View source for Layer order and arranging layers

Layer stack is an ordered set of layers. Each layer contains topics and associations. If two layers contain a topic with a same subject identifier, these topics merge. Wandora views a merged topic that inherits it's properties from both topics. Inherited properties include topic names, occurrences and associations. Wandora's topic map model is limited and a topic may contain only one base and one subject locator in Wandora. Now, what happens if two layers contain a topic with a same subject identifier, and both topics have different base name. Which base name is viewed in the merged topic. The answer relates to the order of layers in the layer stack. Wandora solves topic properties from top to down and a base name locating top layer hides a base name locating lower layer. Same applies to subject locators, occurrences and variant names. If both merged topics carry an occurrence with same scope, the merged topic views only an occurrence from the higher layer. The occurrence in lower layer is hidden.

Saying this, it is clear that the user may want to switch layer order. To arrange topic map layer order in the layer stack Wandora user can
* Drag and drop layers.
* Use submenus in '''Layers > [[ArrangeLayers|Arrange layers]]'''.

== Arranging layers example ==

Lets say we have added two layers in Wandora. These layer are labeled as '''layer1''' and '''layer2'''. Both layers contain a topic with subject identifier of http://www.wandora.net/Finland representing a concept of country Finland. As both topics contain same subject identifier, topics merge in Wandora. Now lets say that both topics have different base name. Topic in '''layer1''' has a base name '''Suomi''' while the topic in layer '''layer2''' has a base name '''Finland'''. Which base name Wandora views for the merged topic? When '''layer1''' is above '''layer2''' the merged topic has a base name '''Suomi''' and when the '''layer1''' is below the '''layer2''' the merged topic has a base name '''Finland'''.

Below is an image of Wandora where '''layer1''' overlays '''layer2'''. Wandora views topic '''Suomi'''. Notice that topic's layer distribution is '''[ 1:1:0 ]'''. Topic's layer distribution is shown in the footer and indicates the topic has a merged companion in '''layer1''' and '''layer2''' but not in the layer Base.


[[Image:arrange_layers_example1.gif|center]]


Ok, now the user switches the layer order and drags the '''layer2''' to the top. Topic's base name changes automatically to '''Finland''' as the topic in '''layer2''' has different base name than the topic in '''layer1'''. 


[[Image:arrange_layers_example2.gif|center]]


Next Wandora user deletes the base name in '''layer2'''. The base name from '''layer1''' pops up in red. Red color indicates the base name doesn't originate current layer and you can't change it directly. Yes, you can override the base name again but the ''lower'' base name stays unchanged and pop ups again if you delete the overlaying name. To change the ''lower'' base name you need to select the layer where base name locates. Topic's layer distribution '''[ 1:1:0 ]''' hints which layer to choose.


[[Image:arrange_layers_example3.gif|center]]


It may be easier to think the layer concept of Wandora similar to the layer concept of modern graphic design applications. Graphic design application views an element drawn to the top layer on top of the elements drawn into the lower layers. If the layer order is changed, the order of graphic elements changes too.

== See also ==

* [[Introduction to Layered Topic Maps]]
* [[Layered topic map]]
* [[Creating new layers]]
* [[Topic's layer distribution]]