Custom structural directives in Angular - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom structural directive selector.

Angular

@Directive({ selector: '[[1]]' })
export class HighlightDirective {}

Drag options to blanks, or click blank then click option'

AmyHighlight

Bhighlight

CappHighlight

DcustomHighlight

Attempts:

3 left

💡 Hint

Common Mistakes

Using a selector without square brackets.

Using a selector that is not a valid attribute name.

The selector for a structural directive is usually a bracketed attribute name. 'highlight' is the correct attribute name here.

2fill in blank

medium

Complete the constructor to inject the TemplateRef needed for the directive.

Angular

constructor(private [1]: TemplateRef<any>) {}

Drag options to blanks, or click blank then click option'

AviewContainer

BelementRef

Crenderer

DtemplateRef

Attempts:

3 left

💡 Hint

Common Mistakes

Injecting ElementRef instead of TemplateRef.

Using a variable name that does not match the injected type.

TemplateRef is injected with a variable name like 'templateRef' to access the template inside the directive.

3fill in blank

hard

Fix the error in the directive to properly create an embedded view.

Angular

constructor(private templateRef: TemplateRef<any>, private [1]: ViewContainerRef) {}

ngOnInit() {
  this.[1].createEmbeddedView(this.templateRef);
}

Drag options to blanks, or click blank then click option'

AviewContainer

BcontainerRef

CviewContainerRef

Dcontainer

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong variable name that does not match the injected type.

Not injecting ViewContainerRef at all.

The correct Angular service to create embedded views is ViewContainerRef, commonly named 'viewContainerRef'.

4fill in blank

hard

Fill both blanks to conditionally render the template based on the input property.

Angular

@Input() set appIf(condition: boolean) {
  if (condition) {
    this.[1].createEmbeddedView(this.[2]);
  } else {
    this.[1].clear();
  }
}

Drag options to blanks, or click blank then click option'

AviewContainerRef

BtemplateRef

CcontainerRef

Dtemplate

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the roles of ViewContainerRef and TemplateRef.

Using incorrect variable names that do not match injected services.

The ViewContainerRef is used to create or clear views, and TemplateRef is the template to render.

5fill in blank

hard

Fill all three blanks to create a structural directive that repeats a template N times.

Angular

@Input() set appRepeat(count: number) {
  this.[1].clear();
  for (let i = 0; i < count; i++) {
    this.[2].createEmbeddedView(this.[3]);
  }
}

Drag options to blanks, or click blank then click option'

AviewContainerRef

CtemplateRef

Attempts:

3 left

💡 Hint

Common Mistakes

Not clearing the container before adding views.

Using wrong variable names for injected services.

Clear the view container, then create embedded views from the template the specified number of times.

Practice

1. What is the main purpose of a custom structural directive in Angular?

easy

A. To style elements with CSS classes

B. To fetch data from a server

C. To handle user input events

D. To add or remove HTML elements dynamically based on conditions

Solution

1. Step 1: Understand structural directives role

   Structural directives change the structure of the DOM by adding or removing elements.

2. Step 2: Identify the main use case

   Custom structural directives let you control when parts of the page appear or disappear dynamically.

3. Final Answer:

   To add or remove HTML elements dynamically based on conditions -> Option D

4. Quick Check:

   Structural directives = dynamic HTML blocks [OK]

Hint: Structural directives control HTML blocks, not styles or events [OK]

Common Mistakes:

• Confusing structural directives with attribute directives
• Thinking they handle styling or events
• Assuming they fetch data

2. Which of the following is the correct way to inject dependencies in a custom structural directive constructor?

easy

A. constructor(private http: HttpClient) {}

B. constructor(private elementRef: ElementRef, private renderer: Renderer2) {}

C. constructor(private templateRef: TemplateRef<any>, private viewContainer: ViewContainerRef) {}

D. constructor(private router: Router) {}

Solution

1. Step 1: Identify dependencies for structural directives

   Structural directives need TemplateRef to access the template and ViewContainerRef to insert or remove views.

