在基于MFC对话框的webviewWindow组件的初始化之外，如何使用“webviewWindow->Navigate()”？

Question

Title says it all. I'll admit that I'm new to MFC and Webview2 but I have a working program that displays the webview already thanks to the "Getting Started" sections of Webview2. But I wanted to add onto this by having code navigate to a different url by a function call. Here's what I have:

BOOL CMFCApplication1Dlg::OnInitDialog(){
    
CDialogEx::OnInitDialog();

// Add "About..." menu item to system menu.

// IDM_ABOUTBOX must be in the system command range.
ASSERT((IDM_ABOUTBOX & 0xFFF0) == IDM_ABOUTBOX);
ASSERT(IDM_ABOUTBOX < 0xF000);

CMenu* pSysMenu = GetSystemMenu(FALSE);
if (pSysMenu != nullptr)
{
    BOOL bNameValid;
    CString strAboutMenu;
    bNameValid = strAboutMenu.LoadString(IDS_ABOUTBOX);
    ASSERT(bNameValid);
    if (!strAboutMenu.IsEmpty())
    {
        pSysMenu->AppendMenu(MF_SEPARATOR);
        pSysMenu->AppendMenu(MF_STRING, IDM_ABOUTBOX, strAboutMenu);
    }
}

// Set the icon for this dialog.  The framework does this automatically
//  when the application's main window is not a dialog
SetIcon(m_hIcon, TRUE);         // Set big icon
SetIcon(m_hIcon, FALSE);        // Set small icon

// TODO: Add extra initialization here   

// this is where the WebView2 "Getting Started" code begins
HRESULT hresult = CoInitializeEx(NULL, COINIT_APARTMENTTHREADED | COINIT_DISABLE_OLE1DDE);

// The main window class name.
static TCHAR szWindowClass[] = _T("DesktopApp");

// The string that appears in the application's title bar.
static TCHAR szTitle[] = _T("WebView sample");

// Pointer to WebViewController
static wil::com_ptr<ICoreWebView2Controller> webviewController;

// Pointer to WebView window
static wil::com_ptr<ICoreWebView2> webviewWindow;

HWND hWnd = GetSafeHwnd();
try {
    // Locate the browser and set up the environment for WebView
    CreateCoreWebView2EnvironmentWithOptions(nullptr, nullptr, nullptr,
        Callback<ICoreWebView2CreateCoreWebView2EnvironmentCompletedHandler>(
            [hWnd](HRESULT result, ICoreWebView2Environment* env) -> HRESULT {

        // Create a CoreWebView2Controller and get the associated CoreWebView2 whose parent is the main window hWnd
        env->CreateCoreWebView2Controller(hWnd, Callback<ICoreWebView2CreateCoreWebView2ControllerCompletedHandler>(
            [hWnd](HRESULT result, ICoreWebView2Controller* controller) -> HRESULT {
            if (controller != nullptr) {
                webviewController = controller;
                webviewController->get_CoreWebView2(&webviewWindow);
            }

            // Add a few settings for the webview
            // The demo step is redundant since the values are the default settings
            ICoreWebView2Settings* Settings;
            webviewWindow->get_Settings(&Settings);
            Settings->put_IsScriptEnabled(TRUE);
            Settings->put_AreDefaultScriptDialogsEnabled(TRUE);
            Settings->put_IsWebMessageEnabled(TRUE);

            // Resize WebView to fit the bounds of the parent window
            RECT bounds = { 0, 0, 1920, 1080 };
            webviewController->put_Bounds(bounds);

            // Schedule an async task to navigate to Bing
            webviewWindow->Navigate(L"https://www.bing.com/");

            // Step 4 - Navigation events


            // Step 5 - Scripting

            // Step 6 - Communication between host and web content

            return S_OK;
        }).Get());
        return S_OK;
    }).Get());
webviewWindow->Navigate(L"http://www.google.com");
}
catch (...) {
    MessageBoxW(L"can i bypass this null pointer? probably not", 0, MB_OK);
}

return TRUE;  // return TRUE  unless you set the focus to a control
}

All of this runs well when I comment

webviewWindow->Navigate(L"http://www.google.com"); 

But the problem is when I try to navigate to google outside of the block "webviewController" is defined. Here's the Error:

Exception thrown: read access violation.
webviewWindow.**m_ptr** was nullptr.

I've look at the documentation and looked around for guides, but I've gotten nothing.

Answer 1

The error diagnostic indicates, that you're trying to dereference a null pointer. Since it conveniently contains both the object name and a reason, it's clear that you're trying to access webviewWindow before it has been assigned.

webviewWindow is assigned to in this line of code:

webviewController->get_CoreWebView2(&webviewWindow);

Pay attention that this code is inside an anonymous function object passed as the ICoreWebView2CreateCoreWebView2ControllerCompletedHandler callback. The code appears to be executed asynchronously, leaving the possibility that code that's visually trailing the creation code is actually executed prior to creation of the WebView2 control completing.

With that out of the way, the earliest time when it's safe to interact with the WebView2 control is from its ICoreWebView2CreateCoreWebView2ControllerCompletedHandler . If you want to navigate to any particular URL, you're going to have to place that code into this callback. If the code in question is running inside a non-static class member you can capture this and call any given class member on that instance.