About Oracle: Creating a mobile app with APEX

Your phone is packed with all kind of sensors. Many of them you can not use in JavaScript, but you can access the GPS sensor to retrieve your location. In this blogpost we will access the GPS location and use it in our application.

On the Activity page the GPS Location will be retrieved. When the record is entered when the user is still on the location of the activity she can check Use location. With this item checked the GPS location is stored with the activity.
When the page is opened to enter a new activity the GPS location is retrieved. It is compared to the GPS locations in the database and if possible it will provide a default value for the Location item.

We will:

update the database to add columns to store the location
import a plug-in to retrieve the GPS location
add items on the Activity page to store the location
add Dynamic Actions on the Activity page to act on the GPS location
use the stored location to provide a default value for the column Location

Update the database objects

There are some changes needed in the database to accommodate the location data.

The columns act_lattitude and act_longitude are added to the table ttm_activity. We also add a column act_sdo_location which is of the type sdo_geometry to be able to use Oracle Spatial functions.

The trigger on ttm_activity is changed to construct the column act_sdo_location from act_lattitude and act_longitude.

The view ttm_activity_vw and it's instead of trigger are changed to support the new columns.

The package ttm_alg is expanded with a function to yield a default value for location.

Download the change script from here and execute it.

Import the Store Location plug-in

Download the Store Location plug-in from APEX World. (Go to the Plug-ins page and search for store location)

Load into the application plug-ins:

Go to Shared Components > Plug-ins
Press Import
Select the plug-in file
Hit Next
Press Next again
Hit the button Install Plug-in

The plug-in is installed and ready for use.

Add items to the Activity page

Items are needed to store the GPS coordinates that are retrieved by the plug-in.

Furthermore we need a check box to indicate whether the record is entered on the location of the activity.

Go to page 15
Go to P15_ACT_LOCATION and press the right mouse button
Select Create Page Item

Name: P15_USE_LOCATION
Type: Checkbox
Appearance > Template: Hidden
List of Values:

Type: Static Values
Static Values: STATIC:Use location;Y

Create another item:

Name: P15_ACT_LATTITUDE
Type: Text Field
Label: Lattitude
Source:

Type: Database Column
Database Column: ACT_LATTITUDE

And another one:

Name: P15_ACT_LONGITUDE
Type: Text Field
Label: Longitude
Source:

Type: Database Column
Database Column: ACT_LONGITUDE

Adding the Dynamic Actions

Two dynamic actions will be added. One to retrieve the GPS location and another to react on the retrieval of the location. These are separate actions because the retrieval of the GPS location is an asynchronous process.

Go to page 15
Go to the Dynamic Actions tab
Right click on Page Load and click Create Dynamic Action

Name: Page Load - Retieve GPS location
Select the Action

Change Identification > Action to Store Location [Plug-In]
In Settings:

Lattitude Item: P15_ACT_LATTITUDE
Longitude Item: P15_ACT_LONGITUDE

Right click on Page Load and click Create Dynamic Action

Name: Location retrieved
When:

Event: Custom
Custom Event: location-retrieved
Selection Type: jQuery Selector
jQuery Selector: html

Select the Action

Action: Execute PL/SQL Code
Settings > PL/SQL Code:

:P15_ACT_LOCATION := nvl(:P15_ACT_LOCATION

, ttm_alg.default_location(:P15_ACT_LATTITUDE,:P15_ACT_LONGITUDE));

Items to Submit: P15_ACT_LATTITUDE,P15_ACT_LONGITUDE
Items to Return: P15_ACT_LOCATION

The last Dynamic Action retrieves a default value for the field Location based on the GPS location. The default value is retrieved in the function ttm_alg.default_location that uses an Oracle Spatial query to find activities that have been recorded close to the current GPS location. The query is discussed in the last paragraphs of this post.

Eating the pudding

The proof is in..., well you know.

So let's try it.

log in to the application on your phone
press New activity
after the page has been opened your permission to use the location is asked. Confirm it.
now scroll down to the bottom
(after some time) you will see the GPS coordinates
fill in the items of the activity
enter in Location: test GPS location
check Use Location, else the GPS location will not be stored
accept the changes
press New again
scroll down to the bottom
(after some time) you will see the GPS coordinates
Location should then contain the value you entered for the previous record: test GPS location

If things do not work as expected use the Chrome Inspector (or any other Developer tools) to examine what is wrong.

Background on selecting the default value

Based on the GPS coordinates the default value for the Location field is determined. This is done using a spatial query:

select act_location

from ttm_activities

where act_lattitude is not null

and act_longitude is not null

and act_use_location = 'Y'

and sdo_geom.within_distance(act_sdo_location,0.2

, sdo_geometry(2001,8307

,sdo_point_type(p_longitude,p_lattitude,null)

,null,null),1,'unit=km')

= 'TRUE'

order by sdo_geom.sdo_distance(act_sdo_location

, sdo_geometry(2001,8307

,sdo_point_type(p_longitude,p_lattitude,null)

,null,null),1,'unit=km')

;

This query uses the Oracle Spatial function sdo_geom.within_distance to find all the records with a GPS location within 200 m's (the value 0.2) from the given GPS coordinates. This proves to be a reasonable distance to correct for the bias in GPS location.

The records are sorted based on the distance to the coordinates which is retrieved using the function sdo_geom.sdo_distance. This way if the query delivers more than one row the one closest to the the given point is selected.

Using Oracle Spatial functions

The Oracle Spatial functions expect arguments in the form of a SDO Geometry objects. The column act_sdo_location is already stored as such an object. The GPS coordinates are numbers and need to be converted to a SDO Geometry object:

sdo_geometry(2001,8307

,sdo_point_type(p_longitude,p_lattitude,null)

,null,null),1,'unit=km')

Without going to deep into the complex geometry subjects:

the first parameter 2001 indicates that the shape is a point. You can also have lines, polygons etc.
the second parameter 8307 references the coordinate system
you notice the values for longitude and lattitude in the call to sdo_point_type
the last parameter indicates that the unit used is kilometers.
more detailed information can be found in the Oracle documentation

To be able to compare values the coordinate system and the unit used should be consistent in database and code.