Android基礎-Activity基本使用

公眾號菜鳥童靴發表於2020-11-24
Android

1. Activity概念

Activity，即活動，是一種可以包含使用者介面的元件，主要用於和使用者進行互動

活動提供應用程式繪製其UI的視窗。此視窗通常填充螢幕，但可能小於螢幕並浮動在其他視窗的頂部。通常，一個活動在應用程式中實現一個螢幕。

多數應用包含多個螢幕，即多個活動，一般會指定一個活動為主要活動，即使用者啟動程式顯示的第一個螢幕。

2. Activity基本用法

2.1 註冊宣告

所有的活動都要在AndroidManifest.xml中進行註冊才能生效活動的註冊宣告要放在<application>標籤內，在<activity>標籤中是用了android：name來指定具體註冊哪一個活動。

**配置主活動的方法**就是在<application>標籤內加入<intent-filter>標籤。這個標籤內包含和兩個可選標籤、。這些標籤元素組合在一起以指定您的活動可以響應的意圖型別。

示例：

<span style="color:#000000"><code class="language-xml"><span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>activity</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>.ExampleActivity<span style="color:#999999">"</span></span> <span style="color:#d19a66">android:icon</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>@drawable/app_icon<span style="color:#999999">"</span></span><span style="color:#999999">></span></span>
    <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>intent-filter</span><span style="color:#999999">></span></span>
        <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>action</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.action.SEND<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
        <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>category</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.category.DEFAULT<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
        <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>data</span> <span style="color:#d19a66">android:mimeType</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>text/plain<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
    <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>intent-filter</span><span style="color:#999999">></span></span>
<span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>activity</span><span style="color:#999999">></span></span>
</code></span>
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如果時配置成主活動的話，為：

<span style="color:#000000"><code class="language-xml"><span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>action</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.action.MAIN<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>

<span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>category</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.category.LAUNCHER<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
</code></span>
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2.2 許可權宣告

除非兩個活動在其清單中具有相同的許可權，否則父活動無法啟動子活動。如果為父活動宣告uses-permission元素，則每個子活動必須具有匹配的uses-permission元素。

示例：

<span style="color:#000000"><code class="language-xml"><span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>manifest</span><span style="color:#999999">></span></span>
<span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>activity</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>....<span style="color:#999999">"</span></span>
   <span style="color:#d19a66">android:permission</span><span style="color:#669900"><span style="color:#999999">=</span>”com.google.socialapp.permission.SHARE_POST”</span>

<span style="color:#999999">/></span></span>
</code></span>
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如果父活動宣告瞭該許可權，則子活動也必須宣告該許可權，否則父活動無法啟動子活動。

2.3 銷燬一個活動

銷燬一個活動只需要按back鍵即可。Activity類提供了一個finish()方法，活動呼叫這個方法就可銷燬當前活動。

2.4 Intent使用

Intent是Android程式中各元件之間進行互動的一種重要方式，不僅可以指明當前元件想要執行的動作，還可以在不同元件之間進行傳遞資料。被用於啟動活動，啟動服務以及傳送廣播等場景。

2.4.1 使用顯式Intent

Activity類中提供一個startActivity()方法，專門用於啟動活動。

顯示啟動的三種方法：

  • 基本用法

    <span style="color:#000000"><code class="language-java">Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span>FirstActivity<span style="color:#999999">.</span><span style="color:#c678dd">this</span><span style="color:#999999">,</span> SecondActivity<span style="color:#999999">.</span><span style="color:#c678dd">class</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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  • 通過Intent的ComponentName啟動

    <span style="color:#000000"><code class="language-java">ComponentName cn <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> ComponentName<span style="color:#999999">(</span><span style="color:#669900">"com.baiheng.activitytest"</span><span style="color:#999999">,</span><span style="color:#669900">"com.baiheng.activitytest.ThirdActivity"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
intent<span style="color:#999999">.</span><span style="color:#61aeee">setComponent</span><span style="color:#999999">(</span>cn<span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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  • 通過初始化Intent時指定包名

