feat(paginator): expose previousPageIndex inside PageEvent (#10759)

* Exposes the previous page index inside the object that is emitted by the `page` event.
* Reworks the `page` event tests to use a spy rather than saving the last result to the test component.

Fixes #10758.

src/lib/paginator/paginator.spec.ts

@@ -1,6 +1,6 @@
 import {async, ComponentFixture, TestBed, inject, tick, fakeAsync} from '@angular/core/testing';
 import {MatPaginatorModule} from './index';
-import {MatPaginator, PageEvent} from './paginator';
+import {MatPaginator} from './paginator';
 import {Component, ViewChild} from '@angular/core';
 import {MatPaginatorIntl} from './paginator-intl';
 import {NoopAnimationsModule} from '@angular/platform-browser/animations';
@@ -111,7 +111,10 @@ describe('MatPaginator', () => {
       dispatchMouseEvent(getNextButton(fixture), 'click');
 
       expect(paginator.pageIndex).toBe(1);
-      expect(component.latestPageEvent ? component.latestPageEvent.pageIndex : null).toBe(1);
+      expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+        previousPageIndex: 0,
+        pageIndex: 1
+      }));
     });
 
     it('should be able to go to the previous page', () => {
@@ -122,7 +125,10 @@ describe('MatPaginator', () => {
       dispatchMouseEvent(getPreviousButton(fixture), 'click');
 
       expect(paginator.pageIndex).toBe(0);
-      expect(component.latestPageEvent ? component.latestPageEvent.pageIndex : null).toBe(0);
+      expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+        previousPageIndex: 1,
+        pageIndex: 0
+      }));
     });
   });
 
@@ -169,7 +175,10 @@ describe('MatPaginator', () => {
       dispatchMouseEvent(getLastButton(fixture), 'click');
 
       expect(paginator.pageIndex).toBe(9);
-      expect(component.latestPageEvent ? component.latestPageEvent.pageIndex : null).toBe(9);
+      expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+        previousPageIndex: 0,
+        pageIndex: 9
+      }));
     });
 
     it('should be able to go to the first page via the first page button', () => {
@@ -180,7 +189,10 @@ describe('MatPaginator', () => {
       dispatchMouseEvent(getFirstButton(fixture), 'click');
 
       expect(paginator.pageIndex).toBe(0);
-      expect(component.latestPageEvent ? component.latestPageEvent.pageIndex : null).toBe(0);
+      expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+        previousPageIndex: 3,
+        pageIndex: 0
+      }));
     });
 
     it('should disable navigating to the next page if at last page', () => {
@@ -189,10 +201,10 @@ describe('MatPaginator', () => {
       expect(paginator.pageIndex).toBe(9);
       expect(paginator.hasNextPage()).toBe(false);
 
-      component.latestPageEvent = null;
+      component.pageEvent.calls.reset();
       dispatchMouseEvent(getNextButton(fixture), 'click');
 
-      expect(component.latestPageEvent).toBe(null);
+      expect(component.pageEvent).not.toHaveBeenCalled();
       expect(paginator.pageIndex).toBe(9);
     });
 
@@ -200,10 +212,10 @@ describe('MatPaginator', () => {
       expect(paginator.pageIndex).toBe(0);
       expect(paginator.hasPreviousPage()).toBe(false);
 
-      component.latestPageEvent = null;
+      component.pageEvent.calls.reset();
       dispatchMouseEvent(getPreviousButton(fixture), 'click');
 
-      expect(component.latestPageEvent).toBe(null);
+      expect(component.pageEvent).not.toHaveBeenCalled();
       expect(paginator.pageIndex).toBe(0);
     });
 
@@ -270,35 +282,37 @@ describe('MatPaginator', () => {
     fixture.detectChanges();
 
     // The first item of the page should be item with index 40
-    let firstPageItemIndex: number | null = paginator.pageIndex * paginator.pageSize;
-    expect(firstPageItemIndex).toBe(40);
+    expect(paginator.pageIndex * paginator.pageSize).toBe(40);
 
     // The first item on the page is now 25. Change the page size to 25 so that we should now be
     // on the second page where the top item is index 25.
+    component.pageEvent.calls.reset();
     paginator._changePageSize(25);
-    let paginationEvent = component.latestPageEvent;
-    firstPageItemIndex = paginationEvent ?
-        paginationEvent.pageIndex * paginationEvent.pageSize : null;
-    expect(firstPageItemIndex).toBe(25);
-    expect(paginationEvent ? paginationEvent.pageIndex : null).toBe(1);
+
+    expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+      pageIndex: 1,
+      pageSize: 25
+    }));
 
     // The first item on the page is still 25. Change the page size to 8 so that we should now be
     // on the fourth page where the top item is index 24.
+    component.pageEvent.calls.reset();
     paginator._changePageSize(8);
-    paginationEvent = component.latestPageEvent;
-    firstPageItemIndex = paginationEvent ?
-        paginationEvent.pageIndex * paginationEvent.pageSize : null;
-    expect(firstPageItemIndex).toBe(24);
-    expect(paginationEvent ? paginationEvent.pageIndex : null).toBe(3);
+
+    expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+      pageIndex: 3,
+      pageSize: 8
+    }));
 
