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// displaying only relevant content not all the functions

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {

  // other stuff of didFinishLaunchingWithOptions inside app delegate

  if #available(iOS 10.0, *) {
    UNUserNotificationCenter.current().delegate = self
    let authOptions: UNAuthorizationOptions = [.alert, .badge, .sound]
    UNUserNotificationCenter.current().requestAuthorization(

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func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    print("Dhruw: Dumping notification payload")
    if application.applicationState != .active {
      let value1: String? = userInfo["key1"] as? String
      let value2: String? = userInfo["key2"] as? String
      let value3: Bool? = userInfo["key3"] as? Bool
      // do something with the received data
    }
  }

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// HTTP URL : https://fcm.googleapis.com/fcm/send
{
  "registration_ids":["XXXXXXXXXXXX:APA91XXXX7ziO-XXXXXXXXXXXXXXXXXXXXXX3B7OzLmNoAfHmZ3ju1M8gk7d-fYwXXXXXXdOHiwNhP6ThXNizSAu-Q0RkywWR8YCEiXf6a4Y803HY1t-XXXXXXXXXXX"],
  "notification": {
      "title":"Venue change for Avengers secret meet",
      "body":"Tap here to open location on your phone"
  },
  "data": {
    "latitude" : "39.204720",
    "longitude" : "-96.564909",

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curl -i -H 'Content-type: application/json' -H 'Authorization: key=<your_server_key>' -XPOST https://fcm.googleapis.com/fcm/send -d '{
  "registration_ids":["registration_ids", "of the", "target", "devices as array"],
  "notification": {
      "title":"Title of your notification",
      "body":"content of your notification"
  },
  "data": {
    "key1" : "value1",
    "key2" : "value2",
    "key3" : 23.56565,

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The introduction to Reactive Programming you've been missing

(by @andrestaltz)

---

This tutorial as a series of videos

If you prefer to watch video tutorials with live-coding, then check out this series I recorded with the same contents as in this article: Egghead.io - Introduction to Reactive Programming.

---

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.monaco-shell {
  font-family: "Operator Mono", "Inconsolata", monospace;
}

/* This makes the dirty tab circle yellow */
.vs-dark
.monaco-workbench
> .part.editor
> .content
> .one-editor-silo

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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>DVTConsoleDebuggerInputTextColor</key>
	<string>0 0 0 1</string>
	<key>DVTConsoleDebuggerInputTextFont</key>
	<string>SFMono-Bold - 11.0</string>
	<key>DVTConsoleDebuggerOutputTextColor</key>
	<string>0 0 0 1</string>

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Bank bank = .... ;
List<Customer> customers = bank.getCustomers(); // Get all bank customers
List<Transaction> = customer.getTransactions(); // Get all transactions by 1 customer
double amount = transaction.getAmount(); // Get transaction amount
String transactionType = transaction.getType(); // Get type of transaction (deposit or withdraw)

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// JAVA WAY - Who has minimum balance
Customer minBalanceCustomer = Collections.min(
  customers, (o1, o2) -> (int) (o1.getBalance() - o2.getBalance())
);

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// THE KOTLIN WAY

// 1. Who did maximum number of transactions
val maxTransactionsBy = customers.maxBy { it.transactions.count() }

// List all transactions irrespective of customers
val transactions = customers.flatMap { it.transactions }

// 2a. Get total amount deposited in the bank (use transactions constant from above)
val totalAmountDeposited = transactions.filter { it.type == "deposit" }.sumByDouble { it.amount }