    <span style="color:#000000"><code class="language-java">Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span><span style="color:#669900">"android.intent.action.MAIN"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
intent<span style="color:#999999">.</span><span style="color:#61aeee">setClassName</span><span style="color:#999999">(</span><span style="color:#669900">"com.baiheng.activitytest"</span><span style="color:#999999">,</span> <span style="color:#669900">"com.baiheng.activitytest.ThirdActivity"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
<span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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2.4.2 使用隱式Intent

不明確指出我們想要啟動哪一個活動，指定一系列抽象的action和category等資訊，交由系統分析這個Intent，這個通過Intent的Intent-filter是實現。

<span style="color:#000000"><code class="language-xml"><span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>activity</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>.SecondActivity<span style="color:#999999">"</span></span><span style="color:#999999">></span></span>
            <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>intent-filter</span><span style="color:#999999">></span></span>
                <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>action</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>com.baiheng.activitytest.action.START<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
                <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>category</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.category.DEFAULT<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
            <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>intent-filter</span><span style="color:#999999">></span></span>
        <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>activity</span><span style="color:#999999">></span></span>
</code></span>
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<span style="color:#000000"><code class="language-java">Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
intent<span style="color:#999999">.</span><span style="color:#61aeee">setAction</span><span style="color:#999999">(</span>Intent<span style="color:#999999">.</span>ACTION_START<span style="color:#999999">)</span><span style="color:#999999">;</span>     intent<span style="color:#999999">.</span><span style="color:#61aeee">addCategory</span><span style="color:#999999">(</span>Intent<span style="color:#999999">.</span>CATEGORY_DEFAULT<span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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每個Intent只能指定一個action，但是卻能指定多個category。

在<intent-filter>標籤中再配置一個<data>標籤，用於更精準地指定當前活動能夠響應什麼型別的資料。<data>標籤中主要可以配置一下內容：

  • android:scheme：用於指定資料的協議部分，如示例：http
  • android:host：用於指定資料的主機名部分，如示例www.baidu.com
  • android:port：用於指定資料的埠部分，一般緊隨在主機名之後
  • android:path:用於指定主機名和埠之後的部分，如一段網址跟在主機名之後的內容
  • android:mimeType：用於指定可以處理的資料型別，允許使用萬用字元的方式進行指定。

只有<data>標籤中指定的內容和Intent中攜帶的Data完全一致，當前活動才能夠相應該Intent。一般不會在<data>標籤中指定過多內容。

2.4.3 使用隱式Intent啟動其他程式

使用隱式Intent不僅可以啟動程式內的活動，還可以啟動其他程式的活動。

示例：

<span style="color:#000000"><code class="language-java">Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span>Intent<span style="color:#999999">.</span>ACTION_VIEW<span style="color:#999999">)</span><span style="color:#999999">;</span>
                intent<span style="color:#999999">.</span><span style="color:#61aeee">setData</span><span style="color:#999999">(</span>Uri<span style="color:#999999">.</span><span style="color:#61aeee">parse</span><span style="color:#999999">(</span><span style="color:#669900">"http://www.google.com.hk"</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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Uri.parse將一個網址字串解析成一個Uri物件。

<span style="color:#000000"><code class="language-xml"><span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>activity</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>.ThirdActivity<span style="color:#999999">"</span></span><span style="color:#999999">></span></span>
            <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>intent-filter</span> <span style="color:#d19a66">tools:ignore</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>AppLinkUrlError<span style="color:#999999">"</span></span><span style="color:#999999">></span></span>
                <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>action</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.action.VIEW<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
                <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>category</span> <span style="color:#d19a66">android:name</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>android.intent.category.DEFAULT<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
                <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"><</span>data</span> <span style="color:#d19a66">android:scheme</span><span style="color:#669900"><span style="color:#999999">=</span><span style="color:#999999">"</span>http<span style="color:#999999">"</span></span> <span style="color:#999999">/></span></span>
            <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>intent-filter</span><span style="color:#999999">></span></span>
        <span style="color:#e06c75"><span style="color:#e06c75"><span style="color:#999999"></</span>activity</span><span style="color:#999999">></span></span>
</code></span>
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3. Activity 間資料傳遞

