精通Xamarin.Forms：实战解决方案

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"Xamarin.Forms Solutions 是一本面向Xamarin平台iOS和Android移动应用开发者的书籍，由Gerald Versluis和Steven Thewissen合著，并由David Ortinau作序。这本书提供了解决日常开发中遇到的Xamarin.Forms常见问题的方案，通过GitHub提供示例代码，便于开发者直接应用到自己的项目中。内容涵盖了用户界面、数据与安全、连接性和外部服务等多个问题领域。书中的解决方案不仅解决具体问题，还强调了最佳实践，以提升代码质量。" 在本书中，开发者可以学习到： 1. 深入理解Xamarin.Forms的基础知识：包括框架的工作原理，这对于初学者来说至关重要。 2. 创建自定义渲染器和依赖服务：这允许开发者根据特定平台的需求定制UI和功能，确保跨平台一致性。 3. 管理用户界面外观：通过样式和主题、布局选项、旋转和动画来实现，使UI更加美观和响应。 4. 构建复杂用户界面：利用各种控件，如PDF查看器、条形码解析、搜索和查找等功能，提高用户体验。 5. 数据和安全性：学习如何存储登录凭据、本地数据缓存以及在HTTP请求中安全地发送授权令牌，以保护用户数据。 6. 安全通信：通过安全的HTTP请求与服务进行交互，确保应用程序的数据传输安全。 7. 应用的签名和部署：优化二进制文件大小，以确保应用的高效分发。本书适合不同水平的Xamarin.Forms开发者，从初学者到专家都能从中受益。初学者可以快速掌握基础知识，中级开发者可以通过解决方案加速项目进度，而经验丰富的开发者则可以发现一些提升工作效率的新技巧。 "Xamarin.Forms Solutions"是一本实用的指南，它为开发者提供了大量实战经验和代码示例，有助于提高Xamarin.Forms应用的开发效率和质量。书中的每个解决方案都经过精心设计，旨在帮助开发者解决实际开发过程中可能遇到的问题，并遵循最佳实践，从而编写出更高质量的代码。

G. Versluis and S. Thewissen, Xamarin.Forms Solutions, https://doi.org/10.1007/978-1-4842-4134-9_1

CHAPTER 1

Fundamentals

In this chapter, we look at some Xamarin.Forms fundamentals. The concepts

explained here will be used throughout this book, so it is important that you have an

understanding of how these fundamentals work. If you already have some knowledge

of Forms and more specifically, custom renderers, the DependencyService, the

MessagingCenter, behaviors and effects, then it is probably safe to skip this chapter.

If not, please keep reading.

Custom Renderers

At the heart of Xamarin.Forms are the renderers. Xamarin.Forms is a library that allows

you to define the user interface (UI) as an abstract layer as opposed to implementing a

UI per platform. To make this work, Xamarin has provided a set of so-called renderers

that will take these abstract controls and translate them into the native counterparts

for you.

Built-in Renderers

For example, if you define a (Xamarin.Forms) Button at runtime, this will be translated

to a UIButton and Android.Widget.Button for iOS and Android, respectively. All the

properties you have set on the Forms button will be mapped by the renderer to the native

equivalent of that property. Or, if a similar property is not available, code is injected to

simulate the intended behavior.

A schematic overview of this process is shown in Figure1-1.

Creating a custom renderer takes roughly three steps:

1. Create a custom control—This needs to be created in the shared

Xamarin.Forms project. Typically you would want to create an

inheritance of the control that you want to create a renderer for.

We will see why in a minute.

2. Use the control in your app—is also happens in the shared

Forms project. You can use your control from either XAML or

code, use it anywhere in your screens and layout.

3. Create a custom renderer for your control—Finally, you have to

implement the custom renderer in each platform that you want to

add something custom. Implementing it for each platform is not

absolutely necessary. We will see this a little later.

The first step isn’t absolutely necessary, but I would recommend it. If you do not

create a custom control, the renderer will be invoked for all the default controls. That

could be something that you would want, but typically it isn’t. Imagine you want

to add color your Entry control’s border. By implementing a custom renderer for

the Entry, all entries in your project will have a colored border. By sub-classing the

Entry into MyEntry and applying the renderer to that, you can apply the color to only

those controls.

Let’s proceed with the first step, creating a custom control. This does not have to be

too hard; it can be just an inheritance of the default control without adding anything

new. At least, if in the future your requirements change, you have the possibility to still

add something to your custom control.

The entry for this example is shown in Listing 1-1 and is called MyEntry.

Listing 1-1. The Custom Entry to Which We Will Apply the Custom Renderer

public class MyEntry : Entry

{

}

We have now simply created a new type with the sole purpose of not rendering all

entry controls, but only the ones of the MyEntry type.

For the second step, we need to implement this control somewhere in our app. Since

we do not have one sample app that we will work with in this book, refer to Listing 1-2 for

a sample code snippet.

CHAPTER 1 FUNDAMENTALS

Listing 1-2. A Basic Page in XAML Using the MyEntry Control

<?xml version="1.0" encoding="utf-8"?>

<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"

xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"

xmlns:local="clr-namespace:SolutionsSampleApp"

x:Class="SolutionsSampleApp.MainPage">

<local:MyEntry Text="Welcome to Xamarin Forms!"

VerticalOptions="Center" HorizontalOptions="Center" />

</ContentPage>

The code in Listing 1-2 is XAML (Extensible Application Markup Language). With

this XML-like syntax you can define your UI design. For this book, we assume that you

already have worked with Xamarin.Forms before and that you know what XAML is.

If not, refer to this link for more information: https://docs.microsoft.com/en-us/

xamarin/xamarin-forms/xaml/xaml-basics/.

Note Throughout the course of this book, we will be defining the UI in XAML.

Everything that can be done in XAML can also be done in code. Whether you want

to use XAML or code is mostly a matter of taste.

There are two things notable in the code in Listing 1-2. First, make sure that you

included a new namespace declaration in the root of the page. In Listing 1-2, it is this

line: xmlns:local="clr-namespace:SolutionsSampleApp". This is similar to adding a

using statement in your class. Now, the local prefix is available to declare controls in the

namespace that is associated to it. You will see this prefixing of tags more often in the

remainder of this book, now you know that this is because of a namespace declaration.

This is what the <local:MyEntry /> tag does. It defines a new control, our custom

control, and takes it from the namespace that we have abbreviated as local. Since our

MyEntry control is just an inheritance of the Entry control, all properties of the Entry are

also available in our control.

Now that we have our own custom control and consumed it in our app, it is time for

the final step: create the custom renderer for it. The renderer classes are to be created for

each platform that we want to support. But note that you are not required to implement

a custom renderer for each platform if you decide to implement it for one platform.

CHAPTER 1 FUNDAMENTALS

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tlforest

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