FHIR平台應用程式開發
====

本文假設前提是除Bundle外，相關資料皆已匯入FHIR Server。以癌要事前審查為例，Bundle必須包含Claim及相關Profile。主要開發工具是[firely .NET SDK](https://docs.fire.ly/projects/Firely-NET-SDK/en/latest/)相關技術包含：
* FHIR Client
* FHIR Search / Bundle parsing
* FHIRPath
* FHIR Read
* FHIR Model
* FHIR Serialization

我們將工作分成三個步驟：

步驟一：取得FHIR Server Claim清單
---
這個步驟的重點在透過FHIR Client連接FHIR Server，設定搜尋條件後，使用FHIR Search取得Claim的清單。還傳值以Bundle封裝，因此需解析Bundle的結構才能列相關資料。

FhirClient client = new FhirClient("http://localhost:8080/fhir");

// Read all Claim resource from the server with a search query
var searchParams = new SearchParams();
searchParams.Add("status", "active");

// read limit to 50 results
searchParams.LimitTo(50);

var bundle = await client.SearchAsync<Claim>(searchParams);

foreach (var entry in bundle.Entry)
{
    var claim = (Claim)entry.Resource;
    Console.WriteLine($"Claim ID: {claim.Id}, Status: {claim.Status}, Type: {claim.Type.Coding[0].Code}");
}

步驟二：取得Claim所對應之Profile(Reference)清單
---
使用FHIR Modle根據Resource id取得特定Claim Resource，同時使用FHIRPath語法，取的Claim相關Profile的Reference清單。這一段FHIRPath`descendants().where($this is Reference).reference`就是取得該Resource物件下，資料格式為Resource的Elemet，並回傳reference。
Claim所包含的Profile也有第二階的Reference，此時可應用相同技術，取的所有Bundle所需要之Reference清單。

// Read a specific Claim by ID
var claimId = "9f76d5a5-30c0-419a-9e32-9605a095de49"; // replace with actual Claim ID

var specificClaim = await client.ReadAsync<Claim>($"Claim/{claimId}");
Console.WriteLine($"Specific Claim ID: {specificClaim.Id}, Status: {specificClaim.Status}, Type: {specificClaim.Type.Coding[0].Code}");

// using FHIR path to get all reference to Claim
var directReferenceValues = specificClaim.Select("descendants().where($this is Reference).reference");

// Print the number of references found
Console.WriteLine($"Number of direct references in Claim {specificClaim.Id}: {directReferenceValues.Count()}");
foreach (var refValue in directReferenceValues)
{
    Console.WriteLine($"Direct Reference: {refValue}");
}

步驟三：產生Bundle
---
首先，新增一個Bundle物件，並根據前一步驟所得的Reference清單，將Claim以及相關Profile，從FHIR Server讀取後加入Bundle.Entry。

產生Bundle後，可使用FHIR Serialization技術，將FHIR Bundle物件轉換為Json格式。

// Create a Bundle to hold the Claim resources
var claimBundle = new Bundle
{
    Type = Bundle.BundleType.Collection,
    Timestamp = DateTimeOffset.Now,
    Id = Guid.NewGuid().ToString(), // Generate a new unique ID for the bundle
    Meta = new Meta
    {
        Profile = new List<string> { "https://nhicore.nhi.gov.tw/pas/StructureDefinition/Bundle-twpas" }
    },
    Entry = new List<Bundle.EntryComponent>()
};

// add specificClaim to the Bundle
claimBundle.Entry.Add(new Bundle.EntryComponent
{
    Resource = specificClaim,
    FullUrl = $"urn:uuid:{specificClaim.Id}"
});

// add directReferenceValues to the Bundle as Reference resources
foreach (var reference in directReferenceValues)
{
    if (reference is Hl7.Fhir.Model.FhirString refString)
    {
        Bundle.EntryComponent entry = new Bundle.EntryComponent
        {
            Resource = await client.ReadAsync<Resource>(refString.Value), // Read the resource from the FHIR server
            FullUrl = $"urn:uuid:{refString.Value}" // Set the full URL to the reference ID
        };
        claimBundle.Entry.Add(entry); // Add the entry to the bundle
    }
}

// Serialize the Bundle to JSON with pretty print
var fhirJsonSerializer = new FhirJsonSerializer(new SerializerSettings { Pretty = true });
var bundleJson = fhirJsonSerializer.SerializeToString(claimBundle);
// print the serialized JSON
Console.WriteLine(bundleJson);