[Android] Fixes cancel Webview Navigation

[paymicro]

Description of Change

Call Navigating event before load Url.

Issues Resolved

• fixes [Android] WebView Navigating Cancel property not working with custom scheme #4891
• fixes Cancel Webview Navigation using "e.cancel = true" not working #4919
• closes Fix [Android] WebView Navigating Cancel property not working with custom scheme #5460

API Changes

None

Platforms Affected

• Android

Behavioral/Visual Changes

None

Testing Procedure

• run UItest4919
• check that the webpage does not load on init
• click on Go to github button
• check that the webpage does not load
• click on Toggle cancel navigation button
• click on Go to github button
• webpage should load
• all links should work
• click on Toggle cancel navigation button
• all links should not work

PR Checklist

• Has automated tests
• Rebased on top of the target branch at time of PR
• Changes adhere to coding standard

[brackert]

This PR only fixes #4919
PR #5460 fixes issues #4919 and #4891

[samhouts]

@paymicro These changes look good. Can you reconcile them to #5460 and see which approach is preferable? Thanks!!

[brackert]

@paymicro @samhouts
I don't think this fix works to fix #4891. The problem is not limited to setting the url with the Source property. The primary issue is when an embedded link in the html of the WebView is clicked on and is canceled using the Navigating event, the navigation is not canceled. In Android 9.0 (API 28), the cancel works (sort of), but in earlier API versions it does not work. See my notes in #5460. Without retesting everything, I'm pretty certain that ShouldOverrideUrlLoading must be used to trigger the Navigating event to properly cancel the navigation to an embedded link.

[brackert]

@paymicro
Great job on incorporating the fixes for #4891. It looks like it should work perfectly.
Thanks!

[kensykora]

Will this be included in a 3.6.X release?

[brackert]

@kensykora
It was included in 4.0 pre8. Doesn't look like it is going to be in 3.6. I upgraded my project to pre8 and haven't had any problems... yet. (Also pre9 is now available, but I haven't installed it.)

[chenjeff622]

it still not work at Xamarin.Forms 4.2.0.709249. any fix please?
(Both Android and IOS)
// can stop load page
private void WebView_nav_Navigating(object sender, WebNavigatingEventArgs e)
{
e.Cancel = true; <-- webview stop load page .
}

// can not stop load page
private void WebView_nav_Navigating(object sender, WebNavigatingEventArgs e)
{
if (e.Url.Equals("https://XXXXX.com"))
{
e.Cancel = true; <-- webview still load page .
}
}

[marthinch]

I also get this issue on XF 3.6 and i'm not sure if i upgrade my XF version then this issue will be fixed.