Correctly handle supported image formats on Android APIs < 23

[okarmazin]

The library declares minSdk 21, but BarcodeReader accesses ImageFormat constants that were added in API 23. This PR guards the access to these constants with a version check.

Also, the explicit API mode was enabled for the Kotlin compiler. This flag instructs the compiler to enforce visibility modifiers and return types be explicitly specified (if they would be public by default).

[axxel]

Just to make I understand the situation regarding the explicit mode: Everything in a kotlin class is public unless specified private, right? So if there is no private visibility modifier, the symbol is (explicitly) public. What is there to gain when littering the code with a bunch of public strings? Preventing an ignorant developer from accidentally adding public symbols to the API?

[okarmazin]

Yes, unfortunately Kotlin visibility is implicitly public by default, unless specified otherwise.

Explicit API mode helps prevent two errors:

1. Makes it harder to accidentally publish implementation details in your public API. The requirement to specify visibility makes you think about your symbols and if they are actually supposed to be accessible by library consumers.

Note: Kotlin also has internal modifier that makes the element accessible within the same compilation unit (Gradle project) but not by its consumers. It's somewhat akin to Java package-only visibility, except enforced by the compiler.

2. Prevents type inference from changing your return types by mistake. Explicit API mode requires that return types be specified explicitly instead of relying on type inference. It's the same as if you disallowed auto in public API in C++.

Take the following example from BarcodeReader:

public fun read(bitmap: Bitmap, cropRect: Rect = Rect(), rotation: Int = 0): Result? {
    return read(bitmap, options, cropRect, rotation)
}

I could also write the function in a single-expression format. Without Explicit API mode I could omit the return type:

public fun read(bitmap: Bitmap, cropRect: Rect = Rect(), rotation: Int = 0) = read(bitmap, options, cropRect, rotation)

Explicit API mode prevents that:

public fun read(bitmap: Bitmap, cropRect: Rect = Rect(), rotation: Int = 0): Result? = read(bitmap, options, cropRect, rotation)

[axxel]

As you can possibly infer from my choice of words in my last comment, I'm not a fan of 'unnecessary' verbosity but I'm also not particularly emotional about the Kotlin code base in general and you clearly have more knowledge and experience here... Thanks for taking the time to engage in the discussion.