feat(connected-position): apply the fallback position that shows the largest area of the element

Switches the `connected-position` to pick the fallback position with the largest visible area, if all of the fallbacks didn't fit into the viewport.

Fixes #2049.

Author: crisbeto

src/lib/core/overlay/position/connected-position-strategy.spec.ts

@@ -211,6 +211,36 @@ describe('ConnectedPositionStrategy', () => {
       expect(overlayRect.right).toBe(originRect.left);
     });
 
+    it('should pick the fallback position that shows the largest area of the element', () => {
+      // Use the fake viewport ruler because we don't know *exactly* how big the viewport is.
+      fakeViewportRuler.fakeRect = {
+        top: 0, left: 0, width: 500, height: 500, right: 500, bottom: 500
+      };
+      positionBuilder = new OverlayPositionBuilder(fakeViewportRuler);
+
+      originElement.style.top = '200px';
+      originElement.style.left = '475px';
+      originRect = originElement.getBoundingClientRect();
+
+      strategy = positionBuilder.connectedTo(
+        fakeElementRef,
+        {originX: 'end', originY: 'center'},
+        {overlayX: 'start', overlayY: 'center'})
+        .withFallbackPosition(
+          {originX: 'end', originY: 'top'},
+          {overlayX: 'start', overlayY: 'bottom'})
+        .withFallbackPosition(
+          {originX: 'end', originY: 'top'},
+          {overlayX: 'end', overlayY: 'top'});
+
+      strategy.apply(overlayElement);
+
+      let overlayRect = overlayElement.getBoundingClientRect();
+
+      expect(overlayRect.top).toBe(originRect.top);
+      expect(overlayRect.left).toBe(originRect.left);
+    });
+
     it('should position a panel properly when rtl', () => {
       // must make the overlay longer than the origin to properly test attachment
       overlayElement.style.width = `500px`;

src/lib/core/overlay/position/connected-position-strategy.ts

@@ -76,28 +76,33 @@ export class ConnectedPositionStrategy implements PositionStrategy {
 
     // We use the viewport rect to determine whether a position would go off-screen.
     const viewportRect = this._viewportRuler.getViewportRect();
-    let firstOverlayPoint: Point = null;
+    let attemptedPositions: OverlayPoint[] = [];
 
     // We want to place the overlay in the first of the preferred positions such that the
     // overlay fits on-screen.
     for (let pos of this._preferredPositions) {
       // Get the (x, y) point of connection on the origin, and then use that to get the
       // (top, left) coordinate for the overlay at `pos`.
       let originPoint = this._getOriginConnectionPoint(originRect, pos);
-      let overlayPoint = this._getOverlayPoint(originPoint, overlayRect, pos);
-      firstOverlayPoint = firstOverlayPoint || overlayPoint;
+      let overlayPoint = this._getOverlayPoint(originPoint, overlayRect, viewportRect, pos);
 
       // If the overlay in the calculated position fits on-screen, put it there and we're done.
-      if (this._willOverlayFitWithinViewport(overlayPoint, overlayRect, viewportRect)) {
+      if (overlayPoint.fitsInViewport) {
         this._setElementPosition(element, overlayPoint);
         this._onPositionChange.next(new ConnectedOverlayPositionChange(pos));
         return Promise.resolve(null);
+      } else {
+        attemptedPositions.push(overlayPoint);
       }
     }
 
-    // TODO(jelbourn): fallback behavior for when none of the preferred positions fit on-screen.
-    // For now, just stick it in the first position and let it go off-screen.
-    this._setElementPosition(element, firstOverlayPoint);
+    // If none of the preferred positions were in the viewport, rank them based on the
+    // visible area that the element would have at that position and take the one with
+    // largest visible area.
+    this._setElementPosition(element, attemptedPositions.sort((a, b) => {
+      return a.visibleArea - b.visibleArea;
+    }).pop());
+
     return Promise.resolve(null);
   }
 
@@ -172,15 +177,14 @@ export class ConnectedPositionStrategy implements PositionStrategy {
 
