问题

I started investigating this issue with this question which was partially resolved in the iOS 9.2 SDK.

However, upon further investigation, I realized that this framework was still not working as expected.

In summary, a GKGraph can be constructed with nodes (GKGraphNode and its subclasses), between which pathfinding costs and paths can be calculated. A GKGraphNode2D is simply a GKGraphNode that sits on and encapsulates its coordinates in a two-dimensional grid. GKGraphNode can be subclassed, and the methods costToNode(_:) and estimatedCostToNode(_:) can be overridden to provide costs between nodes. Once these custom nodes are added to a graph, the graph should use these methods to determine the lowest-cost path between connected nodes.

I am constructing nodes that I'll call GeometricNode, where the cost between nodes is simply the distance (in two-dimensional coordinate space) between two nodes. Nodes are connected to all their neighbors in 2D space, including their diagonal neighbors. I.e., neighboring nodes above, below, to the left and right of a particular node are a distance of 1 away, while diagonally-neighboring nodes are sqrt(2) ~ 1.414 away. These costs are calculated in the overridden methods costToNode(_:) and estimatedCostToNode(_:).

Unfortunately, even though these connections are appropriately set up, and the costs are appropriately calculated, GKGraph does not always calculate the correct path when invoking findPathFromNode(_:toNode:).

Code (Sample project)

@import UIKit;
@import Foundation;
@import GameplayKit;

@interface GeometricNode : GKGraphNode2D
@end

@implementation GeometricNode

- (float)estimatedCostToNode:(GKGraphNode *)node {
    NSAssert(([node isKindOfClass:[GeometricNode class]]), @"Must Use GeometricNode");
    return [self geometricDistanceToNode:(GeometricNode *)node];
}

- (float)costToNode:(GKGraphNode *)node {
    NSAssert(([node isKindOfClass:[GeometricNode class]]), @"Must Use GeometricNode");
    return [self geometricDistanceToNode:(GeometricNode *)node];
}

- (CGFloat)geometricDistanceToNode:(GeometricNode *)node {
    return sqrtf( powf(self.position[0] - node.position[0], 2) + powf(self.position[1] - node.position[1], 2) );
}

@end

And the failing test:

@import XCTest;

#import "GeometricNode.h"

@interface Graph_Node_TestTests : XCTestCase

@property (nonatomic, strong) GKGraph *graph;
@property (nonatomic, strong) NSArray <GeometricNode *>* nodes;

@end


@implementation Graph_Node_TestTests

- (void)setUp {
    /*
     This is the graph we are creating:

     A---B---C
     | X | X |
     F---E---D

     where all of the nodes are `GeometricNode` objects.

     The nodes' cost to each other is determined by simple geometry, assuming
     the nodes are spaced in a grid all one unit away from their top-left-bottom-right
     neighbors. Diagonal nodes are spaced sqrt(2) ~ 1.414 away from one another.
     */

    GeometricNode *nodeA = [[GeometricNode alloc]  initWithPoint:(vector_float2){0, 0}];
    GeometricNode *nodeB = [[GeometricNode alloc]  initWithPoint:(vector_float2){1, 0}];
    GeometricNode *nodeC = [[GeometricNode alloc]  initWithPoint:(vector_float2){2, 0}];
    GeometricNode *nodeD = [[GeometricNode alloc]  initWithPoint:(vector_float2){2, 1}];
    GeometricNode *nodeE = [[GeometricNode alloc]  initWithPoint:(vector_float2){1, 1}];
    GeometricNode *nodeF = [[GeometricNode alloc]  initWithPoint:(vector_float2){0, 1}];

    [nodeA addConnectionsToNodes:@[ nodeB, nodeF, nodeE ] bidirectional:YES];
    [nodeC addConnectionsToNodes:@[ nodeB, nodeD, nodeE ] bidirectional:YES];
    [nodeE addConnectionsToNodes:@[ nodeF, nodeD, nodeB ] bidirectional:YES];
    [nodeB addConnectionsToNodes:@[ nodeF, nodeD ] bidirectional:YES];

    self.nodes = @[ nodeA, nodeB, nodeC, nodeD, nodeE, nodeF ];
    self.graph = [GKGraph graphWithNodes:self.nodes];
}

- (void)testNodeCosts {
    /*
     A---B---C
     | X | X |
     F---E---D

     We would expect, for example, the path from A to C to be A-B-C, at a cost of
     2, to be chosen over a path of A-E-C, at a cost of 2.828.

     Instead, GKGraph chooses this latter incorrect path.
     */
    GeometricNode *nodeA = self.nodes[0];
    GeometricNode *nodeB = self.nodes[1];
    GeometricNode *nodeC = self.nodes[2];
    GeometricNode *nodeE = self.nodes[4];

    XCTAssert([nodeA.connectedNodes containsObject:nodeB], @"Node A is connected to Node B");
    XCTAssert([nodeB.connectedNodes containsObject:nodeC], @"Node B is connected to Node C");
    XCTAssert([nodeA.connectedNodes containsObject:nodeE], @"Node A is connected to Node E");
    XCTAssert([nodeE.connectedNodes containsObject:nodeC], @"Node E is connected to Node C");

    XCTAssertEqualWithAccuracy([nodeA costToNode:nodeB], 1, 0.001, @"Cost from A-B should be 1");
    XCTAssertEqualWithAccuracy([nodeB costToNode:nodeC], 1, 0.001, @"Cost from B-C should be 1");

    XCTAssertEqualWithAccuracy([nodeA costToNode:nodeE], sqrt(2), 0.001, @"Cost from A-E should be sqrt(2) ~ 1.414");
    XCTAssertEqualWithAccuracy([nodeE costToNode:nodeC], sqrt(2), 0.001, @"Cost from E-C should be sqrt(2) ~ 1.414");

    // The actual path calculated by GKGraph, and the expected and actual (incorrect) paths
    NSArray <GeometricNode *>* actualPath    = [self.graph findPathFromNode:nodeA toNode:nodeC];
    NSArray <GeometricNode *>* expectedPath  = @[ nodeA, nodeB, nodeC ];
    NSArray <GeometricNode *>* incorrectPath = @[ nodeA, nodeE, nodeC ];

    // We would expect GKGraph to choose this lowest-cost path: A-B-C
    CGFloat totalExpectedCost = [nodeA costToNode:nodeB] + [nodeB costToNode:nodeC];
    XCTAssertEqualWithAccuracy(totalExpectedCost, 2, 0.001, @"Lowest cost path cost should be 2");

    // GKGraph is actually choosing this more costly path: A-E-C
    CGFloat totalIncorrectCost = [nodeA costToNode:nodeE] + [nodeE costToNode:nodeC];
    CGFloat totalActualCost = 0;
    for (NSInteger i = 0; i < actualPath.count - 1; i++) {
        totalActualCost += [((GeometricNode *)actualPath[i]) costToNode:actualPath[i + 1]];
    }

    XCTAssert([actualPath isEqualToArray:expectedPath], @"Actual found path should be equal to A-B-C");
    XCTAssert(![actualPath isEqualToArray:incorrectPath], @"Actual found path should not be equal to A-E-C");
    XCTAssertGreaterThan(totalIncorrectCost, totalActualCost);
    XCTAssertLessThanOrEqual(totalActualCost, totalExpectedCost);
}

@end