[iOS] Add WKUIDelegate support to PlatformWebViewParams for JavaScript alert/confirm/prompt handling

[gitpushyj]

Problem

Currently, iOS PlatformWebViewParams is an empty class with no properties, making it impossible for developers to provide a custom WKUIDelegate implementation. This means JavaScript alert(), confirm(), and prompt() calls are silently ignored on iOS.

Android allows customization through PlatformWebViewParams:

// webview/src/androidMain/.../WebView.android.kt
actual data class PlatformWebViewParams(
    val chromeClient: AccompanistWebChromeClient? = null,
)

iOS has no such capability:
// webview/src/iosMain/.../WebView.ios.kt

actual class PlatformWebViewParams  // Empty class - no properties!

Additionally, IOSWebView composable doesn't use platformWebViewParams at all, and WKUIDelegate is never set on the WKWebView.

Why This Happens
The key difference between platforms:

• Android - WebChromeClient.onJsAlert() not overridden → System dialog shown automatically
• iOS - WKUIDelegate not set → Silently ignored (no response)

Android's WebChromeClient has built-in default implementations that show system dialogs. iOS's WKWebView has no default behavior - if WKUIDelegate is not set, JavaScript dialogs simply don't work.

Impact

On iOS, when a webpage calls:

alert("message") → Silently ignored
confirm("question") → Returns false immediately
prompt("input") → Returns null immediately
window.open() → Blocked
This creates a significant platform inconsistency between Android and iOS, breaking hybrid apps that rely on JavaScript dialogs.

Proposed Solution

Add uiDelegate property to iOS PlatformWebViewParams:

actual data class PlatformWebViewParams(
    val uiDelegate: WKUIDelegateProtocol? = null,
)

Apply the delegate in IOSWebView:

platformWebViewParams?.uiDelegate?.let {
    this.setUIDelegate(it)
}

Provide a default AccompanistUIDelegate implementation (similar to AccompanistWebChromeClient) that shows native iOS alerts:

class AccompanistUIDelegate : NSObject(), WKUIDelegateProtocol {
    override fun webView(
        webView: WKWebView,
        runJavaScriptAlertPanelWithMessage: String,
        initiatedByFrame: WKFrameInfo,
        completionHandler: () -> Unit,
    ) {
        // Show native iOS alert
    }
    
    override fun webView(
        webView: WKWebView,
        runJavaScriptConfirmPanelWithMessage: String,
        initiatedByFrame: WKFrameInfo,
        completionHandler: (Boolean) -> Unit,
    ) {
        // Show native iOS confirm dialog
    }
    
    override fun webView(
        webView: WKWebView,
        runJavaScriptTextInputPanelWithPrompt: String,
        defaultText: String?,
        initiatedByFrame: WKFrameInfo,
        completionHandler: (String?) -> Unit,
    ) {
        // Show native iOS prompt dialog
    }
}

Environment

Library version: latest (main branch)
Kotlin: 2.1.20
Compose Multiplatform: 1.8.0

Usage Example

// iosMain

@Composable
actual fun getPlatformWebViewParams(): PlatformWebViewParams? {
    val uiDelegate = remember { AccompanistUIDelegate() }
    return PlatformWebViewParams(uiDelegate = uiDelegate)
}

Question

Is there an alternative approach to handle JavaScript alert(), confirm(), and prompt() on iOS within this library that I might have missed? If not, would a PR implementing the proposed solution above be welcome?