Jot

Jot - a .NET library for state persistence

Introduction

Almost every application needs to keep track of its own state, regardless of what it otherwise does. This typically includes:

1. Sizes and locations of movable/resizable elements of the UI (forms, tool windows, draggable toolbars...)
2. Last entered data (e.g. username, selected tab indexes, recently opened files...)
3. Settings and user preferences

A common approach is to store this data in a .settings file and read and update it as needed. This involves writing a lot of boilerplate code to copy that data back and forth. This code is generally tedious, error-prone and no fun to write.

With Jot, you only need to declare which properties of which objects you want to track, and when to persist and apply data. This is a better abstraction for this requirement, resulting in more readable and concise code.

Installation

Jot is available on NuGet and can be installed from the package manager console:

install-package Jot

Example: Persisting the size and location of a Window

To illustrate the basic idea, let's compare two ways of dealing with this requirement: .settings file (Scenario A) versus Jot (Scenario B).

Scenario A (.settings file)

Step 1: Define settings

Step 2: Apply previously stored data

public MainWindow()
{
	InitializeComponent();

	this.Left = MySettings.Default.MainWindowLeft;
	this.Top = MySettings.Default.MainWindowTop;
	this.Width = MySettings.Default.MainWindowWidth;
	this.Height = MySettings.Default.MainWindowHeight;
	this.WindowState = MySettings.Default.MainWindowWindowState;
} 

Step 3: Persist updated data before the window is closed

protected override void OnClosed(EventArgs e)
{
	MySettings.Default.MainWindowLeft = this.Left;
	MySettings.Default.MainWindowTop = this.Top;
	MySettings.Default.MainWindowWidth = this.Width;
	MySettings.Default.MainWindowHeight = this.Height;
	MySettings.Default.MainWindowWindowState = this.WindowState;

	MySettings.Default.Save();

	base.OnClosed(e);
}     

This is a lot of work, even for a single window. If there were 10 resizable/movable elements of the UI, the settings file would become a jungle of similarly named properties, making this code quite tedious and error prone to write.

Also notice that for each property of the window, we need to mention it in five places (in the settings file, twice in the constructor and twice in OnClosed).

Scenario B (Jot)

Step 1: Create and configure the tracker

// Expose services as static class to keep the example simple 
static class Services
{
	// expose the tracker instance
	public static Tracker Tracker = new Tracker();

	static Services()
	{
		// tell Jot how to track Window objects
		Tracker.Configure()
			.Id(w => w.Name)
			.Properties(w => new { w.Height, w.Width, w.Left, w.Top, w.WindowState })
			.PersistOn(nameof(Window.WindowClosed))
	}
}

Step 2: Track the window instance

public MainWindow()
{
	InitializeComponent();
    
	// Start tracking the Window instance.
	// This will apply any previously stored data and start listening for "WindowClosed" event to persist new data. 
	Services.Tracker.Track(this);
}

That's it. We've set up tracking for all window objects in one place, so that all we need to to is call tracker.Track(window) on each window instance to preserve it's size and location. It's concise, the intent is clear, and there's no repetition. Notice also that we've mentioned each property only once, and it would be trivial to track additional properties.

Real world form/window tracking

The above code (both scenarios) works but it doesn't account for a few things. The first one is multiple displays. Screens can be unplugged, and we never want to position a window onto a screen that's no longer there. We can get around this problem very easily if we make the screen resolution part of the identifier. Jot will then track the same window separately for each screen configuration.

WPF Window

Here's how to properly track a WPF window:

// 1. tell the tracker how to track Window objects (this goes in a startup class)
tracker.Configure()
	.Id(w => w.Name, SystemInformation.VirtualScreen.Size) //  new { w.Top, w.Width, w.Height, w.Left, w.WindowState })
	.PersistOn(nameof(Window.Closing))
	.StopTrackingOn(nameof(Window.Closing));

// 2. in the Window constructor
public Window1()
{
	// fetch the tracker instance e.g. via IOC or static property
	var tracker = Services.Tracker;
	tracker.Track(this);
}

The Id method has a params object [] parameter that can be used to define a namespace for the id. These parameters simply get ToString-ed and concatenated to the Id. By using the screen resolution as the namespace, we ensure that we maintain separate configurations for different resolutions.

Windows forms

Winforms have a few additional caveats:

• Forms will return bogus size/location data for maximized/minimized forms, so we have to cancel persisting those
• Tracking needs to be applied during OnLoad since Top and Left properties set in the constructor are ignored

Here's how to properly track (Windows) Forms:

…

You can use this interface to make Jot store data anywhere you like e.g. in the cloud (to share settings for a user between machines) or a database.

