Custom structural directives in Angular - Step-by-Step Execution

Concept Flow - Custom structural directives

Start: Angular template

↓

Detect *directive syntax

↓

Call directive's constructor

↓

Directive receives TemplateRef & ViewContainerRef

↓

Directive decides when/how to add/remove views

↓

ViewContainerRef creates embedded views

↓

Angular renders embedded views in DOM

↓

Directive updates views on input changes

↓

End: DOM reflects directive logic

Angular reads the template, finds the custom structural directive, then uses its logic to add or remove parts of the DOM dynamically.

Execution Sample

Angular

import { Directive, Input, TemplateRef, ViewContainerRef } from '@angular/core';

@Directive({ selector: '[appShow]' })
export class ShowDirective {
  constructor(private tpl: TemplateRef<any>, private vcr: ViewContainerRef) {}

  @Input() set appShow(condition: boolean) {
    this.vcr.clear();
    if (condition) this.vcr.createEmbeddedView(this.tpl);
  }
}

This directive shows or hides a template part based on a boolean input.

Execution Table

Step	Action	Input Condition	ViewContainerRef State	DOM Change
1	Directive instantiated	N/A	Empty	No DOM added
2	Input appShow set to true	true	Cleared then view created	Template inserted in DOM
3	Input appShow set to false	false	Cleared	Template removed from DOM
4	Input appShow set to true again	true	Cleared then view created	Template re-inserted in DOM
5	No further input changes	N/A	View remains	DOM unchanged
6	End of trace	N/A	Final state kept	DOM reflects last input

💡 Execution stops as no more input changes occur; DOM matches directive logic.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
condition	undefined	true	false	true	true
ViewContainerRef state	empty	view created	cleared	view created	view created

Key Moments - 3 Insights

Why does the directive clear the ViewContainerRef before creating a new view?

Clearing removes any previously rendered views to avoid duplicates. See execution_table rows 2 and 3 where clearing happens before creating or removing views.

What happens if the input condition is false?

The directive clears the container and does not create a view, so the template is removed from the DOM. See execution_table row 3.

How does Angular know where to insert the template in the DOM?

Angular uses the ViewContainerRef linked to the directive's host element to insert or remove embedded views dynamically.

Visual Quiz - 3 Questions

Look at the execution_table, what is the ViewContainerRef state after step 3?

A View created

B Unchanged

C Cleared

D Error state

💡 Hint

Check the 'ViewContainerRef State' column in row 3 of the execution_table.

At which step does the template get inserted back into the DOM after being removed?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for when the input condition changes back to true and a view is created again.

If the input condition never changes from true, how would the execution_table change?

A No clearing steps would occur

B Views would be created and cleared repeatedly

C The template would never be inserted

D The directive would throw an error

💡 Hint

Refer to variable_tracker for condition changes and execution_table for clearing actions.

Concept Snapshot

Custom structural directives in Angular
- Use @Directive with selector like '[appShow]'
- Inject TemplateRef and ViewContainerRef
- Control DOM by creating/removing embedded views
- Use input setters to react to changes
- Clear container before adding views to avoid duplicates

Full Transcript

Custom structural directives in Angular let you add or remove parts of the page dynamically. Angular reads the template and finds your directive marked with a star (*). It calls your directive's constructor, giving you the template and a place to insert views. Your directive decides when to add or remove these views based on input values. For example, if the input is true, it creates the view and inserts it into the DOM. If false, it clears the container, removing the view. This process repeats whenever the input changes, keeping the page in sync with your logic.

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