Monday 12 December 2016

Leaflet plugin

I have been trying to replace a long-standing map based on the OpenLayers 2 library to LeafletJS. The main sticking point was that OpenLayers has a useful permalink function that changes the URL to include a query string that identifies the map location and which layers should be shown. 

Leaflet has a neat plugin to show the location as a hash but that doesn't include the layers to show. After a bit of investigation I decided to try to write a Leaflet plugin to make all this easier.

If a map has optional base layers and some overlays it often will use the Layers control to make the choices easy to select. The layers are not easily accessible from the map object as only the currently displayed layers, not the hidden ones, are available. The Layers control has access to all the layers, but that control is not accessible from the central map object. The only way to have access to all of the layers is from the Layers control, so a new control, inherited from the Layers control seemed to be the right answer. HashLayers control was developed.

HashLayers does all that the Layers control does, of course, but adds the functionality of the URL hash. The hash looks like this:
This gets appended to the URL of the page that the map is on. Using the hash means the position and layers can be changed dynamically as the map is zoomed or scrolled around and as different layers are selected. Changing the hash value of a URL doesn't force a reload of the page and doesn't show up in browser history.

The zoom is the current zoom level and longitude and latitude are the centre of the map. The layers is a string representing the base layers and any overlays. If a base layer is selected 'B' is shown, 'O' shows an available base layer not selected. The overlays are shown as a string of 'T' or 'F' (true or false) showing if they are shown or not. A typical hash might be:
 This has the first base layer visible, the second one not showing and four overlays available, but with only the third one visible. These selections will be reflected in the visible selection control too.

If a map is created with the HashLayers control on it and a hash is passed in the URL, the map will centre on the location and zoom specified and show the requested layers. If no hash is passed then the default location and zoom will be used from the setView() or map options as normal. Layers will be shown as they were added to the map. A hash to reflect all this will be appended to the URL which gets updated with every change. There is no need for the OpenLayers permalink control.

I now need to test it more carefully and decide if it is worth publishing as a Leaflet plugin.

Saturday 3 December 2016

Replacing OpenLayers

When OS released their Open Data I created a simple map that showed a few of their offerings as overlays on an OSM base layer At the time my maps were driven by OpenLayers. OpenLayers v2 works well but is a lot harder to use than LeafletJS. As soon as Leaflet arrived I started to use that and it has only got better as time has passed. There is OpenLayers v3 available I think, but I doubt I'll use it.

OpenLayers has a permalink option that changes the URL in the browser to include a query string describing the location of the centre of the map and the zoom level. This lets you bookmark or send a link that will open the map in the right place. Leaflet has a plugin to create a hash on the URL that does a similar thing.  It works well but there is a bit of a problem.

The OpenLayers permalink adds a 'layers=' bit to the URL. An example would be BOTFF. This means the first base layer is shown and not the second one, then the first overlay is visible but not the second or third. This great, because not only does the URL make map show the right place, but the overlay you wanted to show will also appear too. Leaflet's hash plugin doesn't do this and that is a problem for me.

I decided to replace the map showing the OS overlays using Leaflet. It was quick to do and works well, but I can't use it as the replacement for the OpenLayers map because Leaflet doesn't have a built-in way to show the layers that were selected. People link to my map, according to the logs, with the query string to select the place and layers. If I just replaced it with the Leaflet-based map their links would all fail. They couldn't create a new bookmark that includes the layers to show either. I decided to fix this.

I looked into the documentation for the latest version of Leaflet (1.0.2 at the time of writing). The documentation is very useful. If there are overlays on a map a layers control can be added that allows the overlays to be selected. There can also be more than one base layer that the control allows you to select.

I thought about a generic way to create a similar effect to the OpenLayers permalink. For a one-off map I can code up the layers to do as I need, but a generic version would be useful. I can get at the layers that a map is currently showing, but Leaflet's way of hiding a layer is to remove it from the map. The definition of that layer is then not accessible from the map object. I couldn't see how to find the list of layers including the ones not displayed. The layers control has a list of the layers it uses, which would be ideal, but I couldn't find any way to find a a reference to that from the map object. There doesn't seem to be any way to find any controls from the map object. A map method that returns a list of controls might be useful.

So far I have created a bespoke version of the Leaflet map I wanted. It will accept an OpenLayers-style URL, display the map and create a Leaflet-style URL. I'm thinking about how to duplicate the OpenLayers feature more generically. I have ideas ...