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In the last three blog posts, I introduced our new undo/redo feature and described how to customize it. So far, we’ve lived with the assumption that there is only a single global recorder to which all objects would send their changes. In this post, I will show how to cope with several recorders. Then, I will see how recordable objects can react to undo/redo operations by implementing a callback interface.    

This article is part of a series of 5 about undo/redo:

  1. Announcement and introduction
  2. Getting started – tutorial
  3. Logical operations, scopes, naming
  4. Recorders, recorder providers, callbacks
  5. Case study: Visual Designer

NOTE: This blog post is about an available pre-release of PostSharp. You can install PostSharp 4.0 only using NuGet by enabling the “Include pre-release” option. Undo/Redo is implemented in the package PostSharp.Patterns.Model.

Coping with several recorders in an application

In my previous examples, the global recorder was automatically and transparently assigned to all new recordable objects. This behavior is the result of the default value of two settings:

  • The AutoRecord property of the [Recordable] custom attribute (set to true by default,) determines whether the object should be assigned to the ambient recorder upon instantiation.

  • The RecordingServices.RecorderProvider property points to a chain of responsibility that provides recorders to recordable objects. The default implementation unconditionally returns a single global instance.

Note that the RecorderProvider is only used when AutoRecord is set to true. Otherwise, the recorder must be assigned manually, as we will see below.

Overriding the default RecorderProvider

If you want to customize which recorder is being assigned to recordable by default, you need to derive a class from RecorderProvider and implement the GetRecorderImpl method. The parameter contains the object for which the recorder is required. Note that instances of RecorderProvider form a chain of responsibility; if one of the instances return null, the next instance will be invoked.

class CustomRecorderProvider : RecorderProvider
{
    public CustomRecorderProvider(RecorderProvider next) : base(next)
    {
    }

    protected override Recorder GetRecorderImpl(object obj)
    {
    throw new NotImplementedException();
    }
}

The second step is to add the custom provider to the chain of responsibility:

RecordingServices.RecorderProvider = new CustomRecorderProvider(RecordingServices.RecorderProvider);

Assigning recorders manually

Another way to assign recorders to objects is to disable the AutoRecord behavior:

[Recordable(AutoRecord = false)]
class TableBooking
{
    // Details skipped.
}

Then you can attach a recorder to an object using the Recorder.Attach method:

Recorder recorder = new Recorder();
TableBooking booking = new TableBooking();
recorder.Attach(booking);

The symmetric method is Recorder.Detach. Note that invoking any of the Attach or Detach results in adding the attach or detach operation to the list of undoable operations. This seems a bit unintuitive but is important to conserve the ability to undo consistently.

Recordable and Aggregatable

As a rule, an object must always have the same recorder as its parent, unless the parent has no recorder. Therefore, the parent’s recorder is automatically assigned to any child object. Note that the recorder is not detached from a child when the child is detached from its parent.

This rule allows to easily work with real-world object models, where the undo/redo behavior of an object is often derived from its parent.

Undo/redo callbacks

There are situations where you may want to execute custom code before or after an object is affected by an undo or redo operation. For instance, if you have a custom implementation of INotifyPropertyChanged, you may want to raise the PropertyChanged event after undo or redo. This can be done by having your recordable object implement the IRecordableCallback interface.

This is illustrated in the following example, where we want to count the number of operations performed on a child list, but we don’t want this counter to be affected by undo/redo operations.

[Recordable]
class IntList : IRecordableCallback
{
    [NotRecorded]
    private bool replaying;
    [Child]
    public readonly AdvisableCollection<int> List = new AdvisableCollection<int>();
        [NotRecorded]
    public int OperationCount { get; private set; }
    public IntList()
    {
        this.List.CollectionChanged += ListOnCollectionChanged;
    }
    private void ListOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs notifyCollectionChangedEventArgs)
    {
if ( !this.replaying ) this.OperationCount++; } void IRecordableCallback.OnReplaying(ReplayKind kind, ReplayContext context) { this.replaying = true; } void IRecordableCallback.OnReplayed(ReplayKind kind, ReplayContext context) { this.replaying = false; } }

Conclusion

This is the last of a 4-part blog series about the new undo/redo feature found in PostSharp 4.0. We tried to design it to be super easy to get started with, but customizable enough for real-world applications. Thanks to PostSharp, implementing undo/redo in your own applications should become affordable and reasonably easy.

