feat(stepper): Add animation to stepper (angular#6361)

* Add animation

* Implement Angular animation

* Clean up unnecessary code

* Generalize animation so that vertical and horizontal steppers can use the same function

src/cdk/stepper/stepper.ts

@@ -30,8 +30,14 @@ import {AbstractControl} from '@angular/forms';
 /** Used to generate unique ID for each stepper component. */
 let nextId = 0;
 
+/**
+ * Position state of the content of each step in stepper that is used for transitioning
+ * the content into correct position upon step selection change.
+ */
+export type StepContentPositionState = 'previous' | 'current' | 'next';
+
 /** Change event emitted on selection changes. */
-export class CdkStepperSelectionEvent {
+export class StepperSelectionEvent {
   /** Index of the step now selected. */
   selectedIndex: number;
 
@@ -119,7 +125,7 @@ export class CdkStepper {
   }
 
   /** Event emitted when the selected step has changed. */
-  @Output() selectionChange = new EventEmitter<CdkStepperSelectionEvent>();
+  @Output() selectionChange = new EventEmitter<StepperSelectionEvent>();
 
   /** The index of the step that the focus can be set. */
   _focusIndex: number = 0;
@@ -146,11 +152,23 @@ export class CdkStepper {
     return `mat-step-label-${this._groupId}-${i}`;
   }
 
-  /** Returns nique id for each step content element. */
+  /** Returns unique id for each step content element. */
   _getStepContentId(i: number): string {
     return `mat-step-content-${this._groupId}-${i}`;
   }
 
+  /** Returns position state of the step with the given index. */
+  _getAnimationDirection(index: number): StepContentPositionState {
+    const position = index - this._selectedIndex;
+    if (position < 0) {
+      return 'previous';
+    } else if (position > 0) {
+      return 'next';
+    } else {
+      return 'current';
+    }
+  }
+
   private _emitStepperSelectionEvent(newIndex: number): void {
     const stepsArray = this._steps.toArray();
     this.selectionChange.emit({

src/lib/stepper/_stepper-theme.scss

@@ -33,11 +33,11 @@
     background-color: mat-color($background, card);
   }
 
-  .vertical-content-container {
+  .mat-vertical-content-container {
     border-left-color: mat-color($foreground, divider);
   }
 
-  .connector-line {
+  .mat-connector-line {
     border-top-color: mat-color($foreground, divider);
   }
 }

src/lib/stepper/stepper-horizontal.html

@@ -21,14 +21,16 @@
       </div>
     </div>
 
-    <div *ngIf="!isLast" class="connector-line"></div>
+    <div *ngIf="!isLast" class="mat-connector-line"></div>
   </ng-container>
 </div>
-
-<div *ngFor="let step of _steps; let i = index"
-     class="mat-horizontal-stepper-content" role="tabpanel"
-     [id]="_getStepContentId(i)"
-     [attr.aria-labelledby]="_getStepLabelId(i)"
-     [attr.aria-expanded]="selectedIndex == i">
-  <ng-container [ngTemplateOutlet]="step.content"></ng-container>
+<div class="mat-horizontal-content-container">
+  <div *ngFor="let step of _steps; let i = index"
+       class="mat-horizontal-stepper-content" role="tabpanel"
+       [@stepTransition]="_getAnimationDirection(i)"
+       [id]="_getStepContentId(i)"
+       [attr.aria-labelledby]="_getStepLabelId(i)"
+       [attr.aria-expanded]="selectedIndex == i">
+    <ng-container [ngTemplateOutlet]="step.content"></ng-container>
+  </div>
 </div>

src/lib/stepper/stepper-horizontal.ts

@@ -8,6 +8,7 @@
 
 import {Component} from '@angular/core';
 import {MdStepper} from './stepper';
+import {animate, state, style, transition, trigger} from '@angular/animations';
 
 @Component({
   moduleId: module.id,
@@ -19,6 +20,15 @@ import {MdStepper} from './stepper';
     'class': 'mat-stepper-horizontal',
     'role': 'tablist',
   },
+  animations: [
+    trigger('stepTransition', [
+      state('previous', style({transform: 'translate3d(-100%, 0, 0)', visibility: 'hidden'})),
+      state('current', style({transform: 'translate3d(0%, 0, 0)', visibility: 'visible'})),
+      state('next', style({transform: 'translate3d(100%, 0, 0)', visibility: 'hidden'})),
+      transition('* => *',
+          animate('500ms cubic-bezier(0.35, 0, 0.25, 1)'))
+    ])
+  ],
   providers: [{provide: MdStepper, useExisting: MdHorizontalStepper}]
 })
 export class MdHorizontalStepper extends MdStepper { }

