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within the app container. For example, trying to create a jQuery datepicker widget
($("myCalendar").datepicker()) will hurl out this kind of error. You can get around this on the
app level by wrapping the code above with MSApp.execUnsafeLocalFunction, but that doesn’t
solve injections coming from deeper inside the library. In the jQuery example given here, the
control can be created but clicking a date in that control generates another error.
In short, you’re free to use third-party libraries so long as you’re aware that they were
generally written with assumptions that don’t always apply within the app container. Over time, of
course, fully Windows 8–compatible versions of such libraries will emerge.
Here My Am! with ms-appdata
OK! Having endured seven pages of esoterica, let’s play with some real code and return to the Here My
Am! app we wrote in Chapter 2. Here My Am! used the convenient URL.createObjectURL method to
display a picture taken through the camera capture UI in an img element:
captureUI.captureFileAsync(Windows.Media.Capture.CameraCaptureUIMode.photo)
.done(function (capturedFile) {
if (capturedFile) {
that.src = URL.createObjectURL(capturedFile);
}
});
This is all well and good: we just take it on faith that the picture is stored somewhere so long as we
get a URI. Truth is, pictures (and video) from the camera capture API are just stored in a temp file; if you
set a breakpoint in the debugger and look at capturedFile, you’ll see that it has an ugly file path like
C:\Users\kraigb\AppData\Local\Packages\ ProgrammingWin8-JS-CH3-
HereMyAm3a_5xchamk3agtd6\TempState\picture001.png. Egads. Not the friendliest of locations, and
definitely not one that we’d want a typical consumer to ever see!
With an app like this, let’s copy that temp file to a more manageable location, to allow the user, for
example, to select from previously captured pictures (as we’ll do in Chapter 8, “State, Settings, Files, and
Documents”). We’ll make a copy in the app’s local appdata folder and use ms-appdata to set the img src
to that location. Let’s start with the call to captureUI.captureFileAsync as before:
//For use across chained promises
var capturedFile = null;
captureUI.captureFileAsync(Windows.Media.Capture.CameraCaptureUIMode.photo)
.then(function (capturedFileTemp) {
//Be sure to check validity of the item returned; could be null if the user canceled.
if (!capturedFileTemp) { throw ("no file captured"); }
Notice that instead of calling done to get the results of the promise, we’re using then instead. This is
because we need to chain a number of async operations together and then allows errors to propagate
through the chain, as we’ll see in the next section. In any case, once we get a result in capturedFileTemp
(which is in a gnarly-looking folder), we then open or create a “HereMyAm” folder within our local
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