You may wonder how we tested the feature. Of course we have dozens of unit tests, but unit tests don’t make good usability tests. Since as a compiler-building company we don’t have any UI application to dogfood, we chose an open-source project and added the undo/redo feature to it. More in my next blog post.

Happy PostSharping!

-gael

UPDATE: Change product version from PostSharp 3.2 to PostSharp 4.0.

In the previous posts, I described the new undo/redo feature of PostSharp 4.0 and looked at most common use cases. In this post, I will show how you can customize its default behavior.

This article is part of a series of 5 about undo/redo:

  1. Announcement and introduction
  2. Getting started – tutorial
  3. Logical operations, scopes, naming
  4. Recorders, recorder providers, callbacks
  5. Case study: Visual Designer

NOTE: This blog post is about an available pre-release of PostSharp. You can install PostSharp 4.0 only using NuGet by enabling the “Include pre-release” option. Undo/Redo is implemented in the package PostSharp.Patterns.Model.

This blog post covers the following topics:

  • A conceptual introduction to logical operations

  • Defining the operation name declaratively

  • Avoiding automatic scopes

  • Defining atomic scopes declaratively

  • Defining scopes imperatively

  • Customizing scope names

Logical Operations

The Recordable patterns record primitive changes to an object model into a Recorder object. Primitive changes are typically field value changes or primitive collections operations such as Add or Remove. The Recorder itself is basically a list (i.e. a linear collection) of atomic changes. This simple and beautiful structure is sufficient to implement the undo changes and revert the object model to any point in the past.

In practice however, you don’t want to undo to any random point in the past, but only to safe points. Typically, work on an object model is separated into logical operations, which must be undone or redone as a whole. Users always want to undo an operation, not a primitive change. Logical operations are implemented using the concept of recording scope. All changes done within a scope are grouped into a single operation.

By default, any public method of a Recordable object automatically executes inside a scope. That is, all changes done by a single method call will always be undone as a whole.

Note that scopes don’t necessarily define new operations. Because scopes can be nested but operations cannot, only the outermost will open an operation. Nested scope will be ignored unless they are atomic scopes (see below).

Defining the operation name declaratively

Because they are apparent to users, operations must have a name that is meaningful to the user. One way to do that is to use the [RecordingScope] custom attribute. Let’s see this on an example:

[Recordable]
class TableBooking
{
    public string CustomerName { get; set; }
    public DateTime StartTime { get; set; }
    public DateTime EndTime { get; set; }
[RecordingScope("Postpone booking")] public void Postpone(TimeSpan time) { this.StartTime += time; this.EndTime += time; } }

Avoiding automatic scopes

There are situations where you want to prevent the default behavior that executes each public method in a scope. To opt-out, you can use the [RecordingScope] custom attribute and define the RecordingScopeOption.Skip flag.

[Recordable]
class TableBooking
{
    public string CustomerName { get; set; }
    public DateTime StartTime { get; set; }
    public DateTime EndTime { get; set; }

    [RecordingScope(RecordingScopeOption.Skip)]
    public void Postpone(TimeSpan time)
    {
        this.StartTime += time;
        this.EndTime += time;
    }
}

Defining atomic scopes declaratively

An atomic scope is a scope whose changes get rolled back if the code running inside the scope is not successful. Atomic scopes give the feeling that the code runs in a “transaction”. However, unlike real transactional systems, atomic recording scopes don’t provide transaction isolation. Changes are visible from other threads as soon as they are performed; there is no Commit semantic.

In the following code snippet, the [RecordingScope] attribute ensures that any change performed by the LoadFile method to the object will be rolled back to its initial state in case the input file contains a line that cannot be parsed into an integer.

[Recordable]
class IntList
{
    [Child] 
    private readonly AdvisableCollection<int> list = new AdvisableCollection<int>();
[RecordingScope(RecordingScopeOption.Atomic)] public void LoadFile(string file) { foreach (string line in File.ReadAllLines(file)) { this.list.Add(int.Parse(line)); } } }

Defining scopes imperatively

So far, we’ve seen how to define scopes declaratively using the [RecordingScope] custom attribute. Declarative programming is convenient, but sometimes more flexibility is needed. For these situations, the Recorder.OpenScope method must be used. It allows you to open a scope, set its name, and determine whether it should be atomic. The OpenScope method returns a RecordingScope object, which must be disposed at the end of the scope. For atomic scopes, you must invoke the Complete method before disposing the object, otherwise the changes performed in this scope will be rolled back.

