In general new formats are added to OGR by implementing format specific drivers with instantiating a GDALDriver and subclasses of GDALDataset and OGRLayer. The GDALDriver instance is registered with the GDALDriverManager at runtime.
Before following this tutorial to implement an OGR driver, please review the OGR Architecture document carefully.
The tutorial will be based on implementing a simple ascii point format.
The format specific driver class is implemented as a instance of GDALDriver. One instance of the driver will normally be created, and registered with the GDALDriverManager. The instantiation of the driver is normally handled by a global C callable registration function, similar to the following placed in the same file as the driver class.
The SetDescription() sets the name of the driver. This name is specified on the commandline when creating datasources so it is generally good to keep it short and without any special characters or spaces.
SetMetadataItem( GDAL_DCAP_VECTOR, "YES" ) is specified to indicate that the driver will handle vector data.
SetMetadataItem( GDAL_DCAP_VIRTUALIO, "YES" ) is specified to indicate that the driver can deal with files opened with the VSI*L GDAL API. Otherwise this metadata item should not be defined.
The driver declaration generally looks something like this for a format with read or read and update access (the Open() method) and creation support (the Create() method).
The Open() method is called by GDALOpenEx(). It should quietly return NULL if the passed filename is not of the format supported by the driver. If it is the target format, then a new GDALDataset object for the dataset should be returned.
It is common for the Open() method to be delegated to an Open() method on the actual format's GDALDataset class.
The Identify() method is implemented as such :
Examples of the Create() method is left for the section on creation and update.
We will start implementing a minimal read-only datasource. No attempt is made to optimize operations, and default implementations of many methods inherited from GDALDataset are used.
The primary responsibility of the datasource is to manage the list of layers. In the case of the SPF format a datasource is a single file representing one layer so there is at most one layer. The "name" of a datasource should generally be the name passed to the Open() method.
The Open() method below is not overriding a base class method, but we have it to implement the open operation delegated by the driver class.
For this simple case we provide a stub TestCapability() that returns FALSE for all extended capabilities. The TestCapability() method is pure virtual, so it does need to be implemented.
The constructor is a simple initializer to a default state. The Open() will take care of actually attaching it to a file. The destructor is responsible for orderly cleanup of layers.
The Open() method is the most important one on the datasource, though in this particular instance it passes most of it's work off to the OGRSPFLayer constructor if it believes the file is of the desired format.
Note that Open() methods should try and determine that a file isn't of the identified format as efficiently as possible, since many drivers may be invoked with files of the wrong format before the correct driver is reached. In this particular Open() we just test the file extension but this is generally a poor way of identifying a file format. If available, checking "magic header values" or something similar is preferable.
In the case of the SPF format, update in place is not supported, so we always fail if bUpdate is FALSE.
A GetLayer() method also needs to be implemented. Since the layer list is created in the Open() this is just a lookup with some safety testing.
The OGRSPFLayer is implements layer semantics for an .spf file. It provides access to a set of feature objects in a consistent coordinate system with a particular set of attribute columns. Our class definition looks like this:
The layer constructor is responsible for initialization. The most important initialization is setting up the OGRFeatureDefn for the layer. This defines the list of fields and their types, the geometry type and the coordinate system for the layer. In the SPF format the set of fields is fixed - a single string field and we have no coordinate system info to set.
Pay particular attention to the reference counting of the OGRFeatureDefn. As OGRFeature's for this layer will also take a reference to this definition, it is important that we also establish a reference on behalf of the layer itself.
Note that the destructor uses Release() on the OGRFeatureDefn. This will destroy the feature definition if the reference count drops to zero, but if the application is still holding onto a feature from this layer, then that feature will hold a reference to the feature definition and it will not be destroyed here (which is good!).
The GetNextFeature() method is usually the work horse of OGRLayer implementations. It is responsible for reading the next feature according to the current spatial and attribute filters installed.
The while() loop is present to loop until we find a satisfactory feature. The first section of code is for parsing a single line of the SPF text file and establishing the x, y and name for the line.
The next section turns the x, y and name into a feature. Also note that we assign a linearly incremented feature id. In our case we started at zero for the first feature, though some drivers start at 1.
Next we check if the feature matches our current attribute or spatial filter if we have them. Methods on the OGRLayer base class support maintain filters in the OGRLayer member fields m_poFilterGeom (spatial filter) and m_poAttrQuery (attribute filter) so we can just use these values here if they are non-NULL. The following test is essentially "stock" and done the same in all formats. Some formats also do some spatial filtering ahead of time using a spatial index.
If the feature meets our criteria we return it. Otherwise we destroy it, and return to the top of the loop to fetch another to try.
While in the middle of reading a feature set from a layer, or at any other time the application can call ResetReading() which is intended to restart reading at the beginning of the feature set. We implement this by seeking back to the beginning of the file, and resetting our feature id counter.
In this implementation we do not provide a custom implementation for the GetFeature() method. This means an attempt to read a particular feature by it's feature id will result in many calls to GetNextFeature() until the desired feature is found. However, in a sequential text format like spf there is little else we could do anyway.
There! We have completed a simple read-only feature file format driver.