Hi Mike - yeah, reading through I wasn't very clear. Let me try again
with a more fleshed out example:
On the server there is a hierarchy, each node in that hierarchy may
contain some meta data for example:
:id - the unique id of the node
:type - indicating some semantics about that particular node
:current? - indicating whether it is still actively used or not
Imagine there are 5 nodes and node 4, which is a leaf is not :current?.
On the client I have a number of projections of this data:
- an "active" tree which prunes out all nodes that aren't :current?,
so there are 4 nodes
- an "everything" tree which shows everything, e.g. all 5 nodes
In addition, every node in each tree is selected and there are
multiple instances of these trees.
On the client (on a reporting page for example, where one of these
trees are in the filter) I need to know which nodes the user has been
selected. I have a handler, which in response to some event (either
the server indicating new data is available or the user changing some
data) which needs to know which nodes have been selected.
If the user hasn't selected anything then the 'selected nodes' should
be the default set (i.e. all of the nodes). As soon as the user
changes the selection (by deselecting a node in the first instance)
the set of selected nodes is now every node that is selected (e.g.
every node apart from the one they just selected) and that instance of
the tree is no longer tracking the defaults.
The server is free to send new config data at any point in time. When
this happens, the default set should be updated. The non-default set
that the user has changed should also be consolidated as well, but
that is different.
Lets say my app state looks like:
{:page-1 {:some-active-tree {:selected-ids [] :tracking-default? true}
:another-active-tree {:selected-ids [] :tracking-default? true}
:some-all-tree {:selected-ids [] :tracking-default? true}}
The user hasn't done anything so the selected-ids should be the
default sets (4 ids for :some-active-tree and :another-active-tree and
5 ids for :some-all-tree). If the user were now to deselect node 2 in
:another-active-tree then app-state looks like:
{:page-1 {:some-active-tree {:selected-ids [] :tracking-default? true}
:another-active-tree {:selected-ids [0 1 3]
:tracking-default? false}
:some-other-tree {:selected-ids [] :tracking-default? true}}
If they deselect node 3 in some-other-tree:
{:page-1 {:some-active-tree {:selected-ids [] :tracking-default? true}
:another-active-tree {:selected-ids [0 1 3]
:tracking-default? false}
:some-other-tree {:selected-ids [0 1 2 4]
:tracking-default? false}}
Should the server now update the config hierarchy changing node 4 back
to :current? and adding another node then at the very least the
'selected nodes' for :some-active-tree should contain the ids of all 6
nodes. :another-active-tree and :some-other-tree should also be
informed but they might not be updated depending upon the selections
(it gets more complicated...).
At this point it is clear that one solution is to record a delta from
the defaults, but that only works because we are talking about
booleans; there are other non-boolean use-cases unfortunately.
Another solution is to store the sets of defaults in app-state itself
rather than have it be a subscription and then overtime it changes
update the affected parts of app-state (:some-tree and
:some-other-tree in this example).
This would be much easier if it was just dealing with rendering data,
in which case subscriptions are a perfect fit, but the set of data
needs to be sent back to the server periodically in a handler, and
handlers can't see subscriptions.
To be frank, if anyone is still reading, my experience tells me that
if the problem is this hard to explain and requires this much
explanation then _I_ haven't understood it properly :), so I think I
need some more hammock time is in order.
Thanks for anybody who hasn't lost the will to live yet ... :).
On 21 April 2015 at 13:08, Mike Thompson <[email protected]> wrote:
> On Tuesday, April 21, 2015 at 4:52:05 AM UTC+10, Colin Yates wrote:
>> Hi,
>>
>> This is somewhat reframe specific, but how do people handle default-values
>> that can change? My specific use-case is that I have a tree which can be
>> expanded and collapsed. By default the tree should be expanded to a certain
>> level, however, as soon as the user manually expands or collapses a node
>> they should no longer follow the default. The data the tree is displaying
>> can change, meaning the defaults can change over time.
>>
>> I can't simply define the defaults on startup because the defaults will
>> change over time.
>>
>> It can't be a subscription because the values need to be available in the
>> handler.
>>
>> My current thinking is that the app-db state has a 'follow-defaults?' which
>> is true by default but is set to false when the user explicitly changes the
>> state (e.g. by expanding or collapsing). When the underlying hierarchy
>> changes from the server, propagate that change to all of the parts of the
>> app-state that are interested.
>>
>> To be explicit, imagine I have the following template for tree:
>> {:expanded-ids [] :follow-defaults? true}. There are 6 instances of this
>> template in the app-db (i.e. 6 distinct UI trees). When the server informs
>> the client that the source-data has changed it then updates each instance
>> where follow-defaults?.
>>
>> I understand the rationale as to why subscriptions can't be in the handlers
>> but a subscription which switches on follow-defaults? seems ideal.
>>
>> Maybe I could hack a UI-less component which reacts to that subscription
>> change by directly updating the underlying db...
>>
>> What would you all do?
>
>
> Hi Colin,
>
> I'm not sure I'm clear on the problem. Here's my attempt to explain back ...
>
> You have some setting (data) in your app (tree display state) which can be:
> 1. in a server-supplied state (you call this "default" state?)
> 2. optionally, in user-supplied state (if present, overrides 1).
>
> Over time, new versions of 1. arrive. If 2. exists, it always overrides 1.
>
> Have I understood?
>
> --
> Mike
>
>
>
>
>
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