The code snippet below uses the [RecordingScope] attribute to opt-out from the default scope, then uses OpenScope to open a new scope with a custom name.

[Recordable]
class IntList
{
    [Child] 
    readonly AdvisableCollection<int> list = new AdvisableCollection<int>();
[RecordingScope(RecordingScopeOption.Skip)] public void LoadFile(string file) { string scopeName = string.Format("Loading from {0}", file);
using ( RecordingScope scope = RecordingServices.DefaultRecorder.OpenScope(scopeName, RecordingScopeOption.Atomic)) { foreach (string line in File.ReadAllLines(file)) { this.list.Add(int.Parse(line)); } scope.Complete(); } } }

Customizing scope names

In the example above, I’ve shown how to specify a custom scope name when the scope is opened. This approach requires to mix UI code (generation of human-readable names) into domain code. It also demands more boilerplate code and can be cumbersome if you need have a large number of operations to customize. For instance, what if you need to have a proper name for the operation of setting every property in your classes?

This use case is covered by the OperationFormatter class. PostSharp comes with a default formatter, which can be overridden thanks to the RecordingServices.OperationFormatter property. Formatters form a chain of responsibility; if one formatter is not able to provide a name for an operation, it should return null, which causes the next formatter to be invoked.

Formatters have access to the operation descriptor, which is an object describing the operation. The IOperationDescriptor interface has a single property named OperationKind, which informs you to which class the descriptor must be cast.

In the following code snippet, we will show how to create a custom formatter that generates the same name as in the previous example.

class CustomFormatter : OperationFormatter
{
    public CustomFormatter(OperationFormatter next)
        : base(next)
    {
    }
protected override string FormatImpl(IOperationDescriptor operation) { if (operation.OperationKind != OperationKind.Method) return null;
MethodExecutionOperationDescriptor descriptor = (MethodExecutionOperationDescriptor) operation; if (descriptor.Method.DeclaringType == typeof (IntList) && descriptor.Method.Name == "LoadFile") { string file = (string) descriptor.Arguments[0]; return string.Format("Loading from {0}", file); } return null; } }

The following code inserts our custom formatter in the chain of responsibility:

RecordingServices.OperationFormatter = new CustomFormatter(RecordingServices.OperationFormatter);    

Of course, the example above is rather naïve because there is too much coupling between the formatter and the domain code. A more serious implementation would probably rely on other custom attributes such as [DisplayName] of the System.ComponentModel namespace. PostSharp is agnostic about the implementation of custom formatters and I just wanted to keep this example simple.

Summary

Scopes are the mechanism through which logical operations are defined. Logical operations are the “things” that are exposed to the user and that can be undone or redone. Therefore, they must be named. PostSharp provides three ways to define scope: implicitly, declaratively, and imperatively. It also provides, orthogonally, two ways to name operations: by setting the name when the scope is created, or lazily, based on the concept of operation formatter, which allows to better separate UI concerns from business logic. Finally, PostSharp defines the notion of atomic scope, which causes all changes to be rolled back in case of exception.

These few concepts allow to customize how your code maps to undoable operations.

But there is more. In the next post, I will talk about multiple recorders and custom operations.

Happy PostSharping!

-gael

UPDATE: Change product version from PostSharp 3.2 to PostSharp 4.0.

In my previous blog post, I described how to make your model classes recordable, so that they can participate in the undo/redo feature. Today, I will show how you can expose the feature to your application’s user interface. There are two approaches: one is to simply include ready-made UndoButton or RedoButton controls to your XAML code. The other is to interact with the Recorder class in C#. Let’s see how.

This article is part of a series of 5 about undo/redo:

  1. Announcement and introduction
  2. Getting started – tutorial
  3. Logical operations, scopes, naming
  4. Recorders, recorder providers, callbacks
  5. Case study: Visual Designer

NOTE: This blog post is about an available pre-release of PostSharp. You can install PostSharp 4.0 only using NuGet by enabling the “Include pre-release” option. Undo/Redo is implemented in the package PostSharp.Patterns.Model.

