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+# iOS and Cross-Platform Development
+
+![image-20220118173020092](img/cd.png)
+
+|                      |           Android           |              iOS              |
+| -------------------- | :-------------------------: | :---------------------------: |
+| Market Share         |             82%             |             13.9%             |
+| Programming Lang     |      Java<br />Kotlin       | Objective-C<br />Swift (2014) |
+| IDE                  | Eclipse<br />Android Studio |             Xcode             |
+| OS                   |      Windows/Mac/Linux      |         Only on Mac!          |
+| Distribution Process |   Release can take hours    |    Release can take weeks     |
+| Fees                 |         \$24 one-off        |         \$99 per year         |
+
+### iOS
+
+- Based heavily on Mac OS X
+- Closed source
+    - Tools, deployment, app ecosystem controlled by Apple
+- Apps can only be installed from the App Store
+    - Cryptographically signed
+    - Apple runs App Store
+    - Approves all apps available from it
+
+#### iOS Apps
+
+- Written in Objective-C & Swift
+    - An extension of C to add support for objects
+    - Using the Cocoa Touch UI framework
+    - Can also use C/C++ libraries
+    - Compiles to native code via LLVM (not interpreted (JIT) as on Android)
+
+##### Structure of an App
+
+**M**odel **V**iew **C**ontroller Design Pattern
+
+![image-20220118174042986](img/ce.png)
+
+**UI Application Object**: manages event loop & high-level app behaviour
+
+**Application Delegate**: Handles app initialisation and state transitions
+
+**Data Objects**: Store data for the app (images, database, etc.)
+
+**Document**: Manage data objects
+
+**View Controller**: Manages the presentation of the app on the screen, loads views, shows views, rotates views, etc.
+
+**UI Window**: Coordinates the presentation of one or more views on a screen
+
+**Views & UI objects**: Draw content, buttons, etc.
+
+##### iOS Lifecycle
+
+- Apps are sandboxed; inter-app communication is used to pass data
+- Only one application in the foreground / visible at once
+- A main loop processes events for the application
+    - Xcode creates the main function
+- App can be in a number of significant states
+    - Active: foreground
+    - Inactive: foreground but interrupted
+    - Background: same as services in Android
+    - Suspended: main loop no longer running; remains in memory but is potentially killed by the operating system
+
+##### State Transitions
+
+![image-20220118175629095](img/cf.png)
+
+![image-20220118175733832](img/cg.png)
+
+![image-20220118180929130](img/da.png)
+
+#### App Store
+
+- Pre-moderation
+    - Apple checks all applications in advance manually
+    - Versus Android: publish and then revoke
+- A long list of guidelines as to what is appropriate
+
+## Cross-platform
+
+#### Xamarin
+
+- Developed by Microsoft
+- `xamarin.Forms` for native UI code