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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:

Source Code



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}
Transformed Code
1using System;
2using System.ComponentModel;
3
4[TrackChanges]
5[NotifyPropertyChanged]
6public partial class Comment
7: INotifyPropertyChanged, ISwitchableChangeTracking, IChangeTracking
8{
9    public Guid Id { get; }
10private string _author = default!;
11
12    public string Author { get { return this._author; } set { if (value != this._author) { this._author = value; OnPropertyChanged("Author"); } return; } }
13    private string _content = default!;
14
15    public string Content { get { return this._content; } set { if (value != this._content) { this._content = value; OnPropertyChanged("Content"); } return; } }
16
17    public Comment(Guid id, string author, string content)
18    {
19        this.Id = id;
20        this.Author = author;
21        this.Content = content;
22    }
23private bool _isTrackingChanges;
24
25    public bool IsChanged { get; private set; }
26
27    public bool IsTrackingChanges
28    {
29        get
30        {
31            return _isTrackingChanges;
32        }
33        set
34        {
35            if (this._isTrackingChanges != value)
36            {
37                this._isTrackingChanges = value;
38                this.OnPropertyChanged("IsTrackingChanges");
39            }
40        }
41    }
42
43    public void AcceptChanges()
44    {
45        IsChanged = false;
46    }
47    protected void OnChange()
48    {
49        if (IsChanged == false)
50        {
51            IsChanged = true;
52            this.OnPropertyChanged("IsChanged");
53        }
54    }
55    protected virtual void OnPropertyChanged(string name)
56    {
57        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
58        if (name is not ("IsChanged" or "IsTrackingChanges"))
59        {
60            OnChange();
61        }
62    }
63    public event PropertyChangedEventHandler? PropertyChanged;
64}
Source Code
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}
Transformed Code
1public class ModeratedComment : Comment
2{
3    public ModeratedComment(Guid id, string author, string content) : base(id, author, content)
4    {
5    }
6private bool? _isApproved;
7
8    public bool? IsApproved { get { return this._isApproved; } set { if (value != this._isApproved) { this._isApproved = value; OnPropertyChanged("IsApproved"); } return; } }
9}

Aspect implementation

The new aspect implementation is the following:

