Summary of Chapter 6. Button clicks
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
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
Processing the click
Button class defines a
Clicked event that is fired when the user taps 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
Sharing button clicks
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
false when pressing the
Button is no longer valid. A disabled
Button does not generate a
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
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
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:
Propertiesproperty is a dictionary with
objectitems. The contents of the dictionary are automatically saved in application local storage prior to program termination, and reloaded when the program starts up.
Applicationclass defines three protected virtual methods that the program's standard
OnResume. These refer to application lifecycle events.
SavePropertiesAsyncmethod 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
Applicationobject that you can then use to obtain the
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.