Change Tracking example, step 3: integrating with INotifyPropertyChanged
At this point, we have a TrackChanges aspect that implements the IChangeTracking interface, supports hand-written base implementations of this interface, and reports errors if the pattern contract is not respected. However, we have built this aspect in pure isolation. In practice, the TrackChanges aspect must interact with the NotifyPropertyChanged pattern. When the IsChanged property changes, the PropertyChanged event must be raised.
It is essential to understand that the two concepts interacting with each other are not aspects or interfaces but patterns. Aspects, by definition, are executable artifacts that automate the implementation and verification patterns, but patterns can also be implemented manually. Patterns define extension points. The OnPropertyChanged method is a part of the pattern we chose to implement the INotifyPropertyChanged interface, but not a part of the interface itself. Patterns are essentially conventions, and a different implementation pattern can rely on a different triggering mechanism than the OnPropertyChanged method.
Therefore, when you design an aspect, you should first reason about the pattern, think about how the different patterns combine, and how they work with inherited classes or parent-child relationships.
For this example, we decide (and we insist this is a design pattern decision) to invoke the OnChange method from the OnPropertyChanged method. Why? There are two reasons for this. First, the setters of all mutable properties are already supposed to call the OnPropertyChanged method, so adding a new call to OnChange everywhere would be a double pain. This argument is valid if we implement the pattern by hand, but what if we use an aspect? Here comes the second reason: the code generated by Metalama is much less readable when two aspects are added to one property.
Let's see this pattern in action:
1[TrackChanges]
2[NotifyPropertyChanged]
3public partial class Comment
4{
5 public Guid Id { get; }
6 public string Author { get; set; }
7 public string Content { get; set; }
8
9 public Comment( Guid id, string author, string content )
10 {
11 this.Id = id;
12 this.Author = author;
13 this.Content = content;
14 }
15}
1using System;
2using System.ComponentModel;
3
4[TrackChanges]
5[NotifyPropertyChanged]
6public partial class Comment: INotifyPropertyChanged, ISwitchableChangeTracking, IChangeTracking
7{
8 public Guid Id { get; }
9
10
11 private string _author = default!;
12 public string Author
13 {
14 get
15 {
16 return this._author;
17 }
18
19 set
20 {
21 if (value != this._author)
22 {
23 this._author = value;
24 this.OnPropertyChanged("Author");
25 }
26
27 return;
28 }
29 }
30
31 private string _content = default!;
32 public string Content
33 {
34 get
35 {
36 return this._content;
37 }
38
39 set
40 {
41 if (value != this._content)
42 {
43 this._content = value;
44 this.OnPropertyChanged("Content");
45 }
46
47 return;
48 }
49 }
50
51 public Comment( Guid id, string author, string content )
52 {
53 this.Id = id;
54 this.Author = author;
55 this.Content = content;
56 }
57
58 private bool _isTrackingChanges;
59
60 public bool IsChanged { get; private set; }
61
62 public bool IsTrackingChanges
63 {
64 get
65 {
66 return _isTrackingChanges;
67 }
68
69 set
70 {
71 if (this._isTrackingChanges != value)
72 {
73 this._isTrackingChanges = value;
74 this.OnPropertyChanged("IsTrackingChanges");
75 }
76 }
77 }
78
79 public void AcceptChanges()
80 {
81 this.IsChanged = false;
82 }
83
84 protected void OnChange()
85 {
86 if (this.IsChanged == false)
87 {
88 this.IsChanged = true;
89 this.OnPropertyChanged("IsChanged");
90 }
91 }
92
93 protected virtual void OnPropertyChanged(string name)
94 {
95 this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
96 if (name is not ("IsChanged" or "IsTrackingChanges"))
97 {
98 this.OnChange();
99 }
100 }
101
102 public event PropertyChangedEventHandler? PropertyChanged;
103}
104
1public class ModeratedComment : Comment
2{
3 public ModeratedComment( Guid id, string author, string content ) : base( id, author, content )
4 {
5 }
6
7 public bool? IsApproved { get; set; }
8}
1public class ModeratedComment : Comment
2{
3 public ModeratedComment( Guid id, string author, string content ) : base( id, author, content )
4 {
5 }
6
7
8 private bool? _isApproved;
9
10 public bool? IsApproved
11 {
12 get
13 {
14 return this._isApproved;
15 }
16
17 set
18 {
19 if (value != this._isApproved)
20 {
21 this._isApproved = value;
22 this.OnPropertyChanged("IsApproved");
23 }
24
25 return;
26 }
27 }
28}
Aspect implementation
The new aspect implementation is as follows:
1using Metalama.Framework.Advising;
2using Metalama.Framework.Aspects;
3using Metalama.Framework.Code;
4using Metalama.Framework.Diagnostics;
5
6
7#pragma warning disable IDE1005
8
9
10[Inheritable]
11public class TrackChangesAttribute : TypeAspect
12{
13 private static readonly DiagnosticDefinition<INamedType> _mustHaveOnChangeMethod = new(
14 "MY001",
15 Severity.Error,
16 $"The '{nameof(ISwitchableChangeTracking)}' interface is implemented manually on type '{{0}}', but the type does not have an '{nameof(OnChange)}()' method.");
17
18 private static readonly DiagnosticDefinition _onChangeMethodMustBeProtected = new(
19 "MY002",
20 Severity.Error,
21 $"The '{nameof(OnChange)}()' method must be have the 'protected' accessibility.");
22
23 private static readonly DiagnosticDefinition<IMethod> _onPropertyChangedMustBeVirtual = new(
24 "MY003",
25 Severity.Error,
26 "The '{0}' method must be virtual.");
27
28 public override void BuildAspect( IAspectBuilder<INamedType> builder )
29 {
30 // Implement the ISwitchableChangeTracking interface.
