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Chapter 11: Emacs Display 179 One use for image-flush is to tell Emacs about a change in an image file. If an image specification contains a :file property, the image is cached based on the file’s contents when the image is first displayed. Even if the file subsequently changes, Emacs continues displaying the old version of the image. Calling image-flush flushes the image from the cache, forcing Emacs to re-read the file the next time it needs to display that image. Another use for image-flush is for memory conservation. If your Lisp program creates a large number of temporary images over a period much shorter than image-cacheeviction-delay (see below), you can opt to flush unused images yourself, instead of waiting for Emacs to do it automatically. clear-image-cache &optional filter [Function] This function clears an image cache, removing all the images stored in it. If filter is omitted or nil, it clears the cache for the selected frame. If filter is a frame, it clears the cache for that frame. If filter is t, all image caches are cleared. Otherwise, filter is taken to be a file name, and all images associated with that file name are removed from all image caches. If an image in the image cache has not been displayed for a specified period of time, Emacs removes it from the cache and frees the associated memory. [Variable] This variable specifies the number of seconds an image can remain in the cache without being displayed. When an image is not displayed for this length of time, Emacs removes it from the image cache. Under some circumstances, if the number of images in the cache grows too large, the actual eviction delay may be shorter than this. If the value is nil, Emacs does not remove images from the cache except when you explicitly clear it. This mode can be useful for debugging. image-cache-eviction-delay 11.17 Buttons The Button package defines functions for inserting and manipulating buttons that can be activated with the mouse or via keyboard commands. These buttons are typically used for various kinds of hyperlinks. A button is essentially a set of text or overlay properties, attached to a stretch of text in a buffer. These properties are called button properties. One of these properties, the action property, specifies a function which is called when the user invokes the button using the keyboard or the mouse. The action function may examine the button and use its other properties as desired. In some ways, the Button package duplicates the functionality in the Widget package. See Section “Introduction” in The Emacs Widget Library. The advantage of the Button package is that it is faster, smaller, and simpler to program. From the point of view of the user, the interfaces produced by the two packages are very similar. 11.17.1 Button Properties Each button has an associated list of properties defining its appearance and behavior, and other arbitrary properties may be used for application specific purposes. The following properties have special meaning to the Button package:
