Add lesson 18

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+
+Let's create an app that can actually handle input from user.
+
+To do this we will need `ReadKeyStroke` function:
+```
+EFI_SIMPLE_TEXT_INPUT_PROTOCOL.ReadKeyStroke()
+
+Summary:
+Reads the next keystroke from the input device.
+
+Prototype:
+typedef
+EFI_STATUS
+(EFIAPI *EFI_INPUT_READ_KEY) (
+ IN EFI_SIMPLE_TEXT_INPUT_PROTOCOL *This,
+ OUT EFI_INPUT_KEY *Key
+ );
+
+Parameters:
+This 		A pointer to the EFI_SIMPLE_TEXT_INPUT_PROTOCOL instance.
+Key 		A pointer to a buffer that is filled in with the keystroke information
+		for the key that was pressed.
+
+Description:
+The ReadKeyStroke() function reads the next keystroke from the input device.
+
+Status Codes Returned:
+EFI_SUCCESS 		The keystroke information was returned.
+EFI_NOT_READY 		There was no keystroke data available.
+EFI_DEVICE_ERROR 	The keystroke information was not returned due to hardware errors.
+```
+
+The `EFI_INPUT_KEY` function is defined like this:
+```
+typedef struct {
+ UINT16 ScanCode;
+ CHAR16 UnicodeChar;
+} EFI_INPUT_KEY;
+```
+
+
+Create `InteractiveApp` with the following code:
+```
+#include <Library/UefiBootServicesTableLib.h>
+#include <Library/UefiLib.h>
+
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  UINTN Index;
+  EFI_INPUT_KEY Key;
+
+  Print(L"Try to guess the secret symbol!\n");
+  Print(L"To quit press 'q'\n");
+
+  while(TRUE) {
+    gBS->WaitForEvent(1, &(gST->ConIn->WaitForKey), &Index);
+    gST->ConIn->ReadKeyStroke(gST->ConIn, &Key);
+    Print(L"ScanCode = %04x, UnicodeChar = %04x (%c)\n", Key.ScanCode, Key.UnicodeChar, Key.UnicodeChar);
+
+    if (Key.UnicodeChar == 'k') {
+      Print(L"Correct!\n");
+      break;
+    } else if (Key.UnicodeChar == 'q') {
+      Print(L"Bye!\n");
+      break;
+    } else {
+      Print(L"Wrong!\n");
+    }
+  }
+  gST->ConIn->Reset(gST->ConIn, FALSE);
+  return EFI_SUCCESS;
+}
+```
+
+The code is pretty simple, so I hope no detailed explanation is needed.
+App asks for the secret symbol and loops until user presses the correct one ('k'), or presses 'q'.
+
+```
+FS0:\> InteractiveApp.efi
+Try to guess the secret symbol!
+To quit press 'q'
+ScanCode = 0000, UnicodeChar = 0066 (f)
+Wrong!
+ScanCode = 0000, UnicodeChar = 0074 (t)
+Wrong!
+ScanCode = 0000, UnicodeChar = 0073 (s)
+Wrong!
+ScanCode = 0000, UnicodeChar = 0069 (i)
+Wrong!
+ScanCode = 0000, UnicodeChar = 0061 (a)
+Wrong!
+ScanCode = 0000, UnicodeChar = 006E (n)
+Wrong!
+ScanCode = 0000, UnicodeChar = 006B (k)
+Correct!
+FS0:\>
+```
+
+With this app you can also look at how special keys are handled.
+This is the output from "PageUp" key:
+```
+ScanCode = 0017, UnicodeChar = 0000 ( )
+Wrong!
+ScanCode = 0000, UnicodeChar = 005B ([)
+Wrong!
+ScanCode = 0000, UnicodeChar = 0035 (5)
+Wrong!
+ScanCode = 0000, UnicodeChar = 007E (~)
+Wrong!
+```
+