1using Metalama.Framework.Advising;
2using Metalama.Framework.Aspects;
3using Metalama.Framework.Code;
4using Metalama.Framework.Diagnostics;
5
6#pragma warning disable IDE0031, IDE1005
7
8[Inheritable]
9public class TrackChangesAttribute : TypeAspect
10{
11    private static readonly DiagnosticDefinition<INamedType> _mustHaveOnChangeMethod = new(
12        "MY001",
13        Severity.Error,
14        $"The '{nameof(ISwitchableChangeTracking)}' interface is implemented manually on type '{{0}}', but the type does not have an '{nameof(OnChange)}()' method.");
15
16    private static readonly DiagnosticDefinition _onChangeMethodMustBeProtected = new(
17        "MY002",
18        Severity.Error,
19        $"The '{nameof(OnChange)}()' method must be have the 'protected' accessibility.");
20
21    private static readonly DiagnosticDefinition<IMethod> _onPropertyChangedMustBeVirtual = new(
22        "MY003",
23        Severity.Error,
24        "The '{0}' method must be virtual.");
25
26    public override void BuildAspect(IAspectBuilder<INamedType> builder)
27    {
28        // Implement the ISwitchableChangeTracking interface.         
29        var implementInterfaceResult = builder.Advice.ImplementInterface(builder.Target,
30            typeof(ISwitchableChangeTracking), OverrideStrategy.Ignore);
31
32        if (implementInterfaceResult.Outcome == AdviceOutcome.Ignore)
33        {
34            // If the type already implements ISwitchableChangeTracking, it must have a protected method called OnChanged, without parameters, otherwise
35            // this is a contract violation, so we report an error.
36
37            var onChangeMethod = builder.Target.AllMethods.OfName(nameof(this.OnChange))
38                .SingleOrDefault(m => m.Parameters.Count == 0);
39
40            if (onChangeMethod == null)
41            {
42                builder.Diagnostics.Report(
43                    _mustHaveOnChangeMethod.WithArguments(builder.Target));
44            }
45            else if (onChangeMethod.Accessibility != Accessibility.Protected)
46            {
47                builder.Diagnostics.Report(_onChangeMethodMustBeProtected);
48            }
49        }
50        else
51        {
52            builder.Advice.IntroduceField(builder.Target, "_isTrackingChanges", typeof(bool));
53        }
54
55
56        var onPropertyChanged = this.GetOnPropertyChangedMethod(builder.Target);
57
58        if (onPropertyChanged == null) 
59        {
60            // If the type has an OnPropertyChanged method, we assume that all properties
61            // and fields already call it, and we hook into OnPropertyChanged instead of
62            // overriding each setter.
63
64            var fieldsOrProperties = builder.Target.FieldsAndProperties
65                .Where(f =>
66                    !f.IsImplicitlyDeclared && f.Writeability == Writeability.All &&
67                    f.IsAutoPropertyOrField == true);
68
69            foreach (var fieldOrProperty in fieldsOrProperties)
70            {
71                builder.Advice.OverrideAccessors(fieldOrProperty, null,
72                    nameof(this.OverrideSetter));
73            }
74        } 
75        else if
76            (onPropertyChanged.DeclaringType
77             .Equals(builder.Target)) 
78        {
79            // If the OnPropertyChanged method was declared in the current type, override it.
80            builder.Advice.Override(onPropertyChanged, nameof(this.OnPropertyChanged));
81        } 
82        else if (implementInterfaceResult.Outcome ==
83                 AdviceOutcome.Ignore) 
84        {
85            // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
86            // we assume that the type already hooked the OnPropertyChanged method, and
87            // there is nothing else to do.
88        }
89        else
90        {
91            // If the OnPropertyChanged method was defined in a base class, but not overridden
92            // in the current class, and if we implement ISwitchableChangeTracking ourselves,
93            // then we need to override OnPropertyChanged.
94
95            if (!onPropertyChanged.IsVirtual)
96            {
97                builder.Diagnostics.Report(
98                    _onPropertyChangedMustBeVirtual.WithArguments(onPropertyChanged));
99            }
100            else
101            {
102                builder.Advice.IntroduceMethod(builder.Target, nameof(this.OnPropertyChanged),
103                    whenExists: OverrideStrategy.Override);
104            }
105        } 
106    }
107
108
109    private IMethod? GetOnPropertyChangedMethod(INamedType type)
110        => type.AllMethods
111            .OfName("OnPropertyChanged")
112            .SingleOrDefault(m => m.Parameters.Count == 1);
113
114    [InterfaceMember] public bool IsChanged { get; private set; }
115
116
117    [InterfaceMember]
118    public bool IsTrackingChanges
119    {
120        get => meta.This._isTrackingChanges;
121        set
122        {
123            if (meta.This._isTrackingChanges != value)
124            {
125                meta.This._isTrackingChanges = value;
126
127                var onPropertyChanged = this.GetOnPropertyChangedMethod(meta.Target.Type);
128
129                if (onPropertyChanged != null)
130                {
131                    onPropertyChanged.Invoke(nameof(this.IsTrackingChanges));
132                }
133            }
134        }
135    }
136
137    [InterfaceMember]
138    public void AcceptChanges() => this.IsChanged = false;
139
140
141    [Introduce(WhenExists = OverrideStrategy.Ignore)]
142    protected void OnChange()
143    {
144        if (this.IsChanged == false)
145        {
146            this.IsChanged = true;
147
148            var onPropertyChanged = this.GetOnPropertyChangedMethod(meta.Target.Type);
149
150            if (onPropertyChanged != null)
151            {
152                onPropertyChanged.Invoke(nameof(this.IsChanged));
153            }
154        }
155    }
156
157    [Template]
158    private void OverrideSetter(dynamic? value)
159    {
160        meta.Proceed();
161
162        if (value != meta.Target.Property.Value)
163        {
164            this.OnChange();
165        }
166    }
167
168    [Template]
169    protected virtual void OnPropertyChanged(string name)
170    {
171        meta.Proceed();
172
173        if (name is not (nameof(this.IsChanged) or nameof(this.IsTrackingChanges)))
174        {
175            this.OnChange();
176        }
177    }
178}

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.

