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### A.3.6 DataLink Window

DataLink Window

The DataLink Window, unlike the other Table View Windows, is rather specialist, and only applies to a particular type of table, namely the {links}-response table format defined by the DataLink standard. This format is sometimes loosely known as the DataLink format, and is used to represent links of various kinds to external data resources.

Because of its specialist nature, this window does not appear in the Control Window toolbar like the other view windows, so to open it you have to select the DataLink View item from the Views menu in the Control Window. This menu item will only be enabled if the loaded table looks like it has the appropriate form.

The upper panel of this window displays the table content just like the Data Window, but the lower panel contains link-specific information about the row that is currently selected in the upper one.

The Row Link Type sub-panel reports what type of link the given row represents. Its value depends on which table columns are filled in, and may be one of:

- Fixed Access URL: The link is to a URL given in the `access_url` column.
- Service Invocation: The link is to a parameterised URL defined by a service descriptor referenced in the `service_def` column, with parameters supplied by other columns and/or user interaction.
- Link Error: The row reports an error in the `error_message` column indicating why a requested link cannot be supplied.

The Row Detail panel provides additional information on the row. For the rows that actually represent a link, information such as link semantics and resource content type are listed where available, along with a URL sub-panel as below.

The URL sub-panel, if present, shows the actual link URL and provides options to invoke it, i.e. to do something with the listed resource. The parts of this panel are:

URL
  Displays the URL that can be invoked, with per-row parameters substituted in if applicable. You can cut and paste from this field to copy the URL if required.
Type
  Displays TOPCAT's best guess, given the information available in the row, about what kind of resource the link represents. It doesn't always get it right. If it's wrong, you can select one of the options manually from this selector instead. The value in this selector is by default updated every time you select a new row in the table. However, if you want the selected type to be "sticky", i.e. not to be automatically updated, uncheck the checkbox on the left of it. Then the selected value will stay the same until it's changed manually. The purpose of this selector is to give a hint to the Action selection.
Action
  Provides a list of possible actions to perform on the identified URL. The action depends on the value of the Type field; when the Type changes, the Action changes to the value it had last time that Type was selected. Each Type has a default Action, but you can change it by choosing from the selector. If you don't want the action to get updated automatically, uncheck the checkbox on the left as explained above. Most of the action options have behaviour that is similar to a corresponding activation action, as noted below. The options are:
  - Report URL: displays the URL in the Result field
  - View image internally: displays the linked resource (an image in a format like PNG, GIF, JPEG, FITS) in an internal image viewer (like the Display Image action)
  - Load Table: loads the table from the linked resource (a FITS or VOTable table) into this TOPCAT application (like the Load Table action)
  - Plot Table: pops up a plane plot window that allows to perform plots using the table referenced by the linked resource, without actually loading the table into the TOPCAT application (like the Plot Table action)
  - View DataLink Table: displays the linked resource (a DataLink table) in a new window like this one (like the View Datalink Table action)
  - Send FITS Image: sends the linked resource (a FITS image) using SAMP to external image viewers (like the Send FITS Image action)
  - Send Spectrum: sends the linked resource (a spectrum) using SAMP to external spectrum viewers (like the Send Spectrum action)
  - Send Table: sends the linked resource (a VOTable) using SAMP to external table viewers (like the Send VOTable action)
  - Download URL: downloads the linked resource to a local file, chosen using a popup filesystem browser
  - Show Web Page: opens the linked resource in the system web browser (like the View in Web Browser action)
  Note that there currently is not much opportunity to customise the behaviour of the various Action options supplied by this window; these actions are less configurable than their corresponding Activation Actions.
Invoke
  Hitting this button actually invokes the selected Action on the current row.
Result
  Displays the outcome of the most recently invoked URL. An OK or FAIL indicator is displayed, along with a text message that may provide some detail.

In the case of a Service Invocation link type, there may be user-supplied parameters for the link. If so, a Parameters sub-panel appears at the bottom with a list of the available parameters (defaults may or may not be present) to allow you to enter values. Note that the user interface for supplying these parameters is currently very basic, and may be improved in future releases.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk