Why Custom structural directives in Angular? - Purpose & Use Cases

The Big Idea

Discover how to clean up your Angular templates and make your UI logic reusable with just a few lines of code!

The Scenario

Imagine you want to show or hide parts of your webpage based on complex rules, like user roles or feature flags, and you have to write the same logic everywhere in your HTML templates.

The Problem

Manually repeating show/hide logic clutters your templates, makes them hard to read, and if you need to change the rule, you must update many places, risking mistakes and bugs.

The Solution

Custom structural directives let you create reusable, clean, and easy-to-understand commands that control the structure of your page, so you write the logic once and use it everywhere effortlessly.

Before vs After

✗ Before

*ngIf="user.isAdmin && featureEnabled"<br>&nbsp;&nbsp;<!-- content -->

✓ After

*appShowIfAdminFeature<br>&nbsp;&nbsp;<!-- content -->

What It Enables

It enables you to build clear, reusable, and maintainable templates that adapt dynamically without repeating complex logic.

Real Life Example

Showing special admin controls only to users with admin rights and when a feature is enabled, using a simple directive instead of repeating conditions everywhere.

Key Takeaways

Manual logic repetition makes templates messy and error-prone.

Custom structural directives encapsulate complex display rules.

They improve code clarity, reuse, and maintenance.

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