feat: salvage Angular developer skill

skills/angular-developer/references/angular-animations.md

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+# Angular Animations
+
+When animating elements in Angular, **first analyze the project's Angular version** in `package.json`.
+For modern applications (**Angular v20.2 and above**), prefer using native CSS with `animate.enter` and `animate.leave`. For older applications, you may need to use the deprecated `@angular/animations` package.
+
+## 1. Native CSS Animations (v20.2+ Recommended)
+
+Modern Angular provides `animate.enter` and `animate.leave` to animate elements as they enter or leave the DOM. They apply CSS classes at the appropriate times.
+
+### `animate.enter` and `animate.leave`
+
+Use these directly on elements to apply CSS classes during the enter or leave phase. Angular automatically removes the enter classes when the animation completes. For `animate.leave`, Angular waits for the animation to finish before removing the element from the DOM.
+
+`animate.enter` example:
+
+```html
+@if (isShown()) {
+<div class="enter-container" animate.enter="enter-animation">
+  <p>The box is entering.</p>
+</div>
+}
+```
+
+```css
+/* Ensure you have a starting style if using transitions instead of keyframes */
+.enter-container {
+  border: 1px solid #dddddd;
+  margin-top: 1em;
+  padding: 20px;
+  font-weight: bold;
+  font-size: 20px;
+}
+.enter-container p {
+  margin: 0;
+}
+.enter-animation {
+  animation: slide-fade 1s;
+}
+@keyframes slide-fade {
+  from {
+    opacity: 0;
+    transform: translateY(20px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+```
+
+_Note: `animate.leave` may be added to child elements being removed._
+
+### Event Bindings and Third-party Libraries
+
+You can bind to `(animate.enter)` and `(animate.leave)` to call functions or use JS libraries like GSAP.
+
+```html
+@if(show()) {
+<div (animate.leave)="onLeave($event)">...</div>
+}
+```
+
+```ts
+import { AnimationCallbackEvent } from '@angular/core';
+
+onLeave(event: AnimationCallbackEvent) {
+  // Custom animation logic here
+  // CRITICAL: You MUST call animationComplete() when done so Angular removes the element!
+  event.animationComplete();
+}
+```
+
+## 2. Advanced CSS Animations
+
+CSS offers robust tools for advanced animation sequences.
+
+### Animating State and Styles
+
+Toggle CSS classes on elements using property binding to trigger transitions.
+
+```html
+<div [class.open]="isOpen">...</div>
+```
+
+```css
+div {
+  transition: height 0.3s ease-out;
+  height: 100px;
+}
+div.open {
+  height: 200px;
+}
+```
+
+### Animating Auto Height
+
+You can use `css-grid` to animate to auto height.
+
+```css
+.container {
+  display: grid;
+  grid-template-rows: 0fr;
+  transition: grid-template-rows 0.3s;
+}
+.container.open {
+  grid-template-rows: 1fr;
+}
+.container > div {
+  overflow: hidden;
+}
+```
+
+### Staggering and Parallel Animations
+
+- **Staggering**: Use `animation-delay` or `transition-delay` with different values for items in a list.
+- **Parallel**: Apply multiple animations in the `animation` shorthand (e.g., `animation: rotate 3s, fade-in 2s;`).
+
+### Programmatic Control
+
+Retrieve animations directly using standard Web APIs:
+
+```ts
+const animations = element.getAnimations();
+animations.forEach((anim) => anim.pause());
+```
+
+## 3. Legacy Animations DSL (Deprecated)
+
+For older projects (pre v20.2 or where `@angular/animations` is already heavily used), you use the component metadata DSL.
+
+**Important:** Do not mix legacy animations and `animate.enter`/`leave` in the same component.
+
+### Setup
+
+```ts
+bootstrapApplication(App, {
+  providers: [provideAnimationsAsync()],
+});
+```
+
+### Defining Transitions
+
+```ts
+import {signal} from '@angular/core';
+import {trigger, state, style, animate, transition} from '@angular/animations';
+
+@Component({
+  animations: [
+    trigger('openClose', [
+      state('open', style({opacity: 1})),
+      state('closed', style({opacity: 0})),
+      transition('open <=> closed', [animate('0.5s')]),
+    ]),
+  ],
+  template: `<div [@openClose]="isOpen() ? 'open' : 'closed'">...</div>`,
+})
+export class OpenClose {
+  isOpen = signal(true);
+}
+```