Juce guide

Skill kunitoki/sonic-skills/skills/juce-guide

Modular Markdown-based audio skills for AI agents and developers, covering signal processing, synthesis, effects, analysis, and spatial audio.

Install

npx -y skills add kunitoki/sonic-skills --skill juce-guide

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Step-by-step guide for building a JUCE audio plugin from scratch. Use when the user asks "how do I create a JUCE plugin", "set up a JUCE project", "add parameters to my JUCE plugin", "package my VST3", or "how do I wire up APVTS". Trigger on phrases like "create a JUCE plugin", "JUCE CMake setup", "AudioProcessor skeleton", "add APVTS parameters", or "codesign my plugin on macOS".

SKILL.md

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JUCE Audio Plugin — Build Guide

Five steps from empty folder to distributable plugin. Do each step in order; skip none.

Step 1 — Project setup (CMake + JUCE)

Add JUCE as a git submodule or FetchContent target; call add_subdirectory(JUCE).
Use juce_add_plugin(MyPlugin FORMATS VST3 AU Standalone ...) to declare the plugin target.
Set PLUGIN_MANUFACTURER_CODE, PLUGIN_CODE, IS_SYNTH, NEEDS_MIDI_INPUT in the same call.
Link juce::juce_audio_processors juce::juce_audio_utils juce::juce_dsp to the target.

Step 2 — AudioProcessor skeleton

Subclass juce::AudioProcessor; override prepareToPlay, releaseResources, processBlock.
In prepareToPlay: store sampleRate and maximumBlockSize, allocate internal buffers, reset all state (filters, envelopes, delay lines — unconditionally).
In processBlock: read only pre-cached raw parameter pointers; never allocate, never call JUCE String ops or DBG().
Implement getStateInformation / setStateInformation using apvts.copyState() and apvts.replaceState() with a MemoryOutputStream / MemoryBlock.

Step 3 — Parameters via APVTS

Declare a static createParameterLayout() method that returns AudioProcessorValueTreeState::ParameterLayout.
Construct apvts(*this, nullptr, "Parameters", createParameterLayout()) as a member.
After constructing APVTS, cache raw pointers once: gainParam = apvts.getRawParameterValue("Gain").
In processBlock, dereference with gainParam->load() — no APVTS calls on the audio thread.
Use AudioProcessorValueTreeState::ParameterAttachment (or SliderAttachment) in the editor, never in the processor.

Step 4 — Editor and UI

Subclass juce::AudioProcessorEditor; call setSize(w, h) in the constructor.
Override paint for background / labels; override resized to lay out components with setBounds.
Declare juce::Slider members; attach each via juce::AudioProcessorValueTreeState::SliderAttachment.
Keep all UI state in the editor — the processor must function correctly with no editor open.
The editor holds a reference to the processor (passed in constructor); never store a raw editor pointer in the processor.

Step 5 — Build, test, package

Configure: cmake -B build -DCMAKE_BUILD_TYPE=Release
Build: cmake --build build --config Release
Locate artefacts under build/MyPlugin_artefacts/Release/.
macOS codesign: codesign --deep --force --sign "Developer ID Application: ..." MyPlugin.vst3 then spctl --assess --verbose MyPlugin.vst3.
Notarize for distribution: xcrun notarytool submit MyPlugin.pkg --apple-id ... --wait.
Validate in a DAW host or use the JUCE AudioPluginHost utility (extras/AudioPluginHost).

Common pitfalls

Pitfall	Correct approach
Calling `apvts.getParameter()` in `processBlock`	Cache `getRawParameterValue()` once after APVTS construction
Allocating in `processBlock`	Allocate in `prepareToPlay` only
Not resetting state on `prepareToPlay` re-entry	Always reset unconditionally
Storing editor pointer in processor	Use `createEditor()` factory; processor owns no editor reference
Missing `JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR`	Add to every `AudioProcessor` and `AudioProcessorEditor` subclass

Use the juce-review skill to audit finished processor code for thread-safety violations.