一個App一般包含多個Activity，涉及到Activity之間的資料傳遞。關於如何傳遞不同的資料型別在Intent部分詳解，這裡介紹一下兩種傳遞方式

3.1 向下傳遞資料

Intent中提供了一些列putExtra()方法的過載，可以把想要傳遞的資料暫存在Intent中，啟動了另一個活動後，把這些資料從Intent中取出來就可以。

示例：在FirstActivity中：

<span style="color:#000000"><code class="language-java">String data <span style="color:#669900">=</span> <span style="color:#669900">"Hello SecondActivity"</span><span style="color:#999999">;</span>
                Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span>FirstActivity<span style="color:#999999">.</span><span style="color:#c678dd">this</span><span style="color:#999999">,</span> SecondActivity<span style="color:#999999">.</span><span style="color:#c678dd">class</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                intent<span style="color:#999999">.</span><span style="color:#61aeee">putExtra</span><span style="color:#999999">(</span><span style="color:#669900">"extra_data"</span><span style="color:#999999">,</span> data<span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">startActivity</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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SecondActivity：

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>Bundle savedInstanceState<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>savedInstanceState<span style="color:#999999">)</span><span style="color:#999999">;</span>
        <span style="color:#61aeee">setContentView</span><span style="color:#999999">(</span>R<span style="color:#999999">.</span>layout<span style="color:#999999">.</span>second_activity<span style="color:#999999">)</span><span style="color:#999999">;</span>
        Intent intent <span style="color:#669900">=</span> <span style="color:#61aeee">getIntent</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        String data <span style="color:#669900">=</span> intent<span style="color:#999999">.</span><span style="color:#61aeee">getStringExtra</span><span style="color:#999999">(</span><span style="color:#669900">"extra_data"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        Log<span style="color:#999999">.</span><span style="color:#61aeee">d</span><span style="color:#999999">(</span><span style="color:#669900">"SecondActivity"</span><span style="color:#999999">,</span> data<span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
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3.2 返回資料給上一個活動

Activity中還有一個startActivityForResult()方法也適用於啟動活動，但是該方法期望活動銷燬的時候返回一個結果給上一個活動。

該方法接收兩個引數，一個是Intent，第二個引數是請求碼，用於之後在回撥中判斷資料的來源。

FirstActivity：

<span style="color:#000000"><code class="language-java">Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span>FirstActivity<span style="color:#999999">.</span><span style="color:#c678dd">this</span><span style="color:#999999">,</span> SecondActivity<span style="color:#999999">.</span><span style="color:#c678dd">class</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">startActivityForResult</span><span style="color:#999999">(</span>intent<span style="color:#999999">,</span><span style="color:#98c379">1</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
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SecondActivity：

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>Bundle savedInstanceState<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>savedInstanceState<span style="color:#999999">)</span><span style="color:#999999">;</span>
        <span style="color:#61aeee">setContentView</span><span style="color:#999999">(</span>R<span style="color:#999999">.</span>layout<span style="color:#999999">.</span>second_activity<span style="color:#999999">)</span><span style="color:#999999">;</span>
        Button button2 <span style="color:#669900">=</span> <span style="color:#999999">(</span>Button<span style="color:#999999">)</span> <span style="color:#61aeee">findViewById</span><span style="color:#999999">(</span>R<span style="color:#999999">.</span>id<span style="color:#999999">.</span>button_2<span style="color:#999999">)</span><span style="color:#999999">;</span>
        button2<span style="color:#999999">.</span><span style="color:#61aeee">setOnClickListener</span><span style="color:#999999">(</span><span style="color:#c678dd">new</span> View<span style="color:#999999">.</span>OnClickListener<span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>

