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 public string Author { get; set; }
16 public string Content { get; set; }
17}
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 }
7
8 public bool? IsApproved { get; set; }
9}
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)
7 {
8 }
9private bool? _isApproved;
10
11 public bool? IsApproved { get { return this._isApproved; } set { if (value != this._isApproved) { this._isApproved = value; OnPropertyChanged("IsApproved"); } return; } }
12 private bool? _acceptedIsApproved;
13}
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#pragma warning disable IDE0031, IDE1005
8
9[Inheritable]
10public class TrackChangesAttribute : TypeAspect
11{
12 private static readonly DiagnosticDefinition<INamedType> _mustHaveOnChangeMethod = new(
13 "MY001",
14 Severity.Error,
15 $"The '{nameof(ISwitchableChangeTracking)}' interface is implemented manually on type '{{0}}', but the type does not have an '{nameof(OnChange)}()' method.");
16
17 private static readonly DiagnosticDefinition _onChangeMethodMustBeProtected = new(
18 "MY002",
19 Severity.Error,
20 $"The '{nameof(OnChange)}()' method must be have the 'protected' accessibility.");
21
22 private static readonly DiagnosticDefinition<IMethod> _onPropertyChangedMustBeVirtual = new(
23 "MY003",
24 Severity.Error,
25 "The '{0}' method must be virtual.");
26
27 [InterfaceMember(WhenExists = InterfaceMemberOverrideStrategy.Ignore)]
28 public bool IsChanged { get; private set; }
29
30 [InterfaceMember(WhenExists = InterfaceMemberOverrideStrategy.Ignore)]
31 public bool IsTrackingChanges
32 {
33 get => meta.This._isTrackingChanges;
34 set
35 {
36 if (meta.This._isTrackingChanges != value)
37 {
38 meta.This._isTrackingChanges = value;
39
40 var onPropertyChanged = this.GetOnPropertyChangedMethod(meta.Target.Type);
41
42 if (onPropertyChanged != null)
43 {
44 onPropertyChanged.Invoke(nameof(this.IsTrackingChanges));
45 }
46
47 if (value)
48 {
49 this.AcceptChanges();
50 }
51 }
52 }
53 }
54
55 public override void BuildAspect(IAspectBuilder<INamedType> builder)
56 {
57 // Select fields and automatic properties that can be changed.
58 var fieldsOrProperties = builder.Target.FieldsAndProperties
59 .Where(f =>
60 !f.IsImplicitlyDeclared && f.Writeability == Writeability.All &&
61 f.IsAutoPropertyOrField == true)
62 .ToArray();
63
64 //
65 var introducedFields = new Dictionary<IFieldOrProperty, IField>();
66
67 // Create a field for each mutable field or property. These fields
68 // will contain the accepted values.
69 foreach (var fieldOrProperty in fieldsOrProperties)
70 {
71 var upperCaseName = fieldOrProperty.Name.TrimStart('_');
72 upperCaseName =
73 upperCaseName.Substring(0, 1).ToUpper(CultureInfo.InvariantCulture) +
74 upperCaseName.Substring(1);
75 var acceptedField =
76 builder.Advice.IntroduceField(builder.Target, "_accepted" + upperCaseName,
77 fieldOrProperty.Type);
78 introducedFields[fieldOrProperty] = acceptedField.Declaration;
79 }
80
81 // Implement the ISwitchableChangeTracking interface.
82 var implementInterfaceResult = builder.Advice.ImplementInterface(builder.Target,
83 typeof(ISwitchableChangeTracking), OverrideStrategy.Ignore,
84 new { IntroducedFields = introducedFields });
85 //
86
87 // If the type already implements ISwitchableChangeTracking, it must have a protected method called OnChanged, without parameters, otherwise
88 // this is a contract violation, so we report an error.
89 if (implementInterfaceResult.Outcome == AdviceOutcome.Ignore)
90 {
91 var onChangeMethod = builder.Target.AllMethods.OfName(nameof(this.OnChange))
92 .SingleOrDefault(m => m.Parameters.Count == 0);
93
94 if (onChangeMethod == null)
95 {
96 builder.Diagnostics.Report(
97 _mustHaveOnChangeMethod.WithArguments(builder.Target));
98 }
99 else if (onChangeMethod.Accessibility != Accessibility.Protected)
100 {
101 builder.Diagnostics.Report(_onChangeMethodMustBeProtected);
102 }
103 }
104 else
105 {
106 builder.Advice.IntroduceField(builder.Target, "_isTrackingChanges", typeof(bool));
107 }
108
109 // Override all writable fields and automatic properties.
110 // If the type has an OnPropertyChanged method, we assume that all properties
111 // and fields already call it, and we hook into OnPropertyChanged instead of
112 // overriding each setter.
113 var onPropertyChanged = this.GetOnPropertyChangedMethod(builder.Target);
114
115 if (onPropertyChanged == null)
116 {
117 // If the type has an OnPropertyChanged method, we assume that all properties
118 // and fields already call it, and we hook into OnPropertyChanged instead of
119 // overriding each setter.
