Creating Mobile Apps with Xamarin.Forms Preview Edition 2

453 Pages·2015·15.1 MB·English

by  Charles Petzold

Creating
Mobile Apps
with Xamarin.Forms
2
Cross-platform C# programming
for iOS, Android, and Windows Phone
CHARLES PETZOLD
PREVIEW EDITION
This excerpt provides early content from a book currently in development and is still in draft format.
See additional notice below.
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Table of contents
Introduction ....................................................................................................................... viii
Who should read this book ...................................................................................................................................................... viii
Conventions and features in this book ................................................................................................................................ viii
The 1st and 2nd Preview Editions ............................................................................................................................................... ix
System requirements ..................................................................................................................................................................... ix
Downloads: Code samples ............................................................................................................................................................ x
Updating the code samples ..................................................................................................................................................... x
Big changes coming for Windows Phone ........................................................................................................................ xi
Acknowledgments .......................................................................................................................................................................... xi
Free ebooks from Microsoft Press ........................................................................................................................................... xii
We want to hear from you ......................................................................................................................................................... xii
Chapter 1 How does Xamarin.Forms fit in? .............................................................. 1
Cross-platform mobile development ....................................................................................................................................... 2
The mobile landscape................................................................................................................................................................ 2
Problem 1: Different user-interface paradigms .............................................................................................................. 2
Problem 2: Different development environments ......................................................................................................... 3
Problem 3: Different programming interfaces ................................................................................................................ 3
Problem 4: Different programming languages ............................................................................................................... 3
The C# and .NET solution ............................................................................................................................................................. 4
A single language for all platforms ...................................................................................................................................... 5
Sharing code ................................................................................................................................................................................. 6
Introducing Xamarin.Forms ......................................................................................................................................................... 8
The Xamarin.Forms option ...................................................................................................................................................... 8
XAML support ............................................................................................................................................................................. 10
Platform specificity ................................................................................................................................................................... 12
A cross-platform panacea? .................................................................................................................................................... 12
Your development environment ............................................................................................................................................. 13
Table of contents iii
Machines and IDEs ................................................................................................................................................................... 13
Devices and emulators ............................................................................................................................................................ 14
Installation ........................................................................................................................................................................................ 14
Creating an iOS app ................................................................................................................................................................. 15
Creating an Android app ....................................................................................................................................................... 15
Creating a Windows Phone app ......................................................................................................................................... 16
All ready? ...................................................................................................................................................................................... 16
Chapter 2 Anatomy of an app .................................................................................... 17
Say hello ............................................................................................................................................................................................ 17
Inside the files ................................................................................................................................................................................. 20
The iOS Project .......................................................................................................................................................................... 23
The Android project ................................................................................................................................................................. 23
The Windows Phone project ................................................................................................................................................ 24
Nothing special! ........................................................................................................................................................................ 25
PCL or SAP? ...................................................................................................................................................................................... 25
Labels for text .................................................................................................................................................................................. 27
1. Include padding on the page ......................................................................................................................................... 31
2. Include padding just for iOS (Shared Asset Project only) .................................................................................... 32
3. Include padding just for iOS (PCL or SAP) ................................................................................................................. 33
4. Center the label within the page ................................................................................................................................... 35
5. Center the text within the label...................................................................................................................................... 37
Chapter 3 Deeper into text .......................................................................................... 38
Wrapping paragraphs .................................................................................................................................................................. 38
Text and background colors ..................................................................................................................................................... 40
The Color structure ....................................................................................................................................................................... 42
Font sizes and attributes ............................................................................................................................................................. 46
Formatted text ................................................................................................................................................................................ 47
Chapter 4 Scrolling the stack ...................................................................................... 54
Stacks of views ................................................................................................................................................................................ 54
Scrolling content ............................................................................................................................................................................ 58
Table of contents iv
The Expands option ...................................................................................................................................................................... 64
Frame and BoxView ...................................................................................................................................................................... 68
A ScrollView in a StackLayout? ................................................................................................................................................. 76
Chapter 5 Dealing with sizes ...................................................................................... 81
Pixels, points, dps, DIPs, and DIUs ........................................................................................................................................... 81
Metrical sizes .................................................................................................................................................................................... 88
Estimated font sizes ...................................................................................................................................................................... 89
Fitting text to available size ....................................................................................................................................................... 92
A fit-to-size clock ........................................................................................................................................................................... 96
Empirically fitting text .................................................................................................................................................................. 98
Chapter 6 Button clicks .............................................................................................. 103
Processing the click .................................................................................................................................................................... 103
Sharing button clicks ................................................................................................................................................................. 106
Anonymous event handlers .................................................................................................................................................... 109
Distinguishing views with IDs ................................................................................................................................................. 111
Saving transient data ................................................................................................................................................................. 114
Chapter 7 XAML vs. code ........................................................................................... 121
Properties and attributes ......................................................................................................................................................... 122
Property-element syntax.......................................................................................................................................................... 126
Adding a XAML page to your project ................................................................................................................................ 130
Platform specificity in the XAML file ................................................................................................................................... 135
The content property attribute ............................................................................................................................................. 139
Formatted text ............................................................................................................................................................................. 141
Chapter 8 Code and XAML in harmony ................................................................ 145
Passing arguments ..................................................................................................................................................................... 145
Constructors with arguments ............................................................................................................................................ 145
Can I call methods from XAML? ....................................................................................................................................... 148
The x:Name attribute................................................................................................................................................................. 150
Custom XAML-based views .................................................................................................................................................... 156
Events and handlers ................................................................................................................................................................... 160
Table of contents v
Tap gestures...................................................................................................................................................................................163
Chapter 9 Platform-specific API calls .................................................................... 170
Preprocessing in the Shared Asset Project ........................................................................................................................170
Parallel classes and the Shared Asset Project ...................................................................................................................173
DependencyService and the Portable Class Library ......................................................................................................175
Platform-specific sound rendering .......................................................................................................................................179
Chapter 10 XAML markup extensions .................................................................. 186
The code infrastructure .............................................................................................................................................................186
Accessing static members ........................................................................................................................................................188
Resource dictionaries .................................................................................................................................................................194
StaticResource for most purposes ....................................................................................................................................195
A tree of dictionaries .............................................................................................................................................................201
DynamicResource for special purposes .........................................................................................................................205
Lesser-used markup extensions .............................................................................................................................................208
A custom markup extension ...................................................................................................................................................209
Chapter 11 The bindable infrastructure ............................................................... 214
The Xamarin.Forms class hierarchy ......................................................................................................................................215
A peek into BindableObject and BindableProperty ......................................................................................................221
Defining bindable properties ..................................................................................................................................................228
The generic Create method ................................................................................................................................................233
The read-only bindable property .....................................................................................................................................234
Chapter 12 Styles ......................................................................................................... 240
The basic Style ..............................................................................................................................................................................240
Styles in code.................................................................................................................................................................................246
Style inheritance ...........................................................................................................................................................................247
Implicit styles .................................................................................................................................................................................252
Dynamic styles ..............................................................................................................................................................................257
Device styles ..................................................................................................................................................................................264
Chapter 13 Bitmaps ..................................................................................................... 269
Platform-independent bitmaps .............................................................................................................................................270
Table of contents vi
Fit and fill ................................................................................................................................................................................... 273
Embedded resources ............................................................................................................................................................ 275
More on sizing ......................................................................................................................................................................... 281
Browsing and waiting ........................................................................................................................................................... 292
Streaming bitmaps ..................................................................................................................................................................... 296
Accessing the streams .......................................................................................................................................................... 297
Generating bitmaps at run time ....................................................................................................................................... 299
Platform-specific bitmaps ........................................................................................................................................................ 303
Bitmap resolutions ................................................................................................................................................................. 305
Toolbars and their icons ...................................................................................................................................................... 312
Button images ......................................................................................................................................................................... 319
Chapter 14 Absolute layout ...................................................................................... 322
AbsoluteLayout in code ............................................................................................................................................................ 323
Attached bindable properties ................................................................................................................................................ 328
Proportional sizing and positioning .................................................................................................................................... 332
Working with proportional coordinates ............................................................................................................................ 334
AbsoluteLayout and XAML ..................................................................................................................................................... 339
Overlays .......................................................................................................................................................................................... 343
Some fun ........................................................................................................................................................................................ 346
Chapter 15 The interactive interface ...................................................................... 355
View overview............................................................................................................................................................................... 355
Slider and Stepper ...................................................................................................................................................................... 356
Slider basics .............................................................................................................................................................................. 356
Common pitfalls ..................................................................................................................................................................... 359
Slider color selection ............................................................................................................................................................. 361
The Stepper difference ......................................................................................................................................................... 365
Switch and CheckBox ................................................................................................................................................................ 368
Switch basics ............................................................................................................................................................................. 368
A traditional CheckBox......................................................................................................................................................... 370
Typing text ..................................................................................................................................................................................... 375
Table of contents vii
Keyboard and focus ...............................................................................................................................................................376
Choosing the keyboard ........................................................................................................................................................377
Entry properties and events ................................................................................................................................................379
The Editor difference .............................................................................................................................................................385
The SearchBar ...........................................................................................................................................................................389
Date and time selection ............................................................................................................................................................394
The DatePicker .........................................................................................................................................................................395
The TimePicker (or is it a TimeSpanPicker?) ................................................................................................................398
Chapter 16 Data binding ........................................................................................... 402
Binding basics ...............................................................................................................................................................................402
Code and XAML ...........................................................................................................................................................................404
Source and BindingContext ....................................................................................................................................................407
The binding mode .......................................................................................................................................................................414
String formatting .........................................................................................................................................................................419
Why is it called “Path”?..............................................................................................................................................................422
Binding value converters ..........................................................................................................................................................425
Bindings and custom views .....................................................................................................................................................433
Introduction
This is the second Preview Edition of a book about writing applications for Xamarin.Forms, the exciting
new mobile development platform for iOS, Android, and Windows Phone unveiled by Xamarin in May
2014. Xamarin.Forms lets you write shared user-interface code in C# and XAML (the Extensible Applica-
tion Markup Language) that maps to native controls on these three platforms.
This book is a Preview Edition because it’s not complete. It has only 16 chapters and doesn’t cover
some important topics. The final edition of the book will probably be published in the summer of 2015.
All information about this book can be found on the book’s home page at:
http://developer.xamarin.com/guides/cross-platform/xamarin-forms/creating-mobile-apps-xamarin-forms/
Who should read this book
This book is for C# programmers who want to write applications for the three most popular mobile
platforms—iOS, Android, and Windows Phone—with a single code base. Xamarin.Forms also has ap-
plicability for those programmers who want eventually to use C# and the Xamarin.iOS and Xama-
rin.Android libraries to target the native application programming interfaces (APIs) of these platforms.
Xamarin.Forms can be a big help in getting programmers started with these platforms or in construct-
ing a prototype or proof-of-concept application.
This book assumes that you know C# and are familiar with the use of the .NET Framework. However,
when I discuss some C# and .NET features that might be somewhat new to recent C# programmers, I
adopt a somewhat slower pace.
Conventions and features in this book
This book has just a few typographical conventions:
 All programming elements referenced in the text—including classes, methods, properties, varia-
ble names, etc.—are shown in a monospaced font, such as the StackLayout class.
 Items that appear in the user interface of Visual Studio or Xamarin Studio, or the applications
discussed in these chapters, appear in boldface, such as the Add New Project dialog.
 Application solutions and projects also appear in boldface, such as MonkeyTap.
Introduction ix
The 1st and 2nd Preview Editions
This book is intended as a tutorial to learn Xamarin.Forms programming. It is not a replacement for the
online API documentation, which can be found at the Xamarin.Forms Framework link on this page:
http://api.xamarin.com/.
The first Preview Edition of this book was published in October 2014 to coincide with the Xamarin
Evolve conference. It contained six chapters but no coverage of XAML.
This second Preview Edition was reconceived to contain shorter and more focused chapters. After
the book establishes a solid foundation in code-only Xamarin.Forms programning, XAML is introduced
in Chapter 7, and from that point on, XAML is used to define the user interface of most of the sample
programs.
This Preview Edition unfortunately ends without covering some essential topics. The most obvious
missing topics include the Grid, collection views such as ListView, page navigation, triggers, behav-
iors, and maps. However, it is our intention to publish chapters online just about as quickly as I can
write them. New chapters will appear on the Xamarin webpage devoted to this book.
System requirements
This book assumes that you’ll be using Xamarin.Forms to write applications that simultaneously target
all three supported mobile platforms—iOS, Android, and Windows Phone. However, it’s very likely that
many readers will be targeting only one or two platforms in their Xamarin.Forms solutions. The plat-
forms you target—and the Xamarin Platform package you purchase—govern your hardware and soft-
ware requirements. For targeting iOS devices, you’ll need a Mac installed with Apple Xcode as well as
the Xamarin Platform, which includes Xamarin Studio. For targeting Windows Phone, you’ll need Visual
Studio 2012 or later (not an Express Edition) on a PC, and you’ll need to have installed the Xamarin
Platform.
However, you can also use Visual Studio on the PC to target iOS devices if the Mac with Xcode and
the Xamarin Platform are accessible via WiFi. You can target Android devices from Visual Studio on the
PC or from Xamarin Studio on either the PC or Mac.
Chapter 1 has more details on the various configurations you can use and resources for additional
information and support. My setup for creating this book consisted of a Microsoft Surface Pro 2 (with
external monitor, keyboard, and mouse) installed with Visual Studio 2013 and the Xamarin Platform,
connected by WiFi with a MacBook Pro installed with Xcode and the Xamarin Platform.
All the screen shots in this book show an iPhone, an Android phone, and a Windows Phone in that
order. The three devices shown in these screen shots reflect my setup and hardware:
 The iPhone 6 simulator on the MacBook Pro running iOS 8.2