Example steps to obtain a reusable PKCE authentication

[SirVer]

In #63 @wfraser added support for the newer Dropbox OAUTH2 Apis. I am using them in a Android/iOS Toy app that has become very important to me, but is not shipped on the App store. It took me some time to figure out how to use PKCE tokens and serialize them for reuse, so I thought I share the code I came up with for future reference.

I am using this get_key app to receive a serialized Auth object:

// This application showcases how you can acquire a PKCE key that can be reused by apps that
// require background syncing and access.

use dropbox_sdk::default_client::{NoauthDefaultClient};
use dropbox_sdk::oauth2;
use std::io::{self, BufRead};
use structopt::StructOpt;

#[derive(StructOpt, Debug)]
#[structopt(name = "get_key")]
struct Opt {
    #[structopt(long)]
    app_key: String,
}

pub fn main() {
    let opt = Opt::from_args();

    let flow_type = oauth2::Oauth2Type::PKCE(oauth2::PkceCode::new());
    let auth_url = oauth2::AuthorizeUrlBuilder::new(&opt.app_key, &flow_type).build();
    println!(
        "Please authenticate on this URL and paste the resulting token as stdin.\n{}",
        auth_url,
    );

    let token = {
        let stdin = io::stdin();
        let mut iterator = stdin.lock().lines();
        let line1 = iterator.next().unwrap().unwrap();
        line1.trim().to_string()
    };

    // This next step must be done, otherwise `auth.save()` returns None. We receive an access
    // token, though we do not do anything with it here.
    println!("Swapping auth token for access token");
    let mut auth = oauth2::Authorization::from_auth_code(opt.app_key, flow_type, token, None);
    auth.obtain_access_token(NoauthDefaultClient::default()).expect("Could not get access token.");

    println!("Use this serialized token for access to Dropbox. Store it securely.");
    println!("{}", auth.save().unwrap());
}

The string printed as last line by this code can be turned into a valid auth object and hence a client again using this code:

let auth = oauth2::Authorization::load(app_key, serialized_auth_string).unwrap();
let client = UserAuthDefaultClient::new(auth);

I am not sure if I am holding this correctly, but this seems to work for me.