120 foreach (var fieldOrProperty in fieldsOrProperties)
121 {
122 builder.Advice.OverrideAccessors(fieldOrProperty, null,
123 nameof(this.OverrideSetter));
124 }
125 }
126 else if (onPropertyChanged.DeclaringType.Equals(builder.Target))
127 {
128 builder.Advice.Override(onPropertyChanged, nameof(this.OnPropertyChanged));
129 }
130 else if (implementInterfaceResult.Outcome == AdviceOutcome.Ignore)
131 {
132 // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
133 // we assume that the type already hooked the OnPropertyChanged method, and
134 // there is nothing else to do.
135 }
136 else
137 {
138 // If the OnPropertyChanged method was defined in a base class, but not overridden
139 // in the current class, and if we implement ISwitchableChangeTracking ourselves,
140 // then we need to override OnPropertyChanged.
141
142 if (!onPropertyChanged.IsVirtual)
143 {
144 builder.Diagnostics.Report(
145 _onPropertyChangedMustBeVirtual.WithArguments(onPropertyChanged));
146 }
147 else
148 {
149 builder.Advice.IntroduceMethod(builder.Target, nameof(this.OnPropertyChanged),
150 whenExists: OverrideStrategy.Override);
151 }
152 }
153 }
154
155 private IMethod? GetOnPropertyChangedMethod(INamedType type)
156 => type.AllMethods
157 .OfName("OnPropertyChanged")
158 .SingleOrDefault(m => m.Parameters.Count == 1);
159
160 [InterfaceMember]
161 public virtual void AcceptChanges()
162 {
163 if (meta.Target.Method.IsOverride)
164 {
165 meta.Proceed();
166 }
167 else
168 {
169 this.IsChanged = false;
170 }
171
172 var introducedFields =
173 (Dictionary<IFieldOrProperty, IField>)meta.Tags["IntroducedFields"]!;
174
175 foreach (var field in introducedFields)
176 {
177 field.Value.Value = field.Key.Value;
178 }
179 }
180
181 [InterfaceMember]
182 public virtual void RejectChanges()
183 {
184 if (meta.Target.Method.IsOverride)
185 {
186 meta.Proceed();
187 }
188 else
189 {
190 this.IsChanged = false;
191 }
192
193
194 var introducedFields =
195 (Dictionary<IFieldOrProperty, IField>)meta.Tags["IntroducedFields"]!;
196
197 foreach (var field in introducedFields)
198 {
199 field.Key.Value = field.Value.Value;
200 }
201 }
202
203
204 [Introduce(WhenExists = OverrideStrategy.Ignore)]
205 protected void OnChange()
206 {
207 if (this.IsChanged == false)
208 {
209 this.IsChanged = true;
210
211 var onPropertyChanged = this.GetOnPropertyChangedMethod(meta.Target.Type);
212
213 if (onPropertyChanged != null)
214 {
215 onPropertyChanged.Invoke(nameof(this.IsChanged));
216 }
217 }
218 }
219
220 [Template]
221 private void OverrideSetter(dynamic? value)
222 {
223 meta.Proceed();
224
225 if (value != meta.Target.Property.Value)
226 {
227 this.OnChange();
228 }
229 }
230
231 [Template]
232 protected virtual void OnPropertyChanged(string name)
233 {
234 meta.Proceed();
235
236 if (name is not (nameof(this.IsChanged) or nameof(this.IsTrackingChanges)))
237 {
238 this.OnChange();
239 }
240 }
241}
Let's focus on the following part of the BuildAspect method for the moment.
65var introducedFields = new Dictionary<IFieldOrProperty, IField>();
66
67// Create a field for each mutable field or property. These fields
68// will contain the accepted values.
69foreach (var fieldOrProperty in fieldsOrProperties)
70{
71 var upperCaseName = fieldOrProperty.Name.TrimStart('_');
72 upperCaseName =
73 upperCaseName.Substring(0, 1).ToUpper(CultureInfo.InvariantCulture) +
74 upperCaseName.Substring(1);
75 var acceptedField =
76 builder.Advice.IntroduceField(builder.Target, "_accepted" + upperCaseName,
77 fieldOrProperty.Type);
78 introducedFields[fieldOrProperty] = acceptedField.Declaration;
79}
80
81// Implement the ISwitchableChangeTracking interface.
82var implementInterfaceResult = builder.Advice.ImplementInterface(builder.Target,
83 typeof(ISwitchableChangeTracking), OverrideStrategy.Ignore,
84 new { IntroducedFields = introducedFields });First, 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:
160[InterfaceMember]
161public virtual void AcceptChanges()
162{
163 if (meta.Target.Method.IsOverride)
164 {
165 meta.Proceed();
166 }
167 else
168 {
169 this.IsChanged = false;
170 }
171
172 var introducedFields =
173 (Dictionary<IFieldOrProperty, IField>)meta.Tags["IntroducedFields"]!;
174
175 foreach (var field in introducedFields)
176 {
177 field.Value.Value = field.Key.Value;
178 }
179}
180181[InterfaceMember]
182public virtual void RejectChanges()
183{
184 if (meta.Target.Method.IsOverride)
185 {
186 meta.Proceed();
187 }
188 else
189 {
190 this.IsChanged = false;
191 }
192
193
194 var introducedFields =
195 (Dictionary<IFieldOrProperty, IField>)meta.Tags["IntroducedFields"]!;
196
197 foreach (var field in introducedFields)
198 {
199 field.Key.Value = field.Value.Value;
200 }
201}
202
203As 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.