Extend an existing field
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In order to extend an existing field you must subclass a built in field (e.g
FieldTextInput
, FieldColour
) and then modify part of it to fit your needs.
Some parts of a field you can modify are:
If you want to create a
custom field
that does not need behaviour from any built-in field you should subclass Field
.
Common extensions
Most custom fields extend one of these three types:
- Text Input: If you want your users to type into your field, you should extend
FieldTextInput
.
- Number: If you want to store a number, you should extend
FieldNumber
.
- Dropdown: If you want to create a dropdown, but you want it to store a different model
than the default string or image model, you should extend
FieldDropdown
.
- Caution: Before extending
FieldDropdown
, check that the dropdown field's
customization options cannot fulfill your needs.
Under certain circumstances you may wish to extend a different field type. For
example FieldLabelSerializable
extends FieldLabel
.
Subclassing
import * as Blockly from 'blockly';
export class MyCustomTextField extends Blockly.FieldTextInput {
constructor(value, validator, config) {
super(value, validator, config);
}
}
The constructor for a field's subclass looks very similar to the constructor for
a custom field. The signature of the sub-constructor should
generally match the signature of the super-constructor.
JSON and registration
You should also register the field once:
Blockly.fieldRegistry.register('my_custom_text_field', MyCustomTextField);
and provide an implementation of fromJson
in the class so that it works with
the JSON format:
static fromJson(options) {
const value = Blockly.utils.parsing.replaceMessageReferences(options.value);
return new MyCustomTextField(value);
}
For more information about registering a field see the JSON and registration
section in Creating a Custom Field.
Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.
Last updated 2025-08-20 UTC.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Missing the information I need","missingTheInformationINeed","thumb-down"],["Too complicated / too many steps","tooComplicatedTooManySteps","thumb-down"],["Out of date","outOfDate","thumb-down"],["Samples / code issue","samplesCodeIssue","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-20 UTC."],[[["\u003cp\u003eTo extend an existing Blockly field, subclass a built-in field like \u003ccode\u003eFieldTextInput\u003c/code\u003e or \u003ccode\u003eFieldColour\u003c/code\u003e and modify its editor, on-block display, or displayed text.\u003c/p\u003e\n"],["\u003cp\u003eFor custom fields with unique behaviors, subclass the base \u003ccode\u003eField\u003c/code\u003e class instead of extending an existing field.\u003c/p\u003e\n"],["\u003cp\u003eCommon field extensions involve customizing text input (\u003ccode\u003eFieldTextInput\u003c/code\u003e), numbers (\u003ccode\u003eFieldNumber\u003c/code\u003e), or dropdowns (\u003ccode\u003eFieldDropdown\u003c/code\u003e) to meet specific needs.\u003c/p\u003e\n"],["\u003cp\u003eWhen subclassing, ensure your constructor matches the super-constructor's signature and register the field with \u003ccode\u003eBlockly.fieldRegistry.register\u003c/code\u003e.\u003c/p\u003e\n"],["\u003cp\u003eImplement the \u003ccode\u003efromJson\u003c/code\u003e method in your custom field class to enable compatibility with the Blockly JSON format for serialization and deserialization.\u003c/p\u003e\n"]]],["To extend an existing field, subclass a built-in field like `FieldTextInput` or `FieldColour`, modifying its editor, on-block display, or text. For unique fields, subclass `Field`. Common extensions include `FieldTextInput`, `FieldNumber`, and `FieldDropdown`. Subclass constructors should mirror the super-constructor's signature. Register the field using `Blockly.fieldRegistry.register()` and implement `fromJson` for JSON compatibility. Extending different fields such as `FieldLabelSerializable` is also possible.\n"],null,["# Extend an existing field\n\nIn order to extend an existing field you must subclass a built in field (e.g\n`FieldTextInput`, `FieldColour`) and then modify part of it to fit your needs.\nSome parts of a field you can modify are:\n\n- Its [editor](/blockly/guides/create-custom-blocks/fields/anatomy-of-a-field#editor_display).\n- Its [on-block display](/blockly/guides/create-custom-blocks/fields/anatomy-of-a-field#on-block_display).\n- The [text](/blockly/guides/create-custom-blocks/fields/anatomy-of-a-field#text) it displays.\n\nIf you want to [create a\ncustom field](/blockly/guides/create-custom-blocks/fields/customizing-fields/creating)\nthat does not need behaviour from any built-in field you should subclass `Field`.\n\nCommon extensions\n-----------------\n\nMost custom fields extend one of these three types:\n\n- [Text Input](/blockly/guides/create-custom-blocks/fields/built-in-fields/text-input): If you want your users to type into your field, you should extend `FieldTextInput`.\n- [Number](/blockly/guides/create-custom-blocks/fields/built-in-fields/number): If you want to store a number, you should extend `FieldNumber`.\n- [Dropdown](/blockly/guides/create-custom-blocks/fields/built-in-fields/dropdown): If you want to create a dropdown, but you want it to store a different model than the default string or image model, you should extend `FieldDropdown`.\n - Caution: Before extending `FieldDropdown`, check that the dropdown field's [customization options](/blockly/guides/create-custom-blocks/fields/built-in-fields/dropdown#customization) cannot fulfill your needs.\n\nUnder certain circumstances you may wish to extend a different field type. For\nexample `FieldLabelSerializable` extends `FieldLabel`.\n\nSubclassing\n-----------\n\n import * as Blockly from 'blockly';\n\n export class MyCustomTextField extends Blockly.FieldTextInput {\n\n constructor(value, validator, config) {\n super(value, validator, config);\n }\n }\n\nThe constructor for a field's subclass looks very similar to the [constructor for\na custom field](/blockly/guides/create-custom-blocks/fields/customizing-fields/creating#implementing_a_constructor). The signature of the sub-constructor should\ngenerally match the signature of the super-constructor.\n\nJSON and registration\n---------------------\n\nYou should also register the field once: \n\n Blockly.fieldRegistry.register('my_custom_text_field', MyCustomTextField);\n\nand provide an implementation of `fromJson` in the class so that it works with\nthe JSON format: \n\n static fromJson(options) {\n const value = Blockly.utils.parsing.replaceMessageReferences(options.value);\n return new MyCustomTextField(value);\n }\n\nFor more information about registering a field see the [JSON and registration](/blockly/guides/create-custom-blocks/fields/customizing-fields/creating#json_and_registration)\nsection in Creating a Custom Field."]]