feat(modal): add 'beforeDismiss' option

Closes #1598

Closes #1799

src/modal/modal-ref.ts

@@ -50,7 +50,7 @@ export class NgbModalRef {
 
   constructor(
       private _windowCmptRef: ComponentRef<NgbModalWindow>, private _contentRef: ContentRef,
-      private _backdropCmptRef?: ComponentRef<NgbModalBackdrop>) {
+      private _backdropCmptRef?: ComponentRef<NgbModalBackdrop>, private _beforeDismiss?: Function) {
     _windowCmptRef.instance.dismissEvent.subscribe((reason: any) => { this.dismiss(reason); });
 
     this.result = new Promise((resolve, reject) => {
@@ -75,8 +75,10 @@ export class NgbModalRef {
    */
   dismiss(reason?: any): void {
     if (this._windowCmptRef) {
-      this._reject(reason);
-      this._removeModalElements();
+      if (!this._beforeDismiss || this._beforeDismiss() !== false) {
+        this._reject(reason);
+        this._removeModalElements();
+      }
     }
   }
 

src/modal/modal-stack.ts

@@ -53,7 +53,7 @@ export class NgbModalStack {
     this._applicationRef.attachView(windowCmptRef.hostView);
     containerEl.appendChild(windowCmptRef.location.nativeElement);
 
-    ngbModalRef = new NgbModalRef(windowCmptRef, contentRef, backdropCmptRef);
+    ngbModalRef = new NgbModalRef(windowCmptRef, contentRef, backdropCmptRef, options.beforeDismiss);
 
     activeModal.close = (result: any) => { ngbModalRef.close(result); };
     activeModal.dismiss = (reason: any) => { ngbModalRef.dismiss(reason); };

src/modal/modal.spec.ts

@@ -367,6 +367,43 @@ describe('ngb-modal', () => {
     });
   });
 
+  describe('beforeDismiss options', () => {
+
+    it('should not dismiss when the callback returns false', () => {
+      const modalInstance = fixture.componentInstance.openTplDismiss({beforeDismiss: () => { return false; }});
+      fixture.detectChanges();
+      expect(fixture.nativeElement).toHaveModal();
+
+      (<HTMLElement>document.querySelector('button#dismiss')).click();
+      fixture.detectChanges();
+      expect(fixture.nativeElement).toHaveModal();
+
+      modalInstance.close();
+      fixture.detectChanges();
+      expect(fixture.nativeElement).not.toHaveModal();
+    });
+
+    it('should dimiss when the callback does not return false', () => {
+      fixture.componentInstance.openTplDismiss({beforeDismiss: () => {}});
+      fixture.detectChanges();
+      expect(fixture.nativeElement).toHaveModal();
+
+      (<HTMLElement>document.querySelector('button#dismiss')).click();
+      fixture.detectChanges();
+      expect(fixture.nativeElement).not.toHaveModal();
+    });
+
+    it('should dismiss when the callback is not defined', () => {
+      fixture.componentInstance.openTplDismiss({});
+      fixture.detectChanges();
+      expect(fixture.nativeElement).toHaveModal();
+
+      (<HTMLElement>document.querySelector('button#dismiss')).click();
+      fixture.detectChanges();
+      expect(fixture.nativeElement).not.toHaveModal();
+    });
+  });
+
   describe('container options', () => {
 
     it('should attach window and backdrop elements to the specified container', () => {

src/modal/modal.ts

@@ -13,6 +13,12 @@ export interface NgbModalOptions {
    */
   backdrop?: boolean | 'static';
 
+  /**
+   * Function called when a modal will be dismissed.
+   * If this function returns false, the modal is not dismissed.
+   */
+  beforeDismiss?: () => boolean;
+
   /**
    * An element to which to attach newly opened modal windows.
    */