     // The first item on the page is 24. Change the page size to 16 so that we should now be
     // on the first page where the top item is index 0.
+    component.pageEvent.calls.reset();
     paginator._changePageSize(25);
-    paginationEvent = component.latestPageEvent;
-    firstPageItemIndex = paginationEvent ?
-        paginationEvent.pageIndex * paginationEvent.pageSize : null;
-    expect(firstPageItemIndex).toBe(0);
-    expect(paginationEvent ? paginationEvent.pageIndex : null).toBe(0);
+
+    expect(component.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+      pageIndex: 0,
+      pageSize: 25
+    }));
   });
 
   it('should show a select only if there are multiple options', () => {
@@ -363,7 +377,7 @@ function getLastButton(fixture: ComponentFixture<any>) {
                    [hidePageSize]="hidePageSize"
                    [showFirstLastButtons]="showFirstLastButtons"
                    [length]="length"
-                   (page)="latestPageEvent = $event">
+                   (page)="pageEvent($event)">
     </mat-paginator>
   `,
 })
@@ -374,8 +388,7 @@ class MatPaginatorApp {
   hidePageSize = false;
   showFirstLastButtons = false;
   length = 100;
-
-  latestPageEvent: PageEvent | null;
+  pageEvent = jasmine.createSpy('page event');
 
   @ViewChild(MatPaginator) paginator: MatPaginator;
 

src/lib/paginator/paginator.ts

@@ -33,6 +33,12 @@ export class PageEvent {
   /** The current page index. */
   pageIndex: number;
 
+  /**
+   * Index of the page that was selected previously.
+   * @deletion-target 7.0.0 To be made into a required property.
+   */
+  previousPageIndex?: number;
+
   /** The current page size */
   pageSize: number;
 
@@ -144,31 +150,39 @@ export class MatPaginator extends _MatPaginatorBase implements OnInit, OnDestroy
   /** Advances to the next page if it exists. */
   nextPage(): void {
     if (!this.hasNextPage()) { return; }
+
+    const previousPageIndex = this.pageIndex;
     this.pageIndex++;
-    this._emitPageEvent();
+    this._emitPageEvent(previousPageIndex);
   }
 
   /** Move back to the previous page if it exists. */
   previousPage(): void {
     if (!this.hasPreviousPage()) { return; }
+
+    const previousPageIndex = this.pageIndex;
     this.pageIndex--;
-    this._emitPageEvent();
+    this._emitPageEvent(previousPageIndex);
   }
 
   /** Move to the first page if not already there. */
   firstPage(): void {
     // hasPreviousPage being false implies at the start
     if (!this.hasPreviousPage()) { return; }
+
+    const previousPageIndex = this.pageIndex;
     this.pageIndex = 0;
-    this._emitPageEvent();
+    this._emitPageEvent(previousPageIndex);
   }
 
   /** Move to the last page if not already there. */
   lastPage(): void {
     // hasNextPage being false implies at the end
     if (!this.hasNextPage()) { return; }
+
+    const previousPageIndex = this.pageIndex;
     this.pageIndex = this.getNumberOfPages();
-    this._emitPageEvent();
+    this._emitPageEvent(previousPageIndex);
   }
 
   /** Whether there is a previous page. */
@@ -200,10 +214,11 @@ export class MatPaginator extends _MatPaginatorBase implements OnInit, OnDestroy
     // Current page needs to be updated to reflect the new page size. Navigate to the page
     // containing the previous page's first item.
     const startIndex = this.pageIndex * this.pageSize;
-    this.pageIndex = Math.floor(startIndex / pageSize) || 0;
+    const previousPageIndex = this.pageIndex;
 
+    this.pageIndex = Math.floor(startIndex / pageSize) || 0;
     this.pageSize = pageSize;
-    this._emitPageEvent();
+    this._emitPageEvent(previousPageIndex);
   }
 
   /**
@@ -221,19 +236,20 @@ export class MatPaginator extends _MatPaginatorBase implements OnInit, OnDestroy
     }
 
     this._displayedPageSizeOptions = this.pageSizeOptions.slice();
-    if (this._displayedPageSizeOptions.indexOf(this.pageSize) == -1) {
+
+    if (this._displayedPageSizeOptions.indexOf(this.pageSize) === -1) {
       this._displayedPageSizeOptions.push(this.pageSize);
     }
 
     // Sort the numbers using a number-specific sort function.
     this._displayedPageSizeOptions.sort((a, b) => a - b);
-
     this._changeDetectorRef.markForCheck();
   }
 
   /** Emits an event notifying that a change of the paginator's properties has been triggered. */
-  private _emitPageEvent() {
+  private _emitPageEvent(previousPageIndex: number) {
     this.page.emit({
+      previousPageIndex,
       pageIndex: this.pageIndex,
       pageSize: this.pageSize,
       length: this.length