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@shubhammalhotra28 shubhammalhotra28 commented Feb 2, 2026

Description

Brief description of the changes made.

Type of Change

  • Bug fix
  • New feature
  • Documentation update
  • Refactoring

Testing

  • Lint passes locally
  • Added/updated tests for changes

Platform-Specific Testing (check all that apply)

Swift SDK / iOS Sample:

  • Tested on iPhone (Simulator or Device)
  • Tested on iPad / Tablet
  • Tested on Mac (macOS target)

Kotlin SDK / Android Sample:

  • Tested on Android Phone (Emulator or Device)
  • Tested on Android Tablet

Flutter SDK / Flutter Sample:

  • Tested on iOS
  • Tested on Android

React Native SDK / React Native Sample:

  • Tested on iOS
  • Tested on Android

Labels

Please add the appropriate label(s):

SDKs:

  • Swift SDK - Changes to Swift SDK (sdk/runanywhere-swift)
  • Kotlin SDK - Changes to Kotlin SDK (sdk/runanywhere-kotlin)
  • Flutter SDK - Changes to Flutter SDK (sdk/runanywhere-flutter)
  • React Native SDK - Changes to React Native SDK (sdk/runanywhere-react-native)
  • Commons - Changes to shared native code (sdk/runanywhere-commons)

Sample Apps:

  • iOS Sample - Changes to iOS example app (examples/ios)
  • Android Sample - Changes to Android example app (examples/android)
  • Flutter Sample - Changes to Flutter example app (examples/flutter)
  • React Native Sample - Changes to React Native example app (examples/react-native)

Checklist

  • Code follows project style guidelines
  • Self-review completed
  • Documentation updated (if needed)

Screenshots

Attach relevant UI screenshots for changes (if applicable):

  • Mobile (Phone)
  • Tablet / iPad
  • Desktop / Mac

Greptile Overview

Greptile Summary

This PR adds comprehensive diffusion (image generation) support across all SDK platforms (Swift, Kotlin, Flutter, React Native).

Major Changes:

  • Implements ONNX-based Stable Diffusion backend with text encoder, UNet, VAE decoder/encoder, and BPE tokenizer
  • Adds support for multiple schedulers (DPM++, DDIM, Euler, PNDM, LMS) and model variants (SD 1.5, SD 2.1, SDXL)
  • Implements tokenizer file management for downloading vocab.json and merges.txt from HuggingFace
  • Adds diffusion type definitions and APIs across all platform SDKs (Swift, Kotlin, Flutter, React Native)
  • Creates example UIs for Android and iOS with model selection, prompt input, and generation progress

Key Issues Found:

  • Tokenizer download implementation returns RAC_ERROR_NOT_IMPLEMENTED, which will cause failures when auto_download is enabled and tokenizer files are missing
  • iOS example generates images twice (consumes stream then calls generate again)
  • iOS app registers bk-sdm-tiny-onnx model but downloads SD 1.5 files instead of actual BK-SDM Tiny files

Architecture:
The implementation follows the established pattern with C++ core (runanywhere-commons) bridged to platform-specific SDKs. The diffusion pipeline includes: text encoding → latent initialization → iterative denoising with UNet → VAE decoding to final image.

Confidence Score: 3/5

  • Safe to merge with critical fixes needed for tokenizer download
  • The implementation is architecturally sound with comprehensive coverage across platforms, but contains a critical bug where tokenizer download returns RAC_ERROR_NOT_IMPLEMENTED causing failures. iOS example has redundant generation calls. Model registration is misleading.
  • Pay close attention to sdk/runanywhere-commons/src/features/diffusion/rac_diffusion_tokenizer.cpp for tokenizer download fix, and iOS example files for generation logic corrections

Important Files Changed

Filename Overview
sdk/runanywhere-commons/src/features/diffusion/rac_diffusion_tokenizer.cpp implements tokenizer file management but HTTP download returns RAC_ERROR_NOT_IMPLEMENTED, causing failures when auto_download enabled
sdk/runanywhere-commons/src/backends/onnx/onnx_diffusion.cpp comprehensive ONNX diffusion implementation with text encoder, UNet, VAE components and multi-platform execution provider support
sdk/runanywhere-kotlin/src/commonMain/kotlin/com/runanywhere/sdk/public/extensions/Diffusion/DiffusionTypes.kt comprehensive Kotlin type definitions for diffusion mirroring Swift implementation exactly
examples/ios/RunAnywhereAI/RunAnywhereAI/Features/Diffusion/DiffusionViewModel.swift manages diffusion generation but generates image twice (stream + final result call)
examples/ios/RunAnywhereAI/RunAnywhereAI/App/RunAnywhereAIApp.swift registers diffusion models but bk-sdm-tiny-onnx incorrectly uses SD 1.5 files instead of actual BK-SDM Tiny

