In this article, we will implement the ability to revert the object to the last-accepted version. The .NET Framework exposes this ability as the IRevertibleChangeTracking interface. It adds a new RejectChanges method. This method must revert any changes performed since the last call to the AcceptChanges method.
We need to duplicate each field or automatic property: one copy will contain the current value, and the second will contain the accepted value. The AcceptChanges method copies the current values to the accepted ones, while the RejectChanges method copies the accepted values to the current ones.
Let's see this pattern in action:
Source Code
1#pragma warning disable CS8618
2
3[TrackChanges]
4[NotifyPropertyChanged]
5public partial class Comment
6{
7 public Comment( Guid id, string author, string content )
8 {
9 this.Id = id;
10 this.Author = author;
11 this.Content = content;
12 }
13
14 public Guid Id { get; }
15
16 public string Author { get; set; }
17
18 public string Content { get; set; }
19}
Transformed Code
1#pragma warning disable CS8618
2
3using System;
4using System.ComponentModel;
5
6[TrackChanges]
7[NotifyPropertyChanged]
8public partial class Comment
9: INotifyPropertyChanged, ISwitchableChangeTracking, IRevertibleChangeTracking, IChangeTracking
10{
11 public Comment( Guid id, string author, string content )
12 {
13 this.Id = id;
14 this.Author = author;
15 this.Content = content;
16 }
17
18 public Guid Id { get; }
19private string _author = default!;
20
21 public string Author { get { return this._author; } set { if (value != this._author) { this._author = value; OnPropertyChanged("Author"); } return; } }
22 private string _content = default!;
23
24 public string Content { get { return this._content; } set { if (value != this._content) { this._content = value; OnPropertyChanged("Content"); } return; } }
25 private string _acceptedAuthor;
26 private string _acceptedContent;
27 private bool _isTrackingChanges;
28
29 public bool IsChanged { get; private set; }
30
31 public bool IsTrackingChanges
32 {
33 get
34 {
35 return _isTrackingChanges;
36 }
37 set
38 {
39 if (this._isTrackingChanges != value)
40 {
41 this._isTrackingChanges = value;
42 this.OnPropertyChanged("IsTrackingChanges");
43 if (value)
44 {
45 AcceptChanges();
46 }
47 }
48 }
49 }
50
51 public virtual void AcceptChanges()
52 {
53 IsChanged = false;
54 this._acceptedAuthor = this.Author;
55 this._acceptedContent = this.Content;
56 }
57 protected void OnChange()
58 {
59 if (IsChanged == false)
60 {
61 IsChanged = true;
62 this.OnPropertyChanged("IsChanged");
63 }
64 }
65 protected virtual void OnPropertyChanged(string name)
66 {
67 PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
68 if (name is not ("IsChanged" or "IsTrackingChanges"))
69 {
70 OnChange();
71 }
72 }
73 public virtual void RejectChanges()
74 {
75 IsChanged = false;
76 this.Author = this._acceptedAuthor;
77 this.Content = this._acceptedContent;
78 }
79 public event PropertyChangedEventHandler? PropertyChanged;
80}
Source Code
1[TrackChanges]
2public class ModeratedComment : Comment
3{
4 public ModeratedComment( Guid id, string author, string content ) : base( id, author, content ) { }
5
6 public bool? IsApproved { get; set; }
7}
Transformed Code
1using System;
2
3[TrackChanges]
4public class ModeratedComment : Comment
5{
6 public ModeratedComment( Guid id, string author, string content ) : base( id, author, content ) { }
7private bool? _isApproved;
8
9 public bool? IsApproved { get { return this._isApproved; } set { if (value != this._isApproved) { this._isApproved = value; OnPropertyChanged("IsApproved"); } return; } }
10 private bool? _acceptedIsApproved;
11}
Aspect implementation
Here is the complete code of the new version of the TrackChanges aspect:
1using Metalama.Framework.Advising;
2using Metalama.Framework.Aspects;
3using Metalama.Framework.Code;
4using Metalama.Framework.Diagnostics;
5using System.Globalization;
6
7[Inheritable]
8public class TrackChangesAttribute : TypeAspect
9{
10 private static readonly DiagnosticDefinition<INamedType> _mustHaveOnChangeMethod = new(
11 "MY001",
12 Severity.Error,
13 $"The '{nameof(ISwitchableChangeTracking)}' interface is implemented manually on type '{{0}}', but the type does not have an '{nameof(OnChange)}()' method." );
14
15 private static readonly DiagnosticDefinition _onChangeMethodMustBeProtected = new(
16 "MY002",
17 Severity.Error,
18 $"The '{nameof(OnChange)}()' method must be have the 'protected' accessibility." );
19
20 private static readonly DiagnosticDefinition<IMethod> _onPropertyChangedMustBeVirtual = new(
21 "MY003",
22 Severity.Error,
23 "The '{0}' method must be virtual." );
24
25 [InterfaceMember( WhenExists = InterfaceMemberOverrideStrategy.Ignore )]
26 public bool IsChanged { get; private set; }
27
28 [InterfaceMember( WhenExists = InterfaceMemberOverrideStrategy.Ignore )]
29 public bool IsTrackingChanges
30 {
31 get => meta.This._isTrackingChanges;
32 set
33 {
34 if ( meta.This._isTrackingChanges != value )
35 {
36 meta.This._isTrackingChanges = value;
37
38 var onPropertyChanged = this.GetOnPropertyChangedMethod( meta.Target.Type );
39
40 if ( onPropertyChanged != null )
41 {
42 onPropertyChanged.Invoke( nameof(this.IsTrackingChanges) );
43 }
44
45 if ( value )
46 {
47 this.AcceptChanges();
48 }
49 }
50 }
51 }
52
53 public override void BuildAspect( IAspectBuilder<INamedType> builder )
54 {
55 // Select fields and automatic properties that can be changed.
