安卓的启动activity

一、关于安卓的启动

1、安卓的一个主要的xml配置文件是：AndroidManifest.xml

该文件放在应用的根目录，那么具体根目录是哪里需要根据各个项目去确定（不是工程的根目录，可能是对应的app的src->main->的目录下面，具体根据各个工程的设置）

该文件中的application tag下面可以放置activity并且可以有多个activity，但是每个activity对应的android:name均要不一致

    <application
        android:allowBackup="true"
        android:dataExtractionRules="@xml/data_extraction_rules"
        android:fullBackupContent="@xml/backup_rules"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.SystemWidget"
        tools:targetApi="31">
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

            <meta-data
                android:name="android.app.lib_name"
                android:value="" />
        </activity>
        <activity android:name=".MyViewTest" />
        <activity android:name=".TopBarTest" />
    </application>

如上所示，有3个activity，但是第一个比较特殊还有intent-filter, 对应的action是MAIN表明程序将会以MainActivity类先启动，对应的category是LAUNCHER指明了该应用的图标，那么如果把该launcher去掉会发生什么事情呢？看到有别的博客说是可以启动但是没有图标，这里尝试了一下发现报了如下的错误：Could not identify launch activity: Default Activity not found

二、关于activity在学习过程中遇到的问题

1、编写了类class TopBarTest extends Activity， 然后在类MainActivity中进行调用：startActivity(new Intent(this, TopBarTest.class)); 发现报错，报了如下的错误：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.agiledeveloper.systemwidget, PID: 26002
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:446)
        at android.view.View.performClick(View.java:7317)
        at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1219)
        at android.view.View.performClickInternal(View.java:7291)
        at android.view.View.access$3600(View.java:838)
        at android.view.View$PerformClick.run(View.java:28247)
        at android.os.Handler.handleCallback(Handler.java:900)
        at android.os.Handler.dispatchMessage(Handler.java:103)
        at android.os.Looper.loop(Looper.java:219)
        at android.app.ActivityThread.main(ActivityThread.java:8668)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1109)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:441)
        at android.view.View.performClick(View.java:7317) 
        at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1219) 
        at android.view.View.performClickInternal(View.java:7291) 
        at android.view.View.access$3600(View.java:838) 
        at android.view.View$PerformClick.run(View.java:28247) 
        at android.os.Handler.handleCallback(Handler.java:900) 
        at android.os.Handler.dispatchMessage(Handler.java:103) 
        at android.os.Looper.loop(Looper.java:219) 
        at android.app.ActivityThread.main(ActivityThread.java:8668) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1109) 
     Caused by: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.agiledeveloper.systemwidget/com.agiledeveloper.systemwidget.TopBarTest}; have you declared this activity in your AndroidManifest.xml?
        at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:2118)
        at android.app.Instrumentation.execStartActivity(Instrumentation.java:1746)
        at android.app.Activity.startActivityForResult(Activity.java:5412)
        at androidx.activity.ComponentActivity.startActivityForResult(ComponentActivity.java:712)
        at android.app.Activity.startActivityForResult(Activity.java:5337)
        at androidx.activity.ComponentActivity.startActivityForResult(ComponentActivity.java:693)
        at android.app.Activity.startActivity(Activity.java:5819)
        at android.app.Activity.startActivity(Activity.java:5787)
        at com.agiledeveloper.systemwidget.MainActivity.btnTopBar(MainActivity.java:36)
        at java.lang.reflect.Method.invoke(Native Method) 
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:441) 
        at android.view.View.performClick(View.java:7317) 
        at com.google.android.material.button.MaterialButton.performClick(MaterialButton.java:1219) 
        at android.view.View.performClickInternal(View.java:7291) 
        at android.view.View.access$3600(View.java:838) 
        at android.view.View$PerformClick.run(View.java:28247) 
        at android.os.Handler.handleCallback(Handler.java:900) 
        at android.os.Handler.dispatchMessage(Handler.java:103) 
        at android.os.Looper.loop(Looper.java:219) 
        at android.app.ActivityThread.main(ActivityThread.java:8668) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1109) 

刚开始看了错误前面几行，实在是看不出为啥崩溃，后来看到这一行错误提示时候：Caused by: android.content.ActivityNotFoundException: Unable to find explicit activity class {com.agiledeveloper.systemwidget/com.agiledeveloper.systemwidget.TopBarTest}; have you declared this activity in your AndroidManifest.xml?

才发现原来继承Activity,使用Activity需要在AndroidManifest.xml中添加对应的Activity，即上面的xml中的最后一行