onMessage prop is required in some or all environments for injectedJavaScript/injectedJavaScriptBeforeContentLoaded to work

Several users (e.g. in #1260 and #1291) are tripping up on the unintuitive behaviour of an onMessage prop needing to be provided in order for injectedJavaScript to run. It also tripped me up when I first came to use this package.

We currently document the behaviour in docs/Guide.md, in the section The window.ReactNativeWebView.postMessage method and onMessage prop:

However, I see no reason in the first place why lack of onMessage should abrogate JS injection, other than the fact that it’s always behaved like this.

Here is what’s causing it, in case anyone feels like making a PR:

• src/WebView.ios.tsx#L339 messagingEnabled is set to false only if onMessage is defined.
• src/WebView.macos.tsx#L320 messagingEnabled is set to false only if onMessage is defined.
• src/WebView.android.tsx#L329 messagingEnabled is set to false only if onMessage is defined.
• src/WebView.windows.tsx#L235 messagingEnabled is set to false only if onMessage is defined.
• ios/RNCWebView.m#L1249-L1254: the injectedJavaScriptBeforeContentLoaded and injectedJavaScript user scripts do not get added if messagingEnabled is false (my own FIXME recommendation appears above these lines).
• android/src/main/java/com/reactnativecommunity/webview/RNCWebViewManager.java#L1058-L1062: Addition and removal of JS interfaces are toggled by the state of messagingEnabled.
• I can’t find references to messagingEnabled in windows/ReactNativeWebView. Maybe I’m just looking in the wrong places, or maybe JS injection isn’t supported at all on Windows, yet?

Since #1286 was merged, this issue is solved for iOS, but hasn’t to my understanding been addressed for any of the other platforms.

The PR also didn’t touch src/WebView.ios.ts, so it may furthermore be incomplete in any case.

[PrantikMondal]

I am already using ‘onMessage’. Still it is not working to inject javascript. My WebView something like that: <WebView onLoadEnd={() => { this._setstatus() }} onNavigationStateChange={this.on_navigaton_new} injectedJavaScript={custom_script} onMessage={x => this.onMessageGet(x)} javaScriptEnabled={true} source={{ uri: this.state.url, forceReload: this.state.forceReload }} allowsLinkPsreview={true} incognito={true} domStorageEnabled={true} ref={(ref) => this.myWebView = ref}/>

Earlier it was working fine. Suddenly it stop injectjavascript automatically without changing anything.

[zaptrem]

Still an issue as of 10.3.2.