From 534af71f40cfba930737ff48b18be9a6b39ac04e Mon Sep 17 00:00:00 2001
From: crisbeto <crisbeto@abv.bg>
Date: Mon, 24 Dec 2018 16:14:08 +0200
Subject: [PATCH] fix(paginator): update page index if invalid after length
 change

Updates the page index of a paginator if its length changed to something where the index isn't valid anymore (e.g. the user is on the last page and the amount of pages becomes smaller).

Fixes #14520.
---
 src/lib/paginator/paginator.spec.ts | 21 +++++++++++++++++++++
 src/lib/paginator/paginator.ts      | 12 ++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/src/lib/paginator/paginator.spec.ts b/src/lib/paginator/paginator.spec.ts
index d6dfe0e714db..7b3b23af99f1 100644
--- a/src/lib/paginator/paginator.spec.ts
+++ b/src/lib/paginator/paginator.spec.ts
@@ -414,6 +414,27 @@ describe('MatPaginator', () => {
     expect(getLastButton(fixture).hasAttribute('disabled')).toBe(true);
   });
 
+  it('should update the page index when switching to a smaller length', fakeAsync(() => {
+    fixture.componentInstance.length = 50;
+    fixture.componentInstance.pageSize = 10;
+    fixture.componentInstance.pageIndex = 4;
+    fixture.detectChanges();
+
+    expect(paginator.pageIndex).toBe(4);
+
+    fixture.componentInstance.pageEvent.calls.reset();
+
+    fixture.componentInstance.length = 10;
+    fixture.detectChanges();
+    tick();
+
+    expect(paginator.pageIndex).toBe(0);
+    expect(fixture.componentInstance.pageEvent).toHaveBeenCalledWith(jasmine.objectContaining({
+      previousPageIndex: 4,
+      pageIndex: 0
+    }));
+  }));
+
 });
 
 function getPreviousButton(fixture: ComponentFixture<any>) {
diff --git a/src/lib/paginator/paginator.ts b/src/lib/paginator/paginator.ts
index 0445dcd9f502..b9dd464cc24d 100644
--- a/src/lib/paginator/paginator.ts
+++ b/src/lib/paginator/paginator.ts
@@ -100,6 +100,18 @@ export class MatPaginator extends _MatPaginatorBase implements OnInit, OnDestroy
   get length(): number { return this._length; }
   set length(value: number) {
     this._length = coerceNumberProperty(value);
+
+    const maxPageIndex = Math.max(this.getNumberOfPages() - 1, 0);
+    const currentPageIndex = this._pageIndex;
+
+    if (currentPageIndex > maxPageIndex && this.initialized) {
+      // Needs to happen on the next tick, in order to avoid "changed after checked" errors.
+      Promise.resolve().then(() => {
+        this._pageIndex = maxPageIndex;
+        this._emitPageEvent(currentPageIndex);
+      });
+    }
+
     this._changeDetectorRef.markForCheck();
   }
   _length: number = 0;