# 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