56 var fieldsOrProperties = builder.Target.FieldsAndProperties
57 .Where( f =>
58 !f.IsImplicitlyDeclared && f.Writeability == Writeability.All &&
59 f.IsAutoPropertyOrField == true )
60 .ToArray();
61
62 //
63 var introducedFields = new Dictionary<IFieldOrProperty, IField>();
64
65 // Create a field for each mutable field or property. These fields
66 // will contain the accepted values.
67 foreach ( var fieldOrProperty in fieldsOrProperties )
68 {
69 var upperCaseName = fieldOrProperty.Name.TrimStart( '_' );
70
71 upperCaseName =
72 upperCaseName.Substring( 0, 1 ).ToUpper( CultureInfo.InvariantCulture ) +
73 upperCaseName.Substring( 1 );
74
75 var acceptedField =
76 builder.IntroduceField(
77 "_accepted" + upperCaseName,
78 fieldOrProperty.Type );
79
80 introducedFields[fieldOrProperty] = acceptedField.Declaration;
81 }
82
83 // Implement the ISwitchableChangeTracking interface.
84 var implementInterfaceResult = builder.ImplementInterface(
85 typeof(ISwitchableChangeTracking),
86 OverrideStrategy.Ignore,
87 new { IntroducedFields = introducedFields } );
88
89 //
90
91 // If the type already implements ISwitchableChangeTracking, it must have a protected method called OnChanged, without parameters, otherwise
92 // this is a contract violation, so we report an error.
93 if ( implementInterfaceResult.Outcome == AdviceOutcome.Ignore )
94 {
95 var onChangeMethod = builder.Target.AllMethods.OfName( nameof(this.OnChange) )
96 .SingleOrDefault( m => m.Parameters.Count == 0 );
97
98 if ( onChangeMethod == null )
99 {
100 builder.Diagnostics.Report( _mustHaveOnChangeMethod.WithArguments( builder.Target ) );
101 }
102 else if ( onChangeMethod.Accessibility != Accessibility.Protected )
103 {
104 builder.Diagnostics.Report( _onChangeMethodMustBeProtected );
105 }
106 }
107 else
108 {
109 builder.IntroduceField( "_isTrackingChanges", typeof(bool) );
110 }
111
112 // Override all writable fields and automatic properties.
113 // If the type has an OnPropertyChanged method, we assume that all properties
114 // and fields already call it, and we hook into OnPropertyChanged instead of
115 // overriding each setter.
116 var onPropertyChanged = this.GetOnPropertyChangedMethod( builder.Target );
117
118 if ( onPropertyChanged == null )
119 {
120 // If the type has an OnPropertyChanged method, we assume that all properties
121 // and fields already call it, and we hook into OnPropertyChanged instead of
122 // overriding each setter.
123 foreach ( var fieldOrProperty in fieldsOrProperties )
124 {
125 builder.With( fieldOrProperty )
126 .OverrideAccessors(
127 null,
128 nameof(this.OverrideSetter) );
129 }
130 }
131 else if ( onPropertyChanged.DeclaringType.Equals( builder.Target ) )
132 {
133 builder.With( onPropertyChanged ).Override( nameof(this.OnPropertyChanged) );
134 }
135 else if ( implementInterfaceResult.Outcome == AdviceOutcome.Ignore )
136 {
137 // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
138 // we assume that the type already hooked the OnPropertyChanged method, and
139 // there is nothing else to do.
140 }
141 else
142 {
143 // If the OnPropertyChanged method was defined in a base class, but not overridden
144 // in the current class, and if we implement ISwitchableChangeTracking ourselves,
145 // then we need to override OnPropertyChanged.
