poi - Dave's Blog

Folks familiar with JavaScript UWP apps in Win10 have often been confused by what PWAs in Win10 actually are. TLDR: PWAs in Win10 are simply JavaScript UWP apps. The main difference between these JS UWP Apps and our non-PWA JS UWP apps are our target end developer audience, and how we get Win10 PWAs into the Microsoft Store. See this Win10 blog post on PWAs on Win10 for related info.

Web App

On the web a subset of web sites are web apps. These are web sites that have app like behavior - that is a user might call it an app like Outlook, Maps or Gmail. And they may also have a W3C app manifest.

A subset of web apps are progressive web apps. Progressive web apps are web apps that have a W3C app manifest and a service worker. Various OSes are beginning to support PWAs as first class apps on their platform. This is true for Win10 as well in which PWAs are run as a WWA.

Windows Web App

In Win10 a WWA (Windows Web App) is an unofficial term for a JavaScript UWP app. These are UWP apps so they have an AppxManifest.xml, they are packaged in an Appx package, they run in an App Container, they use WinRT APIs, and are installed via the Microsoft Store. Specific to WWAs though, is that the AppxManifest.xml specifies a StartPage attribute identifying some HTML content to be used as the app. When the app is activated the OS will create a WWAHost.exe process that hosts the HTML content using the EdgeHtml rendering engine.

Packaged vs Hosted Web App

Within that we have a notion of a packaged web app and an HWA (hosted web app). There's no real technical distinction for the end developer between these two. The only real difference is whether the StartPage identifies remote HTML content on the web (HWA), or packaged HTML content from the app's appx package (packaged web app). An end developer may create an app that is a mix of these as well, with HTML content in the package and HTML content from the web. These terms are more like ends on a continuum and identifying two different developer scenarios since the underlying technical aspect is pretty much identical.

Win10 PWA

Win10 PWAs are simply HWAs that specify a StartPage of a URI for a PWA on the web. These are still JavaScript UWP apps with all the same behavior and abilities as other UWP apps. We have two ways of getting PWAs into the Microsoft Store as Win10 PWAs. The first is PWA Builder which is a tool that helps PWA end developers create and submit to the Microsoft Store a Win10 PWA appx package. The second is a crawler that runs over the web looking for PWAs which we convert and submit to the Store using an automated PWA Builder-like tool to create a Win10 PWA from PWAs on the web (see Welcoming PWAs to Win10 for more info). In both cases the conversion involves examining the PWAs W3C app manifest and producing a corresponding AppxManifest.xml. Not all features supported by AppxManifest.xml are also available in the W3c app manifest. But the result of PWA Builder can be a working starting point for end developers who can then update the AppxManifest.xml as they like to support features like share targets or others not available in W3C app manifests.

Previously I described Application Content URI Rules (ACUR) parsing and ACUR ordering. This post describes what you get from putting a URI in ACUR.

URIs in the ACUR gain the following which is otherwise unavailable:

• Geoloc API usage
• Audio and video capture API usage
• Pointer lock API usage
• Web notifications API usage
• IndexedDB API usage
• Clipboard API usage
• window.external.notify access from within webview
• window.close the primary window
• Top level navigation in the primary window
• Cross origin XHR and fetch to ms-appx(-web) scheme URIs
• Cross origin dirtied canvas read access if dirtied by ms-appx(-web) scheme URIs
• Cross origin text track for video element for tracks from ms-appx(-web) scheme URIs

URIs in the ACUR that also have full WinRT access additionally gain the following:

• Cross origin XHR and fetch
• Cross origin dirtied canvas read access
• Cross origin text track for video element
• Local audio and video WinRT plugins work with media elements

JavaScript Microsoft Store apps have some details related to activation that are specific to JavaScript Store apps and that are poorly documented which I’ll describe here.

StartPage syntax

The StartPage attributes in the AppxManifest.xml (Package/Applications/Application/@StartPage, Package/Applications/Extensions/Extension/@StartPage) define the HTML page entry point for that kind of activation. That is, Application/@StartPage defines the entry point for tile activation, Extension[@Category="windows.protocol"]/@StartPage defines the entry point for URI handling activation, etc. There are two kinds of supported values in StartPage attributes: relative Windows file paths and absolute URIs. If the attribute doesn’t parse as an absolute URI then it is instead interpreted as relative Windows file path.

This implies a few things that I’ll declare explicitly here. Windows file paths, unlike URIs, don’t have a query or fragment, so if you are using a relative Windows file path for your StartPage attribute you cannot include anything like ‘?param=value’ at the end. Absolute URIs use percent-encoding for reserved characters like ‘%’ and ‘#’. If you have a ‘#’ in your HTML filename then you need to percent-encode that ‘#’ for a URI and not for a relative Windows file path.

If you specify a relative Windows file path, it is turned into an ms-appx URI by changing all backslashes to forward slashes, percent-encoding reserved characters, and combining the result with a base URI of ms-appx:///. Accordingly the relative Windows file paths are relative to the root of your package. If you are using a relative Windows file path as your StartPage and need to switch to using a URI so you can include a query or fragment, you can follow the same steps above.

StartPage validity

The validity of the StartPage is not determined before activation. If the StartPage is a relative Windows file path for a file that doesn’t exist, or an absolute URI that is not in the Application Content URI Rules, or something that doesn’t parse as a Windows file path or URI, or otherwise an absolute URI that fails to resolve (404, bad hostname, etc etc) then the JavaScript app will navigate to the app’s navigation error page (perhaps more on that in a future blog post). Just to call it out explicitly because I have personally accidentally done this: StartPage URIs are not automatically included in the Application Content URI Rules and if you forget to include your StartPage in your ACUR you will always fail to navigate to that StartPage.

StartPage navigation

When your app is activated for a particular activation kind, the StartPage value from the entry in your app’s manifest that corresponds to that activation kind is used as the navigation target. If the app is not already running, the app is activated, navigated to that StartPage value and then the Windows.UI.WebUI.WebUIApplication activated event is fired (more details on the order of various events in a moment). If, however, your app is already running and an activation occurs, we navigate or don’t navigate to the corresponding StartPage depending on the current page of the app. Take the app’s current top level document’s URI and if after removing the fragment it already matches the StartPage value then we won’t navigate and will jump straight to firing the WebUIApplication activated event.

Since navigating the top-level document means destroying the current JavaScript engine instance and losing all your state, this behavior might be a problem for you. If so, you can use the MSApp.pageHandlesAllApplicationActivations(true) API to always skip navigating to the StartPage and instead always jump straight to firing the WebUIApplication activated event. This does require of course that all of your pages all handle all activation kinds about which any part of your app cares.

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.

You can use conditional breakpoints and debugging commands in windbg and cdb that together can amount to effectively patching a binary at runtime. This can be useful if you have symbols but you can't easily rebuild the binary. Or if the patch is small and the binary requires a great deal of time to rebuild.

Skipping code

If you want to skip a chunk of code you can set a breakpoint at the start address of the code to skip and set the breakpoint's command to change the instruction pointer register to point to the address at the end of the code to skip and go. Voila you're skipping over that code now. For example:

bp 0x6dd6879b "r @eip=0x6dd687c3 ; g"

Changing parameters

You may want to modify parameters or variables and this is simple of course. In the following example a conditional breakpoint ANDs out a bit from dwFlags. Now when we run its as if no one is passing in that flag.

bp wiwi!RelativeCrack "?? dwFlags &= 0xFDFFFFFF;g"

Slightly more difficult is to modify string values. If the new string length is the same size or smaller than the previous, you may be able to modify the string value in place. But if the string is longer or the string memory isn't writable, you'll need a new chunk of memory into which to write your new string. You can use .dvalloc to allocate some memory and ezu to write a string into the newly allocated memory. In the following example I then overwrite the register containing the parameter I want to modify:

.dvalloc 100
ezu 000002a9`d4eb0000 "mfcore.dll"
r rcx = 000002a9`d4eb0000

Calling functions

You can also use .call to actually make new calls to methods or functions. Read more about that on the Old New Thing: Stupid debugger tricks: Calling functions and methods. Again, all of this can be used in a breakpoint command to effectively patch a binary.

I've made a Unicode Clock in JavaScript.

Unicode has code points for all 30 minute increments of clock faces. This is a simple project to display the one closest to the current time written in JavaScript.

Because the code points are all above 0xFFFF, I make use of some ES6 additions. I use the \u{XXXXXX} style escape sequence since the old style JavaScript escape sequence \uXXXX only supports code points up to 0xFFFF. I also use the method String.codePointAt rather than String.charCodeAt because the code points larger than 0xFFFF are represented in JavaScript strings using surrogate pairs and charCodeAt gives the surrogate value rather than codePointAt which gives the code point represented by the pair of surrogates.

"🕛".codePointAt(0)
128347
"🕛".charCodeAt(0)
55357

🕐🕑🕒🕓🕔🕕🕖🕗🕘🕙🕚🕛🕜🕝🕞🕟🕠🕡🕢🕣🕤🕥🕦🕧

The ordering of the code points does not make it simple to do this. I initially guessed the first code point in the range would be 12:00 followed by 12:30, 1:00 and so on. But actually 1:00 is first followed by all the on the hour times then all the half hour times.