PassThroughWindow.swift

import AppKit
import SwiftUI
import Combine
import OSLog

/// Full screen window that hosts the SwiftUI view.
/// - Can not become active nor main so user stays in the app they were in.
/// - Ignores all mouse events allowing user to interact with content behind it.
/// - Enables mouse events, only when mouse is over its content view.
@MainActor
final class PassThroughWindow<Content: View>: NSPanel {

    /// Provided ``CurrentValueSubject`` to listen to the hover over SwiftUI view events.
    var isMouseTrackingEnabled: CurrentValueSubject<Bool, Never>

    private let log = Logger(subsystem: "Window Management", category: "PassThroughWindow<Content: View>")
    private var subscriptions: Set<AnyCancellable> = []
    private let content: Content

    // Prevents window to be activated, we don't want that because then we remove focus
    // from the active application and change it to our app, which creates a subpar user experience.
    override var canBecomeKey: Bool { false }
    override var canBecomeMain: Bool { false }

    public init(
        isMouseTrackingEnabled: CurrentValueSubject<Bool, Never>,
        @ViewBuilder _ content: @escaping () -> Content
    ) {
        self.isMouseTrackingEnabled = isMouseTrackingEnabled
        let contentRect = NSRect.zero
        self.content = content()

        super.init(
            contentRect: contentRect,
            styleMask: [
                .nonactivatingPanel,
                .fullSizeContentView,
            ],
            backing: .buffered,
            defer: false
        )

        collectionBehavior = [
            .canJoinAllSpaces,
            .ignoresCycle,
            .transient
        ]

        // Allow the panel to appear in a fullscreen space
        collectionBehavior.insert(.fullScreenAuxiliary)

        // UI tweaks to be invisible.
        isOpaque = false
        hasShadow = false
        backgroundColor = .clear

        // Allow the panel to be on top of almost all other windows
        isFloatingPanel = true
        level = .floating
        hidesOnDeactivate = false
        becomesKeyOnlyIfNeeded = true

        // While we may set a title for the window, don't show it
        titleVisibility = .hidden
        titlebarAppearsTransparent = true
        isMovableByWindowBackground = false

        acceptsMouseMovedEvents = true
        orderFrontRegardless()
        allowsToolTipsWhenApplicationIsInactive = true

        let content = NSHostingView(rootView: self.content)
        content.autoresizingMask = [.height, .width]
        contentView?.addSubview(content)

        // We activate/deactivate based on what the container tells us.
        isMouseTrackingEnabled.sink { isEnabled in
            isEnabled
            ? self.activateInteractions()
            : self.deactivateInteractions()
        }.store(in: &subscriptions)
    }

    /// Activates interaction with the window
    func activateInteractions() {
        ignoresMouseEvents = false
    }

    /// Deactivates interaction with the window
    func deactivateInteractions() {
        ignoresMouseEvents = true
    }
}