feat(model): add card, carousel, and alert Block Kit blocks

[srtaalej]

Summary

• Adds CardBlock — structured content display (hero image, icon, title, subtitle, body, actions)
• Adds CarouselBlock — horizontally scrollable collection of cards (1–10)
• Adds AlertBlock — prominent notice block with severity levels (modals only)
• Follow up to docs: add Block Kit type instructions for slack-api-model #1623

Parity with node-slack-sdk#2567 and python-slack-sdk#1865.

Test plan

• Deserialization tests in BlockKitTest.java (parse + round-trip + builder)
• Static helper tests in BlocksTest.java
• All 66 tests pass

Test app.java

  package blockkit;

  import com.slack.api.bolt.App;
  import com.slack.api.bolt.AppConfig;
  import com.slack.api.bolt.socket_mode.SocketModeApp;
  import com.slack.api.model.block.composition.MarkdownTextObject;
  import com.slack.api.model.block.element.ButtonElement;
  import com.slack.api.model.block.element.ImageElement;

  import java.util.Arrays;

  import static com.slack.api.model.block.Blocks.*;
  import static com.slack.api.model.block.composition.BlockCompositions.*;
  import static com.slack.api.model.view.Views.*;

  public class App {

      public static void main(String[] args) throws Exception {
          var app = new App(AppConfig.builder()
                  .singleTeamBotToken(System.getenv("SLACK_BOT_TOKEN"))
                  .build());

          // /cards — sends a carousel with cards to the channel
          app.command("/cards", (req, ctx) -> {
              ctx.client().chatPostMessage(r -> r
                      .channel(req.getPayload().getChannelId())
                      .text("Carousel and Card blocks demo")
                      .blocks(asBlocks(
                              carousel(c -> c
                                      .elements(Arrays.asList(
                                              card(card -> card
                                                      .heroImage(ImageElement.builder()

  .imageUrl("https://picsum.photos/seed/a/400/200")
                                                              .altText("Card 1 hero")
                                                              .build())
                                                      .slackIcon(slackIcon("rocket"))

  .title(MarkdownTextObject.builder().text("Launch Status").build())

  .subtitle(MarkdownTextObject.builder().text("All systems go").build())
                                                      .body(MarkdownTextObject.builder().text("The 
  deployment completed successfully.").build())
                                                      .actions(Arrays.asList(
                                                              ButtonElement.builder()
                                                                      .text(plainText("View"))
                                                                      .actionId("card_1_btn")
                                                                      .build()
                                                      ))
                                              ),
                                              card(card -> card
                                                      .heroImage(ImageElement.builder()

  .imageUrl("https://picsum.photos/seed/b/400/200")
                                                              .altText("Card 2 hero")
                                                              .build())
                                                      .icon(ImageElement.builder()

  .imageUrl("https://picsum.photos/seed/icon/32/32")
                                                              .altText("Custom icon")
                                                              .build())

  .title(MarkdownTextObject.builder().text("Custom Icon Card").build())
                                                      .body(MarkdownTextObject.builder().text("This 
  card uses a custom image icon.").build())
                                                      .actions(Arrays.asList(
                                                              ButtonElement.builder()
                                                                      .text(plainText("Open"))
                                                                      .actionId("card_2_btn")
                                                                      .build()
                                                      ))
                                              ),
                                              card(card -> card

  .title(MarkdownTextObject.builder().text("Minimal Card").build())
                                                      .body(MarkdownTextObject.builder().text("No 
  hero image, no icon, no actions.").build())
                                              )
                                      ))
                              )
                      ))
              );
              return ctx.ack();
          });

          // /alerts — opens a modal with alert blocks
          app.command("/alerts", (req, ctx) -> {
                                              ),
                                              card(card -> card
                                                      .heroImage(ImageElement.builder()

  .imageUrl("https://picsum.photos/seed/b/400/200")
                                                              .altText("Card 2 hero")
                                                              .build())
                                                      .icon(ImageElement.builder()

  .imageUrl("https://picsum.photos/seed/icon/32/32")
                                                              .altText("Custom icon")
                                                              .build())

  .title(MarkdownTextObject.builder().text("Custom Icon Card").build())
                                                      .body(MarkdownTextObject.builder().text("This
  card uses a custom image icon.").build())
                                                      .actions(Arrays.asList(
                                                              ButtonElement.builder()
                                                                      .text(plainText("Open"))
                                                                      .actionId("card_2_btn")
                                                                      .build()
                                                      ))
                                              ),
                                              card(card -> card

  .title(MarkdownTextObject.builder().text("Minimal Card").build())
                                                      .body(MarkdownTextObject.builder().text("No 
  hero image, no icon, no actions.").build())
                                              )
                                      ))
                              )
                      ))
              );
              return ctx.ack();
          });

          // /alerts — opens a modal with alert blocks
          app.command("/alerts", (req, ctx) -> {
              ctx.client().viewsOpen(r -> r
                      .triggerId(req.getPayload().getTriggerId())
                      .view(view(v -> v
                              .type("modal")
                              .title(viewTitle(vt -> vt.type("plain_text").text("Alert Block
  Demo")))
                              .close(viewClose(vc -> vc.type("plain_text").text("Close")))
                              .blocks(asBlocks(
                                      alert(a -> a.text(plainText("Default
  alert")).level("default")),
                                      alert(a -> a.text(plainText("Info alert")).level("info")),
                                      alert(a -> a.text(plainText("Warning
  alert")).level("warning")),
                                      alert(a -> a.text(plainText("Error alert")).level("error")),
                                      alert(a -> a.text(plainText("Success
  alert")).level("success"))
                              ))
                      ))
              );
              return ctx.ack();
          });

          app.blockAction("card_1_btn", (req, ctx) -> ctx.ack());
          app.blockAction("card_2_btn", (req, ctx) -> ctx.ack());

          new SocketModeApp(System.getenv("SLACK_APP_TOKEN"), app).start();
      }
  }

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.37%. Comparing base (da4ba33) to head (ac33ff6).

Files with missing lines	Patch %	Lines
...api/model/block/composition/BlockCompositions.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1624      +/-   ##
============================================
+ Coverage     73.34%   73.37%   +0.02%     
- Complexity     4521     4531      +10     
============================================
  Files           478      478              
  Lines         14300    14308       +8     
  Branches       1490     1490              
============================================
+ Hits          10489    10499      +10     
  Misses         2920     2920              
+ Partials        891      889       -2     

Flag	Coverage Δ
jdk-14	73.37% <75.00%> (+0.02%)	⬆️

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[WilliamBergamin]

This is looking good 🚀

There is one blocking comment around the slackIcon type

[WilliamBergamin]

From these docs the type for slackIkon should be some sort of composition object with type: icon 🤔

[srtaalej]

ah yes good catch

[WilliamBergamin]

Is there a reason why we are initializing this value as an empty list?
Would not initializing it follow existing patterns?

[srtaalej]

hmm no that was an oversight thanks for catching

[WilliamBergamin]

Nice 🚀