feat(tree/testing): add getTreeStructure for tree harness

Author: annieyw

src/material/tree/testing/shared.spec.ts

@@ -96,6 +96,28 @@ export function runHarnessTests(
     // no-op if already collapsed
     expect(await firstGroup.isExpanded()).toBe(false);
   });
+
+  it ('should correctly get tree structure', async () => {
+    const trees = await loader.getAllHarnesses(treeHarness);
+    const flatTree = trees[0];
+    expect(await flatTree.getTreeStructure()).toEqual(
+      `Flat Group 1
+Flat Group 2`);
+  });
+
+  it('should correctly get tree structure', async () => {
+    const trees = await loader.getAllHarnesses(treeHarness);
+    const nestedTree = trees[1];
+    expect(await nestedTree.getTreeStructure()).toEqual(
+      `Nested Group 1
+\tNested Leaf 1.1
+\tNested Leaf 1.2
+\tNested Leaf 1.3
+Nested Group 2
+\tNested Group 2.1
+\t\tNested Leaf 2.1.1
+\t\tNested Leaf 2.1.2`);
+  });
 }
 
 interface FoodNode {

src/material/tree/testing/tree-harness.ts

@@ -28,4 +28,17 @@ export class MatTreeHarness extends ComponentHarness {
   async getNodes(filter: TreeNodeHarnessFilters = {}): Promise<MatTreeNodeHarness[]> {
     return this.locatorForAll(MatTreeNodeHarness.with(filter))();
   }
+
+  async getTreeStructure(): Promise<string> {
+    let treeString = '';
+    const nodes = await this.getNodes();
+    for (let i = 0; i < nodes.length; i++) {
+      treeString += i === 0 ? '' : '\n';
+      const node = nodes[i];
+      const level = await node.getLevel();
+      treeString += '\t'.repeat(level - 1);
+      treeString += await node.getText();
+    }
+    return treeString;
+  }
 }