Android学习笔记

1.获得全局Context
全局Application

public class MyApplication extends Application {    private static Context mContext;    private static MyApplication mApp;    @Override    public void onCreate() {        super.onCreate();        mContext = getApplicationContext();        mApp = this;    }    /***     * 获取Context对象     * @return     */    public static Context getContext(){        return mContext;    }    /****     * 获取Application对象     * @return     */    public static MyApplication getApp(){        return mApp;    }}

Activity基类

public class BaseActivity extends AppCompatActivity {    public Context mContext;    public BaseActivity(){        mContext = MyApplication.getContext();    }}

使用

public class MainActivity extends BaseActivity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);    }    public void click(View view){        switch (view.getId()){            case R.id.btn:                Toast.makeText(mContext, "点击了按钮", Toast.LENGTH_SHORT).show();                break;            case R.id.rl_parent:                Toast.makeText(mContext, "点击了相对布局", Toast.LENGTH_SHORT).show();                break;        }    }}

2.系统启动流程

加载Linux内核—启动init进程—fork出zygote进程—fork出SystemServer进程
在zygote开启的时候，会调用ZygoteInit.main()进行初始化

  public static void main(String argv[]) {      ......      //为Zygote注册一个服务端Socket,用来与系统服务进行通信       registerZygoteSocket(socketName);    if (argv[1].equals("start-system-server")) {                startSystemServer();            } else if (!argv[1].equals("")) {                throw new RuntimeException(argv[0] + USAGE_STRING);            }    }    ......

 private static boolean startSystemServer()            throws MethodAndArgsCaller, RuntimeException {      ......        int pid;        try {        ......            /* fork SystemServer进程*/            pid = Zygote.forkSystemServer(                    parsedArgs.uid, parsedArgs.gid,                    parsedArgs.gids,                    parsedArgs.debugFlags,                    null,                    parsedArgs.permittedCapabilities,                    parsedArgs.effectiveCapabilities);        } catch (IllegalArgumentException ex) {            throw new RuntimeException(ex);        }        ......

public final class SystemServer {    /**     *zygote的入口     */    public static void main(String[] args) {        new SystemServer().run();    }    // 加载本地服务        System.loadLibrary("android_servers");         // 初始化系统上下文对象        createSystemContext();}//创建SystemServiceManager，后面的其他服务都要通过他来启动 mSystemServiceManager = new SystemServiceManager(mSystemContext);        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

  private void createSystemContext() {  //创建ActivityThread对象        ActivityThread activityThread = ActivityThread.systemMain();        mSystemContext = activityThread.getSystemContext();        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);    }

 public static ActivityThread systemMain() {        if (!ActivityManager.isHighEndGfx()) {            HardwareRenderer.disable(true);        } else {            HardwareRenderer.enableForegroundTrimming();        }        ActivityThread thread = new ActivityThread();        thread.attach(true);        return thread;    }

我们看看attach方法里面

 private void attach(boolean system) { //创建Instrumentation    mInstrumentation = new Instrumentation();     //创建ContextImpl                 ContextImpl context = ContextImpl.createAppContext(                        this, getSystemContext().mPackageInfo);                         //创建Application                mInitialApplication = context.mPackageInfo.makeApplication(true, null);                //调用Application的onCreate方法                mInitialApplication.onCreate();    }

接下来我们继续看SystemService里面，会通过SystemServerManager启动其他服务。

//开启核心服务 private void startBootstrapServices() {        Installer installer = mSystemServiceManager.startService(Installer.class);        // 创建AMS        mActivityManagerService = mSystemServiceManager.startService(                ActivityManagerService.Lifecycle.class).getService();        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);        mActivityManagerService.setInstaller(installer);        // 电源管理服务需要早点启动，因为其他服务需要依赖它        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);        //让Activity初始化电源管理        mActivityManagerService.initPowerManagement();        // 启动灯光服务  mSystemServiceManager.startService(LightsService.class);        //启动显示服务DMS        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);        // 初始化PMS之前我们需要默认的显示        mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);....        //启动PMS        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);        mFirstBoot = mPackageManagerService.isFirstBoot();        mPackageManager = mSystemContext.getPackageManager();        //启动传感器服务        startSensorService();    }

3.App与AMS通过Binder进行IPC通信，AMS(SystemServer进程)与zygote通过Socket进行IPC通信。在Android系统中，任何一个Activity的启动都是由AMS和应用程序进程（主要是ActivityThread）相互配合来完成的。AMS服务统一调度系统中所有进程的Activity启动，而每个Activity的启动过程则由其所属的进程具体来完成。

4.fitsSystemWindows属性的使用
这个一个boolean值的内部属性，让view可以根据系统窗口(如status bar)来调整自己的布局，如果值为true,就会调整view的paingding属性来给system windows留出空间….

实际效果：

 当status bar为透明或半透明时(4.4以上),系统会设置view的paddingTop值为一个适合的值(status bar的高度)让view的内容不被上拉到状态栏，当在不占据status bar的情况下(4.4以下)会设置paddingTop值为0(因为没有占据status bar所以不用留出空间)。

在BaseActivity里面判断

 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {            Window window = getWindow();            // Translucent status bar            window.setFlags(                    WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS,                    WindowManager.LayoutParams.FLAG_TRANSLUCENT_STATUS);        }