2. Step 2: Match constructor parameters

   constructor(private templateRef: TemplateRef<any>, private viewContainer: ViewContainerRef) {} correctly injects TemplateRef and ViewContainerRef, which are essential for custom structural directives.

3. Final Answer:

   constructor(private templateRef: TemplateRef<any>, private viewContainer: ViewContainerRef) {} -> Option C

4. Quick Check:

   TemplateRef + ViewContainerRef = constructor params [OK]

Hint: Use TemplateRef and ViewContainerRef in constructor for structural directives [OK]

Common Mistakes:

• Injecting ElementRef or Renderer2 which are for attribute directives
• Injecting unrelated services like HttpClient or Router
• Missing TemplateRef or ViewContainerRef

3. Given this directive code snippet, what will be the rendered output if appShowIf input is false?

@Directive({ selector: '[appShowIf]' })
export class ShowIfDirective {
  constructor(private templateRef: TemplateRef<any>, private viewContainer: ViewContainerRef) {}
  @Input() set appShowIf(condition: boolean) {
    if (condition) {
      this.viewContainer.createEmbeddedView(this.templateRef);
    } else {
      this.viewContainer.clear();
    }
  }
}

Usage:

<div *appShowIf="false">Hello World</div>

medium

A. Nothing will be rendered inside the div

B. The text 'Hello World' will be displayed

C. An error will occur because of wrong syntax

D. The div will be rendered but empty

Solution

1. Step 1: Analyze the directive behavior when condition is false

   When appShowIf is false, viewContainer.clear() removes any embedded views, so nothing is rendered.

2. Step 2: Understand the usage effect

   The <div> with 'Hello World' is inside the template controlled by the directive, so it won't appear if condition is false.

3. Final Answer:

   Nothing will be rendered inside the div -> Option A

4. Quick Check:

   False condition = no content shown [OK]

Hint: False input clears view container, so no content appears [OK]

Common Mistakes:

• Thinking the div still renders empty
• Assuming an error occurs
• Confusing attribute directives with structural directives

4. Identify the error in this custom structural directive code:

@Directive({ selector: '[appIf]' })
export class IfDirective {
  constructor(private templateRef: TemplateRef<any>) {}
  @Input() set appIf(condition: boolean) {
    if (condition) {
      this.templateRef.createEmbeddedView();
    }
  }
}

medium

A. Incorrect selector syntax in @Directive decorator

B. Missing ViewContainerRef injection and usage to insert the view

C. Wrong input property name, should be 'appIfCondition'

D. No error, code is correct

Solution

1. Step 1: Check constructor dependencies

   The directive injects only TemplateRef but misses ViewContainerRef, which is needed to insert or clear views.

2. Step 2: Analyze method usage

   Calling createEmbeddedView() on TemplateRef alone is invalid; it should be called on ViewContainerRef with TemplateRef as argument.

3. Final Answer:

   Missing ViewContainerRef injection and usage to insert the view -> Option B

4. Quick Check:

   ViewContainerRef required to add views [OK]

Hint: Always inject ViewContainerRef to add or remove views [OK]

Common Mistakes:

• Trying to create views directly from TemplateRef
• Forgetting to inject ViewContainerRef
• Misnaming input properties

5. You want to create a custom structural directive *appUnless that shows content only when a condition is false. Which implementation correctly achieves this behavior?

hard

A. Use if (!condition) to create the embedded view, else clear it

B. Use if (condition) to create the embedded view, else clear it

C. Always create the embedded view regardless of condition

D. Clear the view only when condition is false

Solution

1. Step 1: Understand the directive goal

   *appUnless should show content only when the condition is false, so the view is created when !condition.

2. Step 2: Match logic to code

   Using if (!condition) to create the embedded view and clearing it otherwise matches the requirement.

3. Final Answer:

   Use if (!condition) to create the embedded view, else clear it -> Option A

4. Quick Check:

   Show content when false = if (!condition) create view [OK]

Hint: Invert condition logic to show content only when false [OK]

Common Mistakes:

• Using if (condition) instead of if (!condition)
• Not clearing the view when condition is true
• Creating view unconditionally