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Destroyed API & Defaults Discussion (markdown)

Tim Schaub
2017-09-25 23:30:08 -07:00
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API-&-Defaults-Discussion.md

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**Outdated!**
OpenLayers 3 should provide an API that makes it easy to accomplish common use cases. People who do not need to leverage the full functionality should be provided fast lanes for a leaner and easy to use configuration. Defaults should be set in the right places to avoid duplicated code.
This document is work in progress. At the time of writing, it reflects @ahocevar's thoughts on a few aspects of the library that come to mind when thinking about API and defaults.
## Map with WMS layer (tiled) in default projection (EPSG:3857)
new ol.Map({
layers: [
// suitable layer type can be chosen based on the source
new ol.source.TiledWMS({
url: '/geoserver/wms',
params: {
'LAYERS': 'usa:states' // would be nice without quotes for the key
}
})
],
// would be even nicer to provide center and zoom as direct map config options
view: {
center: [47, 15],
zoom: 9
}
});
## Map with custom projection and a WMS layer
new ol.Map({
layers: [
new ol.source.TiledWMS({
url: '/geoserver/wms',
params: {
'LAYERS': 'usa:states' // would be nice without quotes for the key
}
})
],
view: {
projection: {
code: 'EPSG:31256',
units: 'meters',
// validity extent from http://spatialreference.org/
extent: [-115317.3972, 151511.8020, 64307.1064, 432456.2246]
},
center: [47, 15],
zoom: 3
}
});
The layer source should not need a projection configuration - it should get it from the view. With the projection's extent, the layer source should be able to create a default tile grid. The map's zoom levels will be derived from the projection's extent as well.
## Map with custom projection and single image WMS layer
Exactly the same as above, only with `ol.source.SingleImageWMS` instead of `ol.Source.TiledWMS`. It should not be necessary to configure the layer source with a resolutions array, and a single image source should be able to serve any resolution that the map uses.
## Map with custom projection, WMS layer with custom tile grid
new ol.Map({
layers: [
new ol.source.TiledWMS({
url: '/geoserver/wms',
params: {
'LAYERS': 'usa:states' // would be nice without quotes for the key
},
tileGrid: {
maxExtent: [-100000, 160000, 60000, 430000],
resolutions: [1000, 500, 200, 100, 50, 20, 10, 5, 2, 1]
}
})
],
view: {
projection: {
code: 'EPSG:31256',
units: 'meters',
// validity extent from http://spatialreference.org/
extent: [-115317.3972, 151511.8020, 64307.1064, 432456.2246]
},
center: [47, 15],
zoom: 3
}
});
Since the TileGrid is a very complex thing, and most use cases except ArcGISCache and WMTS use the same grid extent for each resolution, it makes sense to provide a convenience configuration with a common maxExtent and a set of resolutions.
## Thoughts on defaults
Looking at the current code, there are still more than one places with Web Mercator related defaults. The central place for projection related defaults should be the `ol.projection` package. If layer sources can lazily create a TileGrid upon rendering, it will be possible to use map defaults for layer sources.
The concept of user projections works best in the context of a map. Classes like Coordinate or Extent do not necessarily need to know about the user projection if the map (or the view) provides extent and center related getters.