MAUI threading¶

A MAUI BindableObject reads its dispatcher from BindableObject.Dispatcher. MAUI throws InvalidOperationException when it finds no dispatcher, as it does in a view's unit test. The MAUI invoker catches that and treats the object as having no owning thread. CheckAccess then returns true and Post runs the callback at once. The example below posts a callback to a control with no dispatcher, as in a unit test. The callback runs before Post returns.

DispatcherViewThreadInvoker invoker = new();
StorageBrowserView view = new();
List<string> log = [];

invoker.Post(view.UploadProgressBar, static state => ((List<string>)state!).Add(PostedText), log);

Console.WriteLine(string.Join(", ", log));

posted

The three members work like the other platforms. With no dispatcher, as in a unit test, the calling thread owns every control, so CheckAccess is true even on a worker thread. The example below calls all three members and prints the answers. The true from the worker thread is the difference from WPF and WinForms.

DispatcherViewThreadInvoker invoker = new();
StorageBrowserView view = new();
ProgressBar progressBar = view.UploadProgressBar;
List<string> log = [];
object unclaimed = new();
bool accessFromWorker = false;

Thread worker = new Thread(() => accessFromWorker = invoker.CheckAccess(progressBar));
worker.Start();
worker.Join();
invoker.Post(progressBar, static state => ((List<string>)state!).Add(PostedText), log);

Console.WriteLine(invoker.Claims(progressBar));
Console.WriteLine(invoker.Claims(unclaimed));
Console.WriteLine(invoker.CheckAccess(progressBar));
Console.WriteLine(accessFromWorker);
Console.WriteLine(string.Join(", ", log));

True
False
True
True
posted

You can also call the members through the IViewThreadInvoker type. That is how you call any platform's invoker, or one you write. The method below takes the interface, asks whether the invoker claims a progress bar, whether the thread may write and posts a callback. It works the same for every platform.

public static void CallInvokerThroughInterface(IViewThreadInvoker invoker)
{
    StorageBrowserView view = new();
    ProgressBar progressBar = view.UploadProgressBar;
    List<string> log = [];

    bool claimed = invoker.Claims(progressBar);
    bool mayWrite = invoker.CheckAccess(progressBar);
    invoker.Post(progressBar, static state => ((List<string>)state!).Add(PostedText), log);

    Console.WriteLine(claimed);
    Console.WriteLine(mayWrite);
    Console.WriteLine(string.Join(", ", log));
}

True
True
posted

A MAUI BindableObject raises PropertyChanged, so WhenChanged works on a MAUI entry with no module. The example below observes the filter entry's Text and sets it once. The first value is the empty text the entry held when the observation started.

MauiTodoPage view = new();
List<string?> texts = [];

using (view.FilterEntry.WhenChanged(x => x.Text).Subscribe(texts.Add))
{
    view.FilterEntry.Text = CarFilter;
}

Console.WriteLine(string.Join(", ", texts));

, car

API reference¶

Member	What it does	Type and values	Notes
`Maui.DispatcherViewThreadInvoker`	Routes writes to a MAUI object through the dispatcher it carries.	Sealed class that derives from `SequencerViewThreadInvoker<BindableObject, MauiDispatcherSequencer>`.	Claims `BindableObject`. Routes through `MauiDispatcherSequencer.For` the object's dispatcher, so `CheckAccess` is `false` only when the dispatcher requires a dispatch. With no dispatcher, `CheckAccess` is `true` and `Post` runs the callback at once. Any other type throws `InvalidCastException`. A `null` callback throws `ArgumentNullException`.
`Maui.MauiBindingModule`	Registers the MAUI invoker and the two Visibility converters with a dependency resolver.	Sealed class that implements `IModule`. `Configure(IMutableDependencyResolver resolver)`.	The converters go to the resolver only. `ImportFrom` copies them into a converter service. The WinUI build also registers a dependency-property observer. A `null` resolver throws `ArgumentNullException`.
`WithMaui`	Adds the MAUI module to the builder.	Extension method on `IReactiveUIBindingBuilder` and on `IAppBuilder`. Returns `IReactiveUIBindingBuilder`.	Returns the builder for chaining. A `null` builder throws `ArgumentNullException`. The `IAppBuilder` overload casts to `IReactiveUIBindingBuilder`.
`Maui.BooleanToVisibilityHints`	Changes how a `bool` maps to a `Visibility`.	Flags enum. `None = 0`, `Inverse = 2`, `UseHidden = 4`.	Same values as the WPF enum. `UseHidden` is ignored on WinUI, which has no `Hidden`.
`Maui.BooleanToVisibilityTypeConverter`	Converts a `bool` to a MAUI `Visibility`.	Sealed class. `BindingTypeConverter<bool, Visibility>`.	Same rules as the WPF converter. The conversion always succeeds. A hint that is not a `BooleanToVisibilityHints` value counts as `None`. Affinity is 2.
`Maui.VisibilityToBooleanTypeConverter`	Converts a MAUI `Visibility` to a `bool`.	Sealed class. `BindingTypeConverter<Visibility, bool>`.	Only `Visible` gives `true`. `Inverse` flips the result. The conversion always succeeds. Any other hint counts as `None`. Affinity is 2.