38.9.1 Overlay Properties

Overlay properties are like text properties in that the properties that alter how a character is displayed can come from either source. But in most respects they are different. Text properties are considered a part of the text; overlays are specifically considered not to be part of the text. Thus, copying text between various buffers and strings preserves text properties, but does not try to preserve overlays. Changing a buffer's text properties marks the buffer as modified, while moving an overlay or changing its properties does not. Unlike text property changes, overlay changes are not recorded in the buffer's undo list. See section 32.19 Text Properties, for comparison.

These functions are used for reading and writing the properties of an overlay:

Function: overlay-get overlay prop

This function returns the value of property prop recorded in overlay, if any. If overlay does not record any value for that property, but it does have a category property which is a symbol, that symbol's prop property is used. Otherwise, the value is nil.

Function: overlay-put overlay prop value

This function sets the value of property prop recorded in overlay to value. It returns value.

See also the function get-char-property which checks both overlay properties and text properties for a given character. See section 32.19.1 Examining Text Properties.

Many overlay properties have special meanings; here is a table of them:

priority

This property's value (which should be a nonnegative number) determines the priority of the overlay. The priority matters when two or more overlays cover the same character and both specify a face for display; the one whose priority value is larger takes priority over the other, and its face attributes override the face attributes of the lower priority overlay.

Currently, all overlays take priority over text properties. Please avoid using negative priority values, as we have not yet decided just what they should mean.

window

If the window property is non-nil, then the overlay applies only on that window.

category

If an overlay has a category property, we call it the category of the overlay. It should be a symbol. The properties of the symbol serve as defaults for the properties of the overlay.

face

This property controls the way text is displayed--for example, which font and which colors. See section 38.11 Faces, for more information.

In the simplest case, the value is a face name. It can also be a list; then each element can be any of these possibilities:

• A face name (a symbol or string).

• Starting in Emacs 21, a property list of face attributes. This has the form (keyword value ...), where each keyword is a face attribute name and value is a meaningful value for that attribute. With this feature, you do not need to create a face each time you want to specify a particular attribute for certain text. See section 38.11.3 Face Attributes.

• A cons cell of the form (foreground-color . color-name) or (background-color . color-name). These elements specify just the foreground color or just the background color.

  (foreground-color . color-name) is equivalent to (:foreground color-name), and likewise for the background.

mouse-face

This property is used instead of face when the mouse is within the range of the overlay.

display

This property activates various features that change the way text is displayed. For example, it can make text appear taller or shorter, higher or lower, wider or narrower, or replaced with an image. See section 38.12 The display Property.

help-echo

If an overlay has a help-echo property, then when you move the mouse onto the text in the overlay, Emacs displays a help string in the echo area, or in the tooltip window. For details see Text help-echo. This feature is available starting in Emacs 21.

modification-hooks

This property's value is a list of functions to be called if any character within the overlay is changed or if text is inserted strictly within the overlay.

The hook functions are called both before and after each change. If the functions save the information they receive, and compare notes between calls, they can determine exactly what change has been made in the buffer text.

When called before a change, each function receives four arguments: the overlay, nil, and the beginning and end of the text range to be modified.

When called after a change, each function receives five arguments: the overlay, t, the beginning and end of the text range just modified, and the length of the pre-change text replaced by that range. (For an insertion, the pre-change length is zero; for a deletion, that length is the number of characters deleted, and the post-change beginning and end are equal.)

insert-in-front-hooks

This property's value is a list of functions to be called before and after inserting text right at the beginning of the overlay. The calling conventions are the same as for the modification-hooks functions.

insert-behind-hooks

This property's value is a list of functions to be called before and after inserting text right at the end of the overlay. The calling conventions are the same as for the modification-hooks functions.

invisible

The invisible property can make the text in the overlay invisible, which means that it does not appear on the screen. See section 38.5 Invisible Text, for details.

intangible

The intangible property on an overlay works just like the intangible text property. See section 32.19.4 Properties with Special Meanings, for details.

isearch-open-invisible

This property tells incremental search how to make an invisible overlay visible, permanently, if the final match overlaps it. See section 38.5 Invisible Text.

isearch-open-invisible-temporary

This property tells incremental search how to make an invisible overlay visible, temporarily, during the search. See section 38.5 Invisible Text.

before-string

This property's value is a string to add to the display at the beginning of the overlay. The string does not appear in the buffer in any sense--only on the screen.

after-string

This property's value is a string to add to the display at the end of the overlay. The string does not appear in the buffer in any sense--only on the screen.

evaporate

If this property is non-nil, the overlay is deleted automatically if it ever becomes empty (i.e., if it spans no characters).

local-map

If this property is non-nil, it specifies a keymap for a portion of the text. The property's value replaces the buffer's local map, when the character after point is within the overlay. See section 22.6 Active Keymaps.

keymap

The keymap property is similar to local-map but overrides the buffer's local map (and the map specified by the local-map property) rather than replacing it.