            <span style="color:#999999">@Override</span>
            <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onClick</span><span style="color:#999999">(</span>View v<span style="color:#999999">)</span> <span style="color:#999999">{</span>
                Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                intent<span style="color:#999999">.</span><span style="color:#61aeee">putExtra</span><span style="color:#999999">(</span><span style="color:#669900">"data_return"</span><span style="color:#999999">,</span><span style="color:#669900">"Hello FirstActivity"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">setResult</span><span style="color:#999999">(</span>RRSULT_OK<span style="color:#999999">,</span> intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#61aeee">finish</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
            <span style="color:#999999">}</span>
        <span style="color:#999999">}</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
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最後在FirstActivity中重寫這個方法得到返回資料：

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onActivityResult</span><span style="color:#999999">(</span><span style="color:#c678dd">int</span> requestCode<span style="color:#999999">,</span> <span style="color:#c678dd">int</span> resultCode<span style="color:#999999">,</span> <span style="color:#999999">@Nullable</span> Intent data<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">switch</span><span style="color:#999999">(</span>requestCode<span style="color:#999999">)</span> <span style="color:#999999">{</span>
            <span style="color:#c678dd">case</span> <span style="color:#98c379">1</span><span style="color:#669900">:</span>
                <span style="color:#c678dd">if</span> <span style="color:#999999">(</span>resultCode <span style="color:#669900">==</span> RESULT_OK<span style="color:#999999">)</span> <span style="color:#999999">{</span>
                    String returnedData <span style="color:#669900">=</span> data<span style="color:#999999">.</span><span style="color:#61aeee">getStringExtra</span><span style="color:#999999">(</span><span style="color:#669900">"data_return"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
                    Log<span style="color:#999999">.</span><span style="color:#61aeee">d</span><span style="color:#999999">(</span><span style="color:#669900">"FritstActivity"</span><span style="color:#999999">,</span>returnedData<span style="color:#999999">)</span><span style="color:#999999">;</span>
                <span style="color:#999999">}</span>
                <span style="color:#c678dd">break</span><span style="color:#999999">;</span>
            <span style="color:#c678dd">default</span><span style="color:#669900">:</span>
        <span style="color:#999999">}</span>
    <span style="color:#999999">}</span>
</code></span>
  •  

如果是通過back鍵返回，則可以通過在SecondActivity中重寫onBackPressed()來解決：

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onBackPressed</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>
        Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        intent<span style="color:#999999">.</span><span style="color:#61aeee">putExtra</span><span style="color:#999999">(</span><span style="color:#669900">"data_return"</span><span style="color:#999999">,</span><span style="color:#669900">"Hello FirstActivity"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        <span style="color:#61aeee">setResult</span><span style="color:#999999">(</span>RESULT_OK<span style="color:#999999">,</span> intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
        <span style="color:#61aeee">finish</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
  •  

4. Activity生命週期

4.1 返回棧

Android中的活動是可以層疊的。每啟動一個新的活動，就會覆蓋原來的活動之上，點選Back鍵就會銷燬最上面的活動。

Android是使用任務（Task）來管理活動的，一個任務就是一組存放在棧裡的活動的集合，這個棧也稱作返回棧（Back Stack）。系統總是會顯示處於棧頂活動給使用者。

4.2 活動狀態

每個活動在其生命週期中最多可能會有4中狀態：

  • 執行狀態：

    活動位於返回棧的棧頂時，這時活動就處於執行狀態。

  • 暫停狀態：

    活動不處於棧頂時，但是仍然可見，活動就進入暫停狀態。

  • 停止狀態：

    活動不處於棧頂位置，且完全不可見，就進入停止狀態。處於停止狀態的活動有可能會被系統回收

  • 銷燬狀態：

    活動從返回棧中移除後就變成了銷燬狀態。

4.3 生命週期

Activity類中定義了7種回撥方法，覆蓋了活動生命週期的每一個環節：

  • onCreate()：完成活動的初始化操作，比如載入佈局，繫結事件
  • onStart():在活動由不可見變為可見的時候呼叫
  • onResume()：在活動準備好和使用者進行互動時呼叫。該活動一定處於返回棧棧頂，且處於執行狀態
  • onPause()：系統準備去啟動或者恢復另一個活動的時候呼叫。該方法的執行速度一定要快，否則會影響到新的棧頂活動的使用
  • onStop：在活動完全不可見時呼叫，與前者的區別時啟動的新活動是一個對話方塊式的活動，那麼onPause()方法會得到執行，而該方法不會
  • onDestory()：在活動被銷燬之前呼叫，之後活動變為銷燬狀態
  • onRestart()：由停止狀態變為執行時狀態之前呼叫，活動被重新啟動。

