Macintosh Disk Images

Or… Where Mac OS X really shines over Windows.

On Mac OS X, the preferred installer is none at all. Instead, applications are self-contained bundles that, under the covers, are folder/directory structures that the operating system treats as a single file. Although these entities are analogous to a subdirectory under ‘Program Files’ in Windows, they appear in Finder (the Mac equivalent to Windows Explorer) as a single icon. The bundles should be considered read-only, like the aforementioned Windows folder, and there are known writable areas for user and configuration data.

To “install” a program, a Mac user expects to simply drag the bundle/icon to a location. Done. That’s it. This process is so simple and straightforward that I expect it to be copied by Microsoft at any point. Of course, some applications (think that they) require additional steps and actually use an installer, but short of a major application that touches several areas of the system, like the Xcode Developer Tools, it should be unnecessary.

So, the question becomes: How does one actually distribute applications for Mac OS X? The answer: A publisher provides a downloadable disk image (.dmg) file which contains a complete folder in a single file, readable by any OS X system. When a disk image is opened, it is “mounted” as a new volume with the contents (and name) of the original folder, and from there, the user just drags the application to the desktop (or other folder) or can even just double-click it to run in-situ.

In order to create a disk image, start by creating a distribution folder with the desired name. In our case, with an early version of Pretty Good Solitaire Mac Edition, the folder was named “Pretty Good Solitaire 1.01“. Next, after building a final release version of the application bundle, copy (Alt-drag) it to this folder, along with any other separate files, such as documentation, you want to appear. In our case, for the trial version, we had only the application bundle, named “Pretty Good Solitaire trial[.app]“.

Once the folder is set up as desired, it is time to actually create the disk image file, which is done using the ‘Disk Utility’, which is found in the Utilities folder (Shift-Command-U). Launch this application, and then select ‘File->New->Disk Image from Folder’ from the menu (or Shift-Command-N). Navigate to the folder and press the ‘Image’ button. On the next window, type an appropriate name, make sure that ‘Image Format’ is “compressed” and ‘Encryption’ is “none”, and then press ‘Save’. (We used “pgsme101trial[.dmg]” as a name to eliminate spaces, which are inherently problematic for downloads.) The disk image is then created in the same folder that contains the distribution folder. Quit ‘Disk Utility’.

The disk image is ready to ship, but one can internet-enable the image (by setting a flag) to make it even easier for the user. Normally, when downloaded, a disk image is automatically mounted so the contents can be seen and copied. When internet-enabled, the contents of a disk image are copied to the downloads folder and the disk image itself is unmounted and removed, resulting in just the application in that folder, without the user having to do anything with the image file. An image can be internet-enabled from ‘Terminal’, by navigating to the disk image and typing, “hdiutil internet-enable [filename].dmg“.

This is the basic method for creating a distributable package for Mac OS X. Note that, inexplicably, the Apple Store actually requires ZIP files, so the usual practice is to place the disk image file into a ZIP file, but allowing direct downloads of the DMG file from the product site requires fewer user steps. We have used the above practice for Pretty Good Solitaire Mac Edition 1.0 with success; however, we are modifying it to add “curb appeal” for version 2.0, which will be described later this week.

OT: This weekend produced good NCAA basketball results:
MSU (men) 77, Robert Morris 62.
MSU (women) 60, Middle Tennessee State 59.
MSU (men) 74, USC 69.

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