On the insanity of prompts

Some of the worst code I’ve had the misfortune of touching in the Mozilla codebase is code related to the handling of prompts. Specifically, the prompts involved with following interfaces: nsIPromptService, nsIAuthPrompt, nsIPromptFactory, nsIAuthPromptProvider, nsIPromptService2, nsIAuthPromptAdapterFactory.idl, etc. Just the plethora of confusing interfaces (for what is a fairly basic feature) is the first sign that something is amiss, and if you look at the actual methods on these interfaces you’ll quickly become lost in a maze of twisty little passages, all alike.

The implementation is spaghetti, too. As an example, let’s take a look at the sequence of functions called when JavaScript calls a simple little alert("Hello")

0) We begin our adventure in nsGlobalWindow::Alert().
1) Obtain an nsIPrompt via a do_GetInterface call on the docshell.
2) Ah, docshell doesn’t implement any of that, it just delegates to an nsIPrompt that it obtains from nsIWindowWatcher’s GetNewPrompter().
3) But GetNewPrompter() is actually just a one line function that calls NS_NewPrompter().
4) That’s over in nsPrompt.cpp, a short function that creates a new nsPrompt and initializes it.
5) Ok, now that we have an nsIPrompt (back from step 1), we can set up some strings and call its Alert() method.
6) …which doesn’t actually do anything, except to hand-off to nsIPromptService’s Alert().
7) This Alert() does some basic string twiddling, and next we end up in…
8) DoDialog(), which does more more twiddling, and FINALLY calls…
9) nsIWindowWatcher’s OpenWindow(), which opens a XUL window containing commonDialog.xul to do the prompt.

This is a little silly. And for outright insane, try tracing through the sequence of functions performed for showing an HTTP authentication prompt.

The good news is that I’ve been working on a large patch to simplify all this, and pave the way for some other interesting changes. More on this next time. :-)

About Justin Dolske

Mostly harmless.
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5 Responses to On the insanity of prompts

  1. David Baron says:

    Sounds like at least some of this was designed so that embedders (i.e., the AOL client) could override some of this (and have non-XUL prompts from Gecko). But it would seem better to have one cleaner point for that…

  2. Cheery says:

    I’d love the irony if you replied to https://bugzilla.mozilla.org/show_bug.cgi?id=160144#c131 with
    “i will do, but wait me”

  3. Mossop says:

    I am excited!

  4. Sounds vcool! IS there a bug number for this work?

    /me wants to follow ;-)

  5. jmdesp says:

    I hope you can consider including the ssl client identification prompts in the list also …

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