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[controls] fix memory leak in
VisualElement.Background (dotnet#13656)
* [controls] fix memory leak in `VisualElement.Background` Fixes: dotnet#12344 Fixes: dotnet#13557 Context: https://github.com/dotnet-presentations/dotnet-maui-workshop While testing the `Monkey Finder` sample, I found the following scenario causes an issue: 1. Declare a `{StaticResource}` `Brush` at the `Application` level, with a lifetime of the entire application. 2. Set `Background` on an item in a `CollectionView`, `ListView`, etc. 3. Scroll a lot, navigate away, etc. 4. The `Brush` will hold onto any `View`'s indefinitely. The core problem here being `VisualElement` does: void NotifyBackgroundChanges() { if (Background is ImmutableBrush) return; if (Background != null) { Background.Parent = this; Background.PropertyChanged += OnBackgroundChanged; if (Background is GradientBrush gradientBrush) gradientBrush.InvalidateGradientBrushRequested += InvalidateGradientBrushRequested; } } If no user code sets `Background` to `null`, these events remain subscribed. To fix this: 1. Create a `WeakNotifyCollectionChangedProxy` type for event subscription. 2. Don't set `Background.Parent = this` ~~ General Cleanup ~~ Through doing other fixes related to memory leaks & C# events, we've started to gain a collection of `WeakEventProxy`-related types. I created some core `internal` types to be reused: * `abstract WeakEventProxy<TSource, TEventHandler>` * `WeakNotifyCollectionChangedProxy` The following classes now make use of these new shared types: * `BindingExpression` * `BindableLayout` * `ListProxy` * `VisualElement` This should hopefully reduce mistakes and reuse code in this area. ~~ Concerns ~~ Since, we are no longer doing: Background.Parent = this; This is certainly a behavior change. It is now replaced with: SetInheritedBindingContext(Background, BindingContext); I had to update one unit test that was asserting `Background.Parent`, which can assert `Background.BindingContext` instead. As such, this change is definitely sketchy and I wouldn't backport to .NET 7 immediately. We might test it out in a .NET 8 preview first. * Auto-format source code * Add test & subscribe to InvalidateGradientBrushRequested --------- Co-authored-by: GitHub Actions Autoformatter <[email protected]>
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152
src/Controls/src/Core/Internals/MaybeNullWhenAttribute.cs
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| // Licensed to the .NET Foundation under one or more agreements. | ||
| // The .NET Foundation licenses this file to you under the MIT license. | ||
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| // This was copied from https://github.com/dotnet/runtime/blob/39b9607807f29e48cae4652cd74735182b31182e/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs | ||
| // and updated to have the scope of the attributes be internal. | ||
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| #nullable disable | ||
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| namespace System.Diagnostics.CodeAnalysis | ||
| { | ||
| #if !NETCOREAPP | ||
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| /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)] | ||
| internal sealed class AllowNullAttribute : Attribute { } | ||
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| /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)] | ||
| internal sealed class DisallowNullAttribute : Attribute { } | ||
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| /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)] | ||
| internal sealed class MaybeNullAttribute : Attribute { } | ||
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| /// <summary>Specifies that an output will not be null even if the corresponding type allows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)] | ||
| internal sealed class NotNullAttribute : Attribute { } | ||
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| /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Parameter, Inherited = false)] | ||
| internal sealed class MaybeNullWhenAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with the specified return value condition.</summary> | ||
| /// <param name="returnValue"> | ||
| /// The return value condition. If the method returns this value, the associated parameter may be null. | ||
| /// </param> | ||
| public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue; | ||
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| /// <summary>Gets the return value condition.</summary> | ||
| public bool ReturnValue { get; } | ||
| } | ||
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| /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary> | ||
| [AttributeUsage(AttributeTargets.Parameter, Inherited = false)] | ||
| internal sealed class NotNullWhenAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with the specified return value condition.</summary> | ||
| /// <param name="returnValue"> | ||
| /// The return value condition. If the method returns this value, the associated parameter will not be null. | ||
| /// </param> | ||
| public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue; | ||
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| /// <summary>Gets the return value condition.</summary> | ||
| public bool ReturnValue { get; } | ||
| } | ||
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| /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary> | ||
| [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)] | ||
| internal sealed class NotNullIfNotNullAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with the associated parameter name.</summary> | ||
| /// <param name="parameterName"> | ||
| /// The associated parameter name. The output will be non-null if the argument to the parameter specified is non-null. | ||
| /// </param> | ||
| public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName; | ||
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| /// <summary>Gets the associated parameter name.</summary> | ||
| public string ParameterName { get; } | ||
| } | ||
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| /// <summary>Applied to a method that will never return under any circumstance.</summary> | ||
| [AttributeUsage(AttributeTargets.Method, Inherited = false)] | ||
| internal sealed class DoesNotReturnAttribute : Attribute { } | ||
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| /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary> | ||
| [AttributeUsage(AttributeTargets.Parameter, Inherited = false)] | ||
| internal sealed class DoesNotReturnIfAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with the specified parameter value.</summary> | ||
| /// <param name="parameterValue"> | ||
| /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to | ||
| /// the associated parameter matches this value. | ||
| /// </param> | ||
| public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue; | ||
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| /// <summary>Gets the condition parameter value.</summary> | ||
| public bool ParameterValue { get; } | ||
| } | ||
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| #endif | ||
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| #if !NETCOREAPP || NETCOREAPP3_1 | ||
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| /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values.</summary> | ||
| [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)] | ||
| internal sealed class MemberNotNullAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with a field or property member.</summary> | ||
| /// <param name="member"> | ||
| /// The field or property member that is promised to be not-null. | ||
| /// </param> | ||
| public MemberNotNullAttribute(string member) => Members = new[] { member }; | ||
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| /// <summary>Initializes the attribute with the list of field and property members.</summary> | ||
| /// <param name="members"> | ||
| /// The list of field and property members that are promised to be not-null. | ||
| /// </param> | ||
| public MemberNotNullAttribute(params string[] members) => Members = members; | ||
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| /// <summary>Gets field or property member names.</summary> | ||
| public string[] Members { get; } | ||
| } | ||
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| /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values when returning with the specified return value condition.</summary> | ||
| [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)] | ||
| internal sealed class MemberNotNullWhenAttribute : Attribute | ||
| { | ||
| /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary> | ||
| /// <param name="returnValue"> | ||
| /// The return value condition. If the method returns this value, the associated parameter will not be null. | ||
| /// </param> | ||
| /// <param name="member"> | ||
| /// The field or property member that is promised to be not-null. | ||
| /// </param> | ||
| public MemberNotNullWhenAttribute(bool returnValue, string member) | ||
| { | ||
| ReturnValue = returnValue; | ||
| Members = new[] { member }; | ||
| } | ||
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| /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.</summary> | ||
| /// <param name="returnValue"> | ||
| /// The return value condition. If the method returns this value, the associated parameter will not be null. | ||
| /// </param> | ||
| /// <param name="members"> | ||
| /// The list of field and property members that are promised to be not-null. | ||
| /// </param> | ||
| public MemberNotNullWhenAttribute(bool returnValue, params string[] members) | ||
| { | ||
| ReturnValue = returnValue; | ||
| Members = members; | ||
| } | ||
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| /// <summary>Gets the return value condition.</summary> | ||
| public bool ReturnValue { get; } | ||
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| /// <summary>Gets field or property member names.</summary> | ||
| public string[] Members { get; } | ||
| } | ||
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| #endif | ||
| } |
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