feat(input): add directive for displayed error messages

Adds the `md-error` directive that can be utilised to display validation errors to the user. Example:

```
<md-input-container>
  <input mdInput placeholder="email" [formControl]="emailFormControl">
  <md-error *ngIf="emailFormControl.errors.required">This field is required</md-error>
  <md-error *ngIf="emailFormControl.errors.pattern">
    Please enter a valid email address
  </md-error>
</md-input-container>
```

The `md-input-container` behavior is as follows:
* If there is an error and the user interacted with the input, the errors will be shown.
* If there is an error and the user submitted the a form that wraps the input, the errors will be shown.
* If there are errors to be shown on an input container that has `md-hint`-s, the hints will be hidden.
* If an input with hints and errors becomes valid, the hint won't be displayed anymore.

**Note:** At the moment, all hints will be hidden when an error is shown. This might not be intended for some cases (e.g. with a character counter). It might make sense to only hide the one in the `start` slot, but this could be addressed separately.

Author: crisbeto

src/demo-app/input/input-demo.html

@@ -48,6 +48,49 @@
   </md-card-content>
 </md-card>
 
+<md-card class="demo-card demo-basic">
+  <md-toolbar color="primary">Error messages</md-toolbar>
+  <md-card-content>
+    <h4>Regular</h4>
+
+    <p>
+      <md-input-container>
+        <input mdInput placeholder="example" [(ngModel)]="errorMessageExample1" required>
+        <md-error>This field is required</md-error>
+      </md-input-container>
+
+      <md-input-container>
+        <input mdInput placeholder="email" [formControl]="emailFormControl">
+        <md-error *ngIf="emailFormControl.errors.required">This field is required</md-error>
+        <md-error *ngIf="emailFormControl.errors.pattern">
+          Please enter a valid email address
+        </md-error>
+      </md-input-container>
+    </p>
+
+    <h4>With hint</h4>
+
+    <md-input-container>
+      <input mdInput placeholder="example" [(ngModel)]="errorMessageExample2" required>
+      <md-error>This field is required</md-error>
+      <md-hint>Please type something here</md-hint>
+    </md-input-container>
+
+
+    <form novalidate>
+      <h4>Inside a form</h4>
+
+      <md-input-container>
+        <input mdInput name="example" placeholder="example"
+          [(ngModel)]="errorMessageExample3" required>
+        <md-error>This field is required</md-error>
+      </md-input-container>
+
+      <button color="primary" md-raised-button>Submit</button>
+    </form>
+  </md-card-content>
+</md-card>
+
 <md-card class="demo-card demo-basic">
   <md-toolbar color="primary">Prefix + Suffix</md-toolbar>
   <md-card-content>

src/demo-app/input/input-demo.ts

@@ -4,6 +4,8 @@ import {FormControl, Validators} from '@angular/forms';
 
 let max = 5;
 
+const EMAIL_REGEX = /^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/;
+
 @Component({
   moduleId: module.id,
   selector: 'input-demo',
@@ -17,6 +19,9 @@ export class InputDemo {
   ctrlDisabled = false;
 
   name: string;
+  errorMessageExample1: string;
+  errorMessageExample2: string;
+  errorMessageExample3: string;
   items: any[] = [
     { value: 10 },
     { value: 20 },
@@ -26,6 +31,7 @@ export class InputDemo {
   ];
   rows = 8;
   formControl = new FormControl('hello', Validators.required);
+  emailFormControl = new FormControl('', [Validators.required, Validators.pattern(EMAIL_REGEX)]);
   model = 'hello';
 
   addABunch(n: number) {

src/lib/input/_input-theme.scss

@@ -8,12 +8,12 @@
   $warn: map-get($theme, warn);
   $background: map-get($theme, background);
   $foreground: map-get($theme, foreground);
-  
+
   // Placeholder colors. Required is used for the `*` star shown in the placeholder.
   $input-placeholder-color: mat-color($foreground, hint-text);
   $input-floating-placeholder-color: mat-color($primary);
   $input-required-placeholder-color: mat-color($accent);
-  
+
   // Underline colors.
   $input-underline-color: mat-color($foreground, divider);
   $input-underline-color-accent: mat-color($accent);
@@ -64,7 +64,7 @@
     }
   }
 