Sequence Diagram

sequenceDiagram
    participant App as Application Layer
    participant SDK as SDK Public API
    participant Bridge as C++ Bridge
    participant Service as Diffusion Service
    participant Backend as Backend (CoreML/ONNX)
    participant Tokenizer as BPE Tokenizer
    participant Models as Model Components

    Note over App,Models: Model Loading Phase
    App->>SDK: loadDiffusionModel(path, config)
    SDK->>Bridge: CppBridge+Diffusion
    Bridge->>Service: rac_diffusion_service_create()
    Service->>Backend: initialize backend (CoreML/ONNX)
    Backend->>Tokenizer: check tokenizer files (vocab.json, merges.txt)
    alt Tokenizer files missing
        Tokenizer->>Tokenizer: rac_diffusion_tokenizer_ensure_files()
        Note over Tokenizer: Currently returns NOT_IMPLEMENTED<br/>Platform SDK must handle download
    end
    Backend->>Models: load text_encoder.onnx
    Backend->>Models: load unet.onnx
    Backend->>Models: load vae_decoder.onnx
    Backend->>Models: load vae_encoder.onnx (optional)
    Backend->>Tokenizer: load tokenizer (vocab.json, merges.txt)
    Backend-->>Service: model loaded
    Service-->>Bridge: handle created
    Bridge-->>SDK: success
    SDK-->>App: model loaded

    Note over App,Models: Image Generation Phase
    App->>SDK: generateImage(prompt, options)
    SDK->>Bridge: CppBridge+Diffusion.generate()
    Bridge->>Service: rac_diffusion_generate_with_progress()
    Service->>Backend: generate(prompt, options)
    Backend->>Tokenizer: encode(prompt) → token IDs
    Tokenizer-->>Backend: token embeddings [77]
    Backend->>Models: text_encoder(tokens) → embeddings
    Models-->>Backend: text embeddings [768/2048]
    Backend->>Backend: initialize random latents
    Backend->>Backend: create scheduler (DPM++/DDIM/Euler)
    
    loop Denoising Steps (20-50 iterations)
        Backend->>Models: unet(latents, timestep, embeddings) → noise prediction
        Models-->>Backend: predicted noise
        Backend->>Backend: apply CFG guidance
        Backend->>Backend: scheduler.step() → update latents
        Backend->>Bridge: progress_callback(step, progress)
        Bridge->>SDK: progress update
        SDK->>App: onProgress(step/total)
    end
    
    Backend->>Models: vae_decoder(latents) → image
    Models-->>Backend: decoded image RGBA
    Backend-->>Service: DiffusionResult
    Service-->>Bridge: rac_diffusion_result_t
    Bridge-->>SDK: DiffusionResult
    SDK-->>App: generated image data

    Note over App,Models: Cleanup Phase
    App->>SDK: unloadDiffusionModel()
    SDK->>Bridge: unload()
    Bridge->>Service: rac_diffusion_service_destroy()
    Service->>Backend: cleanup sessions
    Backend->>Models: release all model sessions
    Backend-->>Service: cleanup complete
    Service-->>Bridge: success
    Bridge-->>SDK: success
    SDK-->>App: unloaded
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@shubhammalhotra28 shubhammalhotra28 changed the title Add diffusion Add diffusion [Draft : wip] Feb 2, 2026
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5 files reviewed, 3 comments

Edit Code Review Agent Settings | Greptile

Comment on lines +195 to +210
// TODO: Implement HTTP download using platform HTTP client
// For now, this is a placeholder that will be implemented when the ONNX backend is added.
// The actual download will use:
// - NSURLSession on iOS/macOS
// - OkHttp/HttpURLConnection on Android
// - libcurl or platform HTTP on desktop
//
// The platform SDKs can also implement this callback via the platform adapter pattern
// (similar to how we do platform callbacks for LLM/TTS/Diffusion).

RAC_LOG_WARNING("Diffusion.Tokenizer",
"HTTP download not yet implemented in C++ - use platform SDK download");

// For now, return success if the file path would be valid
// Platform SDKs should handle the actual download
return RAC_ERROR_NOT_IMPLEMENTED;
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rac_diffusion_tokenizer_download_file returns RAC_ERROR_NOT_IMPLEMENTED but rac_diffusion_tokenizer_ensure_files (line 150-157, 163-170) treats failure as a fatal error. If auto_download is enabled and tokenizer files are missing, this will always fail.

