Preview Pane is an existing feature in the File Explorer which shows a lightweight, rich, read-only preview of the file's contents in the view's reading pane. To enable it, you just click the View tab in the ribbon and then click `Preview Pane`. Below is an example of Markdown and Svg files previews in File Explorer with PowerToys.
Preview handlers are called when an item is selected to show a lightweight, rich, read-only preview of the file's contents in the view's reading pane. This is done without launching the file's associated application. Please follow this [documentation](https://docs.microsoft.com/en-us/archive/msdn-magazine/2007/january/windows-vista-and-office-writing-your-own-preview-handlers) to start developing a preview handler, when done, continue with this documentation to learn how to integrate a preview handler into PowerToys.
We have already done most of the development work in the [PreviewHandlerCommon](./common/cominterop/IPreviewHandler.cs) common project. To add a preview for the file type of .xyz:
- Add a new .NET project in the preview pane folder.
- Add a reference to the `PreviewHandlerCommon` common project.
- Create your preview handler class and extend the FileBasedPreviewHandler class. See an example below:
```csharp
using System;
using System.Runtime.InteropServices;
using Common;
namespace XYZPreviewHandler
{
/// <summary>
/// Implementation of preview handler for .xyz files.
/// GUID = CLSID / CLASS ID.
/// </summary>
[Guid("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx")]
[ClassInterface(ClassInterfaceType.None)]
[ComVisible(true)]
public class XYZPreviewHandler : FileBasedPreviewHandler
this.xyzPreviewHandlerControl = new xyzPreviewHandlerControl();
return this.xyzPreviewHandlerControl;
}
}
}
```
Create a separate Preview Handler Control class and extend the `FormHandlerControl` Class.
```csharp
using Common;
namespace XYZPreviewHandler
{
public class XYZPreviewHandlerControl : FormHandlerControl
{
public XYZPreviewHandlerControl()
{
// ... do your initializations here.
}
public override void DoPreview<T>(T dataSource)
{
// ... add your preview rendering code here.
}
}
}
```
#### Integrate the Preview Handler into PowerToys Settings:
Navigate to the [powerpreview](../previewpane/powerpreview/powerpreview.h) project and edit the `powerpreview.h` file. Add the following Settings Object instance to `m_previewHandlers` settings objects array in the constructor initialization:
```cpp
// XYZ Preview Handler Settings Object.
FileExplorerPreviewSettings(
false,
L"<--YOUR_TOGGLE_CONTROL_ID-->",
L"<--Adescriptionofyourpreviewhandler-->",
L"xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx", // your preview handler CLSID.
To add a new Previewer update the `Product.wxs` file in `PowerToysSetup` similar to existing Previewer to register the Preview Handler. More details about registration of Preview Handlers can be [found here.](https://docs.microsoft.com/en-us/windows/win32/shell/how-to-register-a-preview-handler)
Warning: There are known issues([Issue - 1446](https://github.com/microsoft/PowerToys/issues/1446), [Issue - 1545](https://github.com/microsoft/PowerToys/issues/1545)) with MSIX Installation of Preview Handlers and it's not fully supported.
To add a new Previewer with MSIX update the `appxmanifest.xml` file to add file type association for preview handler and add `SurrogateServer` element with class registration. MSIX currently doesn't support .Net Assembly activation with `SurrogateServer` the logic used is shim the activation by using native dll. `dllmain.cpp` in `powerpreview` project expose the `DLLGetClassObject` method which is used to activate .net Assembly by using `CoGetClassObject`.
Add the actual Clsid of the new preview handler in `CLSID.h` in `powerpreview` project and class registration registry changes in `registry.dat` similar to existing preview handler. To update the `registry.dat` mount the registry hive on a empty key in registry by using `reggedit.exe` and add registry key for class registartion for the new preview handler similar to MSI class registration and existing preview handlers. And export the updated `registry.dat` hive also export the `registry.reg` file for making the contents in `registy.dat` visible in source code.
```cpp
// CLSID used in manifest file for Preview Handler.
After successful integration, your preview handler should appear in the PowerToys settings UI under the `File Explorer Preview` Tab. In here you should be able to enable and disable your preview handler.
