Opening and Closing a Layer
===========================
This code shows a simple way to load a layer, get information about it, and then close it using Python scripting for
Global Mapper. The variable "file_path" is a string providing a path to a file which should contain data for one or more
layers (eg. a .shp or .gmw file). The three values returned by :doc:`LoadLayerList <../load/globalmapper.LoadLayerList>`
are the error code of the operation (which isn't used here but can be helpful to verify a successful function call), a
pointer to the start of an array of the layers, and the size of that array. The layers will be represented as Python
ints (pointers), which will be interpreted as a GM_LayerHandle_t32 by Global Mapper functions. For example,
:doc:`GetLayerInfo <../load/globalmapper.GetLayerInfo>` and :doc:`CloseLayer <../load/globalmapper.CloseLayer>` both use
these ints as inputs to work with the loaded layers.

::

    import globalmapper as gm

    err, array_ptr, array_size = gm.LoadLayerList(file_path, gm.GM_LoadFlags_UseDefaultLoadOpts)
    loaded_array = gm.GM_LayerHandle_array_frompointer(array_ptr)
    print("Layers opened: %d" % array_size)

    for i in range(array_size):
        info = gm.GetLayerInfo(loaded_array[i])
        print("Layer %d, 0x%x:" % (i, loaded_array[i]))
        print("    Description: %s" % info.mDescription)
        print("    Type: %s" % info.mTypeName)
        # there are many other properties of 'info' that can also be retrieved this way - see documentation for GM_LayerInfo_t.

    for i in range(array_size):
        err = gm.CloseLayer(loaded_array[i])
        if err == 0:
            print("Closed layer %d successfully" % i)


When working with the type GM_LayerHandle_t32, you may notice that that you can't actually create an instance of it
yourself, or even call 'help' on it. This is because GM_LayerHandle_t32 and all other *_t32 types are functionally just
ints. Refer to the page about :doc:`Global Mapper integer types <globalmapper.DemoInts>` for more information about how
to interpret types like GM_LayerHandle_t32. Note, however, that the variables *array_ptr* and *loaded_array* in the
above example are pointers to arrays, and are their own types, rather than just integers.

If you are working with layers that are already opened, you can retrieve the layer handles using
:doc:`GetLoadedLayerList <../load/globalmapper.GetLoadedLayerList>`. Unlike many other functions, it does not return an
error code, and instead just returns the array pointer and size. The next code example shows a simple way to confirm
that :doc:`GetLoadedLayerList <../load/globalmapper.GetLoadedLayerList>` has successfully found layers before trying
to operate on the array it returns.

::

    import globalmapper as gm

    result = gm.GetLoadedLayerList()
    if not result:
        # no layers are open...
    else:
        list_ptr, list_size = result
        loaded_array = gm.GM_LayerHandle_array_frompointer(list_ptr)
        # do something with the layers...