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File Dialog
Volume Number:3
Issue Number:2
Column Tag:ToolBox Tricks

Not So Standard File Dialogs

By Paul Snively, Contributing Editor, Icom Simulations, Inc.

Last month I discussed how INIT resources could be debugged using TMON, as I did in developing my Set/Boot Paths desk accessory, used in TML Pascal 2.0. Set Paths allows the user to specify what path to use via what I prefer to call the Not-So-Standard File dialog. It's Not-So-Standard because it doesn't display file names at all, only folder names. That's not so weird (nor is it particularly difficult to code), but the "Open" button has become "Select" and, the tricky part, double-clicking on a folder name opens it exactly the way you'd expect, whereas highlighting a folder name and clicking on "Select" returns you to your program with pertinent information about the selection.

Experienced Standard File hackers may knot their brows at this (as I did when I was asked to do it that way) because they know that the Standard File internally coerces double-clicks on names to be a single click followed by a click on the "Open" button, so that the Standard File hook can filter double-clicks. The trick, then, is being able to use a user-written Standard File hook to distinguish between a double-clicked name and a selected name followed by a "Select" click.

Note that my solution is a kludge by just about any definition of the word, and would be universally spurned (even by me) were it not for one simple, overriding fact: it does NOT rely on internal knowledge of Standard File's workings AT ALL, and it was simple and obvious enough to have gone from concept to implementation in less than twelve hours (an important consideration for me; I modemed the resultant files to TML Systems Sunday evening; TML Pascal 2.0 started shipping the next day)!

To start off on an embarrassing note, in the process of writing Set Paths I discovered a bug in my McAssembly Pascal interface macros. Those were published in MacTutor Vol. 2, No. 3, in March 1986, which says two things to me: I need to be more careful in my testing of things before I publish them, and no one has done anything significant with those macros, otherwise they would have encountered the bug and, hopefully, written a letter to the editor about it! I'm profoundly disappointed

Anyway, the bug is in the "Exit" macro. There's a line which is responsible for adjusting the stack back to normal by dropping input parameters. It says:

add.l    #.fsize-.parms,sp

This is a definite boo-boo, because it totally neglects the space that we allocated for the stacked A6 register and the return address of the function. The line SHOULD read:

add.l   #.fsize-.parms-8,sp.

Boy, do I feel appropriately chastized!

Having cleared that up, I can talk safely about using those macros to implement the Not-So-Standard File dialog.

First we need to come up with a way to display NO files at all. The first time I tackled this, ol' Paul thought he'd be tricky and just set the numTypes parameter to _SFGetFile to zero, thereby forcing _SFGetFile to ignore all files, right? Wrong! Even though I had a typeList consisting of all zeros and a numTypes of zero, _SFGetFile insisted on displaying ALL files on the volume, and it took forever to do it, too!

I decided to do something about the INCREDIBLY slow response I was getting from the above scheme. Besides, it didn't work! I changed numTypes to one and used a fileType of '????' to get as few files as possible (theoretically zero). Since every so often you will indeed see a file of type '????' it behooved me to ensure that they didn't show up in the file list. To do that I had to use that fileFilter that I had tried to avoid.

The fileFilter proc is, like most such things for the Macintosh, designed with the expectation that it will be written conforming to Pascal-style parameter passing conventions. It is (in Pascal terms, anyway) a function, not a procedure, since it returns a value. It takes one parameter, a pointer to a low-level parameter block a lá the file system, and returns a boolean. It seems a little backwards: the boolean should be TRUE if the file is NOT to be displayed, and FALSE if the file IS to be displayed. So, since we want to display NO files, we should simply set the boolean to TRUE and we are done! In Pascal this would look something like this:

FUNCTION MyFileFilter(PB : ParamBlockRec;) : BOOLEAN;

BEGIN {MyFileFilter} 
 MyFileFilter := TRUE;
END; {MyFileFilter}

In McAssembly, with my Pascal macros, it's almost as easy: (See MacTutor Vol. 2, No. 3 March 1986 for the definition of these Macros.)

 MyFileFilter  EQU *
 WordResult MyFilterResult
 Long   MyParamBlock
 move.w #-1,MyFilterResult

The combination of the file type of '????' and the above fileFilter works like a charm; by golly, no files show up!


The remaining magic is in an underdiscussed piece of code called a SFHook. Apple describes the SFHook as something that can be used to make a non-standard get or put file (which I almost do) or to make the normal one behave in non-standard ways (which is EXACTLY what I do)!

The SFHook is also a Pascal-like entity; it takes two parameters; the item number (an integer), and the DialogPtr to the Standard File dialog. It returns an integer (an item number also, although it may or may not be the same as the one that was passed to it)!

The SFHook is called constantly throughout the operation of the Standard File dialog; it's called before the dialog is drawn, it's called when there are significant events, and it's even called when there are NO significant events!

The key to understanding the Standard File dialog as it applies to writing a SFHook is the item number. The item number is ordinarily simply the item number of whatever the user clicked on in the dialog box. In addition to that there have always been a few "phony" items numbers, such as item # 100 (which is what was passed when nothing interesting was going on) or keystrokes (item # 1000 plus the ASCII value of the key). Another useful value is -1, which is the value passed before the dialog is displayed. By trapping on this value we can do neat things like changing the title of the "Open" button to "Select." You'll see an example of that a bit later.