109private IMethod? GetOnPropertyChangedMethod(INamedType type)
110    => type.AllMethods
111        .OfName("OnPropertyChanged")
112        .SingleOrDefault(m => m.Parameters.Count == 1);
113

We call GetOnPropertyChangedMethod from BuildAspect.

If we do not find any OnPropertyChanged, we have to override all fields and automatic properties ourselves:

58if (onPropertyChanged == null) 
59{
60    // If the type has an OnPropertyChanged method, we assume that all properties
61    // and fields already call it, and we hook into OnPropertyChanged instead of
62    // overriding each setter.
63
64    var fieldsOrProperties = builder.Target.FieldsAndProperties
65        .Where(f =>
66            !f.IsImplicitlyDeclared && f.Writeability == Writeability.All &&
67            f.IsAutoPropertyOrField == true);
68
69    foreach (var fieldOrProperty in fieldsOrProperties)
70    {
71        builder.Advice.OverrideAccessors(fieldOrProperty, null,
72            nameof(this.OverrideSetter));
73    }
74} 

However, if the closest OnPropertyChanged method is in the base type, the logic is more complex:

58if (onPropertyChanged == null) 
59{
60    // If the type has an OnPropertyChanged method, we assume that all properties
61    // and fields already call it, and we hook into OnPropertyChanged instead of
62    // overriding each setter.
63
64    var fieldsOrProperties = builder.Target.FieldsAndProperties
65        .Where(f =>
66            !f.IsImplicitlyDeclared && f.Writeability == Writeability.All &&
67            f.IsAutoPropertyOrField == true);
68
69    foreach (var fieldOrProperty in fieldsOrProperties)
70    {
71        builder.Advice.OverrideAccessors(fieldOrProperty, null,
72            nameof(this.OverrideSetter));
73    }
74} 

If the closest OnPropertyChanged is in the current type, we override it:

83         AdviceOutcome.Ignore) 
84{
85    // If we have an OnPropertyChanged method but the type already implements ISwitchableChangeTracking,
86    // we assume that the type already hooked the OnPropertyChanged method, and
87    // there is nothing else to do.
88}
89else
90{
91    // If the OnPropertyChanged method was defined in a base class, but not overridden
92    // in the current class, and if we implement ISwitchableChangeTracking ourselves,
93    // then we need to override OnPropertyChanged.
94
95    if (!onPropertyChanged.IsVirtual)
96    {
97        builder.Diagnostics.Report(
98            _onPropertyChangedMustBeVirtual.WithArguments(onPropertyChanged));
99    }
100    else
101    {
102        builder.Advice.IntroduceMethod(builder.Target, nameof(this.OnPropertyChanged),
103            whenExists: OverrideStrategy.Override);
104    }
105} 

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:

141[Introduce(WhenExists = OverrideStrategy.Ignore)]
142protected void OnChange()
143{
144    if (this.IsChanged == false)
145    {
146        this.IsChanged = true;
147
148        var onPropertyChanged = this.GetOnPropertyChangedMethod(meta.Target.Type);
149
150        if (onPropertyChanged != null)
151        {
152            onPropertyChanged.Invoke(nameof(this.IsChanged));
153        }
154    }
155}
156

If the OnPropertyChanged method is present, we invoke it using the Invoke method. Note, to be precise, that Invoke does not invoke really 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 pattern. In the following article, we will add the ability to revert changes done to the object.