Block Types

Purpose and Scope

This page documents the six concrete block types available in Slack's Block Kit implementation within the hypervel/notifications package. Blocks are the visual components that compose a Slack message layout. Each block type serves a specific purpose and has distinct validation rules.

For information about the contract interfaces these blocks implement, see Block Kit Contracts. For the elements and composites used within blocks (such as TextObject, ButtonElement), see Elements and Composites. For overall Slack message construction, see Slack Messages.

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Block Type Architecture

All block types implement the BlockContract interface and reside in the src/Slack/BlockKit/Blocks/ namespace. Each block type corresponds to a specific Slack Block Kit layout component and serializes to JSON via the toArray() method.

Sources: src/Slack/BlockKit/Blocks/ActionsBlock.php14 src/Slack/BlockKit/Blocks/ContextBlock.php15 src/Slack/BlockKit/Blocks/DividerBlock.php10 src/Slack/BlockKit/Blocks/HeaderBlock.php12 src/Slack/BlockKit/Blocks/ImageBlock.php12 src/Slack/BlockKit/Blocks/SectionBlock.php14

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Common Block Properties

Block Identifier

All block types support an optional blockId property set via the id() method. This identifier is used by Slack to track interaction sources and must not exceed 255 characters.

Block Type	Has id() Method	Validation
ActionsBlock	Yes	Max 255 characters
ContextBlock	Yes	Max 255 characters
DividerBlock	Yes	Max 255 characters
HeaderBlock	Yes	Max 255 characters
ImageBlock	Yes	Max 255 characters
SectionBlock	Yes	Max 255 characters

Sources: src/Slack/BlockKit/Blocks/ActionsBlock.php37-42 src/Slack/BlockKit/Blocks/ContextBlock.php38-43 src/Slack/BlockKit/Blocks/DividerBlock.php24-29 src/Slack/BlockKit/Blocks/HeaderBlock.php45-50 src/Slack/BlockKit/Blocks/ImageBlock.php60-65 src/Slack/BlockKit/Blocks/SectionBlock.php53-58

Serialization

All blocks implement the Arrayable contract through their toArray() method, which validates constraints and produces the JSON-compatible array structure expected by Slack's API.

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ActionsBlock

The ActionsBlock contains interactive elements such as buttons, select menus, overflow menus, or date pickers. It enables user interaction within the message.

Properties

• elements (ElementContract[]): Array of interactive element objects
• blockId (?string): Optional unique identifier

Constraints

Constraint	Value	Exception Type
Minimum elements	1	LogicException
Maximum elements	25	LogicException
block_id max length	255 characters	InvalidArgumentException

Methods

• button(string $text): ButtonElement - Adds a button element to the block and returns it for further configuration

Output Structure

Sources: src/Slack/BlockKit/Blocks/ActionsBlock.php14-80

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ContextBlock

The ContextBlock displays contextual information using images and text objects in a compact format, typically used for metadata or supplementary details.

Properties

• elements (ElementContract[]): Array of image elements and text objects
• blockId (?string): Optional unique identifier

Constraints

Constraint	Value	Exception Type
Minimum elements	1	LogicException
Maximum elements	10	LogicException
block_id max length	255 characters	InvalidArgumentException

Methods

• image(string $imageUrl, ?string $altText = null): ImageElement - Adds an image element
• text(string $text): TextObject - Adds a text object

Output Structure

Sources: src/Slack/BlockKit/Blocks/ContextBlock.php15-91

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DividerBlock

The DividerBlock renders a visual separator line, used to segment content within a message. This is the simplest block type with no configurable content.

Properties

• blockId (?string): Optional unique identifier

Constraints

Constraint	Value	Exception Type
block_id max length	255 characters	InvalidArgumentException

Methods

The DividerBlock has no content-specific methods beyond id().

Output Structure

Sources: src/Slack/BlockKit/Blocks/DividerBlock.php10-48

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HeaderBlock

The HeaderBlock displays large, prominent text at the top of a message or section, similar to an H1 heading.

Properties

• text (PlainTextOnlyTextObject): The header text (plain text only)
• blockId (?string): Optional unique identifier

Constructor

The constructor creates a PlainTextOnlyTextObject with a 150 character limit and applies an optional callback for further configuration.

