fix: Can't load target="_blank" links on iOS

Author: rydein

ios/Classes/FlutterWebviewPlugin.m

@@ -3,7 +3,7 @@
 static NSString *const CHANNEL_NAME = @"flutter_webview_plugin";
 
 // UIWebViewDelegate
-@interface FlutterWebviewPlugin() <WKNavigationDelegate, UIScrollViewDelegate> {
+@interface FlutterWebviewPlugin() <WKNavigationDelegate, UIScrollViewDelegate, WKUIDelegate> {
     BOOL _enableAppScheme;
     BOOL _enableZoom;
 }
@@ -105,6 +105,7 @@ - (void)initWebview:(FlutterMethodCall*)call {
     }
 
     self.webview = [[WKWebView alloc] initWithFrame:rc];
+    self.webview.UIDelegate = self;
     self.webview.navigationDelegate = self;
     self.webview.scrollView.delegate = self;
     self.webview.hidden = [hidden boolValue];
@@ -260,6 +261,15 @@ - (void)webView:(WKWebView *)webView decidePolicyForNavigationAction:(WKNavigati
     }
 }
 
+- (WKWebView *)webView:(WKWebView *)webView createWebViewWithConfiguration:(WKWebViewConfiguration *)configuration
+    forNavigationAction:(WKNavigationAction *)navigationAction windowFeatures:(WKWindowFeatures *)windowFeatures {
+
+    if (!navigationAction.targetFrame.isMainFrame) {
+        [webView loadRequest:navigationAction.request];
+    }
+
+    return nil;
+}
 
 - (void)webView:(WKWebView *)webView didStartProvisionalNavigation:(WKNavigation *)navigation {
     [channel invokeMethod:@"onState" arguments:@{@"type": @"startLoad", @"url": webView.URL.absoluteString}];