Your app can use a web map while it has no internet connection, by first downloading an offline map from the web map. As soon as the offline map is downloaded, a mobile worker can disconnect their device and work with the map offline.
Mobile users can specify a geographic area of a web map to download as an offline map. Offline maps defined and created this way are known as on-demand offline maps. On-demand offline maps are useful for mobile users who don't necessarily know in advance where they will be working.
The main advantage of on-demand offline maps is that your app can specify parameters to ensure that only essential web map content is included in the offline map, such as:
- The area of interest
- The max and min scales
- Which layers to include
- Which features to include
- Which related records to include
- Whether to include attachments
- Editing characteristics
The steps to use on-demand offline maps are:
- Create an offline map task
- Examine the web map's offline capabilities
- Create parameters to specify offline map content
- Create a job to generate and download an offline map
- Run the job
Create an offline map task
Create an OfflineMapTask from either an online Map or from a PortalItemrepresenting a web map.
// Create a new web map portal item using its ID (arcgis.com is used by default if a portal is not specified).
PortalItem* portalItem = new PortalItem("acc027394bc84c2fb04d1ed317aac674", this);
// Create a new map from the web map portal item.
Map* map = new Map(portalItem, this);
// Create a new offline map task from the map.
OfflineMapTask* offlineMapTask = new OfflineMapTask(map, this);
// Provide some feedback.
qDebug() << offlineMapTask->portalItem()->averageRating();
Examine the web map's offline capabilities
You should check which layers and tables in the web map can be taken offline by examining the web map's offline capabilities. This can help identify layers or tables that are missing in the offline map that is generated.
Get the OfflineMapCapabilities object by calling the offline method on OfflineMapTask.
When true, the has property indicates that one or more layers or tables cannot be taken offline due to an error. You can check the layer and table to determine which layer or table cannot be taken offline, and why.
Some layers do not support being taken offline (either they have not been offline-enabled, or the type of layer does not support being taken offline). An offline map can still include references to these online-only layers, allowing the layers to be accessed and displayed whenever a network connection is available. This capability is set by set on GenerateOfflineMapParameters.
See Retain online services for more information.
// Execute the offline map tasks's offline map capabilities async method. It returns an offline map
// capabilities object.
offlineMapTask->offlineMapCapabilitiesAsync(generateOfflineMapParameters).then(this, [](
const OfflineMapCapabilities& offlineMapCapabilities)
{
// Test if the offline map capabilities has any errors.
if (offlineMapCapabilities.hasErrors())
{
// Get the QMap (aka dictionary) of layer errors from the offline map capabilities.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=qmap-with-pointer-key
QMap<Layer*, OfflineCapability> layerCapabilities = offlineMapCapabilities.layerCapabilities(); // clazy:exclude=qmap-with-pointer-key
// Loop thru any layer capabilities errors.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=range-loop-reference
for (OfflineCapability layerCapability : layerCapabilities) // clazy:exclude=range-loop-reference
{
// Report layer errors...
}
// Get the QMap (aka dictionary) of feature table errors from the offline map capabilities.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end opf the code line: // clazy:exclude=qmap-with-pointer-key
QMap<FeatureTable*, OfflineCapability> featureTableCapabilities = offlineMapCapabilities.tableCapabilities(); // clazy:exclude=qmap-with-pointer-key
// Loop thru any feature table capabilities errors.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=range-loop-reference
for (OfflineCapability featureTableCapability : featureTableCapabilities) // clazy:exclude=range-loop-reference
{
// Report layer errors...
}
}
});
Create parameters to specify offline map content
When you generate and download an offline map, it should contain content relevant to the mobile user for the geographic area in which they will be working. Take care not to include more content than needed, because this can impact the time it takes to generate and download the offline map. Different parameters are available to control the geographic coverage area and the content of the generated offline map.
-
Create the
GenerateOfflineMapParametersby passing an area of interest to thecreatemethod on theDefault Generate Offline Map Parameters OfflineMapTask. -
Get and examine the returned parameters. These default parameters represent the advanced offline settings configured by the web map author.