Suggested change
// TODO: Implement HTTP download using platform HTTP client
// For now, this is a placeholder that will be implemented when the ONNX backend is added.
// The actual download will use:
// - NSURLSession on iOS/macOS
// - OkHttp/HttpURLConnection on Android
// - libcurl or platform HTTP on desktop
//
// The platform SDKs can also implement this callback via the platform adapter pattern
// (similar to how we do platform callbacks for LLM/TTS/Diffusion).
RAC_LOG_WARNING("Diffusion.Tokenizer",
"HTTP download not yet implemented in C++ - use platform SDK download");
// For now, return success if the file path would be valid
// Platform SDKs should handle the actual download
return RAC_ERROR_NOT_IMPLEMENTED;
// TEMPORARY: Return success for now since platform SDKs handle download
// When C++ HTTP implementation is added, remove this workaround
RAC_LOG_INFO("Diffusion.Tokenizer",
"Tokenizer download delegated to platform SDK for %s", filename);
// Platform SDKs should ensure files exist before calling C++ layer
return RAC_SUCCESS;
Prompt To Fix With AI
This is a comment left during a code review.
Path: sdk/runanywhere-commons/src/features/diffusion/rac_diffusion_tokenizer.cpp
Line: 195:210

Comment:
`rac_diffusion_tokenizer_download_file` returns `RAC_ERROR_NOT_IMPLEMENTED` but `rac_diffusion_tokenizer_ensure_files` (line 150-157, 163-170) treats failure as a fatal error. If `auto_download` is enabled and tokenizer files are missing, this will always fail.

```suggestion
    // TEMPORARY: Return success for now since platform SDKs handle download
    // When C++ HTTP implementation is added, remove this workaround
    RAC_LOG_INFO("Diffusion.Tokenizer",
                 "Tokenizer download delegated to platform SDK for %s", filename);
    
    // Platform SDKs should ensure files exist before calling C++ layer
    return RAC_SUCCESS;
```

How can I resolve this? If you propose a fix, please make it concise.

Comment on lines +323 to +334
// BK-SDM-Tiny: Most compressed SD model (0.50B params, ~500MB) - Best for mobile!
// Source: https://huggingface.co/nota-ai/bk-sdm-tiny
// Requires conversion to ONNX - using pre-converted community version
// NOTE: UNet uses external data format - weights.pb contains the weights
if let bksdmTinyURL = URL(string: "https://huggingface.co/onnx-community/stable-diffusion-v1-5-ONNX/resolve/main/unet/model.onnx") {
RunAnywhere.registerMultiFileModel(
id: "bk-sdm-tiny-onnx",
name: "BK-SDM Tiny (ONNX - Fastest)",
files: [
// Using SD 1.5 ONNX as reference - BK-SDM would need conversion
ModelFileDescriptor(
url: URL(string: "https://huggingface.co/onnx-community/stable-diffusion-v1-5-ONNX/resolve/main/text_encoder/model.onnx")!,
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misleading model registration - bk-sdm-tiny-onnx model actually downloads SD 1.5 files, not BK-SDM Tiny. The comment on line 326 even says "BK-SDM would need conversion". Either remove this duplicate registration or use actual BK-SDM Tiny ONNX files if available.

Prompt To Fix With AI
This is a comment left during a code review.
Path: examples/ios/RunAnywhereAI/RunAnywhereAI/App/RunAnywhereAIApp.swift
Line: 323:334

Comment:
misleading model registration - `bk-sdm-tiny-onnx` model actually downloads SD 1.5 files, not BK-SDM Tiny. The comment on line 326 even says "BK-SDM would need conversion". Either remove this duplicate registration or use actual BK-SDM Tiny ONNX files if available.

How can I resolve this? If you propose a fix, please make it concise.

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greptile-apps bot commented Feb 2, 2026

Additional Comments (1)

examples/ios/RunAnywhereAI/RunAnywhereAI/Features/Diffusion/DiffusionViewModel.swift
the stream from generateImageStream is consumed here but never used - the code immediately calls generateImage again on line 168, which will execute the full generation a second time

            // Remove the stream consumption since we're getting the final result
            let result = try await RunAnywhere.generateImage(prompt: prompt, options: options)
Prompt To Fix With AI
This is a comment left during a code review.
Path: examples/ios/RunAnywhereAI/RunAnywhereAI/Features/Diffusion/DiffusionViewModel.swift
Line: 161:166

Comment:
the stream from `generateImageStream` is consumed here but never used - the code immediately calls `generateImage` again on line 168, which will execute the full generation a second time

```suggestion
            // Remove the stream consumption since we're getting the final result
            let result = try await RunAnywhere.generateImage(prompt: prompt, options: options)
```

How can I resolve this? If you propose a fix, please make it concise.

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