除了**onRestart()**方法之外，其他都是兩兩相對的，活動分為3中生存期：

  • 完整生存期：活動在onCreate()和onDestrory()之間經歷的就是完整生存期
  • 可見生存期：活動在onStart()和onStop之間經歷的就是可見生存期。活動對於使用者總是可見的，即便有可能無法和使用者進行互動。
  • 前臺生存期：活動在onResume()和onPause()之間經歷的就是前臺生存期。活動總是處於執行狀態，此時活動可以和使用者進行互動

[外鏈圖片轉存失敗(img-SiLfeKn5-1565323366159)(https://developer.android.com/guide/components/images/activity_lifecycle.png)]

4.4 活動被回收

如果一個活動進入停止狀態，有可能被系統回收，其中的資料在回收時可能得不到儲存。

Activity中提供了一個onSaveInstanceState()回撥方法，這個方法可以保證活動被回收之前一定被呼叫，可以通過這個方法來儲存資料。

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onSaveInstanceState</span><span style="color:#999999">(</span>Bundle outState<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onSaveInstanceState</span><span style="color:#999999">(</span>outState<span style="color:#999999">)</span><span style="color:#999999">;</span>
        String tempData <span style="color:#669900">=</span> <span style="color:#669900">"Something you just typed"</span><span style="color:#999999">;</span>
        outState<span style="color:#999999">.</span><span style="color:#61aeee">putString</span><span style="color:#999999">(</span><span style="color:#669900">"data_key"</span><span style="color:#999999">,</span> tempData<span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
  •  

資料的取值可以在onCreate()方法中取出

<span style="color:#000000"><code class="language-java"><span style="color:#c678dd">if</span> <span style="color:#999999">(</span>savedInstanceState <span style="color:#669900">!=</span> null<span style="color:#999999">)</span> <span style="color:#999999">{</span>
            String tempData <span style="color:#669900">=</span> savedInstanceState<span style="color:#999999">.</span><span style="color:#61aeee">getString</span><span style="color:#999999">(</span><span style="color:#669900">"data_key"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
            Log<span style="color:#999999">.</span><span style="color:#61aeee">d</span><span style="color:#999999">(</span>TAG<span style="color:#999999">,</span> tempData<span style="color:#999999">)</span><span style="color:#999999">;</span>
        <span style="color:#999999">}</span>
</code></span>
  •  

5. 橫豎屏切換

App橫豎屏切換的時候會銷燬當前的Activity然後重新建立一個，橫豎屏切換時Activity走下述生命週期：

onPause-> onStop-> onDestory-> onCreate->onStart->onResume

禁止橫豎屏自動切換的話，在AndroidManifest.xml中為Activity新增一個屬性：android:screenOrientaiton

6. 啟動模式

啟動模式一共有4種，分別是：standard，singleTop，singleTask和singleInstance。可以再AndroidManifest.xml中通過<activity>標籤指定android:launchMode屬性來選擇啟動模式。

6.1 standard

預設模式，每當啟動一個新活動，就會在返回棧中入棧，並處於棧頂位置。該模式的活動無論這個活動是否在返回棧中存在，每次啟動都會建立該活動的新例項。

6.2 singleTop

當活動指定為singleTop模式，啟動活動時如果發現返回棧的棧頂已經是該活動，則認為可以直接使用它，不會再建立新的活動例項。

6.2 singleTask

當活動的啟動模式指定為singleTask，每次啟動該活動系統首先會在返回棧中檢查是否存在該活動的例項，如果發現就直接使用，並把這個活動之上的所有活動統統出棧。如果沒有發現就建立一個新的活動例項。

