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 > 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.
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.
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.
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.
