Zip Builder V2 Plan (#4341)

Author: paulb777

ZipBuilder/ZipBuilderV2Plan.md

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+V2 will add an option to generate a zip distribution of binary frameworks from an arbitrary list
+of source and binary CocoaPods.
+
+## Introduction
+
+The current [Zip Builder](https://github.com/firebase/firebase-ios-sdk/tree/master/ZipBuilder)
+is Firebase specific. This is a proposal and initial plan to evolve the Zip Builder into a
+multi-purpose Apple binary framework creation tool.
+
+It would be useful to have a generic
+Zip Builder for other Google SDKs and other open source projects. In addition,
+providing a generic Zip Builder would enable Firebase users to generate binary
+frameworks for configurations outside the standard zip and Carthage distributions
+released in the standard Firebase release process.
+
+With the extra flexibility, the Zip Builder will be useful for both SDK distributors
+to package binary distributions and app developers who want a customized binary
+distribution that maps exactly to their app's requirements and provides a clean
+build time speed up.
+
+
+## Background
+
+A more flexible Zip Builder would enable the following scenarios:
+
+  * Creating a zip distribution from an arbitrary set of pods
+  * Support different building different sets of platform slices
+  * Support building with different Xcode versions
+  * Build only the subspecs needed for use case
+  * [Existing solutions](https://github.com/firebase/firebase-ios-sdk/issues/4284#issuecomment-552677044)
+  are intermittently maintained and written in Ruby. A Swift implementation is
+  more accessible and maintainable by the Apple community.
+  * The Swift implementation will be easier to add Swift Package Manager support for which
+  there will likely be a need since Swift Package Manager is even more source-centric than
+  CocoaPods.
+
+## Plan
+
+  1. Add `--zipPods {JSON file}` option. The JSON file contains a list of CocoaPods
+  along with an optional version specifier. If the version is not specified, a CocoaPods
+  install will determine the version - typically the latest, unless another pod requires
+  something lower.
+
+  1. Add `--minimumIOSVersion {version}` option. Specify the minimum iOS version to support.
+  Default is 8.0.
+
+  1. Add `--archs {archs list}` option. Default is "arm64, arm64e, armv7, i386, x86_64"
+
+Unlike the Firebase zip build which builds a two-level zip file with a configurable set of
+installation, when `--zipPods` is specified a single zip file of frameworks will be created.
+
+## Future Directions
+
+  * Binary support for Catalyst
+  * Migrate to building `.xcframeworks` instead of `.frameworks`
+  * Other Apple platforms besides iOS
+  * Swift Package Manager
+  * Dynamic Framework support
+  * More option customization