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+# Example - Tree Explorer
+
+**Note**:
+
+**This is a proposed API. It's experimental in nature and subject to change or removal without notice.**
+
+**Currently, this API is only available when developing extensions using [Insiders](https://code.visualstudio.com/insiders) version of VS Code. Packaged extensions using this API won't run. Please refrain from publishing any extensions using this API.**
+
+The Tree Explorer API allows you to create customized Tree Explorers, similar to the File Explorer that comes with VS Code. A Tree Explorer extension adds an icon to the Activity Bar and creates a custom View when the icon is clicked.
+
+From a high level, extension defines an `explorer` contribution, where it specifies a View label, an icon and a provider id. During the activation, extension calls `vscode.window.registerTreeExplorerNodeProvider` with the specified provider id. VS Code then asks the registered provider to provide necessary information for rendering the Tree Explorer View.
+
+The remaining document assumes that you are familiar with normal [extension development](https://code.visualstudio.com/docs/extensions/overview) for VS Code.
+
+## Example - Dependency Tree Explorer
+
+We'll show you an example of a Dependency Tree Explorer:
+
+![tree-explorer](tree-explorer.png)
+
+To get it running, do the following:
+
+- Clone this repository
+- `cd tree-explorer-sample`
+- Install dependencies with `npm install`
+- Run `Launch Extension` in the Debug View
+
+By default the Explorer wouldn't be displayed on Activity Bar. Find `View: Toggle Custom Explorer` in Command Palette and choose your Explorer to enable it.
+
+### The Structure of Tree Explorer extension
+
+First, let's take a look at `package.json`. There are a few interesting bits:
+
+```json
+{
+  "enableProposedApi": true,
+  "contributes": {
+    "explorer": {
+      "treeLabel": "Dependencies",
+      "icon": "media/dep.svg",
+      "treeExplorerNodeProviderId": "depTree"
+    }
+  }
+}
+```
+
+- `enableProposedApi` tells VS Code we acknowledge its experimental nature and would like to play with it.
+- `contributes.explorer` specifies
+    - A label that will appear at the top of the View
+    - The path to a svg icon that will appear at the Activity Bar
+    - A provider id which we'll use in `vscode.window.registerTreeExplorerNodeProvider`
+
+`typings/vscode.proposed.d.ts` defines the preview API, and is pulled from VS Code's GitHub repository: https://github.com/Microsoft/vscode/blob/master/src/vs/vscode.proposed.d.ts
+
+Let's jump to `src/extension.ts` and take a look at the code.
+
+### The Implementation of a TreeExplorerNodeProvider
+
+A `TreeExplorerNodeProvider` implements two required methods and three optional methods:
+
+```ts
+interface TreeExplorerNodeProvider<T> {
+  provideRootNode(): T | Thenable<T>;
+  resolveChildren(node: T): T[] | Thenable<T[]>;
+
+  getLabel?(node: T): string;
+  getHasChildren?(node: T): boolean;
+  getClickCommand?(node: T): string;
+}
+```
+
+The type `T` is the class of each node. `provideRootNode` provides the invisible node that represents the root of the tree. `resolveChildren` will be invoked when an expandable node is clicked upon, or invoked with the root node as argument when the Activity Bar icon is clicked.
+
+The three optional methods are used for extracting relevant information used for rendering the tree. `getLabel` returns the label for each node, `getHasChildren` determines if each node is expandable, and `getClickCommand` returns a string that represents the command that should be executed when a node is clicked upon. When these functions are not provided, VS Code will use these default values:
+
+- `node.toString()` as `label`
+- `true` as `hasChildren`
+- `null` as `clickCommand`
+
+In our dependency example, the node is defined as an union type:
+
+```ts
+type DepNode = Root // Root node
+             | Node // A dependency installed to `node_modules`
+             | Leaf // A dependency not present in `node_modules`
+             ;
+```
+
+A `Node` is a dependency that still has `dependencies` or `devDependencies` in its `package.json`. A `Leaf` is a dependency that depends on no other packages.
+
+In our `DepNodeProvider`:
+
+```ts
+provideRootNode(): DepNode {
+  return new Root();
+}
+
+resolveChildren(node: DepNode): Thenable<DepNode[]> {
+  return new Promise((resolve) => {
+    switch(node.kind) {
+      case 'root':
+        const packageJsonPath = path.join(this.workspaceRoot, 'package.json');
+        if (this.fileExists(packageJsonPath)) {
+          resolve(this.getDepsInPackageJson(packageJsonPath));
+        } else {
+          vscode.window.showInformationMessage('Workspace has no package.json');
+          resolve([]);
+        }
+        break;
+      case 'node':
+        resolve(this.getDepsInPackageJson(path.join(this.workspaceRoot, 'node_modules', node.moduleName, 'package.json')));
+        break;
+      case 'leaf':
+        resolve([]);
+    }
+  });
+}
+```
+
+`provideRootNode` simply provides a `Root` node. `resolveChildren` behaves differently accordingly to the type of the node to resolves children for:
+
+- `Root`: read `package.json` in `${workspaceRoot}`, and return its `dependencies` and `devDependencies`.
+- `Node`: read `package.json` in `${workspaceRoot}/node_modules/${node.moduleName}`, and return its `dependencies` and `devDependencies`.
+- `Leaf`: `Leaf` isn't expandable and this case shouldn't happen. We return an empty array.
+
+We also implement `getLabel`, `getHasChildren`, `getClickCommand` to:
+
+- Let each `Node` and `Leaf` display its `moduleName` as the label on the tree.
+
+  ```ts
+    getLabel(node: DepNode): string {
+      return node.kind === 'root' ? '' : node.moduleName;
+    }
+  ```
+
+- Make `Leaf` unexpandable.
+
+  ```ts
+  getHasChildren(node: DepNode): boolean {
+    return node.kind !== 'leaf';
+  }
+  ```
+
+- Invoke `extension.openPackageOnNpm` when a `Leaf` is clicked.
+
+  ```ts
+  getClickCommand(node: DepNode): string {
+    return node.kind === 'leaf' ? 'extension.openPackageOnNpm' : null;
+  }
+  ```
+
+## Implementation Considerations
+
+#### Icon
+
+To ensure UI consistency with other Views, you should provide a 32x32 SVG icon with transparent background as `explorer.icon` in package.json.
+
+#### Long running `resolveChildren`
+
+If `resolveChildren` could take very long, you should let it return a promise, and VS Code will show a progress bar as the promise gets resolved.
+
+#### Error handling
+
+We suggest that you handle errors promptly and notify the user using `vscode.window.showInformationMessage` if needed. In our Dependency example, we check for package.json in the root of the workspace. If it doesn't exist, we'll show an information message `Workspace has no package.json`.
+
+## Common Questions
+
+**Q: Will it be possible to access the DOM and add an arbitrary UI using this API?**
+
+**A:** No.
+
+**Q: Can I add an Outline View/Symbol Tree View using this API?**
+
+**A:** We plan to include an API where language services could register a provider for Outline View items, similar to the existing [`DocumentSymbolProvider`](https://code.visualstudio.com/docs/extensionAPI/vscode-api#DocumentSymbolProvider). Please refrain from using this API to make Outline Views.
+
+**Q: Can I have functionality X?**
+
+**A:** We realize this API in its current form is limiting, and we are planning to evolve it. You can track our progress in this [GitHub issue](https://github.com/Microsoft/vscode/issues/15485). If you have ideas about how we could improve this API, please feel free to chime in.
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