Android

Android 版本历史如表 1 所示。

参考：https://en.wikipedia.org/wiki/Android_version_history

Android 系统有下面四大组件：
1、活动（Activities），An activity is the entry point for interacting with the user. It represents a single screen with a user interface.
2、服务（Services），A service is a general-purpose entry point for keeping an app running in the background for all kinds of reasons. It is a component that runs in the background to perform long-running operations or to perform work for remote processes.
3、广播接收器（Broadcast receivers），A broadcast receiver is a component that enables the system to deliver events to the app outside of a regular user flow, allowing the app to respond to system-wide broadcast announcements.
4、内容提供器（Content providers），A content provider manages a shared set of app data that you can store in the file system, in a SQLite database, on the web, or on any other persistent storage location that your app can access. Through the content provider, other apps can query or modify the data if the content provider allows it. For example, the Android system provides a content provider that manages the user's contact information. As such, any app with the proper permissions can query the content provider, such as ContactsContract.Data, to read and write information about a particular person. It is tempting to think of a content provider as an abstraction on a database, because there is a lot of API and support built in to them for that common case.

参考：https://developer.android.com/guide/components/fundamentals

使用 Android Studio 的菜单创建工程，选择“Empty Activity”，不用手写一行代码就可以实现显示 Hello World 的 App。点击“Build”->“Make Project”，可以得到 apk 安装包，安装打开后如图 1 所示。

这个工程中，自动生成的源码主要为“app/src/main/java/com/example/myapplication/MainActivity.java”，其内容为：

package com.example.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //                     ^
        //                     |
        //                     |
        //                 对应文件
        //  app/src/main/res/layout/activity_main.xml
    }
}

上面源码中，并没有“Hello World”字样，那它是从哪里来呢？“Hello World”保存在文件“app/src/main/res/layout/activity_main.xml”中，这个文件中定义了 UI 布局相关内容，其完整内容为：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

此外，还有一个重要的 Manifest 文件 “app/src/main/AndroidManifest.xml”，包含了 App 配置信息。这个例子中其内容为：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.myapplication">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

</manifest>

列出当前连接的所有设备（包含真机和模拟器）：

$ adb devices
List of devices attached
f0146a44	device
emulator-5554	device

$ adb devices -l            # -l 可显示更详细的信息
List of devices attached
f0146a44               device usb:336592896X product:oxygen model:MI_MAX_2 device:oxygen transport_id:4
emulator-5554          device product:sdk_gphone_x86 model:Android_SDK_built_for_x86 device:generic_x86 transport_id:2

当只有一个连接设备时，adb 命令都默认针对这个设备，不需要显式地指定设备。当有多个连接设备时，通过 -s SERIAL （其中 SERIAL 就是 adb devices 的第 1 列输出）可以选择哪个设备，如在设备 f0146a44 上安装软件：

$ adb -s f0146a44 install ./your-app.apk

安装软件：

$ adb install ./your-app.apk
Performing Streamed Install
Success

卸载软件：

$ adb uninstall com.example.MyApp

卸载软件时，需要指定 package 名。通过下面命令可以列出设备中安装的第 3 方软件的 package 名：

$ adb shell pm list packages -3
package:com.baidu.input_mi
package:com.google.android.marvin.talkback
package:com.baidu.duersdk.opensdk

使用 adb pull 和 adb push 可以在设备和主机之间复制文件：

$ adb pull remote local               # 从设备复制文件到本地
$ adb push local remote               # 从本地复制文件到设备

例如：

$ adb push foo.txt /sdcard/foo.txt    # 把本地文件 foo.txt 复制到设备中

执行 adb shell 可以得到一个交互式的 shell。

进入到工程根目录，执行 ./gradlew assembleRelease 可以得到 apk 安装程序，如果还想得到更详细的输出可以增加 --info 或者 --debug 选项。如：

$ ./gradlew assembleRelease

BUILD SUCCESSFUL in 44s
27 actionable tasks: 26 executed, 1 up-to-date

$ find . -name "*.apk"
./app/build/outputs/apk/release/app-release-unsigned.apk   # 这里新生成的apk文件

进入到工程根目录，执行 ./gradlew assembleDebug 可以得到 Debug 版本的 apk 安装程序。如：

$ ./gradlew assembleDebug

BUILD SUCCESSFUL in 4s
27 actionable tasks: 26 executed, 1 up-to-date

$ find . -name "*.apk"
./app/build/outputs/apk/release/app-release-unsigned.apk
./app/build/outputs/apk/debug/app-debug.apk

运行 ./gradlew clean 可以清除编译生成的文件。

$ ./gradlew clean

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

运行 ./gradlew tasks 可以查看所有支持的任务。如：

$ ./gradlew tasks

> Task :tasks

------------------------------------------------------------
All tasks runnable from root project
------------------------------------------------------------

Android tasks
-------------
androidDependencies - Displays the Android dependencies of the project.
signingReport - Displays the signing info for each variant.
sourceSets - Prints out all the source sets defined in this project.

Build tasks
-----------
assemble - Assembles all variants of all applications and secondary packages.
assembleAndroidTest - Assembles all the Test applications.
assembleDebug - Assembles all Debug builds.
assembleRelease - Assembles all Release builds.
build - Assembles and tests this project.
buildDependents - Assembles and tests this project and all projects that depend on it.
buildNeeded - Assembles and tests this project and all projects it depends on.
bundleDebug - Creates all Debug bundles.
bundleRelease - Creates all Release bundles.
clean - Deletes the build directory.
cleanBuildCache - Deletes the build cache directory.
compileDebugAndroidTestSources
compileDebugSources
compileDebugUnitTestSources
compileReleaseSources
compileReleaseUnitTestSources

......

参考：https://developer.android.com/studio/build/building-cmdline

执行 emulator -list-avds 可以列出系统中已经存在的 Android Virtual Devices (AVD)，如：

$ ~/Library/Android/sdk/emulator/emulator -list-avds                # 列出 avd，目前只有一个 avd
Nexus_5X_API_28_x86

执行 emulator -avd name 可以启动相应的 AVD，如：

$ ~/Library/Android/sdk/emulator/emulator -avd Nexus_5X_API_28_x86  # 启动 avd Nexus_5X_API_28_x86
$ ~/Library/Android/sdk/emulator/emulator @Nexus_5X_API_28_x86      # 同上

把 apk 文件拖拽到 AVD 屏幕窗口里，就可以安装 apk 了。

参考：https://developer.android.com/studio/run/emulator-commandline