ng-spy

A lightweight, dependecy-free scrollspy for angular. Use this library to spy on HTML elements on your page when the window is scrolled or resized.

This library provides directives and services to help you subscribe to new elements entering the browser window and possibly adds or remove CSS classes to these elements.

This library does not provide utilities to scrollTo spied elements and certainly does not handle link clicks. You have to handle these on your own.

Table of Contents

1. Example
2. Usage
3. Documentation
   • Directives
   • Services
   • Interfaces
   • Injection Tokens

Example

Checkout this example

Code for the previous example is also in this repository under the src folder.

Usage

1. npm install --save ng-spy

2. Import the module

   @NgModule({
     declarations: [],
     imports: [
       // ...
       ScrollSpyModule
     ],
     providers: [],
     bootstrap: [AppComponent]
   })
   export class AppModule { }

3. In your container component:

   ngAfterViewInit() {
     this.spyService.spy({ thresholdBottom: 50 });
   }

4. Use SpyTarget directive to mark element as a target:

   <h1 spyTarget="target-1">Hello, World!!</h1>

5. Optional: Use SpyOn directive to spy on a target:

   <a spyOn="target-1" activeClass="navbar__link--active" class="navbar__link" href="#target-1">target 1</a>

6. Alternatively, you can subscribe to activeSpyTarget from ScrollSpyService, which will emit every time a new element is active.

   this.scrollspyService.activeSpyTarget.subscribe(
     (activeTargetName: string) => console.log(activeTargetName)
   );

7. To start multiple spys you can scope the ScrollSpyService to a specific component or module in the providers array.

   // In component or module metadata:
   {
     // ...
     providers: [ ScrollSpyService ]
   }

Documentation

Directives

SpyOn

A directive to spy on a previously added target.

Input

Name	Type	Description	Default value
activeClass	string	a class to be added to host element if it is active
spyOn	string	the name of the target to spy on

SpyTarget

A directive to mark an element as a spy target.

Input

Name	Type	Description	Default value
spyTarget	string	the name of the target to be added. This also adds an id attribute to the host element with the spyTarget value

Services

ScrollSpyService

Functions

spy(spyOptions: SpyOptions): void

Starts spying on the targets. Call this on the top level container of your targets in AfterViewInit lifecycle hook. Without calling this function, nothing will happen.

Parameters:

Name	Type	Optional?	Description	Default value
spyOptions	SpyOptions	Yes	Options to override some default behaviors. See SpyOptions for more details.	{ thresholdTop = 0 , thresholdBottom = 0 }

addTarget(target: SpyTarget): void

adds the target to the targets collection for spying. This action will also trigger a check to re-evaluate the active target.

Parameters:

Name	Type	Optional?	Description	Default value
target	SpyTarget	No	The target to be added.

removeTarget(target: string): void

Removes a previously added target from the targets collection. This action will also trigger a check to re-evaluate the active target.

Parameters:

Name	Type	Optional?	Description	Default value
target	string	No	The target name to be removed

checkActiveElement(scrollContainer?: ElementRef): void

Tries to find the active target in the targets collection. If found it will emit the target name using the activeSpyTarget observable, Otherwise it will emit null.

Parameters:

Name	Type	Optional?	Description	Default value
scrollContainer	ElementRef	Yes	The scroll container in which your targets are present.

isElementActive(element: ElementRef, scrollContainer?: ElementRef, currentActiveElement?: ElementRef): boolean

Checks if the element is inside the browser window and optionally inside the scrollContainer.

If a currentActiveElement is provided, it will also check if the element's offsetTop is greater than the currentActiveElement. Meaning, if two targets are active, only the last one on the screen will be emitted.

If a scrollContainer is provided, it will ignore thresholdBottom and thresholdTop in the window check and will instead check for the threshold inside the scrollContainer.

Parameters:

Name	Type	Optional?	Description	Default value
element	ElementRef	No	The element to be checked.
scrollContainer	ElementRef	Yes	The scroll container which the element is inside.
currentActiveElement	ElementRef	Yes	An element that is guaranteed to be active.

stopSpying(): void

Stops spying on the targets, completes the activeSpyTarget subject and clears the targets collection.

Getters

activeSpyTarget: Observable<string>

Use this to subscribe to new active spy targets. Value can be null if no target is found to be active.

Interfaces

SpyOptions

Name	Type	Optional?	Description
scrollContainer	ElementRef	Yes	The scroll container in which your targets are present.
thresholdTop	number	Yes	A number added to the top of your scroll container while checking in isElementActive
thresholdBottom	number	Yes	A number added to the bottom of your scroll container while checking in isElementActive

SpyTarget

Name	Type	Optional?	Description
name	string	No	The name of the spy target. This is also may be used as an id for the html element. So make sure this is unique.
element	ElementRef	No	A reference to the target element.

InjectionTokens

RESIZE_TIME_THRESHOLD

Used in the resize event with auditTime.

Default Value: 300

SCROLL_TIME_THRESHOLD

Used in the scroll events with auditTime.

Default Value: 10