A SqueakPage is holder for a page of morphs that live on the disk or on a server.
A URLMorph is a thumbnail sized stand-in for the page. Clicking on it gets the page.
An ObjectOut is a fake object that stands for an object that is out on the disk. (Like ObjectTracer or ObjectViewer.)
A MorphObjectOut is a subclass that stands for a Morph that is out on the disk.

To find out how to make the pages of any BookMorph go out to the disk (or a server), see http://minnow.cc.gatech.edu/SqueakDoc.1 then go to 'SqueakPages'.

A SqueakPage is always in-memory. Its contentsMorph will be 'become-ed' to a MorphObjectOut tombstone when it goes out. (A page may or may not be in the cache. First put it in, then ask it for the data.) Sending any message to the contentsMorph triggers the fetch. Many URLMorphs may hold onto one page. A page has a thumbnail. A URLMorph points at its page object.

States of a SqueakPage, and the transitions to another state:
1) have a url as a string. Then: (URLMorph grabURL: 'file://Ted''s/books/tryThis/p1').
Drop it into any morph.
2) have a URLMorph, with page==nil. Click it. (makes an empty page, installs
it in the global page cache)
3) have a URLMorph with a SqueakPage, with contentsMorph==nil,
but page is not in the cache (this is a rare case). ask page contentsMorph.
4) OUT: have a URLMorph with a SqueakPage, with contentsMorph being a MorphObjectOut,
and its page is in the cache. Sending the contentsMorph any message brings it in and
becomes it to the morph. (fix up morph's pointer to the page.)
5) Totally IN: a morph, owned by a SqueakPage, has a page in the cache.
The morph is clean.
Whenever someone triggers a purge (when?), contentsMorph is becomed
to a MorphObjectOut. (go to 4)
Causing the morph to execute layoutChanged marks the morph as dirty.
(morph's property #pageDirty is set to true) (go to 6)
6) Totally IN and dirty.
Whenever any other page is fetched from the disk or the net, all other
dirty pages are written and marked clean. (go to 5)

Note that the entire tree of submorphs goes out -- hundreds of objects. Bringing the object back in brings in the SqueakPage, installs it in the cache. Classes other than PasteUpMorph can easily be made to send their contents out if there is any need.

Note that every book is now automatically a WebBook. We simply give a page a url and tell it to purge.

url a string
title
comment
thumbnail
contentsMorph (1) a pasteUpMorph with other morphs in it.
(2) a MorphObjectOut. Sending any message brings it in.
(3) nil if the page has never been in this image.
creationTime
creationAuthor
lastChangeTime
lastChangeAuthor
policy #alwaysWrite, #neverWrite, #ask. (cache can override with a global policy)
(Explicit writing by user has policy #neverWrite)
dirty (Morph>>layoutChanged sends changed: #SqueakPage. If policy==#check,
then the page sets dirty_true.)
(If policy==#alwaysWrite, then set dirty when the page is retrieved from the cache.)

Class MorphObjectOut has an instance variable called page.
All messages to an MorphObjectOut cause it to be brought in. Except the messages needed to write the MorphObjectOut on the disk as part of a parent's being sent out. (size, class, instSize, instVar:at:. Can rename these and call from its own version of the writing routine.)
To purge, go through the clean pages, and any that have world not equal to this world, entomb them.
(If an object in the subtree is held by an object outside the tree, it will remain, And will be duplicated when the tree comes back in. This is a problem already in normal uses of SmartRefStream.)

	private
	computeThumbnail postChangeNotification updateThumbnail

	saving
	dirty: policy policy: prePurge purge urlNoOverwrite: write

	accessing	top
	asMorph
	comment
	comment:
	contentsMorph Return what it is now. If the morph is out on the disk, return nil. Use fetchContents to get the data for sure.
	contentsMorph:
	copyForSaving Make a copy and configure me to be put out on the disk. When it is brought in and touched, it will turn into the object at the url.
	fetchContents Make every effort to get contentsMorph.
	fetchContentsIfAbsent: Make every effort to get contentsMorph. Assume I am in the cache already.
	fetchInformIfError Make every effort to get contentsMorph. Put up a good notice if can't get it. Assume page is in the cache already. Overwrite the contentsMorph no matter what.
	isContentsInMemory Is my contentsMorph in memory, or is it an ObjectOut tombstone? Be careful not to send it any message.
	lastChangeTime
	saveMorph:author: Save the given morph as this page's contents. Update its thumbnail and inform references to this URL that the page has changed.
	thumbnail
	title
	title:
	url
	url:

	private	top
	computeThumbnail Make a thumbnail from my morph.
	postChangeNotification Inform all thumbnails and books that this page has been updated.
	updateThumbnail Update my thumbnail from my morph.

	saving	top
	dirty:
	policy
	policy:
	prePurge Return self if ready to be purged, or nil if not
	purge Replace my morph with a tombstone, if I am not in a world that is being shown.
	urlNoOverwrite: Look in the directory. If there is a file of this name, create a new name. Keep track of highest numbers used as a hint.
	write Decide whether to write this page on the disk.

	as yet unclassified	top
	initialize SqueakPage initialize
	newURLAndPageFor: Create a new SqueakPage whose contents is the given morph. Assign a URL for that page, record it in the page cache, and answer its URL.
	stemUrl: Peel off the 'x5.sp' or '.bo' from the end of a url of a SqueakPage or a BookMorph index file