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Small improvement to Here My Am (Chapters 2 through 7)

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Thanks to M. Dirksma on the Building Windows Store Apps with HTML5/JS Forum for this one. It applies to the first edition of Programming Windows 8 Apps in HTML, CSS, and JavaScript.

When running versions of Here My Am! that use an iframe element to host the Bing Maps web control (Chapters 2-7; in Chapter 8 this is replaced with the Bing Maps SDK control), there is a possibility that the iframe isn’t instantiated before the app attempts to set the geolocation. This can throw an exception.

The solution to this–a good practice for any situation where you might depend on iframe content being loaded, is to place that dependent code inside a handler for the iframe’s load event. M. Dirksma’s solution is as follows:

document.getElementById("map").addEventListener("load", function () {
    var gl = new Windows.Devices.Geolocation.Geolocator();
    gl.getGeopositionAsync().done(function (position) {
        lastPosition = { latitude: position.coordinate.latitude,
            longitude: position.coordinate.longitude };
        callFrameScript(document.frames["map"], "pinLocation",
            [position.coordinate.latitude, position.coordinate.longitude]);
    }, function (error) { console.log("Unable to get location."); })
    ;
}, false);

This should prevent the error and avoid any odd timing issues.

A few bad links (and a link checker for Word)

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A couple of links in my first edition book (out of 1400+) managed to get past my process for checking and validating them. Pages numbers are in the PDF:

For checking links, I found a very helpful add-in for Microsoft Word (2010…now waiting for one that works with Office 2013!) from ablebits.com. See http://www.ablebits.com/word-links-manager/index.php for details. The product as a 15-day free trial, and a license costs $9.95. This was well worth it for how much time it saved me in the last weeks of production!

The purpose of this..bind(this)

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In various pieces of sample code, within the SDK samples as well as those in my book, you’ll often see the following syntax that appears somewhat cryptic (as does much of JavaScript!) to the uninitiated:

this.<some_function>.bind(this));

For example, when wiring up app bar buttons in the book’s Here My Am! examples, you’ll see this code:

var appbar = document.getElementById(“appbar”).winControl;
appbar.getCommandById(“cmdRefreshLocation”).addEventListener(“click”, this.tryRefresh.bind(this));
appbar.getCommandById(“cmdPickFile”).addEventListener(“click”, this.pickFile.bind(this));
appbar.getCommandById(“cmdRecentPictures”).addEventListener(“click”, this.recentPictures.bind(this));

The use of .bind also appears in other contexts, again from Here My Am!:

var image = document.getElementById(“photo”);
image.addEventListener(“click”, capturePhoto.bind(image));

So what’s happening here? Let’s take a step back and talk about the this keyword more generally.

this is an intrinsic variable that typically refers to the current object context on which a method is called. For example, if I have an object named obj1 and I call its obj1.doSomething method, then this inside doSomething will refer to obj1.

If I just have a global function, on the other hand, this in that context will typically be the same as window (the global context).

Event handlers present a particular difficultly with getting the this context set correctly. If you’ve defined an event handler as a member of some object, say obj1.eventHandler, and pass that function to something like addEventListener or assign it to someOtherObject.onEvent, the element/object raising that event doesn’t have your obj1 object through which to call the handler. That is, when JavaScript resolves an identifier like obj1.eventHandler, the result is just a standalone function object which has no context of where it came from.

As a result, if you do something like document.getElementById(“myButton”, obj1.eventHandler) then the function will be called, all right, but the this object within that handler will contain the global context (window) and not obj1.

The bind method solves this problem. By calling it you say, “here’s the object context through which I want this method called,” which is to say, “make this refer to a particular object when you call the method.” So if you document.getElementById(“myButton”, obj1.eventHandler.bind(obj1)) then the this object within eventHandler will, in fact, be set to obj1.

When you see the this.<event>.bind(this) syntax then, you’re just seeing an object wire one of its own methods up as an event handler and making sure that this will remain the same in the event handler.

So in the Here My Am! examples above, the event handler for the app bar buttons exist as part of the page control where this code appears. The this variable thus points to that page control object, so when the events are triggered, the handlers like tryRefresh, defined in that same page control, will have their this values set to the page control as we want.

In the second example, capturePhoto.bind(image), what we’re doing instead is making sure that the specific image element that was tapped shows up as the this object within the capturePhoto function. There are other ways to do this (getting the originating element for the event, for example), but using .bind here works nicely.

Simultaneously debugging script and managed/native code in Windows 8

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As described in Chapter 16 of Programming Windows 8 Apps in HTML, CSS, and JavaScript, Visual Studio can only debug one type of code at a time. Here’s the relevant excerpt:

When you hit that [JavaScript] breakpoint, step through the code using Visual Studio’s Step Into feature (F11 or Debug > Step Into). Notice that you do not step into the C# code, an effect of the fact that Visual Studio can debug either script or managed (C#)/native (C++) code in one debugging session but unfortunately not both. Clearly, using some console output will be your friend with such a limitation.

To set the debug mode, right-click your app project in solution explorer, select Debugging on the left, and then choose a Debugger Type on the right as shown in Figure 16-3. For debugging C#, select Managed Only or Mixed (Managed and Native). Then set breakpoints in your component code and restart the app (you have to restart for this change to take effect). When you trigger calls to the component from JavaScript, you’ll hit those component breakpoints.

Fortunately, there is a way to debug both types of code at the same time using two instances of Visual Studio:

  1. Launch the app (F5) with Script Only debugging selected. This is the instance of Visual Studio in which you’ll be stepping through JavaScript.
  2. Load the project in a second instance of Visual Studio. Ignore warnings about IntelliSense files being already open, but prepare to wait a little while as VS will build a new copy of that database for this second instance.
  3. In this second instance, set the debugging mode to Native Only, Managed Only, or Mixed, depending on your needs. This is where you’ll be debugging C#/VB/C++ code, as in a WinRT component.
  4. Select the Debug -> Attach to Process… menu command. (You do this instead of running the app a second time.)
  5. Find and select the line for WWAHost.exe in the process list that has your app name in the title column.
  6. Above the list, check the Attach to value. If it says “Automatic: Script code”, press the Select… button and indicate Managed and Native instead. Close that dialog.
  7. Click the Attach button.

Then go set breakpoints in this second instance of VS where you want to step through managed/native code. While this still won’t allow you to step across the language boundaries in a single debugger, it will allow you to step through each code type in the different instances.

Thanks for Rob Paveza for this tip.