[1/2] Swift Concurrency migration - Financial Connections API client

[mats-stripe]

Summary

This PR introduces Swift Concurrency to the Financial Connections API client, laying the groundwork for a full migration. The changes are implemented in a way that allows for a gradual rollout without disrupting existing functionality. To do this seamlessly, we introduce two new classes;

1. FinancialConnectionsAsyncAPIClient: A new API client fully powered by Swift Concurrency.
2. FinancialConnectionsAPIClientFacade: A wrapper that encapsulates both the new async client and the existing Futures/Promises-based client.

Some implementation details:

• The facade implements the FinancialConnectionsAPI protocol, maintaining the current interface for SDK consumers.
• This approach allows us to isolate changes and continue using the existing API client in production while testing the new async implementation.

This work also provides a shape for the new async API. The current API is shaped as:

// Futures-based API
func synchronize(clientSecret: String, returnURL: String?) -> Future<FinancialConnectionsSynchronize>

While the new API is shaped as:

// Async-based API
func synchronize(clientSecret: String, returnURL: String?) async throws -> FinancialConnectionsSynchronize

A toggle has been added to the FC example app to switch between API client implementations. This is currently the only way to test the async API client:

Motivation

Move towards migration to Swift Concurrency

Next Steps: Part 2 will focus on migrating existing consumers to use the new async API client.

Testing

Unit tests added, and manual testing done.

Screen.Recording.2025-01-08.at.1.47.18.PM.mov

Changelog

N/a

[emerge-tools]

8 builds increased size

Name	Version	Download	Change	Install	Change	Approval
StripeSize com.stripe.StripeSize	1.0 (1)	2.1 MB	⬇️ 152 B	7.0 MB	⬆️ 100 B	N/A
StripePaymentsSize com.stripe.StripePaymentsSize	1.0 (1)	1.2 MB	⬇️ 159 B (-0.01%)	4.2 MB	⬆️ 100 B	N/A
StripePaymentsUISize com.stripe.StripePaymentsUISize	1.0 (1)	1.9 MB	⬆️ 306 B (0.02%)	6.4 MB	⬆️ 100 B	N/A
StripePaymentSheetSize com.stripe.StripePaymentSheetSize	1.0 (1)	3.8 MB	⬆️ 527 B (0.01%)	11.1 MB	⬆️ 476 B	N/A
StripeIdentitySize com.stripe.StripeIdentitySize	1.0 (1)	1.4 MB	⬇️ 66 B	4.3 MB	⬆️ 100 B	N/A
StripeApplePaySize com.stripe.StripeApplePaySize	1.0 (1)	482.8 kB	⬆️ 158 B (0.03%)	1.7 MB	⬆️ 100 B	N/A
⚠️ StripeFinancialConnectionsSize com.stripe.StripeFinancialConnectionsSize	1.0 (1)	1.5 MB	⬆️ 67.0 kB (4.84%)	4.7 MB	⬆️ 197.0 kB (4.43%)	N/A
⚠️ StripeConnectSize com.stripe.StripeConnectSize	1.0 (1)	1.6 MB	⬆️ 67.5 kB (4.37%)	5.2 MB	⬆️ 193.6 kB (3.91%)	N/A

StripeSize 1.0 (1)
com.stripe.StripeSize

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Total install size change: ⬆️ 100 B
Total download size change: ⬇️ 152 B

Largest size changes

Item	Install Size Change
Other	⬆️ 100 B

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StripePaymentsSize 1.0 (1)
com.stripe.StripePaymentsSize

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Total install size change: ⬆️ 100 B
Total download size change: ⬇️ 159 B (-0.01%)

Largest size changes

Item	Install Size Change
Other	⬆️ 100 B

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StripePaymentsUISize 1.0 (1)
com.stripe.StripePaymentsUISize

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Total install size change: ⬆️ 100 B
Total download size change: ⬆️ 306 B (0.02%)

Largest size changes

Item	Install Size Change
Other	⬆️ 100 B

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StripePaymentSheetSize 1.0 (1)
com.stripe.StripePaymentSheetSize

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Total install size change: ⬆️ 476 B
Total download size change: ⬆️ 527 B (0.01%)

Largest size changes

Item	Install Size Change
Other	⬆️ 476 B

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StripeIdentitySize 1.0 (1)
com.stripe.StripeIdentitySize

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Total install size change: ⬆️ 100 B
Total download size change: ⬇️ 66 B

Largest size changes

Item	Install Size Change
Other	⬆️ 100 B

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StripeApplePaySize 1.0 (1)
com.stripe.StripeApplePaySize

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Total install size change: ⬆️ 100 B
Total download size change: ⬆️ 158 B (0.03%)

Largest size changes

Item	Install Size Change
Other	⬆️ 100 B

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StripeFinancialConnectionsSize 1.0 (1)
com.stripe.StripeFinancialConnectionsSize

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Total install size change: ⬆️ 197.0 kB (4.43%)
Total download size change: ⬆️ 67.0 kB (4.84%)

Largest size changes

Item	Install Size Change
Code Signature	⬆️ 4.7 kB
🗑 StripeFinancialConnections.NativeFlowAPIDataManager.manifest	⬇️ -2.0 kB
StripeFinancialConnections.NativeFlowAPIDataManager.init(manifest...	⬇️ -1.8 kB
StripeFinancialConnectionsSize	⬆️ 1.3 kB
StripeFinancialConnections.PartnerAuthViewController.handleAuthSe...	⬆️ 1.2 kB

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StripeConnectSize 1.0 (1)
com.stripe.StripeConnectSize

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Total install size change: ⬆️ 193.6 kB (3.91%)
Total download size change: ⬆️ 67.5 kB (4.37%)

Largest size changes

Item	Install Size Change
Code Signature	⬆️ 5.4 kB
🗑 StripeFinancialConnections.NativeFlowAPIDataManager.manifest	⬇️ -2.0 kB
StripeFinancialConnections.NativeFlowAPIDataManager.init(manifest...	⬇️ -1.8 kB
StripeConnectSize	⬆️ 1.3 kB
StripeFinancialConnections.PartnerAuthViewController.handleAuthSe...	⬆️ 1.2 kB

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Comment trigger: Size diff threshold of 100.00kB exceeded

[github-actions]

🚨 New dead code detected in this PR:

FinancialConnectionsAsyncAPIClient.swift:144 warning: Protocol 'FinancialConnectionsAsyncAPI' is redundant as it's never used as an existential type
FinancialConnectionsAsyncAPIClient.swift:316 warning: Protocol 'FinancialConnectionsAsyncAPI' conformance is redundant

Please remove the dead code before merging.

If this is intentional, you can bypass this check by adding the label skip dead code check to this PR.

ℹ️ If this comment appears to be left in error, double check that the flagged code is actually used and/or make sure your branch is up-to-date with master.