6.4 singleInstance

指定為singleInstance模式的活動會啟用一個新的返回棧來管理這個活動（如果singleTask模式指定了不同的taskAffinity，也會啟動一個新的返回棧）。這種模式解決了其他程式和我們的程式共享一個活動的例項

7. 簡單技巧

7.1 快速定位某個介面對應的活動

新建一個Java類：BaseActivity，繼承AppCompatActivity,重寫onCreate()方法。

<span style="color:#000000"><code class="language-java"><span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span><span style="color:#999999">@Nullable</span> Bundle savedInstanceState<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>savedInstanceState<span style="color:#999999">)</span><span style="color:#999999">;</span>
        Log<span style="color:#999999">.</span><span style="color:#61aeee">d</span><span style="color:#999999">(</span><span style="color:#669900">"BaseActivity"</span><span style="color:#999999">,</span> <span style="color:#61aeee">getClass</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">.</span><span style="color:#61aeee">getSimpleName</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
  •  

在oncreate()方法中獲取了當前例項的類名，然後將專案中所有活動的父類都不再繼承AppCompatActivity，而是繼承BaseActivity，這樣檢視日誌，，每當進入一個活動的介面，該活動的類名就被列印出來。

7.2 隨時退出程式

按home鍵只是將程式掛起，並沒有退出程式，如果程式需要一個登出或者退出的功能，只需要一個專門的集合類對所有的活動進行管理就可以。

新建一個ActivityCollector類作為活動管理器：

<span style="color:#000000"><code class="language-java"><span style="color:#c678dd">public</span> <span style="color:#c678dd">class</span> ActivityCollector <span style="color:#999999">{</span>

    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> List<span style="color:#61aeee"><span style="color:#999999"><</span>Activity<span style="color:#999999">></span></span> activities <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> ArrayList<span style="color:#669900"><</span><span style="color:#669900">></span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>

    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">addActivity</span><span style="color:#999999">(</span>Activity activity<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        activities<span style="color:#999999">.</span><span style="color:#61aeee">add</span><span style="color:#999999">(</span>activity<span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>

    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">removeActivity</span><span style="color:#999999">(</span>Activity activity<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        activities<span style="color:#999999">.</span><span style="color:#61aeee">remove</span><span style="color:#999999">(</span>activity<span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>

    <span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">finishAll</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">for</span> <span style="color:#999999">(</span>Activity activity <span style="color:#669900">:</span> activities<span style="color:#999999">)</span> <span style="color:#999999">{</span>
            <span style="color:#c678dd">if</span> <span style="color:#999999">(</span><span style="color:#669900">!</span>activity<span style="color:#999999">.</span><span style="color:#61aeee">isFinishing</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>
                activity<span style="color:#999999">.</span><span style="color:#61aeee">finish</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
            <span style="color:#999999">}</span>
        <span style="color:#999999">}</span>
    <span style="color:#999999">}</span>
<span style="color:#999999">}</span>
</code></span>
  •  

修改BaseActivity

<span style="color:#000000"><code class="language-java"><span style="color:#c678dd">public</span> <span style="color:#c678dd">class</span> BaseActivity <span style="color:#c678dd">extends</span> AppCompatActivity <span style="color:#999999">{</span>

    <span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span><span style="color:#999999">@Nullable</span> Bundle savedInstanceState<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onCreate</span><span style="color:#999999">(</span>savedInstanceState<span style="color:#999999">)</span><span style="color:#999999">;</span>
        Log<span style="color:#999999">.</span><span style="color:#61aeee">d</span><span style="color:#999999">(</span><span style="color:#669900">"BaseActivity"</span><span style="color:#999999">,</span> <span style="color:#61aeee">getClass</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">.</span><span style="color:#61aeee">getSimpleName</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        ActivityCollector<span style="color:#999999">.</span><span style="color:#61aeee">addActivity</span><span style="color:#999999">(</span><span style="color:#c678dd">this</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>

