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Summary of Chapter 6. Button clicks

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Note

This book was published in the spring of 2016, and has not been updated since then. There is much in the book that remains valuable, but some of the material is outdated, and some topics are no longer entirely correct or complete.

The Button is the view that allows the user to initiate a command. A Button is identified by text (and optionally an image as demonstrated in Chapter 13, Bitmaps). Consequently, Button defines many of the same properties as Label:

Button also defines three properties that govern the appearance of its border, but the support of these properties and their mutual independence is platform specific:

Button also inherits all the properties of VisualElement and View, including BackgroundColor, HorizontalOptions, and VerticalOptions.

Processing the click

The Button class defines a Clicked event that is fired when the user taps the Button. The Click handler is of type EventHandler. The first argument is the Button object generating the event; the second argument is an EventArgs object that provides no additional information.

The ButtonLogger sample demonstrates simple Clicked handling.

Sharing button clicks

Multiple Button views can share the same Clicked handler, but the handler generally needs to determine which Button is responsible for a particular event. One approach is to store the various Button objects as fields and check which one is firing the event in the handler.

The TwoButtons sample demonstrates this technique. The program also demonstrates how to set the IsEnabled property of a Button to false when pressing the Button is no longer valid. A disabled Button does not generate a Clicked event.

Anonymous event handlers

It is possible to define Clicked handlers as anonymous lambda functions, as the ButtonLambdas sample demonstrates. However, anonymous handlers can't be shared without some messy reflection code.

Distinguishing views with IDs

Multiple Button objects can also be distinguished by setting the StyleId property or AutomationId property to a string. This property is defined by Element but it is not used within Xamarin.Forms. It is intended to be used solely by application programs.

The SimplestKeypad sample uses the same event handler for all 10 number keys on a numeric keypad and distinguishes between them with the StyleId property:

Triple screenshot of simplest keypad

Saving transient data

Many applications need to save data when a program is terminated and to reload that data when the program starts up again. The Application class defines several members that help your program save and restore transient data:

  • The Properties property is a dictionary with string keys and object items. The contents of the dictionary are automatically saved in application local storage prior to program termination, and reloaded when the program starts up.
  • The Application class defines three protected virtual methods that the program's standard App class overrides: OnStart, OnSleep, and OnResume. These refer to application lifecycle events.
  • The SavePropertiesAsync method saves the contents of the dictionary.

It is not necessary to call SavePropertiesAsync. The contents of the dictionary are automatically saved prior to program termination and retrieved before program startup. It's useful during program testing to save data if the program crashes.

Also useful is:

  • Application.Current, a static property that returns the current Application object that you can then use to obtain the Properties dictionary.

The first step is to identify all the variables on the page that you want to persist when the program terminates. If you know all the places where those variables change, you can simply add them to the Properties dictionary at that point. In the page's constructor, you can set the variables from the Properties dictionary if the key exists.

A larger program will probably need to deal with application lifecycle events. The most important is the OnSleep method. A call to this method indicates that the program has left the foreground. Perhaps the user has pressed the Home button on the device, or displayed all the applications, or is shutting down the phone. A call to OnSleep is the only notification that a program receives before it is terminated. The program should take this opportunity to ensure that the Properties dictionary is up to date.

A call to OnResume indicates that the program did not terminate following the last call to OnSleep but is now running in the foreground again. The program might use this opportunity to refresh internet connections (for example).

A call to OnStart occurs during program startup. It is not necessary to wait until this method call to access the Properties dictionary because the contents have already been restored when the App constructor is called.

The PersistentKeypad sample is very similar to SimplestKeypad except that the program uses the OnSleep override to save the current keypad entry, and the page constructor to restore that data.

Note

Another approach to saving program settings is provided by the Xamarin.Essentials Preferences class.