add tests reproing issues

Author: wtholliday

Tests/AudioKitEXTests/FaderGainBugs.swift

@@ -0,0 +1,74 @@
+//
+//  FaderGainBugs.swift
+//  AudioKitEX
+//
+//  Created by Taylor Holliday on 3/23/26.
+//
+
+import Testing
+import AudioKit
+import AudioKitEX
+import AVFoundation
+import Accelerate
+import CAudioKitEX
+
+
+/// Minimal repro: Fader.init calls AudioUnitSetParameter on an AU that hasn't been
+/// added to an engine, so the C-level AudioUnit doesn't recognise the parameter address
+/// and returns kAudioUnitErr_InvalidParameter (-10878).
+///
+/// CheckError logs but doesn't throw, so we call the C API directly to prove
+/// the parameter write fails on an unconnected AudioUnit.
+@MainActor
+@Test
+func testFaderInitWithoutEngine() throws {
+    let mixer = Mixer()
+    let fader = Fader(mixer, gain: 1.0)
+
+    // The Fader's underlying AudioUnit is not connected to any engine yet.
+    // Prove that setting a parameter on it returns kAudioUnitErr_InvalidParameter.
+    let avAudioUnit = fader.avAudioNode as! AVAudioUnit
+    let au = avAudioUnit.audioUnit
+    let rightGainAddress = akGetParameterAddress("FaderParameterRightGain")
+    let status = AudioUnitSetParameter(au, AudioUnitParameterID(rightGainAddress), kAudioUnitScope_Global, 0, 1.0, 0)
+    #expect(status == kAudioUnitErr_InvalidParameter,
+            "Expected kAudioUnitErr_InvalidParameter (-10878) but got \(status)")
+}
+
+/// Prove that a non-default gain passed to Fader.init is silently lost at the
+/// C level. The Swift-side parameter.value holds 0.5, but after the node is
+/// connected to a running engine the DSP kernel still sees the default (1.0).
+@MainActor
+@Test
+func testFaderGainLostAfterConnection() throws {
+    let engine = AudioEngine()
+    let masterMixer = Mixer(name: "Master")
+    engine.output = masterMixer
+
+    let inputMixer = Mixer()
+    let fader = Fader(inputMixer, gain: 0.5)
+
+    // Swift side thinks gain is 0.5
+    #expect(fader.$leftGain.value == 0.5, "Swift-side leftGain should be 0.5")
+
+    // Now connect to the running engine — this calls makeAVConnections
+    masterMixer.addInput(fader)
+    try engine.start()
+
+    // Read back from both the Swift AUParameter and C-level AudioUnit
+    let avAudioUnit = fader.avAudioNode as! AVAudioUnit
+    let au = avAudioUnit.audioUnit
+    let leftGainAddress = akGetParameterAddress("FaderParameterLeftGain")
+
+    // Swift AUParameter thinks it's 0.5
+    let swiftValue = fader.$leftGain.value
+    #expect(swiftValue == 0.5, "Swift-side leftGain should be 0.5 — got \(swiftValue)")
+
+    // C-level AudioUnit does NOT have 0.5
+    var cValue: AudioUnitParameterValue = -1
+    AudioUnitGetParameter(au, AudioUnitParameterID(leftGainAddress), kAudioUnitScope_Global, 0, &cValue)
+    #expect(cValue != 0.5,
+            "C-level gain should NOT be 0.5 — the init-time set was lost (got \(cValue))")
+
+    engine.stop()
+}