift method to generate Base 64 of the previous variable.

And as a result you have:

VlRrVXpTWDRZcXpsemx1Nk1CTGRjSUpvUDoxVGRra3Vib0JJSmpFT1pzT2s3TklpRmxRYkx0ZkFmMUxidmdcxDI3cUxmR0xYREFDZg==

Now, in the headers you need to pass:

key: Authorization
value: Basic VlRrVXpTWDRZcXpsemx1Nk1CTGRjSUpvUDoxVGRra3Vib0JJSmpFT1pzT2s3TklpRmxRYkx0ZkFmMUxidmdcxDI3cUxmR0xYREFDZg==

In the body of the request:

key: grant_type
value: client_credentials

Using Postman I make the request and here I introduce you to your token:

{
  "token_type": "bearer",
  "access_token": "AAAAAAAAAAAAAAAAAAAAAA160QAAAAAAuxypuPwkbAfAqtdE2Px7ps%2By3%2FM%3DPKX1Cs6lhrm8Pk7HZH7YkeEH0V6vEPeHWrnraIuaDBziwIIWqR"
}

Reset a new key for security reasons. If this example does not work for you, it's because I changed the content of the keys.

Finally to use some function of the twitter API, in the documentation it says that we must add the Bearer token.

key: Authorization
value: Bearer AAAAAAAAAAAAAAAAAAAAAA160QAAAAAAuxypuPwkbAfAqtdE2Px7ps%2By3%2FM%3DPKX1Cs6lhrm8Pk7HZH7YkeEH0V6vEPeHWrnraIuaDBziwIIWqR

Note that there is a space between Bearer and {token}

PS: A while ago I also played with the Twittera API resulting in an engine that feeds my library of bookmarks that I get from twitter: link

Through OAUTH2 on SWIFT:

It turns out that you only had to modify the class OAuth2CodeGrant to add in the client_secret that was not happening during the code exchange for the token:

open func accessTokenRequest(with code: String, params: OAuth2StringDict? = nil) throws -> OAuth2AuthRequest {
    guard let clientId = clientConfig.clientId, !clientId.isEmpty else {
        throw OAuth2Error.noClientId
    }
    guard let redirect = context.redirectURL else {
        throw OAuth2Error.noRedirectURL
    }

    guard let clientSecret = clientConfig.clientSecret else {
        throw OAuth2Error.noClientSecret
    }

    let req = OAuth2AuthRequest(url: (clientConfig.tokenURL ?? clientConfig.authorizeURL), method: .GET)

    req.params["code"] = code
    req.params["grant_type"] = type(of: self).grantType
    req.params["redirect_uri"] = redirect
    req.params["client_id"] = clientId
    req.params["client_secret"] = clientSecret

    return req
}

Reference of the latter: link