src/lib/stepper/stepper-vertical.html

@@ -19,12 +19,15 @@
     </div>
 
   </div>
-  <div class="vertical-content-container">
+  <div class="mat-vertical-content-container">
     <div class="mat-vertical-stepper-content" role="tabpanel"
+         [@stepTransition]="_getAnimationDirection(i)"
          [id]="_getStepContentId(i)"
          [attr.aria-labelledby]="_getStepLabelId(i)"
          [attr.aria-expanded]="selectedIndex == i">
-      <ng-container [ngTemplateOutlet]="step.content"></ng-container>
+      <div class="mat-vertical-content">
+        <ng-container[ngTemplateOutlet]="step.content"></ng-container>
+      </div>
     </div>
   </div>
 </div>

src/lib/stepper/stepper-vertical.ts

@@ -8,6 +8,7 @@
 
 import {Component} from '@angular/core';
 import {MdStepper} from './stepper';
+import {animate, state, style, transition, trigger} from '@angular/animations';
 
 @Component({
   moduleId: module.id,
@@ -19,6 +20,14 @@ import {MdStepper} from './stepper';
     'class': 'mat-stepper-vertical',
     'role': 'tablist',
   },
+  animations: [
+      trigger('stepTransition', [
+        state('previous', style({height: '0px', visibility: 'hidden'})),
+        state('next', style({height: '0px', visibility: 'hidden'})),
+        state('current', style({height: '*', visibility: 'visible'})),
+        transition('* <=> current', animate('225ms cubic-bezier(0.4, 0.0, 0.2, 1)'))
+      ])
+  ],
   providers: [{provide: MdStepper, useExisting: MdVerticalStepper}]
 })
 export class MdVerticalStepper extends MdStepper { }

src/lib/stepper/stepper.scss

@@ -1,9 +1,11 @@
+@import '../core/style/variables';
+
 $mat-horizontal-stepper-header-height: 72px !default;
 $mat-label-header-height: 24px !default;
 $mat-stepper-label-min-width: 50px !default;
 $mat-stepper-side-gap: 24px !default;
 $mat-vertical-stepper-content-margin: 12px !default;
-$mat-vertical-stepper-content-padding: 16px 0 32px $mat-stepper-side-gap !default;
+$mat-vertical-content-padding-bottom: 32px !default;
 $mat-vertical-content-container-padding: 8px !default;
 $mat-connector-line-width: 1px !default;
 $mat-connector-line-gap: 8px !default;
@@ -43,6 +45,7 @@ $mat-vertical-stepper-margin-top: $mat-stepper-side-gap - $mat-connector-line-ga
   line-height: $mat-horizontal-stepper-header-height;
   overflow: hidden;
   align-items: center;
+  outline: none;
 
   .mat-stepper-index {
     margin-right: $mat-connector-line-gap;
@@ -55,13 +58,14 @@ $mat-vertical-stepper-margin-top: $mat-stepper-side-gap - $mat-connector-line-ga
   display: flex;
   align-items: center;
   margin: $mat-connector-line-gap 0;
+  outline: none;
 
   .mat-stepper-index {
     margin-right: $mat-vertical-stepper-content-margin;
   }
 }
 
-.connector-line {
+.mat-connector-line {
   border-top-width: $mat-connector-line-width;
   border-top-style: solid;
   width: $mat-horizontal-connector-line-size;
@@ -70,11 +74,19 @@ $mat-vertical-stepper-margin-top: $mat-stepper-side-gap - $mat-connector-line-ga
   height: 0;
 }
 
-.mat-horizontal-stepper-content[aria-expanded='false'] {
-    display: none;
+.mat-horizontal-stepper-content {
+  overflow: hidden;
+
+  &[aria-expanded='false'] {
+    height: 0;
+  }
+}
+
+.mat-horizontal-content-container {
+  overflow: hidden;
 }
 
-.vertical-content-container {
+.mat-vertical-content-container {
   border-left-width: $mat-connector-line-width;
   border-left-style: solid;
   margin-left: $mat-vertical-stepper-content-margin;
@@ -83,21 +95,18 @@ $mat-vertical-stepper-margin-top: $mat-stepper-side-gap - $mat-connector-line-ga
 
 .mat-vertical-stepper-content {
   padding-left: $mat-stepper-side-gap;
+  overflow: hidden;
+}
 
-  &[aria-expanded='false'] {
-    display: none;
-  }
-
-  &[aria-expanded='true'] {
-    padding: $mat-vertical-stepper-content-padding;
-  }
+.mat-vertical-content {
+  padding-bottom: $mat-vertical-content-padding-bottom;
 }
 
 .mat-step {
   margin-top: $mat-connector-line-gap;
 
   &:last-child {
-    .vertical-content-container {
+    .mat-vertical-content-container {
       border: none;
     }
   }