【Xamarin挖牆腳系列：Xamarin.Android的API設計準則】

原文: 【Xamarin挖牆腳系列：Xamarin.Android的API設計準則】

前言

樓主也是看着Xamarin的官方文檔來的。基本也是照貓畫虎。英語勉強湊合。翻譯的不對的地方，你們多多指教。（這些東西估計弄不完整，呵呵因此別報過高的指望，樓主也很忙） 

原文地址：http://developer.xamarin.com/guides/android/advanced_topics/api_design/

API Design

 

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Overview

In addition to the core Base Class Libraries that are part of Mono, Xamarin.Android ships with bindings for various Android APIs to allow developers to create native Android applications with Mono.

At the core of Xamarin.Android there is an interop engine that bridges the C# world with the Java world and provides developers with access to the Java APIs from C# or other .NET languages.

除了一些基本的核心的BCL類庫是Mono的組成部分，Xamarin.Android 這個類庫爲各個版本的Android APIs作了完整的綁定，這就容許使用Mono的開發人員開發原生的安卓程序。Xamarin.Android的設計核心是爲C#等其餘.net語言的開發者提供了通向Java世界的橋樑。

Design Principles

（略了，一些無非痛癢的東西，就直接忽略了，就挑重要的說）

 

Assemblies

Xamarin.Android includes a number of assemblies that constitute the MonoMobile Profile. The Assemblies page has more information.

The bindings to the Android platform are contained in the Mono.Android.dll assembly. This assembly contains the entire binding for consuming Android APIs and communicating with the Android runtime VM.

Xamarin.Android 項目包含了許多支撐Mono開發移動應用的組件程序集。其中，Mono.Android.dll提供了對Android平臺的對應的SDK的一對一的影射綁定。這個程序集中，包含了全部的安卓API，實現能夠與Android虛擬機通訊交互。

 

Binding Design

Collections

The Android APIs utilize the java.util collections extensively to provide lists, sets, and maps. We expose these elements using the System.Collections.Generic interfaces in our binding. The fundamental mappings are:

Collection mappings

Java Type	System Type	Helper Class
java.util.Set<E>	ICollection<T>	Android.Runtime.JavaSet<T>
java.util.List<E>	IList<T>	Android.Runtime.JavaList<T>
java.util.Map<K,V>	IDictionary<TKey,TValue>	Android.Runtime.JavaDictionary<K,V>
java.util.Collection<E>	ICollection<T>	Android.Runtime.JavaCollection<T>

 

 

We have provided helper classes to facilitate faster copyless marshaling of these types. When possible, we recommend using these provided collections instead of the framework provided implementation, like List<T> or Dictionary<TKey, TValue>. The Android.Runtime implementations utilize a native Java collection internally and therefore do not require copying to and from a native collection when passing to an Android API member.

You can pass any interface implementation to an Android method accepting that interface, e.g. pass a List<int> to theArrayAdapter<int>(Context, int, IList<int>) constructor. However, for all implementations except for the Android.Runtime implementations, this involves copying the list from the Mono VM into the Android runtime VM. If the list is later changed within the Android runtime (e.g. by invoking the ArrayAdapter<T>.Add(T) method), those changes will not be visible in managed code. If a JavaList<int> were used, those changes would be visible.

Rephrased, collections interface implementations that are not one of the above listed Helper Classes only marshal [In]:

 1 // This fails:
 2 var badSource  = new List<int> { 1, 2, 3 };
 3 var badAdapter = new ArrayAdapter<int>(context, textViewResourceId, badSource);
 4 badAdapter.Add (4);
 5 if (badSource.Count != 4) // true
 6     throw new InvalidOperationException ("this is thrown");
 7 
 8 // this works:
 9 var goodSource  = new JavaList<int> { 1, 2, 3 };
10 var goodAdapter = new ArrayAdapter<int> (context, textViewResourceId, goodSource);
11 goodAdapter.Add (4);
12 if (goodSource.Count != 4) // false
13     throw new InvalidOperationException ("should not be reached.");

 

 我以爲這段很重要，可是篇幅有點長，不一一翻譯了。意思是：雖然Mono項目提供了對Java對應的API的影射綁定，可是，在實際應用中，咱們推薦使用 Android.Runtime 命名空間下對應的集合類，這些輔助類可讓咱們既能實現對集合的複雜操做

又能夠跟android運行時交互。也就是在ADT 安卓運行時操做這些事例的時候，在託管代碼 Mono 中也能夠被影響。若是使用System.Collection下的集合，那麼這些集合其實在ADT中是被進行了複製操做。轉化成了對應的ADT下的類型

注意：這樣就不能進行兩邊的雙向通知。好比上面的那個例子。UI組件接受的參數接受任意的集合類型，可是，若是使用的是List IList，那麼當UI組件中的元素髮送變化的時候，並不能通知託管代碼中的集合。相反，使用JavaList是能夠的。因此：針對集合的操做，推薦使用Android.Runtime 下面的集合工具類。

 

Properties

 

Java methods are transformed into properties, when appropriate:

 

• The Java method pair T getFoo() and void setFoo(T) are transformed into the Foo property. Example: Activity.Intent .
• The Java method getFoo() is transformed into the read-only Foo property. Example: Context.PackageName .
• Set-only properties are not generated.

• Properties are not generated if the property type would be an array.

就是把類中的 【屬性】成員  set foo() get foo() 設計爲 C#中的 {get;set}

只讀成員 設計爲C#的  read only

只讀成員是非泛型的

除非是泛型的數組類型，不然屬性不會是泛型成員屬性。

 

Events and Listeners

UI中的組件的事件，對應到C#中的就是 委託！！！delegate 或者 Action 或者 Handler 委託類型的設計 。

var button = new Android.Widget.Button (context) {  Text = string.Format ("{0} clicks!", this.count), }; button.Click += (sender, e) => {  button.Text = string.Format ("{0} clicks!", ++this.count); };

Runnables

Java utilizes the java.lang.Runnable interface to provide a delegation mechanism. The java.lang.Thread class is a notable consumer of this interface. Android has employed the interface in the API as well. Activity.runOnUiThread() and View.post() are notable examples.

The Runnable interface contains a single void method, run(). It therefore lends itself to binding in C# as a System.Actiondelegate. We have provided overloads in the binding which accept an Action parameter for all API members which consume aRunnable in the native API, e.g. Activity.RunOnUiThread() and View.Post().

We left the IRunnable overloads in place instead of replacing them since several types implement the interface and can therefore be passed as runnables directly.

java.lang.Runnable 接口提供了委託實現機制。額，不清楚啥意思。應該是C#中的委託類型 ，是Java中的Runnable接口類型的的一種綁定。好比C#中的Action委託。

 

Inner Classes

（略）

Interfaces

接口類型

Java interfaces can contain three sets of members, two of which cause problems from C#:

1. Methods.
2. Types.
3. Fields.

這個比較操蛋的設計，Java語言中的接口類型能夠有 方法 ，類型 Type，字段。可是Type 和字段是不容許出如今C#中的！因此，這種接口類型被轉化成了兩種候選方案

1 一個同名的接口類型，僅僅包含方法

2 一個靜態的類型，包含全部的字段

（下面就略了）

 

 

Resources

因此的圖片 佈局 文件  String.xml文件，都放置到項目的此目錄下。每次添加 或者從新命名文件，都會在 Resource.Designer.cs的類中，生成或修改對應的資源標識。

Constants and Enumerations

 

常亮和枚舉

這些就更不用說了，徹底一對一的從Android的API中，影射綁定到Mono.Android中了。