   /**
    * Gets the (x, y) coordinate of the top-left corner of the overlay given a given position and
-   * origin point to which the overlay should be connected.
-   * @param originPoint
-   * @param overlayRect
-   * @param pos
+   * origin point to which the overlay should be connected, as well as how much of the element
+   * would be inside the viewport at that position.
    */
   private _getOverlayPoint(
       originPoint: Point,
       overlayRect: ClientRect,
-      pos: ConnectionPositionPair): Point {
+      viewportRect: ClientRect,
+      pos: ConnectionPositionPair): OverlayPoint {
     // Calculate the (overlayStartX, overlayStartY), the start of the potential overlay position
     // relative to the origin point.
     let overlayStartX: number;
@@ -199,31 +203,26 @@ export class ConnectedPositionStrategy implements PositionStrategy {
       overlayStartY = pos.overlayY == 'top' ? 0 : -overlayRect.height;
     }
 
-    return {
-      x: originPoint.x + overlayStartX + this._offsetX,
-      y: originPoint.y + overlayStartY + this._offsetY
-    };
-  }
+    // The (x, y) coordinates of the overlay.
+    let x = originPoint.x + overlayStartX + this._offsetX;
+    let y = originPoint.y + overlayStartY + this._offsetY;
 
+    // How much the overlay would overflow at this position, on each side.
+    let leftOverflow = viewportRect.left - x;
+    let rightOverflow = (x + overlayRect.width) - viewportRect.right;
+    let topOverflow = viewportRect.top - y;
+    let bottomOverflow = (y + overlayRect.height) - viewportRect.bottom;
 
-  /**
-   * Gets whether the overlay positioned at the given point will fit on-screen.
-   * @param overlayPoint The top-left coordinate of the overlay.
-   * @param overlayRect Bounding rect of the overlay, used to get its size.
-   * @param viewportRect The bounding viewport.
-   */
-  private _willOverlayFitWithinViewport(
-      overlayPoint: Point,
-      overlayRect: ClientRect,
-      viewportRect: ClientRect): boolean {
+    // Visible parts of the element on each axis.
+    let visibleWidth = this._subtractOverflows(overlayRect.width, leftOverflow, rightOverflow);
+    let visibleHeight = this._subtractOverflows(overlayRect.height, topOverflow, bottomOverflow);
 
-    // TODO(jelbourn): probably also want some space between overlay edge and viewport edge.
-    return overlayPoint.x >= viewportRect.left &&
-        overlayPoint.x + overlayRect.width <= viewportRect.right &&
-        overlayPoint.y >= viewportRect.top &&
-        overlayPoint.y + overlayRect.height <= viewportRect.bottom;
-  }
+    // The area of the element that's within the viewport.
+    let visibleArea = visibleWidth * visibleHeight;
+    let fitsInViewport = (overlayRect.width * overlayRect.height) === visibleArea;
 
+    return {x, y, fitsInViewport, visibleArea};
+  }
 
   /**
    * Physically positions the overlay element to the given coordinate.
@@ -234,8 +233,29 @@ export class ConnectedPositionStrategy implements PositionStrategy {
     element.style.left = overlayPoint.x + 'px';
     element.style.top = overlayPoint.y + 'px';
   }
-}
 
+  /**
+   * Subtracts the amount that an element is overflowing on an axis from it's length.
+   */
+  private _subtractOverflows(length: number, ...overflows: number[]): number {
+    return overflows.reduce((currentValue: number, currentOverflow: number) => {
+      return currentValue - Math.max(currentOverflow, 0);
+    }, length);
+  }
+}
 
 /** A simple (x, y) coordinate. */
-type Point = {x: number, y: number};
+interface Point {
+  x: number;
+  y: number;
+};
+
+/**
+ * Expands the simple (x, y) coordinate by adding info about whether the
+ * element would fit inside the viewport at that position, as well as
+ * how much of the element would be visible.
+ */
+interface OverlayPoint extends Point {
+  visibleArea?: number;
+  fitsInViewport?: boolean;
+}