The x-ms-webview HTML element has the void addWebAllowedObject(string name, any value) method and the webview XAML element has the void AddWebAllowedObject(String name, Object value) method. The object parameter is projected into the webview’s top-level HTML document’s script engine as a new property on the global object with property name set to the name parameter. It is not injected into the current document but rather it is projected during initialization of the next top-level HTML document to which the webview navigates.
If AddWebAllowedObject is called during a NavigationStarting event handler the object will be injected into the document resulting from the navigation corresponding to that event.
If AddWebAllowedObject is called outside of the NavigationStarting event handler it will apply to the navigation corresponding to the next explicit navigate method called on the webview or the navigation corresponding to the next NavigationStarting event handler that fires, whichever comes first.
To avoid this potential race, you should use AddWebAllowedObject in one of two ways: 1. During a NavigationStarting event handler, 2. Before calling a Navigate method and without returning to the main loop.
If called both before calling a navigate method and in the NavigationStarting event handler then the result is the aggregate of all those calls.
If called multiple times for the same document with the same name the last call wins and the previous are silently ignored.
If AddWebAllowedObject is called for a navigation and that navigation fails or redirects to a different URI, the AddWebAllowedObject call is silently ignored.
After successfully adding an object to a document, the object will no longer be projected once a navigation to a new document occurs.
If AddWebAllowedObject is called for a document with All WinRT access then projection will succeed and the object will be added.
If AddWebAllowedObject is called for a document which has a URI which has no declared WinRT access via ApplicationContentUriRules then Allow for web only WinRT access is given to that document.
If the document has Allow for web only WinRT access then projection will succeed only if the object’s runtimeclass has the Windows.Foundation.Metadata.AllowForWeb metadata attribute.
The object must implement the IAgileObject interface. Because the XAML and HTML webview elements run on ASTA view threads and the webview’s content’s JavaScript thread runs on another ASTA thread a developer should not create their non-agile runtimeclass on the view thread. To encourage end developers to do this correctly we require the object implements IAgileObject.
The name parameter must be a valid JavaScript property name, otherwise the call will fail silently. If the name is already a property name on the global object, that property is overwritten if the property is configurable. Non-configurable properties on the global object are not overwritten and the AddWebAllowedObject call fails silently. On success, the projected property is writable, configurable, and enumerable.
Some errors as described above fail silently. Other issues, such as lack of IAgileObject or lack of the AllowForWeb attribute result in an error in the JavaScript developer console.
The other day I had to debug a JavaScript UWA that was failing when trying to use an undefined property. In a previous OS build this code would run and the property was defined. I wanted something similar to windbg/cdb's ba command that lets me set a breakpoint on read or writes to a memory location so I could see what was creating the object in the previous OS build and what that code was doing now in the current OS build. I couldn't find such a breakpoint mechanism in Visual Studio or F12 so I wrote a little script to approximate JavaScript data breakpoints.
The script creates a stub object with a getter and setter. It actually performs the get or set but also calls debugger; to break in the debugger. In order to handle my case of needing to break when window.object1.object2 was created or accessed, I further had it recursively set up such stub objects for the matching property names.
Its not perfect because it is an enumerable property and shows up in hasOwnProperty and likely other places. But for your average code that checks for the existence of a property via if (object.property) it works well.
If you want to represent a value larger than 32bits in an enum in MSVC++ you can use C++0x style syntax to tell the compiler exactly what kind of integral type to store the enum values. Unfortunately by default an enum is always 32bits, and additionally while you can specify constants larger than 32bits for the enum values, they are silently truncated to 32bits.
For instance the following doesn't compile because Lorem::a and Lorem::b have the same value of '1':
enum Lorem {
a = 0x1,
b = 0x100000001
} val;
switch (val) {
case Lorem::a:
break;
case Lorem::b:
break;
}
Unfortunately it is not an error to have b's constant truncated, and the previous without the switch statement does compile just fine:
enum Lorem {
a = 0x1,
b = 0x100000001
} val;
But you can explicitly specify that the enum should be represented by a 64bit value and get expected compiling behavior with the following:
enum Lorem : UINT64 {
a = 0x1,
b = 0x100000001
} val;
switch (val) {
case Lorem::a:
break;
case Lorem::b:
break;
}
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