PropertyRequiredMixin #4272
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| # PropertyRequiredMixin | ||
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| This mixin will make a component's property "required". If a value is not provided for a required property, an exception will be thrown asynchronously. | ||
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| Required properties are useful in situations where a missing value would put the component into an invalid state, such as an accessibility violation. | ||
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| Only `String` properties can be required. | ||
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| ## Making a Property Required | ||
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| To require a property, simply set `required` to `true` in the reactive Lit property definition: | ||
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| ```javascript | ||
| import { PropertyRequiredMixin } from '@brightspace-ui/core/mixins/property-required/property-required-mixin.js'; | ||
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| class MyElem extends PropertyRequiredMixin(LitElement) { | ||
| static properties = { | ||
| prop: { type: String, required: true } | ||
| }; | ||
| } | ||
| ``` | ||
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| If a non-empty `String` value for `prop` is not provided, an exception will be thrown after a few seconds. | ||
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| ## Custom Validation | ||
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| By default, validating a required property involves ensuring that it's a non-empty `String`. To customize this logic, set `required` to an object and provide a `validator` delegate. | ||
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| ```javascript | ||
| static properties = { | ||
| mustBeFoo: { | ||
| required: { | ||
| validator: (value, elem, hasValue) => { | ||
| return value === 'foo'; | ||
| } | ||
| }, | ||
| type: String | ||
| } | ||
| }; | ||
| ``` | ||
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| The `validator` will be passed the current property `value`, a reference to the element and the result of the default validation. | ||
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| ## Custom Messages | ||
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| To customize the exception message that gets thrown when validation fails, set `required` to an object and provide a `message` delegate. | ||
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| ```javascript | ||
| static properties = { | ||
| prop: { | ||
| required: { | ||
| message: (value, elem, defaultMessage) => { | ||
| return `"prop" on "${elem.tagName}" is required.`; | ||
| } | ||
| }, | ||
| type: String | ||
| } | ||
| }; | ||
| ``` | ||
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| The `message` delegate will be passed the current property `value`, a reference to the element and the default message. | ||
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| ## Dependent Properties | ||
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| The mixin will automatically listen for updates to a required property and re-run the `validator` when the value changes. If custom validation logic relies on multiple properties, list them in `dependentProps`. | ||
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| ```javascript | ||
| static properties = { | ||
| prop: { | ||
| required: { | ||
| dependentProps: ['isReallyRequired'] | ||
| validator: (value, elem, hasValue) => { | ||
| return elem.isReallyRequired && hasValue; | ||
| } | ||
| }, | ||
| type: String | ||
| }, | ||
| isReallyRequired: { type: Boolean } | ||
| }; | ||
| ``` | ||
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| ## Unit Testing | ||
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| If no custom `validator` is used, tests that assert the correct required property errors are thrown are unnecessary -- `@brightspace-ui/core` already covers those tests. | ||
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| Required property exceptions are thrown asynchronously multiple seconds after the component has rendered. For unit tests, this makes catching them challenging. | ||
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| To help, use the `flushRequiredPropertyErrors()` method. | ||
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| ```javascript | ||
| const tag = defineCE( | ||
| class extends PropertyRequiredMixin(LitElement) { | ||
| static properties = { | ||
| prop: { | ||
| type: String, | ||
| required: { | ||
| validator: (value) => value === 'valid' | ||
| } | ||
| } | ||
| }; | ||
| } | ||
| ); | ||
| const elem = await fixture(`<${tag} prop="invalid"></${tag}>`); | ||
| expect(() => elem.flushRequiredPropertyErrors()).to.throw(); | ||
| ``` |
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This use case forced me to add support for "dependent properties" -- essentially other properties to watch and re-validate when they change.