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38.9.2 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. See Text Properties, for comparison.

Text properties are considered a part of the text; overlays and their properties 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 property changes are not recorded in the buffer's undo list.

Since more than one overlay can specify a property value for the same character, Emacs lets you specify a priority value of each overlay. You should not make assumptions about which overlay will prevail when there is a conflict and they have the same priority.

These functions read and set 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.

— Function: overlay-properties overlay

This returns a copy of the property list of overlay.

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

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

priority
This property's value (which should be a non-negative integer number) determines the priority of the overlay. No priority, or nil, means zero.

The priority matters when two or more overlays cover the same character and both specify the same property; the one whose priority value is larger overrides the other. For the face property, the higher priority overlay's value does not completely override the other value; instead, its face attributes override the face attributes of the lower priority face property.

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 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:


mouse-face
This property is used instead of face when the mouse is within the range of the overlay. However, Emacs ignores all face attributes from this property that alter the text size (e.g., :height, :weight, and :slant). Those attributes are always the same as in the unhighlighted text.
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 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.
field
Consecutive characters with the same field property constitute a field. Some motion functions including forward-word and beginning-of-line stop moving at a field boundary. See Fields.
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.)

If these functions modify the buffer, they should bind inhibit-modification-hooks to t around doing so, to avoid confusing the internal mechanism that calls these hooks.

Text properties also support the modification-hooks property, but the details are somewhat different (see Special Properties).

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 Invisible Text, for details.
intangible
The intangible property on an overlay works just like the intangible text property. See Special Properties, 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 Invisible Text.
isearch-open-invisible-temporary
This property tells incremental search how to make an invisible overlay visible, temporarily, during the search. See 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.
line-prefix
This property specifies a display spec to prepend to each non-continuation line at display-time. See Truncation.
wrap-prefix
This property specifies a display spec to prepend to each continuation line at display-time. See Truncation.
evaporate
If this property is non-nil, the overlay is deleted automatically if it becomes empty (i.e., if its length becomes zero). If you give an empty overlay a non-nil evaporate property, that deletes it immediately.
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 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.

The local-map and keymap properties do not affect a string displayed by the before-string, after-string, or display properties. This is only relevant for mouse clicks and other mouse events that fall on the string, since point is never on the string. To bind special mouse events for the string, assign it a local-map or keymap text property. See Special Properties.