Undo/Redo Fast Track

For this post, I will use a popular sample application from Visual Studio Gallery: “Easy MVVM Example”. You can create a new project based on this project yourself easily; just open the New Project dialog in Visual Studio, select the Online tab, and search for the sample named “Easy MVVM Example”.

Step 1. Install NuGet packages

Go to the NuGet package manager dialog and add the pre-release package named PostSharp.Patterns.Model.Controls. This will add all dependent packages. Remember to display pre-release packages and not only stable ones.

Step 2. Clean up INotifyPropertyChanged

The sample was not written for PostSharp, so it has its custom INotifyPropertyChanged (INPC) implementation. We would need to write additional code to have our undo/redo feature work with their hand-written INPC, so let’s just clean the project and replace their implementation with our own.

To proceed, find the file Person.cs and replace it with the following content:

using PostSharp.Patterns.Model;

namespace MvvmExample.Model
{
    [NotifyPropertyChanged]
    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
    }
}

This incidentally removes 75% of boilerplate code.

Step 3. Make the Person class recordable

Add the [Recordable] attribute to the Person class.

The code becomes as follows:

using PostSharp.Patterns.Model;
using PostSharp.Patterns.Recording;

namespace MvvmExample.Model
{
    [NotifyPropertyChanged]
    [Recordable]
    public class Person
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Age { get; set; }
    }
}

Step 4. Add undo/redo buttons to XAML

  1. Open the MainWindow.xaml file.

  2. Add the following namespace definition to the root <Window> tag:

    xmlns:model="clr-namespace:PostSharp.Patterns.Model.Controls;assembly=PostSharp.Patterns.Model.Controls"   
  3. Add the following controls before the closing </Window> tag.

<model:UndoButton HorizontalAlignment="Left" Margin="22,24,0,0" VerticalAlignment="Top" />
<model:RedoButton HorizontalAlignment="Left" Margin="64,24,0,0" VerticalAlignment="Top"/>

At this point, you can start the application and try to modify a data grid cell and try clicking the undo and redo buttons.

Interacting with the Recorder manually

In the previous section, we just inserted the UndoButton and RedoButton controls into our XAML code. If you want more control, you can access the Recorder class directly from C# code.

In this example, we are using a global Recorder object. You can get a reference to this instance from the RecordingServices.DefaultProvider class. This class provides the following members:

  • UndoOperations and RedoOperations collections: give access to operations that can be undone or redone. Operations are objects derived from the Operation class. This class has a Name property, which is the human-readable name of the operation.

  • Clear : remove all operations both from the UndoOperations and RedoOperations collection.

  • CanUndo and CanRedo properties: determines whether the Undo or Redo commands are available.

  • Undo and Redo methods: undo or redo one operation

  • AddRestorePoint method: creates a restore point, which basically is an empty operation that serves as a marker.

  • CanAddRestorePoint property: determines if the AddRestorePoint command is available

  • UndoTo and RedoTo methods: undo or redo all operations from the now to a given operation or restore point.

  • MaximumOperationsCount property: maximum number of operations in the UndoOperations collection before old operations are removed.

The Recorder class also has other methods and properties that will be explained in a later post.

Example

Let’s see how we can manually interact with the Recorder in practice.

If you click on the arrow of the Undo button just after the application started, you will see that the operation list is already populated with nine items. These operations were created during the application initialization.

If we don’t want users to be able to undo these operations, we can clear the Recorder at initialization. Go to the ViewModelMain class and append the following instruction to the constructor:

RecordingServices.DefaultRecorder.Clear();

Now, when you start the demo application, you will see that the undo and redo buttons are initially disabled.

Summary

Adding the undo/redo feature to your application can be dead easy if you already have a well-structure code, with a nice separation between the UI and the model. Basically, all you need to do is to make your model recordable, then drop the undo and redo buttons to your toolbar.

Of course, no real-world piece of software is so simple than a demo application, and I would not like you to believe that implementing Recordable is always so trivial. Yet, our implementation of the Recordable pattern makes it possible to customize the behavior to your needs.

In the next blog post, we’ll go deeper and see how you can control the creation and naming of operations.

Happy PostSharping!

-gael

UPDATE: Change product version from PostSharp 3.2 to PostSharp 4.0.