Value conversions and cancellation

Jot lets you hook into the Apply and Persist operations. You can use this to perform value conversion and cancel persisting or applying data. As we've seen in the WinForms example, we can cancel applying size/location properties for Forms that are maximized or minimized:

tracker.Configure()
	.Id(...)
	.Properties(...)
	.WhenPersistingProperty((f, p) => p.Cancel = (f.WindowState != FormWindowState.Normal && (p.Property == nameof(Form.Height) || p.Property == nameof(Form.Width) || p.Property == nameof(Form.Top) || p.Property == nameof(Form.Left))))

There are four hooks you can use: WhenPersistingProperty, WhenApplyingProperty, WhenAppliedState and WhenPersisted.

Tracking and inheritance

Tracking is configured per-type, meaning that a separate TrackingConfiguration object will need to be defined for each type of object we track. This configuration object tells Jot how to track objects of that type, but it also applies to objects of derived types.

When configuring tracking for a derived type, Jot will examine the inheritance hierarchy of that type and look for the closest ancestor type for which a tracking configuration already exists. If it finds one, it will first create a copy of the base type's tracking configuration which you can then further customize.

For example, let's suppose you define a class called MyForm that derives from Form. In addition to tracking the size and location, you also want to track the selected tab of a TabControl that's part of MyForm. Here's what that might look like:

// configure tracking for Form
tracker.Configure()
	.Id(f => f.Name, SystemInformation.VirtualScreen.Size)
	.Properties(f => new { f.Height, f.Width, f.Left, f.Top, f.WindowState})
	.PersistOn(nameof(Form.Closing))
	.StopTrackingOn(nameof(Form.Closed))
	.WhenPersistingProperty((f, p) => p.Cancel = (f.WindowState != FormWindowState.Normal && p.Property != nameof(Form.WindowState)))

// add the selected tab index for MyForm (everything else is already copied from the configuration for Form)
tracker.Configure()
	.Properties(f => f.tabControl1.SelectedIndex);

We do not have to repeat the tracking configuration for size and location. Since MyForm derives from Form, the configuration for MyForm will be copied from the configuration for Form and we only need to add the additional f.tabControl1.SelectedTabIndex property.

Furthermore, if we configure tracking for Form but not for MyForm, Jot will track MyForm instances using the tracking configuration for Form.

The ITrackingAware interface

Sometimes we cannot know at compile time which properties to track. In those situations, we need to configure tracking on a per-instance basis at runtime. To do this, our tracked objects can implement the ITrackingAware interface.

public interface ITrackingAware
{
	void ConfigureTracking(TrackingConfiguration configuration);
}

In the ConfigureTracking method, the object can dynamically specify which properties to track. The configuration parameter is specific to that instance (and not the type) so each instance can independently adjust its tracking configuration.

For example, let's assume we have a form that has a datagrid, and we want to track the widths of grid columns. We could track each grid column object as a separate object, but we can also track those columns as part of tracking the form. Here's what that might look like:

public class MyFormWithDataGrid : ITrackingAware
{
	protected override void OnLoad(EventArgs e)
	{
		Services.Tracker.Track(this);
	}

	public void InitConfiguration(TrackingConfiguration configuration)
	{
		// include data grid column widths when tracking this form
		for (int i = 0; i  f.dataGridView1.Columns[idx].Width);
		}
	}
} 

IOC integration

Once we've explained to Jot how to track different types of objects, all that's needed in order for Jot to track instances of those types is to call:

tracker.Track(obj);

Here's the really cool part... When using an IOC container, many objects in the application will be created by the container. This gives us an opportunity to automatically track all created objects by hooking into the container.

For example, with SimpleInjector we can do this quite easily, with a single line of code:

var tracker = new Jot.Tracker();
var container = new SimpleInjector.Container();

//configure tracking and apply previously stored data to all created objects
container.RegisterInitializer(d => { tracker.Track(d.Instance); }, cx => true);

With this in place, we can easily make any property of any object persistent, just by modifying the tracking configuration for its type. Neat!

Demos

Demo projects for WPF and WinForms are included in the repository.

Contributing

You can contribute to this project in the usual way:

1. First of all, don't forget to star the project
2. Fork the project
3. Push your commits to your fork
4. Make a pull request

TODO

• Async support
• IOC demos
• aspnet core (demo + readme section)
• readme topics: unity ioc integration, attribute based configuration