31 var implementInterfaceResult = builder.Advice.ImplementInterface( builder.Target,
32 typeof(ISwitchableChangeTracking), OverrideStrategy.Ignore );
33
34 if ( implementInterfaceResult.Outcome == AdviceOutcome.Ignored )
35 {
36 // If the type already implements ISwitchableChangeTracking, it must have a protected method called OnChanged, without parameters, otherwise
37 // this is a contract violation, so we report an error.
38
39 var onChangeMethod = builder.Target.AllMethods.OfName( nameof(this.OnChange) )
40 .Where( m => m.Parameters.Count == 0 ).SingleOrDefault();
41
42 if ( onChangeMethod == null )
43 {
44 builder.Diagnostics.Report( _mustHaveOnChangeMethod.WithArguments( builder.Target ) );
45 }
46 else if ( onChangeMethod.Accessibility != Accessibility.Protected )
47 {
48 builder.Diagnostics.Report( _onChangeMethodMustBeProtected );
49 }
50 }
51 else
52 {
53 builder.Advice.IntroduceField( builder.Target, "_isTrackingChanges", typeof(bool) );
54 }
55
56
57 var onPropertyChanged = this.GetOnPropertyChangedMethod( builder.Target );
58
59 if ( onPropertyChanged == null )
60 {
61 // If the type has an OnPropertyChanged method, we assume that all properties
62 // and fields already call it, and we hook into OnPropertyChanged instead of
63 // overriding each setter.
64
65 var fieldsOrProperties = builder.Target.FieldsAndProperties
66 .Where( f =>
67 !f.IsImplicitlyDeclared && f.Writeability == Writeability.All && f.IsAutoPropertyOrField == true );
68
69 foreach ( var fieldOrProperty in fieldsOrProperties )
70 {
71 builder.Advice.OverrideAccessors( fieldOrProperty, null, nameof(this.OverrideSetter) );
72 }
73 }
74 else if ( onPropertyChanged.DeclaringType.Equals( builder.Target ) )
75 {
76 // If the OnPropertyChanged method was declared in the current type, override it.
77 builder.Advice.Override( onPropertyChanged, nameof(this.OnPropertyChanged) );
78 }
79 else if ( implementInterfaceResult.Outcome == AdviceOutcome.Ignored )
80 {
81 // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
82 // we assume that the type already hooked the OnPropertyChanged method, and
83 // there is nothing else to do.
84 }
85 else
86 {
87 // If the OnPropertyChanged method was defined in a base class, but not overridden
88 // in the current class, and if we implement ISwitchableChangeTracking ourselves,
89 // then we need to override OnPropertyChanged.
90
91 if ( !onPropertyChanged.IsVirtual )
92 {
93 builder.Diagnostics.Report( _onPropertyChangedMustBeVirtual.WithArguments( onPropertyChanged ) );
94 }
95 else
96 {
97 builder.Advice.IntroduceMethod( builder.Target, nameof(this.OnPropertyChanged),
98 whenExists: OverrideStrategy.Override );
99 }
100 }
101 }
102
103
104 private IMethod? GetOnPropertyChangedMethod( INamedType type )
105 => type.AllMethods
106 .OfName( "OnPropertyChanged" )
107 .Where( m => m.Parameters.Count == 1 )
108 .SingleOrDefault();
109
110 [InterfaceMember]
111 public bool IsChanged { get; private set; }
112
113
114 [InterfaceMember]
115 public bool IsTrackingChanges
116 {
117 get => meta.This._isTrackingChanges;
118 set
119 {
120 if ( meta.This._isTrackingChanges != value )
121 {
122 meta.This._isTrackingChanges = value;
123
124 var onPropertyChanged = this.GetOnPropertyChangedMethod( meta.Target.Type );
125
126 if ( onPropertyChanged != null )
127 {
128 onPropertyChanged.Invoke( nameof(this.IsTrackingChanges) );
129 }
130 }
131 }
132 }
133
134 [InterfaceMember]
135 public void AcceptChanges() => this.IsChanged = false;
136
137
138 [Introduce( WhenExists = OverrideStrategy.Ignore )]
139 protected void OnChange()
140 {
141 if ( this.IsChanged == false )
142 {
143 this.IsChanged = true;
144
145 var onPropertyChanged = this.GetOnPropertyChangedMethod( meta.Target.Type );
146
147 if ( onPropertyChanged != null )
148 {
149 onPropertyChanged.Invoke( nameof(this.IsChanged) );
150 }
151 }
152 }
153
154 [Template]
155 private void OverrideSetter( dynamic? value )
156 {
157 meta.Proceed();
158
159 if ( value != meta.Target.Property.Value )
160 {
161 this.OnChange();
162 }
163 }
164
165 [Template]
166 protected virtual void OnPropertyChanged( string name )
167 {
168 meta.Proceed();
169
170 if ( name is not (nameof(this.IsChanged) or nameof(this.IsTrackingChanges)) )
171 {
172 this.OnChange();
173 }
174 }
175}
Notice the new GetOnPropertyChangedMethod method. It looks for the OnPropertyChanged method in the AllMethods collection. This collection contains methods defined by the current type and the non-private ones of the base classes. Therefore, GetOnPropertyChangedMethod may return an IMethod from the current type, from the base class, or null.