146
147 if ( !onPropertyChanged.IsVirtual )
148 {
149 builder.Diagnostics.Report( _onPropertyChangedMustBeVirtual.WithArguments( onPropertyChanged ) );
150 }
151 else
152 {
153 builder.IntroduceMethod(
154 nameof(this.OnPropertyChanged),
155 whenExists: OverrideStrategy.Override );
156 }
157 }
158 }
159
160 private IMethod? GetOnPropertyChangedMethod( INamedType type )
161 => type.AllMethods
162 .OfName( "OnPropertyChanged" )
163 .SingleOrDefault( m => m.Parameters.Count == 1 );
164
165 [InterfaceMember]
166 public virtual void AcceptChanges()
167 {
168 if ( meta.Target.Method.IsOverride )
169 {
170 meta.Proceed();
171 }
172 else
173 {
174 this.IsChanged = false;
175 }
176
177 var introducedFields =
178 (Dictionary<IFieldOrProperty, IField>) meta.Tags["IntroducedFields"]!;
179
180 foreach ( var field in introducedFields )
181 {
182 field.Value.Value = field.Key.Value;
183 }
184 }
185
186 [InterfaceMember]
187 public virtual void RejectChanges()
188 {
189 if ( meta.Target.Method.IsOverride )
190 {
191 meta.Proceed();
192 }
193 else
194 {
195 this.IsChanged = false;
196 }
197
198 var introducedFields =
199 (Dictionary<IFieldOrProperty, IField>) meta.Tags["IntroducedFields"]!;
200
201 foreach ( var field in introducedFields )
202 {
203 field.Key.Value = field.Value.Value;
204 }
205 }
206
207 [Introduce( WhenExists = OverrideStrategy.Ignore )]
208 protected void OnChange()
209 {
210 if ( this.IsChanged == false )
211 {
212 this.IsChanged = true;
213
214 var onPropertyChanged = this.GetOnPropertyChangedMethod( meta.Target.Type );
215
216 if ( onPropertyChanged != null )
217 {
218 onPropertyChanged.Invoke( nameof(this.IsChanged) );
219 }
220 }
221 }
222
223 [Template]
224 private void OverrideSetter( dynamic? value )
225 {
226 meta.Proceed();
227
228 if ( value != meta.Target.Property.Value )
229 {
230 this.OnChange();
231 }
232 }
233
234 [Template]
235 protected virtual void OnPropertyChanged( string name )
236 {
237 meta.Proceed();
238
239 if ( name is not (nameof(this.IsChanged) or nameof(this.IsTrackingChanges)) )
240 {
241 this.OnChange();
242 }
243 }
244}
Let's focus on the following part of the BuildAspect method for the moment.
63var introducedFields = new Dictionary<IFieldOrProperty, IField>();
64
65// Create a field for each mutable field or property. These fields
66// will contain the accepted values.
67foreach ( var fieldOrProperty in fieldsOrProperties )
68{
69 var upperCaseName = fieldOrProperty.Name.TrimStart( '_' );
70
71 upperCaseName =
72 upperCaseName.Substring( 0, 1 ).ToUpper( CultureInfo.InvariantCulture ) +
73 upperCaseName.Substring( 1 );
74
75 var acceptedField =
76 builder.IntroduceField(
77 "_accepted" + upperCaseName,
78 fieldOrProperty.Type );
79
80 introducedFields[fieldOrProperty] = acceptedField.Declaration;
81}
82
83// Implement the ISwitchableChangeTracking interface.
84var implementInterfaceResult = builder.ImplementInterface(
85 typeof(ISwitchableChangeTracking),
86 OverrideStrategy.Ignore,
87 new { IntroducedFields = introducedFields } );
88First, the method introduces new fields into the type for each mutable field or automatic property using the IntroduceField method. For details about this practice, see Introducing members.
Note that we are building the introducedFields dictionary, which maps the current-value field or property to the
accepted-value field. This dictionary will be passed to
the ImplementInterface call as a tag. The collection of tags is
an
anonymous object. For more details about this technique, see Sharing state with advice.
The field dictionary is read from the implementation of AcceptChanges and RejectChanges:
165[InterfaceMember]
166public virtual void AcceptChanges()
167{
168 if ( meta.Target.Method.IsOverride )
169 {
170 meta.Proceed();
171 }
172 else
173 {
174 this.IsChanged = false;
175 }
176
177 var introducedFields =
178 (Dictionary<IFieldOrProperty, IField>) meta.Tags["IntroducedFields"]!;
179
180 foreach ( var field in introducedFields )
181 {
182 field.Value.Value = field.Key.Value;
183 }
184}
185186[InterfaceMember]
187public virtual void RejectChanges()
188{
189 if ( meta.Target.Method.IsOverride )
190 {
191 meta.Proceed();
192 }
193 else
194 {
195 this.IsChanged = false;
196 }
197
198 var introducedFields =
199 (Dictionary<IFieldOrProperty, IField>) meta.Tags["IntroducedFields"]!;
200
201 foreach ( var field in introducedFields )
202 {
203 field.Key.Value = field.Value.Value;
204 }
205}
206As you can see, the (Dictionary<IFieldOrProperty, IField>) meta.Tags["IntroducedFields"] expression gets
the IntroducedFields tag, which was passed to
the ImplementInterface method. We cast it back to its original
type
and iterate it. We use the Value property to generate the run-time expression
that represents the field or property. In the AcceptChanges method, we copy the current values to the accepted ones
and do the opposite in the RejectChanges method.