    <span style="color:#999999">@Override</span>
    <span style="color:#c678dd">protected</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onDestroy</span><span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>
        <span style="color:#c678dd">super</span><span style="color:#999999">.</span><span style="color:#61aeee">onDestroy</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        ActivityCollector<span style="color:#999999">.</span><span style="color:#61aeee">removeActivity</span><span style="color:#999999">(</span><span style="color:#c678dd">this</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
<span style="color:#999999">}</span>
</code></span>

如果需要在某個地方退出程式，呼叫ActivityCollector.finishAll()方法就可以。

可以再銷燬所有活動程式碼的後面加上殺掉當前程式，確保程式完全退出：

<span style="color:#000000"><code class="language-java">android<span style="color:#999999">.</span>os<span style="color:#999999">.</span>Process<span style="color:#999999">.</span><span style="color:#61aeee">killProcess</span><span style="color:#999999">(</span>android<span style="color:#999999">.</span>os<span style="color:#999999">.</span>Process<span style="color:#999999">.</span><span style="color:#61aeee">myPid</span><span style="color:#999999">(</span><span style="color:#999999">)</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
</code></span>
  • 1

7.2啟動活動最佳寫法

如果你的模組需要將一些重要資料傳遞給另一個模組，例如兩個字串引數，但是你不清楚啟動這個活動需要傳遞哪些資料，只需在啟動活動新增一個actionStart()方法。

<span style="color:#000000"><code class="language-java"><span style="color:#c678dd">public</span> <span style="color:#c678dd">static</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">actionStart</span><span style="color:#999999">(</span>Context context<span style="color:#999999">,</span> String data1<span style="color:#999999">,</span> String data2<span style="color:#999999">)</span> <span style="color:#999999">{</span>
        Intent intent <span style="color:#669900">=</span> <span style="color:#c678dd">new</span> Intent<span style="color:#999999">(</span>context<span style="color:#999999">,</span> SecondActivity<span style="color:#999999">.</span><span style="color:#c678dd">class</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
        intent<span style="color:#999999">.</span><span style="color:#61aeee">putExtra</span><span style="color:#999999">(</span><span style="color:#669900">"param1"</span><span style="color:#999999">,</span> data1<span style="color:#999999">)</span><span style="color:#999999">;</span>
        intent<span style="color:#999999">.</span><span style="color:#61aeee">putExtra</span><span style="color:#999999">(</span><span style="color:#669900">"param2"</span><span style="color:#999999">,</span> data2<span style="color:#999999">)</span><span style="color:#999999">;</span>
        context<span style="color:#999999">.</span><span style="color:#61aeee">startActivities</span><span style="color:#999999">(</span>intent<span style="color:#999999">)</span><span style="color:#999999">;</span>
    <span style="color:#999999">}</span>
</code></span>
  •  

如果你想要啟動該活動，只需：

<span style="color:#000000"><code class="language-java">button1<span style="color:#999999">.</span><span style="color:#61aeee">setOnClickListener</span><span style="color:#999999">(</span><span style="color:#c678dd">new</span> View<span style="color:#999999">.</span>OnClickListener<span style="color:#999999">(</span><span style="color:#999999">)</span> <span style="color:#999999">{</span>

            <span style="color:#999999">@Override</span>
            <span style="color:#c678dd">public</span> <span style="color:#c678dd">void</span> <span style="color:#61aeee">onClick</span><span style="color:#999999">(</span>View v<span style="color:#999999">)</span> <span style="color:#999999">{</span>
            	SecondActivity<span style="color:#999999">.</span><span style="color:#61aeee">actionStart</span><span style="color:#999999">(</span>FirstActivity<span style="color:#999999">.</span><span style="color:#c678dd">this</span><span style="color:#999999">,</span> <span style="color:#669900">"data1"</span><span style="color:#999999">,</span> <span style="color:#669900">"data2"</span><span style="color:#999999">)</span><span style="color:#999999">;</span>
            <span style="color:#999999">}</span>
 <span style="color:#999999">}</span></code></span>

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