-  .mat-input-container.ng-invalid.ng-touched:not(.mat-focused) {
+  .mat-input-invalid {
     .mat-input-placeholder,
     .mat-placeholder-required {
       color: $input-underline-color-warn;
@@ -73,5 +73,13 @@
     .mat-input-underline {
       border-color: $input-underline-color-warn;
     }
+
+    .mat-input-ripple {
+      background-color: $input-underline-color-warn;
+    }
+  }
+
+  .mat-input-error {
+    color: $input-underline-color-warn;
   }
 }

src/lib/input/index.ts

@@ -1,5 +1,5 @@
 import {NgModule, ModuleWithProviders} from '@angular/core';
-import {MdPlaceholder, MdInputContainer, MdHint, MdInputDirective} from './input-container';
+import {MdPlaceholder, MdInputContainer, MdHint, MdInputDirective, MdErrorDirective} from './input-container';
 import {MdTextareaAutosize} from './autosize';
 import {CommonModule} from '@angular/common';
 import {FormsModule} from '@angular/forms';
@@ -12,7 +12,8 @@ import {PlatformModule} from '../core/platform/index';
     MdInputContainer,
     MdHint,
     MdTextareaAutosize,
-    MdInputDirective
+    MdInputDirective,
+    MdErrorDirective
   ],
   imports: [
     CommonModule,
@@ -24,7 +25,8 @@ import {PlatformModule} from '../core/platform/index';
     MdInputContainer,
     MdHint,
     MdTextareaAutosize,
-    MdInputDirective
+    MdInputDirective,
+    MdErrorDirective
   ],
 })
 export class MdInputModule {

src/lib/input/input-container.html

@@ -36,6 +36,14 @@
           [class.mat-warn]="dividerColor == 'warn'"></span>
   </div>
 
-  <div *ngIf="hintLabel != ''" [attr.id]="_hintLabelId" class="mat-hint">{{hintLabel}}</div>
-  <ng-content select="md-hint, mat-hint"></ng-content>
+  <div class="mat-input-hint-wrapper" [ngSwitch]="_getDisplayedMessages()">
+    <div *ngSwitchCase="'error'" [@transitionMessages]="_messageAnimationState">
+      <ng-content select="md-error, mat-error"></ng-content>
+    </div>
+
+    <div *ngSwitchCase="'hint'" class="mat-input-hint-clearfix" [@transitionMessages]="_messageAnimationState">
+      <div *ngIf="hintLabel != ''" [attr.id]="_hintLabelId" class="mat-hint">{{hintLabel}}</div>
+      <ng-content select="md-hint, mat-hint"></ng-content>
+    </div>
+  </div>
 </div>

src/lib/input/input-container.scss

@@ -3,6 +3,7 @@
 
 
 $mat-input-floating-placeholder-scale-factor: 0.75 !default;
+$mat-input-wrapper-spacing: 1em !default;
 
 // Gradient for showing the dashed line when the input is disabled.
 $mat-input-underline-disabled-background-image:
@@ -32,7 +33,7 @@ $mat-input-underline-disabled-background-image:
 // Global wrapper. We need to apply margin to the element for spacing, but
 // cannot apply it to the host element directly.
 .mat-input-wrapper {
-  margin: 1em 0;
+  margin: $mat-input-wrapper-spacing 0;
   // Account for the underline which has 4px of margin + 2px of border.
   padding-bottom: 6px;
 }
@@ -211,29 +212,53 @@ $mat-input-underline-disabled-background-image:
   }
 }
 
-// The hint is shown below the underline. There can be more than one; one at the start
-// and one at the end.
-.mat-hint {
-  display: block;
+// Wrapper for the hints and error messages. Provides positioning and text size.
+// Note that we're using `top` in order to allow for stacked children to flow downwards.
+.mat-input-hint-wrapper {
   position: absolute;
   font-size: 75%;
-  bottom: 0;
+  top: 100%;
+  width: 100%;
+  margin-top: -$mat-input-wrapper-spacing;
+}
+
+// The hint is shown below the underline. There can be
+// more than one; one at the start and one at the end.
+.mat-hint {
+  display: block;
+  float: left;
 
   &.mat-right {
-    right: 0;
+    float: right;
   }
 
   [dir='rtl'] & {
-    right: 0;
-    left: auto;
+    float: right;
 
     &.mat-right {
-      right: auto;
-      left: 0;
+      float: left;
     }
   }
 }
 
+// Clears the floats on the hints. Necessary for the `transform` animation to work.
+.mat-input-hint-clearfix {
+  &::before,
+  &::after {
+    content: '';
+    display: table;
+  }
+
+  &::after {
+    clear: both;
+  }
+}
+
+// Single errror message displayed beneath the input.
+.mat-input-error {
+  display: block;
+}
+
 .mat-input-prefix, .mat-input-suffix {
   // Prevents the prefix and suffix from stretching together with the container.
   width: 0.1px;