Groovy script in post function #1 Updating Custom Field values (option) dependent on Custom Field value

Updating CF option with CF value dependency

Use Case

Groovy script

import com.atlassian.jira.issue.CustomFieldManager;
import com.atlassian.jira.issue.fields.CustomField;
import com.atlassian.jira.issue.util.DefaultIssueChangeHolder;
import com.atlassian.jira.ComponentManager;
import com.atlassian.jira.issue.customfields.option.Options;
import com.atlassian.jira.issue.customfields.option.Option;
import com.atlassian.jira.issue.ModifiedValue;
import com.atlassian.jira.component.ComponentAccessor;

// Custom field ID's
int sourceCfID = 10101;
int targetCfID = 10102;

// Option ID's of target custom field
int optionFirst = 99998;
int optionSecond = 99999;

CustomFieldManager customFieldManager = ComponentManager.getInstance().getCustomFieldManager();

String sourceCfValue = customFieldManager.getCustomFieldObject(sourceCfID).getValue(issue).toString();

CustomField targetCf = customFieldManager.getCustomFieldObject(targetCfID);

// If value of source custom field is XXX, then select first option, if NOT select second option
// This can be replaced by switch statement if you need to check more options!!!!
int targetCfOption = (sourceCfValue == 'XXX') ? optionFirst : optionSecond;

Options targetCfOptions = ComponentAccessor.getOptionsManager().getOptions(targetCf.getRelevantConfig(issue));

Option ChoosenOption = targetCfOptions.getOptionById(targetCfOption);

targetCf.updateValue(null, issue, new ModifiedValue(issue.getCustomFieldValue(targetCf), ChoosenOption), new DefaultIssueChangeHolder());

Pay attention to obtaining custom field object. We are using CustomFieldManager and getCustomFieldObject(Long id) method to get custom field object, if you need to write it with custom field names, you are able to use getCustomFieldObjectByName(String name).
Next important point in this script is option selecting. At first you have to get all Options via ComponentAccessor and then find option by selected ID. Updating value of custom field containing options is realized by updateValue(FieldLayoutItem fieldLayoutItem, Issue issue, ModifiedValue modifiedValue, IssueChangeHolder issueChangeHolder). We set FieldLayoutItem to null, use global variable issue to get current issue instance, new ModifiedValue where we get customfield and new selected option and as last create new DefaultIssueChangeHolder by default(empty) constructor.