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Here's how to get started w/ Calendar util in Android ICU: https://developer.android.com/reference/android/icu/util/Calendar.

i. Here is an explanation of how to build a calendar UI component using Jetpack Compose:

- Use a Column to arrange the calendar vertically.

- Display the month/year text at the top using Text composables.

- Use a Row with 7 Box composables for the day of week header row. Set text alignments.

- Create a Grid composable for the days. Make 7 columns for days of the week.

- Use your date logic to determine number of rows needed for the month.

- Populate each Grid cell with a Text composable showing the day number.

- Style the text for current day differently, like bold and highlighted background.

- Disable interaction and style unselectable days like the previous/next month days.

- Let the user select a day by clicking with Modifier.clickable and updating state.

- Handle gestures like swipes to change months. Animate month transition.

- Write a ViewModel to store and process calendar state using Calendar APIs.

- Expose month/year info from ViewModel as state for the top Text comps.

- Expose list of days from ViewModel to populate Grid cells.

- Add decorators like events using Box composables overlayed on relevant dates.

The key is using Columns, Rows, Grids and Text to structure the calendar UI and drive it from calendar data in the ViewModel. Compose's declarative style makes building custom calendar views straightforward.

ii. The Android Studio calendar app is a simple calendar app template that comes built-in to Android Studio. Here are some details:

- It provides a basic calendar UI with month view, ability to add/edit events, etc.

- It uses Room database and ViewModel from Android Architecture components.

- The UI is built using ViewBinding, RecyclerView, Fragments, and other Jetpack components.

- It follows Material Design principles and has a responsive UI.

To customize this template app:

- Edit styles and themes in styles.xml to change colors, fonts, sizes etc.

- Modify or add layouts in the res/layout/ folder for new screens.

- Change app icons, images and other drawable assets.

- Edit text strings, labels and other text in res/values/strings.xml.

- Add UI elements like buttons, inputs, dialogs etc. and link to code.

- Change RecyclerView layouts and ViewHolders to show different event views.

- Add functionality for new features like reminders, recurring events etc.

- Plug into Google Calendar API to sync with online calendar.

- Modify Room database schema and entities for custom event data.

- Update ViewModels and repository pattern for architecture changes.

Overall, the calendar template is highly customizable as a starting point. You can modify it to create a production-ready calendar app tailored to your needs.

iii. The "call requires API level 24" error occurs when your Android app is trying to use an API that was introduced in Android 7.0 (API level 24) but your app's minimum SDK version is set lower than that.

This happens because newer APIs and features added in later Android versions are not available in earlier versions. If you try to call a method or class added in API 24 while running on a device with an older Android version, it will crash with this error.

To resolve it:

1. Find where in your app the problematic method is being called. It will be highlighted in the error.

2. Check what API level that method was added in. You can check the documentation for that method.

3. In your app module's build.gradle file, update the minSdkVersion to match the method's API level or higher.

For example:

```
android {
minSdkVersion 24
}
```

4. Compile and run your app again. It should now work on API 24+ without crashing.

5. If you need to support lower API levels, you will have to provide an alternate implementation or code path for those versions where you avoid calling the newer method.

So in summary, the error indicates a mismatch between the newer API calls in your code and your app's lower minSDKVersion. Raising the minSDKVersion resolves it.

Learn more@ https://www.youtube.com/c/ITGuides/search?query=Android.