Access Salesforce Data through REST APIs

Using the RestClient Publisher

To access Salesforce data, you send REST requests to Salesforce and receive the responses in your app. How do you code network requests, send them, and process their responses?

Swift UI apps use view-model architecture. A view class defines screen layouts in SwiftUI and, at runtime, calls an associated model class for the data that populates their interfaces. In the Mobile SDK Swift template project, model classes are in the Models project folder. For example, the ContactsForAccountModel.swift file defines the code that requests a list of contacts for an account. When the response arrives, the ContactsForAccountModel class processes the returned records to send a list of contact information to the ContactsForAccountListView class. 

RestClient is your entry point to all REST services. Mobile SDK apps use RestClient methods to manufacture and send REST requests. You always access this class only through its shared instance:

RestClient.shared()

1. In Xcode, open the ContactsForAccountModel.swift file.
2. Search for the fetchContactsForAccount(_:)method. The implementation reads something like the following:

   class ContactsForAccountModel: ObservableObject {
       @Published var contacts: [Contact] = []
       var account: Account?
       private var contactsCancellable: AnyCancellable?
       func fetchContactsForAccount(){
           guard let acct = self.account else {return}
           let request = RestClient.shared.request(forQuery: 
               "SELECT id, firstName, lastName, phone, 
               email, mailingStreet, mailingCity, 
               mailingState, mailingPostalCode 
               FROM Contact 
               WHERE AccountID = '\(acct.id)'", apiVersion: nil)
           contactsCancellable = RestClient.shared.publisher(for: request)
               .receive(on: RunLoop.main)
               .tryMap({ (response) -> Data in
                   response.asData()
               })
               .decode(type: ContactResponse.self, decoder: JSONDecoder())
               .map({ (record) -> [Contact] in
                   record.records
               })
               .catch( { error in
                   Just([])
               })
               .assign(to: \.contacts, on:self)
           }
       }
   }

let request = RestClient.shared.request(forQuery:... 

You can see the publisher call in the line that begins:

contactsCancellable = RestClient.shared.publisher(for: request)

You might recognize the sequence of calls that follows as a call chain, which is a common and useful construction for Swift Combine publishers. This call chain handles the asynchronous network response as a Future object that eventually either finishes or fails. In each of the chain of calls, the action is performed on the current default object. This object can be reset by the return value of the previous call in the chain. If the network call failed or Salesforce returned an error, the processing falls through to the .catch block. 

For a detailed sense of how the call chain works, here's a generously commented listing of the sequence.

// Use the RestClient publisher to send the request and return the raw REST response
contactsCancellable = RestClient.shared.publisher(for: request)
    // Receive the raw REST response object
    .receive(on: RunLoop.main)
    // Try to convert the raw REST response to Data
    // Throw an error in the event of failure
    .tryMap({ (response) -> Data in
        // Return the response as a Data byte buffer
        response.asData()
    })
    // Decode the Data object as JSON to produce an array
    // of Contacts formatted as a ContactResponse object
    .decode(type: ContactResponse.self, decoder: JSONDecoder())
    // Map the JSON array of Contacts
    .map({ (record) -> [Contact] in
        // Copy the Contact array to the records
        // member of ContactResponse
        record.records
    })
    // If an error is thrown, publish an empty array
    .catch( { error in
        Just([]) // Combine publisher that rescues a failed publisher
    })
    // Store the array of contacts in the model’s published contacts
    // property for use by ContactsForAccountListView
    .assign(to: \.contacts, on:self)
}