「Android」四大组件，你真的都掌握了？

lateinit var textView: TextView [注1]
var gameState: String? = null

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    gameState = savedInstanceState?.getString(GAME_STATE_KEY)
    setContentView(R.layout.activity_main)
    textView = findViewById(R.id.text_view)
}

override fun onRestoreInstanceState(savedInstanceState: Bundle?) {
    textView.text = savedInstanceState?.getString(TEXT_VIEW_KEY)
}

override fun onSaveInstanceState(outState: Bundle?) {
    outState?.run {
        putString(GAME_STATE_KEY, gameState)
        putString(TEXT_VIEW_KEY, textView.text.toString())
    }
    super.onSaveInstanceState(outState)
}

private var name0: String //报错
private var name1: String = "xiaoming" //不报错
private var name2: String? = null //不报错

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    ActivityInfo aInfo = r.activityInfo;
    if (r.packageInfo == null) {
        //step 1: 创建LoadedApk对象
        r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                Context.CONTEXT_INCLUDE_CODE);
    }
    ... //component初始化过程

    java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
    //step 2: 创建Activity对象
    Activity activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);
    ...

    //step 3: 创建Application对象
    Application app = r.packageInfo.makeApplication(false, mInstrumentation);

    if (activity != null) {
        //step 4: 创建ContextImpl对象
        Context appContext = createBaseContextForActivity(r, activity);
        CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
        Configuration config = new Configuration(mCompatConfiguration);
        //step5: 将Application/ContextImpl都attach到Activity对象
        activity.attach(appContext, this, getInstrumentation(), r.token,
                r.ident, app, r.intent, r.activityInfo, title, r.parent,
                r.embeddedID, r.lastNonConfigurationInstances, config,
                r.referrer, r.voiceInteractor);

        ...
        int theme = r.activityInfo.getThemeResource();
        if (theme != 0) {
            activity.setTheme(theme);
        }

        activity.mCalled = false;
        if (r.isPersistable()) {
            //step 6: 执行回调onCreate
            mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
        } else {
            mInstrumentation.callActivityOnCreate(activity, r.state);
        }

        r.activity = activity;
        r.stopped = true;
        if (!r.activity.mFinished) {
            activity.performStart(); //执行回调onStart
            r.stopped = false;
        }
        if (!r.activity.mFinished) {
            //执行回调onRestoreInstanceState
            if (r.isPersistable()) {
                if (r.state != null || r.persistentState != null) {
                    mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                            r.persistentState);
                }
            } else if (r.state != null) {
                mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
            }
        }
        ...
        r.paused = true;
        mActivities.put(r.token, r);
    }

    return activity;
}

@UnsupportedAppUsage
private void handleCreateService(CreateServiceData data) {
    ···
    LoadedApk packageInfo = getPackageInfoNoCheck(
            data.info.applicationInfo, data.compatInfo);
    Service service = null;
    try {
        java.lang.ClassLoader cl = packageInfo.getClassLoader();
        service = packageInfo.getAppFactory()
                .instantiateService(cl, data.info.name, data.intent);
    } 
    ···

    try {
        if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);

        ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
        context.setOuterContext(service);

        Application app = packageInfo.makeApplication(false, mInstrumentation);
        service.attach(context, this, data.info.name, data.token, app,
                ActivityManager.getService());
        service.onCreate();
        mServices.put(data.token, service);
        try {
            ActivityManager.getService().serviceDoneExecuting(
                    data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    } 
    ··· 
}

private void handleBindService(BindServiceData data) {
    Service s = mServices.get(data.token);
    ···
    if (s != null) {
        try {
            data.intent.setExtrasClassLoader(s.getClassLoader());
            data.intent.prepareToEnterProcess();
            try {
                if (!data.rebind) {
                    IBinder binder = s.onBind(data.intent);
                    ActivityManager.getService().publishService(
                            data.token, data.intent, binder);
                } else {
                    s.onRebind(data.intent);
                    ActivityManager.getService().serviceDoneExecuting(
                            data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
                }
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        } 
        ···
    }
}

<uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>

Notification notification = new Notification(icon, text, System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ExampleActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, title, mmessage, pendingIntent);
startForeground(ONGOING_NOTIFICATION_ID, notification);

LocalBroadcastManager.getInstance(MainActivity.this).registerReceiver(receiver, filter);

public class MyBroadCastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        Log.d("MyBroadCastReceiver", "收到信息,内容是 :　" + intent.getStringExtra("info") + "");
    }
}

<receiver android:name=".MyBroadCastReceiver">
    <intent-filter>
        <action android:name="myBroadcast.action.call"/>
    </intent-filter>
</receiver>

//onCreate创建广播接收者对象
mReceiver = new MyBroadCastReceiver();              

//注册按钮
public void click(View view) {
    IntentFilter intentFilter = new IntentFilter();
    intentFilter.addAction("myBroadcast.action.call");
    registerReceiver(mReceiver, intentFilter);
}

@Override
protected void onDestroy() {
    unregisterReceiver(mReceiver);
    super.onDestroy();
}

// Queries the user dictionary and returns results
mCursor = getContentResolver().query(
    UserDictionary.Words.CONTENT_URI,   // The content URI of the words table
    mProjection,                        // The columns to return for each row
    mSelectionClause                    // Selection criteria
    mSelectionArgs,                     // Selection criteria
    mSortOrder);                        // The sort order for the returned rows

public class Installer extends ContentProvider {

    @Override
    public boolean onCreate() {
        return true;
    }

    @Nullable
    @Override
    public Cursor query(@NonNull Uri uri, @Nullable String[] projection, @Nullable String selection, @Nullable String[] selectionArgs, @Nullable String sortOrder) {
        return null;
    }

    @Nullable
    @Override
    public String getType(@NonNull Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(@NonNull Uri uri, @Nullable ContentValues values) {
        return null;
    }

    @Override
    public int delete(@NonNull Uri uri, @Nullable String selection, @Nullable String[] selectionArgs) {
        return 0;
    }

    @Override
    public int update(@NonNull Uri uri, @Nullable ContentValues values, @Nullable String selection, @Nullable String[] selectionArgs) {
        return 0;
    }
}