TeXShop can be configured for an external editor in several ways. Some features below will be useful and others not depending on how your editor works. In the future, your editor may come with configuration instructions.
There is a menu item named "Open for Preview..." When this item is chosen, you will be asked to select a .tex source file. But only the associated pdf preview window opens. If the source has not yet been typeset or if the pdf is out of date, TeXShop will typeset the file as it is opened. You can open your source file in any editor. When it is time to typeset, save the changes in your editor, switch to the preview window, and typeset. In this mode, TeXShop never opens or modifies the source file. It simply passes this file to the Unix TeX or LaTeX process.
The preview window can be configured to contain a typesetting button. If such a button is not there, use "Customize Toolbar..." in the Windows menu to add one.
For users who prefer external editors most of the time, there is a preference item called "Configure for External Editor." When this preference is chosen, the "Open" and "Open Recent..." menus open tex source files in the above manner for editing with an external editor. Moreover, the "Open for Preview..." menu becomes "Open for Editing..." and opens the source file in TeXShop's internal editor for those rare occasions when the internal editor is desired. The "Open" and "Open Recent..." menus can also open jpg, tiff, ps, pdf, dvi, log, and other files; selecting the new preference does not change this behavior.
Some editors are able to call Unix typesetting commands directly. A new preference item, "Automatic Preview Update", has been added to improve your experience with these editors. When this item is active and a .tex file is opened using "Open for Preview", the pdf preview display automatically updates whenever the pdf file is rewritten. Thus you can typeset with an external editor and the preview window will automatically show changes. The preference also applies to .pdf files opened in TeXShop, and if TeXShop is configured to use an external editor, the preference applies to .tex files opened with "Open".
The following applescript commands have been added to TeXShop so editors can call TeXShop directly:
If the external editor modifies the pdf output directly, it can call "refreshpdf" to refresh the pdf display. This is only needed if "Automatic Preview Update" is off. Finally "open_for_externaleditor" opens a tex file by calling "Open for Preview".
The Tcl code to make these calls from AlphaX in included in the AlphaTcl folder inside the TeXShop_Folder of the TeXShop distribution. Read the first lines of this file to see how to configure AlphaX for this purpose.