-
To override default parameters see Advanced parameters. For example, you can automatically update and display online-only layers when a network connection is available (see Retain online services). You can also set the max and min scale, use a local basemap, or set a definition expression on features.
// Execute the offline map tasks's create default generate offline map parameters async method. It returns
// a generate offline map parameter object.
offlineMapTask->createDefaultGenerateOfflineMapParametersAsync(geometry).then(this,
[this](const GenerateOfflineMapParameters& generateOfflineMapParameter)
{
// Copy the returned parameters to a member variable for use outside this scope.
m_generateOfflineMapParameters = generateOfflineMapParameter;
// Limit the maximum scale to 5000 but take all the scales above (use default of 0 as the MinScale).
m_generateOfflineMapParameters.setMaxScale(5000.0);
// Set a couple of attachment options on the generate offline map parameters member variable.
m_generateOfflineMapParameters.setAttachmentSyncDirection(AttachmentSyncDirection::Upload);
m_generateOfflineMapParameters.setReturnLayerAttachmentOption(ReturnLayerAttachmentOption::EditableLayers);
// Get the offline map item info from the generate offline map paramters member variable.
OfflineMapItemInfo offlineMapItemInfo = m_generateOfflineMapParameters.itemInfo();
// Set the title on the offline map info.
offlineMapItemInfo.setTitle(generateOfflineMapParameter.itemInfo().title() + " (Central)");
// Set the item info on the generate offline map parameters member variable.
m_generateOfflineMapParameters.setItemInfo(offlineMapItemInfo);
});
Start by creating the task.
// Create a new offline map task (it is a member variable, global in scope).
m_offlineMapTask = new OfflineMapTask(m_map, this);
Then, generate the default parameters for the task. Set some of the parameters to values that you want for your offline map. Finally, see if the map can be taken offline.
// After the task loads, get the default parameters.
connect(m_offlineMapTask, &OfflineMapTask::doneLoading, this, [this]()
{
// Get the viewpoint from the map.
const Viewpoint viewpoint = m_map->initialViewpoint();
// Get the geometry from the viewpoint.
const Geometry extent = viewpoint.targetGeometry();
// Execute the offline map tasks's create default generate offline map parameters async method. It
// returns a generate offline map parameter object.
m_offlineMapTask->createDefaultGenerateOfflineMapParametersAsync(extent).then(this,
[this](const GenerateOfflineMapParameters& generateOfflineMapParameters)
{
// Create a modifiable generate offline map parameters object from what was returned from the
// async operation.
GenerateOfflineMapParameters GenerateOfflineMapParametersModifiable = generateOfflineMapParameters;
// Set the maximum scale on the the generate offline map parameters.
GenerateOfflineMapParametersModifiable.setMaxScale(5000.0);
// Set the attachment sync direction on the the generate offline map parameters.
GenerateOfflineMapParametersModifiable.setAttachmentSyncDirection(AttachmentSyncDirection::Upload);
// Set the return layers attachment option on the the generate offline map parameters.
GenerateOfflineMapParametersModifiable.setReturnLayerAttachmentOption(
ReturnLayerAttachmentOption::EditableLayers);
// Set the return schema only for editable layers on the the generate offline map parameters.
GenerateOfflineMapParametersModifiable.setReturnSchemaOnlyForEditableLayers(true);
// Get the offline map item info from the generate offline map parameters.
OfflineMapItemInfo offlineMapItemInfo = GenerateOfflineMapParametersModifiable.itemInfo();
// Get the title from the offline map item info.
QString oldTitle = offlineMapItemInfo.title();
// Create a new title.
