[QGIS-Developer] Vertex tool and cache invalidation

Martin Dobias wonder.sk at gmail.com
Mon Jan 7 01:22:21 PST 2019

Hi Hugo

Are the issues you mentioned solved now by your PR
https://github.com/qgis/QGIS/pull/8724 or that was some other separate

For your open question with "click first" behavior of vertex tool as
an option - I don't know. There has been some interest to address some
ergonomic issues in the new implementation, but I am not sure yet what
we would end up doing...


On Tue, Dec 18, 2018 at 9:30 AM Hugo Mercier <hugo.mercier at oslandia.com> wrote:
> Hi,
> I am facing some issues with the vertex tool of 3.x.
> In 2.18 the "node tool" was working by clicking first on a geometry to
> select it or its vertices and then displace them. With the new "vertex
> tool", there is no more first click involved and geometries are
> "selected" when the mouse position is close enough.
> This behaviour and ergonomic change (which I like !) implied an
> implementation that needs, for descent performances, to build a cache of
> geometries and maintain it.
> Problems arise when a modification on a layer may trigger modifications
> on others. For example when two layers have triggers in database (to
> maintain a correct network topology in database for instance), or even
> when the same layer is opened twice.
> QGIS has no way to know that the modification of a layer can modify
> another layer. So it easily leads to problems where "phantom" geometries
> are still selectable but actual (modified by side effect) geometries are
> not.
> I introduced in a previous version a way to express "data dependencies"
> between layers, with this issue in mind. For PostgreSQL databases, there
> is probably also a way to circumvent this by using the LISTEN/NOTIFY
> mechanism and some lines of Python.
> However I find these solutions very obscure and unfriendly.
> Open question: what about re-introducing the old "click first" behaviour
> of the vertex tool (and probably other tools) in a next version ? Not as
> a replacement, but as a possible alternative.
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