Constraints

Constraint	Value	Exception Type
text max length	150 characters	Enforced by PlainTextOnlyTextObject
block_id max length	255 characters	InvalidArgumentException

Output Structure

Sources: src/Slack/BlockKit/Blocks/HeaderBlock.php12-70

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ImageBlock

The ImageBlock displays an image with optional title and alt text. This is distinct from the ImageElement used within other blocks.

Properties

• url (string): The image URL (max 3000 characters)
• altText (?string): Plain-text image description (max 2000 characters, required)
• title (?PlainTextOnlyTextObject): Optional title (max 2000 characters)
• blockId (?string): Optional unique identifier

Constructor

Constructor validates URL length immediately. Alt text can be set in constructor or via alt() method.

Constraints

Constraint	Value	Exception Type
url max length	3000 characters	InvalidArgumentException
altText max length	2000 characters	InvalidArgumentException
altText required	Must be set	LogicException
title max length	2000 characters	Enforced by PlainTextOnlyTextObject
block_id max length	255 characters	InvalidArgumentException

Methods

• alt(string $altText): static - Sets or updates the alt text
• title(string $title): PlainTextOnlyTextObject - Sets the title and returns the text object for further configuration

Output Structure

Sources: src/Slack/BlockKit/Blocks/ImageBlock.php12-115

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SectionBlock

The SectionBlock is the most versatile block type, supporting text content, fields in a two-column layout, and an optional accessory element (such as a button or image).

Properties

• text (?TextObject): Main text content (max 3000 characters)
• fields (TextObject[]): Array of text objects displayed in two columns (max 10 fields, 2000 chars each)
• accessory (?ElementContract): Optional interactive element displayed alongside content
• blockId (?string): Optional unique identifier

Constraints

Constraint	Value	Exception Type
text max length	3000 characters	Enforced by TextObject
field max length	2000 characters each	Enforced by TextObject
field minimum length	1 character	Enforced by TextObject
Maximum fields	10	LogicException
Content requirement	text OR fields must be present	LogicException
block_id max length	255 characters	InvalidArgumentException

Methods

• text(string $text): TextObject - Sets the main text and returns the text object for further configuration
• field(string $text): TextObject - Adds a field and returns the text object for configuration
• accessory(ElementContract $element): static - Sets an accessory element (button, image, etc.)

Output Structure

Sources: src/Slack/BlockKit/Blocks/SectionBlock.php14-118

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Block Type Validation Summary

The following table summarizes validation rules enforced across all block types during toArray() serialization:

Block Type	Required Fields	Optional Fields	Element Limits	Text Length Limits
ActionsBlock	elements (1-25)	block_id	1-25 elements	block_id: 255
ContextBlock	elements (1-10)	block_id	1-10 elements	block_id: 255
DividerBlock	none	block_id	n/a	block_id: 255
HeaderBlock	text	block_id	n/a	text: 150, block_id: 255
ImageBlock	url, altText	block_id, title	n/a	url: 3000, altText: 2000, title: 2000, block_id: 255
SectionBlock	text OR fields	block_id, accessory	max 10 fields	text: 3000, field: 2000, block_id: 255

Sources: src/Slack/BlockKit/Blocks/ActionsBlock.php58-79 src/Slack/BlockKit/Blocks/ContextBlock.php68-90 src/Slack/BlockKit/Blocks/DividerBlock.php34-47 src/Slack/BlockKit/Blocks/HeaderBlock.php55-69 src/Slack/BlockKit/Blocks/ImageBlock.php94-114 src/Slack/BlockKit/Blocks/SectionBlock.php93-117

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Block Type Selection Guide

Choose the appropriate block type based on your content and interaction requirements:

Key Selection Criteria:

• ActionsBlock: When users need to click buttons or interact with select menus
• ContextBlock: For small text labels, timestamps, or icons (max 10 items)
• DividerBlock: To visually separate sections of content
• HeaderBlock: For prominent section titles (limited to 150 characters)
• ImageBlock: For standalone images with alt text and optional title
• SectionBlock: For primary text content, potentially with accessory elements

Sources: All block implementation files in src/Slack/BlockKit/Blocks/