QString newTitle = oldTitle + "(Central)";
// Set the title on the offline map item info with the new title (QString).
offlineMapItemInfo.setTitle(newTitle);
// Execute the offline map tasks's offline map capabilities async method. It returns
// an offline map capabilities object.
m_offlineMapTask->offlineMapCapabilitiesAsync(GenerateOfflineMapParametersModifiable).then(this,
[](const OfflineMapCapabilities& offlineMapCapabilities)
{
// Test if the offline map capabilities has any errors.
if (!offlineMapCapabilities.hasErrors())
{
qDebug() << "This map may be taken offline.";
}
else
{
qDebug() << "This map may NOT be taken offline.";
}
});
});
});
Create a job to generate and download an offline map
To generate and download the offline map, you must create a GenerateOfflineMapJob by providing the GenerateOfflineMapParameters to the generate method on OfflineMapTask. You must also provide a directory on the device to store the offline map. If this download directory already exists, it must be empty. If the directory doesn't exist, it will be created by the job.
// Create a job to download the map area.
GenerateOfflineMapJob* offlineMapJob = offlineMapTask->generateOfflineMap(
m_generateOfflineMapParameters, packagePath);
If you want to control the individual layer and table content, you also need to provide the GenerateOfflineMapParameterOverrides as well as the GenerateOfflineMapParametersto the generate method on OfflineMapTask. For more details see Create offline map parameter overrides.
// Create the job to generate an offline map. Pass in the parameters, a path to store the map package,
// and overrides.
GenerateOfflineMapJob* offlineMapJobWithOverrides = offlineMapTask->generateOfflineMap(
m_generateOfflineMapParameters, packagePath, generateOfflineMapParameterOverrides);
See the Tasks and jobs topic for more details on how to work with jobs in general.
Run the job
To generate the offline map and download it to your device, start the GenerateOfflineMapJob. When complete, the job returns a GenerateOfflineMapResult. If one or more tables or layers fails to be taken offline, the has property may be true (however a layer may be configured for "online-only", which is not an error). You can check the layer and table dictionaries to identify problems.
If the is property of GenerateOfflineMapParameters is false, the job terminates if any layer or table fails to be taken offline.
If you want to display the map immediately, assign the GenerateOfflineMapResultoffline to map property of the map view.
// Create a slot for when the offline map job is done.
connect(generateOfflineMapJob, &GenerateOfflineMapJob::jobDone, this, [generateOfflineMapJob, this]()
{
// Get the generate offline map result from the generate offline map job.
GenerateOfflineMapResult* generateOfflineMapResult = generateOfflineMapJob->result();
// Test if the generate offline map result has any errors.
if (!generateOfflineMapResult->hasErrors())
{
// Broadcast that the job completed successfully and all content was generated.
qDebug() << "Map " + generateOfflineMapResult->mobileMapPackage()->item()->title() +
" was saved to " + generateOfflineMapResult->mobileMapPackage()->path();
// Show the offline map in a map view.
m_mapView->setMap(generateOfflineMapResult->offlineMap());
}
else
{
// Get the QMap (aka dictionary) of layer errors from the generate offline map result.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=qmap-with-pointer-key
const QMap<Layer* , Error> layerErrors = generateOfflineMapResult->layerErrors(); // clazy:exclude=qmap-with-pointer-key
// The generate offline map job is finished but one or more layers has errors.
for (const Error& layerError : layerErrors)
{
// Broadcast the layer errors.
qWarning() << layerError.message() << layerError.additionalMessage();
}
// Get the QMap (aka dictionary) of feature table errors from the generate offline map result.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=qmap-with-pointer-key
const QMap<FeatureTable* , Error> featureTableErrors = generateOfflineMapResult->tableErrors(); // clazy:exclude=qmap-with-pointer-key
// Get the QMap (aka dictionary) of feature table errors from the generate offline map result.
// Note: to supress the Clazy analyzer warning bundled with Qt Creator, add the following comment
// to the end of the code line: // clazy:exclude=qmap-with-pointer-key
for (const Error& featureTableError : featureTableErrors)
{
// Broadcast the layer errors.
qWarning() << featureTableError.message() << featureTableError.additionalMessage();
}
}
});
// Start the job to generate the offline map.
generateOfflineMapJob->start();
Offline maps created by the on-demand workflow are stored in an unpacked mobile map package. When your app goes into the field, you need to open the map directly from this mobile map package download stored on your device.
