Building Custom Connector for Xero API

Hi CalebGuthrie 
I did manage to get the connector working, I think the below is the updated code. I'll also update in Github shortly. 
Since then, I switched to using OData feeds for Xero data as my connector could only retrieve one payslip at a time. The provider of these feeds is ODataLink. https://odatalink.com/

// This file contains your Data Connector logic
section JaimesXeroConnector;

//JaimesXeroConnector OAuth2 values;
client_id = "3";
client_secret = "v";
redirect_uri = "https://oauth.powerbi.com/views/oauthredirect.html";
token_uri = "https://identity.xero.com/connect/token?=";
authorize_uri = "https://login.xero.com/identity/connect/authorize";
logout_uri = "https://login.microsoftonline.com/logout.srf";
connection_uri = "https://api.xero.com/connections"; //Jaime added 6.1.21
tenantid = Text.From(GetTenantId(connection_uri));

// Login modal window dimensions
windowWidth = 720;
windowHeight = 1024;

//OAuth2 Scopes
scope_prefix = "";
scopes = {
"offline_access",
"openid",
"profile",
"email",
"accounting.transactions.read",
"accounting.settings.read",
"accounting.reports.read",
"accounting.journals.read",
"accounting.contacts.read",
"assets.read",
"payroll.employees.read",
"payroll.payruns.read",
"payroll.payslip.read",
"payroll.settings.read",
"payroll.timesheets.read"
};


GetTenantId = (url as text) =>
let
Source = Web.Contents(url),
ImportedJSON = Json.Document(Source,1252),
ConvertedtoTable = Table.FromList(ImportedJSON, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
ExpandedColumn1 = Table.ExpandRecordColumn(ConvertedtoTable, "Column1", {"id", "authEventId", "tenantId", "tenantType", "tenantName", "createdDateUtc", "updatedDateUtc"}, {"id", "authEventId", "tenantId", "tenantType", "tenantName", "createdDateUtc", "updatedDateUtc"}),
ChangedType = Table.TransformColumnTypes(ExpandedColumn1,{{"id", type text}, {"authEventId", type text}, {"tenantId", type text}, {"tenantType", type text}, {"tenantName", type text}, {"createdDateUtc", type datetime}, {"updatedDateUtc", type datetime}}),
tenantId = Text.From(ChangedType{0}[tenantId])
in
tenantId;

[DataSource.Kind="JaimesXeroConnector", Publish="JaimesXeroConnector.Publish"] 
shared JaimesXeroConnector.Contents = (url as text) => 
    let 
        source = Json.Document(Web.Contents(url,[Headers = [#"xero-tenant-id"=tenantid,#"Accept" = "application/json"]])) 
    in 
        source;

// Data Source Kind description
JaimesXeroConnector= [
    TestConnection = (DataSourcePath) => 
        let
            json = Json.Document(DataSourcePath),
            server = json[server],
            database = json[database]
        in   
            try
                { "JaimesXeroConnector.Contents", server, database }
            otherwise
                let
                    message = Text.Format("Couldn't find entity.")
                in 
                    Diagnostics.Trace(TraceLevel.Error,  message, () => error message, true )
            ,

    Authentication = [
        OAuth = [
            StartLogin=StartLogin,
            FinishLogin=FinishLogin,
            Refresh=Refresh,
            Logout=Logout
        ]
    ],
    Label = Extension.LoadString("DataSourceLabel")
];

// Data Source UI publishing description
JaimesXeroConnector.Publish = [
    Beta = true,
    Category = "Other",
    ButtonText = { Extension.LoadString("ButtonTitle"), Extension.LoadString("ButtonHelp") },
    LearnMoreUrl = "https://powerbi.microsoft.com/",
    SourceImage = JaimesXeroConnector.Icons,
    SourceTypeImage = JaimesXeroConnector.Icons
];

// Helper functions for OAuth2: StartLogin, FinishLogin, Refresh, Logout
StartLogin = (resourceUrl, state, display) =>
    let
        authorizeUrl = authorize_uri & "?" & Uri.BuildQueryString([
            response_type = "code",
            client_id = client_id,  
            redirect_uri = redirect_uri,
            state = state,
            scope = GetScopeString(scopes, scope_prefix)
        ])
    in
        [
            LoginUri = authorizeUrl,
            CallbackUri = redirect_uri,
            WindowHeight = 720,
            WindowWidth = 1024,
            Context = null
        ];

FinishLogin = (context, callbackUri, state) =>
    let
        // parse the full callbackUri, and extract the Query string
        parts = Uri.Parts(callbackUri)[Query],
        // if the query string contains an "error" field, raise an error
        // otherwise call TokenMethod to exchange our code for an access_token
        result = if (Record.HasFields(parts, {"error", "error_description"})) then 
                    error Error.Record(parts[error], parts[error_description], parts)
                 else
                    TokenMethod("authorization_code", parts[code])
    in
        result;

Refresh = (resourceUrl, refresh_token) => TokenMethod("refresh_token", "refresh_token", refresh_token);

       

Logout = (token) => logout_uri;


        
// see "Exchange code for access token: POST /oauth/token" for details
TokenMethod = (grantType, code) =>
//TokenMethod = (grantType, tokenField, code) =>
    let
        query = [
//        queryString = [
            grant_type = grantType,
//            grant_type = "authorization_code",
            redirect_uri = redirect_uri,
            client_id = client_id,
            client_secret = client_secret
        ],
        queryWithCode = if(grantType = "refresh_token") then [refresh_token = code] else [code = code],
//        queryWithCode = Record.AddField(queryString, tokenField, code),

        tokenResponse = Web.Contents(token_uri, [
            Content = Text.ToBinary(Uri.BuildQueryString(query & queryWithCode)),
//            Content = Text.ToBinary(Uri.BuildQueryString(queryWithCode)),
//            Headers = [
//                #"authorization" = Text.Combine({"Basic ", "base64encode(", client_id, ":", client_secret, ")"} ), // JB
//                #"Content-type" = "application/x-www-form-urlencoded",
//                #"Accept" = "application/json"
//            ],
            Headers= [#"Content-type" = "application/x-www-form-urlencoded",#"Accept" = "application/json"],
            ManualStatusHandling = {400} 
        ]),
        body = Json.Document(tokenResponse),
                 result = if (Record.HasFields(body, {"error", "error_description"})) then 
                 error Error.Record(body[error], body[error_description], body) 
                         else
                    body
    in
        result;

Value.IfNull = (a, b) => if a <> null then a else b;

GetScopeString = (scopes as list, optional scopePrefix as text) as text =>
    let
        prefix = Value.IfNull(scopePrefix, ""),
        addPrefix = List.Transform(scopes, each prefix & _),
        asText = Text.Combine(addPrefix, " ")
    in
        asText;
        
        

JaimesXeroConnector.Icons = [
    Icon16 = { Extension.Contents("JaimesXeroConnector16.png"), Extension.Contents("JaimesXeroConnector20.png"), Extension.Contents("JaimesXeroConnector24.png"), Extension.Contents("JaimesXeroConnector32.png") },
    Icon32 = { Extension.Contents("JaimesXeroConnector32.png"), Extension.Contents("JaimesXeroConnector40.png"), Extension.Contents("JaimesXeroConnector48.png"), Extension.Contents("JaimesXeroConnector64.png") }
];