79 lines
3.9 KiB
Markdown
79 lines
3.9 KiB
Markdown
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{@a attribute-directive}
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# Testing Attribute Directives
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An _attribute directive_ modifies the behavior of an element, component or another directive.
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Its name reflects the way the directive is applied: as an attribute on a host element.
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<div class="alert is-helpful">
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For the sample app that the testing guides describe, see the <live-example embedded-style noDownload>sample app</live-example>.
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For the tests features in the testing guides, see <live-example stackblitz="specs" noDownload>tests</live-example>.
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</div>
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## Testing the `HighlightDirective`
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The sample application's `HighlightDirective` sets the background color of an element
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based on either a data bound color or a default color (lightgray).
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It also sets a custom property of the element (`customProperty`) to `true`
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for no reason other than to show that it can.
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<code-example path="testing/src/app/shared/highlight.directive.ts" header="app/shared/highlight.directive.ts"></code-example>
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It's used throughout the application, perhaps most simply in the `AboutComponent`:
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<code-example path="testing/src/app/about/about.component.ts" header="app/about/about.component.ts"></code-example>
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Testing the specific use of the `HighlightDirective` within the `AboutComponent` requires only the techniques explored in the ["Nested component tests"](guide/testing-components-scenarios#nested-component-tests) section of [Component testing scenarios](guide/testing-components-scenarios).
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<code-example path="testing/src/app/about/about.component.spec.ts" region="tests" header="app/about/about.component.spec.ts"></code-example>
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However, testing a single use case is unlikely to explore the full range of a directive's capabilities.
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Finding and testing all components that use the directive is tedious, brittle, and almost as unlikely to afford full coverage.
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_Class-only tests_ might be helpful,
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but attribute directives like this one tend to manipulate the DOM.
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Isolated unit tests don't touch the DOM and, therefore,
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do not inspire confidence in the directive's efficacy.
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A better solution is to create an artificial test component that demonstrates all ways to apply the directive.
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<code-example path="testing/src/app/shared/highlight.directive.spec.ts" region="test-component" header="app/shared/highlight.directive.spec.ts (TestComponent)"></code-example>
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<div class="lightbox">
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<img src='generated/images/guide/testing/highlight-directive-spec.png' alt="HighlightDirective spec in action">
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</div>
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<div class="alert is-helpful">
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The `<input>` case binds the `HighlightDirective` to the name of a color value in the input box.
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The initial value is the word "cyan" which should be the background color of the input box.
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</div>
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Here are some tests of this component:
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<code-example path="testing/src/app/shared/highlight.directive.spec.ts" region="selected-tests" header="app/shared/highlight.directive.spec.ts (selected tests)"></code-example>
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A few techniques are noteworthy:
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- The `By.directive` predicate is a great way to get the elements that have this directive _when their element types are unknown_.
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- The <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/:not">`:not` pseudo-class</a>
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in `By.css('h2:not([highlight])')` helps find `<h2>` elements that _do not_ have the directive.
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`By.css('*:not([highlight])')` finds _any_ element that does not have the directive.
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- `DebugElement.styles` affords access to element styles even in the absence of a real browser, thanks to the `DebugElement` abstraction.
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But feel free to exploit the `nativeElement` when that seems easier or more clear than the abstraction.
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- Angular adds a directive to the injector of the element to which it is applied.
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The test for the default color uses the injector of the second `<h2>` to get its `HighlightDirective` instance
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and its `defaultColor`.
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- `DebugElement.properties` affords access to the artificial custom property that is set by the directive.
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<hr>
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