The introduction of HFS added a whole new wrinkle to the issue of the Standard File dialog. It had to be expanded in a way that retained the power and flexibility of the original design, yet also remain upwardly compatible. WDRefNum's and a few new phony item numbers fit the bill rather nicely.

As all users of the Standard File dialog are aware of at one level or another, "Opening" a folder doesn't return you to the application, it simply makes that folder the current directory and shows you the files in it, etc. You must open a FILE to get back from _SFGetFile.

Internally what happens is that opening a folder passes a phony item # 103 to the SFHook, as opposed to a file open, which passes item # 1 (the item # of the "Open" button). So we could catch the 103, coerce it to a 1, and pass it back. The problem there, of course, is the one mentioned before: double-clicks on filenames and filename select/"Open" click sequences are equivalent by the time you get to the SFHook! The result is that double-clicking on a foldername returns from _SFGetFile just as surely as selecting one and clicking "Open" does!


Ok, enough beating around the bush. Obviously the solution to the problem is to find out which item we clicked on, the file list or the "Select" button. I decided that the Mac was fast enough that I could determine within the SFHook where the mouse was, check to see if it was over the file list and, if it was not, coerce the 103 to a 1. Fortunately, one of the things passed to the SFHook is the DialogPtr, making calls to _GetDItem possible. Among other things, _GetDItem returns the viewRect of the item - just what the doctor ordered! _GetMouse gives us our mouse location in local coordinates (another stroke of luck) and _PtInRect answers the burning question: are we pointing to this item or aren't we?

Pulling it all together into a nice, not-so-neat package gives us the following:

UseSF  move.w  #100,-(sp) Top = 100
 move.w #100,-(sp) Left = 100
 pea  PromptString Use promptstring
 pea  myFileFilter Use brief fileFilter
 move.w #1,-(sp) No. of filetypes
 pea  MyListPoint to my list
 pea  myDlgHook  Point to our dlgHook
 pea  myReplyRec Point to reply rec
 _SFGetFile Use std file dialog
 sfbeginDeclare stack frame
 wordresult myDlgResult Function result
 word mySFItem Which item was hit?
 long theDialog  SFGetFile dialog ptr
 sfend  That's all!
 enter  Set up stack frame
 move.w mySFItem,d0Get the item#
 cmp.w  #-1,d0 Is this first time?
 bne  NotFirst Go if not
 sub.l  #14,sp Room for VAR params
 move.l theDialog,-(sp) Stack DialogPtr
 move.w #getOpen,-(sp)  Stack item # of "Open"
 pea  18(sp)VAR itemType
 pea  18(sp)VAR itemHandle
 pea  14(sp)VAR dispRect
 _GetDItemTell me about button
 movea.l8(sp),a0 Get handle
 add.l  #14,sp Drop data structure
 move.l a0,-(sp) Stack item handle
 pea  myTitle  Pass title address
 _SetCTitle Make title = STR
 moveq  #-1,d0 Make sure we're here
NotFirstequ *
 cmp.w  #103,d0  Was it "Select?"
 bne.s  NotSelectGo if not
;***  This is where we will do all kinds of nifty magic ***
 sub.l  #14,sp Room for VAR params
 move.l theDialog,-(sp) Stack DialogPtr
 move.w #7,-(sp) Stack File List item#
 pea  18(sp)VAR itemType
 pea  18(sp)VAR itemHandle
 pea  14(sp)VAR dispRect
 _GetDItemTell me about File List
 clr.l  -(sp)  Room for pt
 pea  (sp)Point to the point
 _GetMouseHere, mousie, mousie...
 pea  4(sp) Point to the rect
 _PtInRectAre we in File List?
 move.w #103,d0  Just to make sure
 move.b (sp)+,d1 Get answer
 add.l  #12,sp Drop remaining data
 bne.s  NotSelectIf in File List, leave
 move.w #1,d0  Treat like file open
NotSelect equ  *
 move.w d0,myDlgResult  Store function result
 exit   Exit the function
myFileFilterequ  *
 loc  New locals here; needed for McAssembly
 sfbeginStack Frame Begin
 wordresult myFilterResultA boolean
 long parmBlkPtr A pointer
 sfend  Stack Frame End
 enter  Set up stack frame
 move.w #-1,myFilterResultTRUE;
 exit   Clean up and leave
PromptStringequ  *
 text # "Highlight a folder and click /"Select/""
myTitle equ *
 text # "Select" title for "Open" button
myList  equ *
 text "????"Any old file type will do
 dcb.b  12,0Remaining are nulls

Now that I've given the code that does the trick, I should probably say a few words about how the replyRec looks when you've opened a folder instead of a file.

First of all, you can forget about the fName field. It won't be valid. Instead, vRefNum will contain the vRefNum of the folder, just like it does for a file, and - get this - fType will return the DirID of the folder ("Sorry about the type conflict," says Apple in the software supplement...obviously talking to Pascal programmers). Of course, the vRefNum and DirID are sufficient for all HFS OSTraps that deal with folders or files within a specific folder. Another thing that you can do if you want or need to is convert the vRefNum and DirID to a pathname extending from the root to the folder (or more precisely, from the folder to the root). One implementation of that algorithm appeared in MacTutor's special HFS issue (January '86). It was written in C by Mike Schuster. Translating it to the language of the reader's choice is an exercise left to the reader (the assembler version is actually rather simple).

That just about covers it. Feel free to use the Not-So-Standard File dialog anytime you have a reason to want to select a folder instead of a file!


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