feat(material/datepicker): allow custom pickers with date inputs

Author: MichaelJamesParsons

src/material/datepicker/date-range-input.ts

@@ -103,7 +103,7 @@ export class MatDateRangeInput<D> implements MatFormFieldControl<DateRange<D>>,
   get rangePicker() { return this._rangePicker; }
   set rangePicker(rangePicker: MatDateRangePicker<D>) {
     if (rangePicker) {
-      this._model = rangePicker._registerInput(this);
+      this._model = rangePicker.registerInput(this);
       this._rangePicker = rangePicker;
       this._registerModel(this._model!);
     }

src/material/datepicker/datepicker-base.ts

@@ -232,10 +232,19 @@ export interface MatDatepickerControl<D> {
   stateChanges: Observable<void>;
 }
 
+/** A picker panel that can be associated with a {@link MatDatepickerControl}. */
+export interface MatDatepickerPanel<C extends MatDatepickerControl<D>, S,
+    D = ExtractDateTypeFromSelection<S>> {
+  id: string;
+  opened: boolean;
+  open(): void;
+  registerInput(input: C): MatDateSelectionModel<S, D>;
+}
+
 /** Base class for a datepicker. */
 @Directive()
 export abstract class MatDatepickerBase<C extends MatDatepickerControl<D>, S,
-  D = ExtractDateTypeFromSelection<S>> implements OnDestroy, OnChanges {
+  D = ExtractDateTypeFromSelection<S>> implements MatDatepickerPanel<C, S, D>, OnDestroy, OnChanges {
   private _scrollStrategy: () => ScrollStrategy;
   private _inputStateChanges = Subscription.EMPTY;
 
@@ -450,7 +459,7 @@ export abstract class MatDatepickerBase<C extends MatDatepickerControl<D>, S,
    * @param input The datepicker input to register with this datepicker.
    * @returns Selection model that the input should hook itself up to.
    */
-  _registerInput(input: C): MatDateSelectionModel<S, D> {
+  registerInput(input: C): MatDateSelectionModel<S, D> {
     if (this._datepickerInput && (typeof ngDevMode === 'undefined' || ngDevMode)) {
       throw Error('A MatDatepicker can only be associated with a single input.');
     }

src/material/datepicker/datepicker-input.ts

@@ -78,7 +78,7 @@ export class MatDatepickerInput<D> extends MatDatepickerInputBase<D | null, D>
   set matDatepicker(datepicker: MatDatepicker<D>) {
     if (datepicker) {
       this._datepicker = datepicker;
-      this._registerModel(datepicker._registerInput(this));
+      this._registerModel(datepicker.registerInput(this));
     }
   }
   _datepicker: MatDatepicker<D>;