Advanced parameters
You can set properties on GenerateOfflineMapParameters to control the offline map content. For example:
- The map's max and min scale
- Whether basemaps are included in the map
- Whether layers that reference online services are retained in the map
- Whether to allow synchronization of the map's geodatabases
- Whether to apply definition expression filters
- Whether to return all rows from a related table
- Whether to continue downloading the map if a single layer fails to be taken offline
- Including feature attachments
- Whether only the schema is provided for editable feature layers
- Update or replace the map's metadata
Scale range
A web map might include image tiled layers, which are composed of many tiles at different levels of detail (similar to a “zoom level”). The amount of space, generation time, and download time required to download tiled layers in a web map will increase with every level of detail. To increase performance, you should only download the levels of detail that are relevant for your app. You can control this by setting the minimum and maximum scale parameters max and min.
If possible, choose a maximum scale that is not too “zoomed in” to prevent generating a large number of unnecessary tiles. Each service limits the number of tiles that can be taken offline. Make sure to set a scale range that avoids hitting this limit.
Include a map's basemap
A web map author can define whether offline maps should:
-
Download the basemap defined by the web map. This is the default and ensures that a tile package is downloaded with the map.
-
Use a tile package that is already on the device. The tile package must be downloaded or copied onto the device separately and can be referenced with an absolute file path or a path relative to the map. Make sure the tile package covers the areas required by your map area. The benefits of this option are that the map file will be smaller, the download time may be faster, and you can use the tile package in many different maps and apps.
To use the tile package on your device, you must set the
GenerateOfflineMapParametersreferenceto the directory that contains the tile package. You should confirm that the tile package file,Basemap Directory reference, exists on the device before running theBasemap Filename GenerateOfflineMapJob. This job will add the tile package, as a basemap, to the offline map. -
Avoid using the basemap. Developers can choose to override this configured behavior when they download an offline map from a web map. To do this, set the
GenerateOfflineMapParameters'sincludeproperty toBasemap false. In this case theGenerateOfflineMapJobwill not download any layers included as part of the map'sBasemap. This task will not use the local tile package, even if you have specified one.
Retain online services
Live, online services are supported in offline maps. You can take a web map offline that has a mix of local on-device content as well as live, online service content (such as weather or traffic information). When network connectivity is available, users can use data from online services. Otherwise, only the local (offline) content is available.
A value of Include for GenerateOfflineMapParameters online means that any data that can't be taken offline will be accessed via the URL in the offline map. Your offline map retains all of the information from the original web map, but requires a network connection and may require authentication.
Manage synchronization of the map's geodatabases
Typically, the updateon the GenerateOfflineMapParameters is set to Generate. This mode allows you to synchronize any geodatabase changes with their online feature services.
If you want to avoid receiving any geodatabase updates, set the update on the GenerateOfflineMapParameters to no. This disables data synchronization on the map’s geodatabases and prevents the corresponding feature services from creating synchronization replicas. The benefits of this option are that the burden on the feature server is reduced and you will not need to unregister geodatabases when they are no longer required.
Apply feature layer definition expression filters
When taking a map offline, the GenerateOfflineMapJob applies the feature layer's definition expression by default. Applying the definition expression may reduce the number of features taken offline for display and sync. If you do not want to apply the definition expression, set is to false.
Return all rows from a related table
If a map contains a layer or table with a relationship to another table, you can choose to download all rows or only related rows from the destination table. The default is to download only the related rows. If you want to return all rows, then you must set the value of DestinationTableRowFilter to be All.
If the table is a standalone table or the source of a relationship, all rows are returned.
Continue downloading the map if a single layer or table fails
By default, the GenerateOfflineMapJob continues to take layers and tables offline even if a layer or table fails. While this ensures that the map is taken offline, data may be missing. When the job completes, you should examine the job's result (discussed under Run the job) to identify if any layers have failed and determine whether or not to continue working with the map.
