evaluator:applicable_contexts:functions:evaluation

Evaluation Clauses in Functions

The @eval and @evalalways command modifiers let you run evaluation expressions inline in your functions.

The main purpose of this is to set variables that can then be used later on in the function by other modifiers like @if, @icon, @label etc.

Variables set via @eval can only be seen by other modifiers. They're executed when Opus performs dynamic updates of the toolbar (e.g. to update the label or icon), but not when the function is actually run.
Variables set by @evalalways can also been seen by the main function itself, when it is run. That means they can also be accessed via evaluation insertion code or by generated command lines.

For example, you might want a button that behaves differently in a normal filesystem folder to other locations. You can use an @eval clause to calculate the folder type once, and then refer to that in later lines.

@eval:=type = pathtype(source); fIsFileSystem = (type == "shell" || type == "filesys")
@enableif:=fIsFileSystem
@label:=fIsFileSystem ? ("Open " + filepart(displayname(source)) + " in Explorer") : ("Disabled (" + type + ")")
explorer.exe "{sourcepath}"

A number of variables are available to the evaluator in this context. Note that some are only applicable to toolbars in the standalone image viewer, and some only apply to Lister toolbars.

Variable	Type	Description
dest	path	The current destination path.
dest_shell	bool	True if the destination is a shell (virtual) folder hosted by Explorer, otherwise false.
expanded	uint	Returns the number of expanded directories.
fullscreen	bool	Viewer only. True if the viewer is fullscreen, otherwise false.
key_repeat	bool	True if the function was launched from a repeated key press (i.e. the key was held down).
lister_title	string	Returns the current title string of the Lister.
preparse_dummy	bool	True if the evaluator is being called with a dummy filename as part of pre-parsing. Opus will do this for evaluator expressions inside a @perfile block. You should generally respond normally (Opus is querying to find out which values the evaluator expression requires), but you may want to filter out some behaviour (e.g. logging) for pre-parsing.
selbytes	uint64	Returns the number of selected bytes (files + directories that have had their sizes calculated).
seldirs	uint	Returns the number of selected directories.
selfiles	uint	Returns the number of selected files.
selimage	bool	Viewer only. True if an area of the image has been selected, false otherwise.
selitems	uint	Returns the total number of selected items.
selpath	string	Provides the full pathname of the currently selected file. In a Lister this means the selected file with input focus (e.g. the file you right-click on). In the viewer this means the currently viewed image. Note that main toolbars in the Lister are not necessarily refreshed when the file selection changes, and so a button on the main toolbar using this may not always show the correct information.
source	path	The current source path.
source_shell	bool	True if the source is a shell (virtual) folder hosted by Explorer, otherwise false.
totalbytes	uint64	Returns the total number of bytes (files + directories that have had their sizes calculated).
totaldirs	uint	Returns the total number of directories.
totalfiles	uint	Returns the total number of files.
totalitems	uint	Returns the total number of items.
viewmode	str	Returns the current view mode in the file display. Values are largeicons, smallicons, list, details, power, thumbnails and tiles.

As well as the above variables, you can also use evaluator functions like IsChecked and IsEnabled to query other information about the state of the Lister.

The return value from the evaluation expression will be used to determine the result of the condition. Return true if the lines following the modifier should be executed, or false to skip to the next @if / @ifset modifier.