104 private IMethod? GetOnPropertyChangedMethod( INamedType type )
105 => type.AllMethods
106 .OfName( "OnPropertyChanged" )
107 .Where( m => m.Parameters.Count == 1 )
108 .SingleOrDefault();We call GetOnPropertyChangedMethod from BuildAspect.
If we do not find any OnPropertyChanged, we have to override all fields and automatic properties ourselves:
59 if ( onPropertyChanged == null )
60 {
61 // If the type has an OnPropertyChanged method, we assume that all properties
62 // and fields already call it, and we hook into OnPropertyChanged instead of
63 // overriding each setter.
64
65 var fieldsOrProperties = builder.Target.FieldsAndProperties
66 .Where( f =>
67 !f.IsImplicitlyDeclared && f.Writeability == Writeability.All && f.IsAutoPropertyOrField == true );
68
69 foreach ( var fieldOrProperty in fieldsOrProperties )
70 {
71 builder.Advice.OverrideAccessors( fieldOrProperty, null, nameof(this.OverrideSetter) );
72 }
73 } However, if the closest OnPropertyChanged method is in the base type, the logic is more complex:
59 if ( onPropertyChanged == null )
60 {
61 // If the type has an OnPropertyChanged method, we assume that all properties
62 // and fields already call it, and we hook into OnPropertyChanged instead of
63 // overriding each setter.
64
65 var fieldsOrProperties = builder.Target.FieldsAndProperties
66 .Where( f =>
67 !f.IsImplicitlyDeclared && f.Writeability == Writeability.All && f.IsAutoPropertyOrField == true );
68
69 foreach ( var fieldOrProperty in fieldsOrProperties )
70 {
71 builder.Advice.OverrideAccessors( fieldOrProperty, null, nameof(this.OverrideSetter) );
72 }
73 } If the closest OnPropertyChanged is in the current type, we override it:
79 else if ( implementInterfaceResult.Outcome == AdviceOutcome.Ignored )
80 {
81 // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
82 // we assume that the type already hooked the OnPropertyChanged method, and
83 // there is nothing else to do.
84 }
85 else
86 {
87 // If the OnPropertyChanged method was defined in a base class, but not overridden
88 // in the current class, and if we implement ISwitchableChangeTracking ourselves,
89 // then we need to override OnPropertyChanged.
90
91 if ( !onPropertyChanged.IsVirtual )
92 {
93 builder.Diagnostics.Report( _onPropertyChangedMustBeVirtual.WithArguments( onPropertyChanged ) );
94 }
95 else
96 {
97 builder.Advice.IntroduceMethod( builder.Target, nameof(this.OnPropertyChanged),
98 whenExists: OverrideStrategy.Override );
99 }
100 } If both the OnPropertyChanged method and the ISwitchableChangeTracking interface are defined in the base type, we do not have to hook OnPropertyChanged because it is the responsibility of the base type. We rely on the outcome of the ImplementInterface method to know if ISwitchableChangeTracking was already implemented.
However, if the base type defines an OnPropertyChanged method but no ISwitchableChangeTracking interface, we need to override the OnPropertyChanged method. It's only possible if the base method is virtual. Otherwise, we report an error. To override a base class method, we need to use IntroduceMethod instead of Override.
Finally, we also need to change the implementations of IsTrackingChanges and OnChange to call OnPropertyChanged. Let's see, for instance, OnChange:
138 [Introduce( WhenExists = OverrideStrategy.Ignore )]
139 protected void OnChange()
140 {
141 if ( this.IsChanged == false )
142 {
143 this.IsChanged = true;
144
145 var onPropertyChanged = this.GetOnPropertyChangedMethod( meta.Target.Type );
146
147 if ( onPropertyChanged != null )
148 {
149 onPropertyChanged.Invoke( nameof(this.IsChanged) );
150 }
151 }
152 }If the OnPropertyChanged method is present, we invoke it using the Invoke method. Note, to be precise, that Invoke does not invoke the method because the code runs at compile time. What it actually does is generate the code that will invoke the method at run time. Note also that we cannot use the conditional ?. operator in this case. We must use an if statement to check if the OnPropertyChanged method is present.
Summary
In this article, we briefly discussed the philosophy of pattern interactions. We then integrated the TrackChanges and the NotifyPropertyChanges patterns. In the following article, we will add the ability to revert changes done to the object.