If you want the job to stop immediately when a layer or table fails, set the GenerateOfflineMapParameters's Continue property to false. This ensures that if a map is successfully taken offline it contains all of its layers and tables.
Failure to take a layer or table offline may be due to an intermittent network connection, loss of the service, or an unsupported layer type.
Inclusion of feature attachments
Some feature services contain attachments (pictures, videos, and other documents) for individual features. Because these files can be large, you should consider your app's offline workflow to determine whether the attachments need to be taken offline, and whether they need to be synchronized with the service when the app is connected. These two behaviors are defined using the return and attachment properties on the GenerateOfflineMapParameters class.
-
The return layer attachment property defines which layers should contain attachments in the offline map. The options are:
-
Return- None of the layers contain attachments.Layer Attachment Option ::None -
Return- All layers have their attachments.Layer Attachment Option ::All Layers -
Return- Layers without editing enabled have attachments.Layer Attachment Option ::Read Only Layers -
Return- Layers with editing enabled have attachments.Layer Attachment Option ::Editable Layers
-
-
The attachment sync direction defines how the attachments are synchronized with the service. The options are:
-
Attachment- Attachments are not synchronized as part of the synchronization operation.Sync Direction ::None -
Attachment- Attachments are uploaded from the client to the service, but any changes on the service are not downloaded to the client.Sync Direction ::Upload -
Attachment- Attachments are uploaded from client to the service, and changes on the service are pulled down to the client.Sync Direction ::Bidirectional
-
Inclusion of features from editable feature layers
Here are some workflows that describe how these two parameters affect each other:
-
Workflow 1 — Download attachments for all layers in the map, allow the user to add or remove attachments from the layers, and then synchronize these changes between the service and the client when online. For example: multiple users collect data on the same area and they want to synchronize all the changes with the centralized services as well as sharing changes with other people in the field.
-
ReturnLayer Attachment Option ::All Layers -
AttachmentSync Direction ::Bidirectional
-
-
Workflow 2 — Download attachments for all read-only layers and update these layers when online. For example: users are offline and viewing a layer of buildings with photos that show how the buildings look. If there are any new photos added to the service, these will be downloaded to the client during synchronization when online.
-
ReturnLayer Attachment Option ::Read Only Layers -
AttachmentSync Direction ::Bidirectional
-
-
Workflow 3 — Download attachments for editable layers only and upload them to the service when online. For example: users are offline and only need to view attachments for editable layers. If there are any read-only layers that provide context for the map, their attachments aren’t included to the local map. If users remove or add any new attachments, these changes can be synchronized to the service when online.
-
ReturnLayer Attachment Option ::Editable Layers -
AttachmentSync Direction ::Bidirectional
-
-
Workflow 4 — Do not download any attachments but allow any new attachments to be uploaded to the service when online. For example: users are offline and collecting new attachments in the field but do not need to view existing attachments.
-
ReturnLayer Attachment Option ::None -
AttachmentSync Direction ::Upload
-
If users are collecting new information in the field where they do not need to access previously created features, you can create an offline map with empty editable feature layers. Do this by setting the GenerateOfflineMapParameters's set property to true.
Update or replace the map's metadata
Access an online map's metadata from the item property. It includes portal item properties such as the title, description, short description, and thumbnail. This information is populated from the portal item that contains the map. You can override any of these metadata properties before you take the map offline. For example, if you are creating offline maps of different areas of interest on the same map, you may want to change the map's title to indicate which area it contains.
You can also create a new OfflineMapItemInfo object and manually set all the details.
// Execute the map view's export image async method. It returns a QImage object.
m_mapView->exportImageAsync().then(this, [this, offlineMapItemInfo](const QImage& thumbnailImage)
{
// Create an offline map item info object (that is modifiable) to store metadata for the map.
OfflineMapItemInfo offlineMapItemInfoModifiable = m_generateOfflineMapParameters.itemInfo();
// Set the QImage thumbnail on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setThumbnail(thumbnailImage);
// Set the title on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setTitle("Water network (Central)");
// Set the snippet on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setSnippet(offlineMapItemInfo->snippet());
// Set the description on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setDescription(offlineMapItemInfo->description());
// Set the access information on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setAccessInformation(offlineMapItemInfo->accessInformation());
// Set some tags on the offline map item info (modifiable).
offlineMapItemInfoModifiable.setTags({"Water network", "Data validation"});
// Apply the metadata to the generate offline map parameters member (gloabl scope) variable.
m_generateOfflineMapParameters.setItemInfo(offlineMapItemInfoModifiable);
});
Create offline map parameter overrides
You may want to control how individual layers or tables are taken offline to do things like:
- Reduce the amount of data (for example, tile data) for a given layer
- Alter the spatial extent of a given layer (for example, to give coverage beyond the study area)
- Filter features (for example, with a where clause) to only take those that are relevant to your fieldwork
- Take features with null geometry (for example, where the attributes are populated in the office but the geometry needs to be captured on-site)
- Omit individual layers
- Define which layers should reference online content
The GenerateOfflineMapParameterOverrides object provides control for these behaviors. It includes three dictionaries containing the generate geodatabase parameters (for feature layers), export vector tile parameters (for vector tile layers), and export tile cache parameters (for image tile layers), as well as two lists of layers and tables that will retain online access in the offline map (see Retain online services). Adjust any of these parameters and create the GenerateOfflineMapJob using the overrides object.
To control how individual layers and tables are taken offline, follow these steps:
-
Generate and modify the
GenerateOfflineMapParametersas described in Create parameters to specify offline map content above. -
Generate the parameter overrides object (
GenerateOfflineMapParameterOverrides) using thecreateGenerateOfflineMapParameterOverrides()method on theOfflineMapTask. Provide theGenerateOfflineMapParametersgenerated from the previous step. -
When the task completes, a pre-populated
GenerateOfflineMapParameterOverridesobject will be provided. This is a modifiable layer-by-layer representation of the suppliedGenerateOfflineMapParametersand includes three dictionaries containing detailed parameters for each layer and table:GenerateGeodatabaseParametersfor feature layers.ExportVectorTilesParametersfor vector tile layers.ExportTileCacheParametersfor image tile layers.
// Create a dictionary key using the custom function called 'setMapParametersKey'.
const OfflineMapParametersKey setMapParametersKey(featureLayer);
// Get a QMap (aka dictionary) of generate geodatabase parameters with associated keys.
QMap<OfflineMapParametersKey, GenerateGeodatabaseParameters> dictonaryOfflineMapParametersKey =
generateOfflineMapParameterOverrides->generateGeodatabaseParameters();
// Get one generate geodatabase parameter from the QMap via its key.
GenerateGeodatabaseParameters generateGeodatabaseParameters = dictonaryOfflineMapParametersKey.
value(setMapParametersKey);
// Set the return attachment on the generate geodatabase parameters.
generateGeodatabaseParameters.setReturnAttachments(false);
To override specific parameters for a layer, create an OfflineMapParametersKey for the layer and use it to access the pre-populated parameters for that layer in the appropriate dictionary of the GenerateOfflineMapParameterOverrides. You can then modify individual properties for taking that layer offline.
The GenerateOfflineMapParameterOverrides also includes two lists which can be modified to specify which Layer and ServiceFeatureTable objects should not have content downloaded but should instead access the original online data whenever a network connection is available (see Retain online services for more details).
After defining your overrides, you can create a GenerateOfflineMapJob by calling generateOfflineMapJob on the offline map task, supplying the parameters and the overrides.
Considerations
-
Advanced symbols are supported only if they are defined in the original service. Any overrides with advanced symbols will result in empty symbols in an offline map.
-
Area-of-interest geometries that cross the dateline are not currently supported.
-
If more than one feature layer in a map refers to the same feature service endpoint, only one feature layer will be taken offline. The other feature layers will raise an error.