详解Android的四大应用程序组件

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什么是四大应用程序组件

Android的四大应用程序组件包括：

Activity（活动）
Service（服务）
ContentProvider（内容提供者）
BroadcastReceiver（广播接收器）

这些组件结合起来，可以实现一个完整的Android应用。

Activity（活动）

Activity（活动）是Android系统中应用界面的核心组件，负责展示用户界面和接受用户输入。

可以理解为一个页面，在一个应用中可能会有多个Activity，通过activity之间的跳转实现应用的入口和不同功能的实现。

每个Activity有自己的生命周期，这个生命周期包括：OnCreate、OnStart、OnResume、OnPause、OnStop和OnDestroy方法。其中OnCreate、OnStart、OnResume和OnPause方法是必须实现的。

示例：以下是一个简单的计算器应用的MainActivity代码示例：

public class MainActivity extends AppCompatActivity{
    EditText etValue1, etValue2;
    Button btnAdd, btnSub, btn_mul, btn_div;
    TextView tvResult;
    int value1, value2;
    int result = 0;
    String operator = "";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        etValue1 = (EditText) findViewById(R.id.et_value1);
        etValue2 = (EditText) findViewById(R.id.et_value2);

        btnAdd = (Button) findViewById(R.id.btn_add);
        btnSub = (Button) findViewById(R.id.btn_sub);
        btn_mul = (Button) findViewById(R.id.btn_mul);
        btn_div = (Button) findViewById(R.id.btn_div);

        tvResult = (TextView) findViewById(R.id.tv_result);

        btnAdd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                operator = "+";
                calculate();
            }
        });
        btnSub.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                operator = "-";
                calculate();
            }
        });
        btn_mul.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                operator = "*";
                calculate();
            }
        });
        btn_div.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                operator = "/";
                calculate();
            }
        });
    }

    private void calculate() {
        value1 = Integer.parseInt(etValue1.getText().toString());
        value2 = Integer.parseInt(etValue2.getText().toString());

        switch (operator) {
            case "+":
                result = value1 + value2;
                break;
            case "-":
                result = value1 - value2;
                break;
            case "*":
                result = value1 * value2;
                break;
            case "/":
                result = value1 / value2;
                break;
        }

        tvResult.setText(String.valueOf(result));
    }
}

Service（服务）

Service（服务）是在后台执行长期运行操作的组件，即使在用户退出应用或者锁屏的情况下，服务依然在后台运行。

示例：以下是一个简单的Android服务的示例代码：

public class MyService extends Service {
    private Timer timer;

    @Override
    public void onCreate() {
        super.onCreate();
        timer = new Timer();
        timer.scheduleAtFixedRate(new TimerTask() {
            @Override
            public void run() {
                Log.d("MyService", "Service is running...");
            }
        }, 0, 1000);
    }

    @Override
    public IBinder onBind(Intent arg0) {
        return null;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        timer.cancel();
        Log.d("MyService", "Service is destroyed...");
    }
}

ContentProvider（内容提供者）

ContentProvider（内容提供者）是Android提供的一种组件，用于在不同应用之间共享数据，类似于使用数据库操作数据。

示例：以下是一个简单的Android内容提供者的示例代码：

public class MyProvider extends ContentProvider {
    private static final UriMatcher uriMatcher;
    private static final int BOOK_DIR = 0;
    private static final int BOOK_ITEM = 1;
    private static final int CATEGORY_DIR = 2;
    private static final int CATEGORY_ITEM = 3;
    private static final String AUTHORITY = "com.example.myprovider";
    private static MyDatabaseHelper dbHelper;
    private static UriMatcher getUriMatcher() {
        UriMatcher uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        uriMatcher.addURI(AUTHORITY, "book", BOOK_DIR);
        uriMatcher.addURI(AUTHORITY, "book/#", BOOK_ITEM);
        uriMatcher.addURI(AUTHORITY, "category", CATEGORY_DIR);
        uriMatcher.addURI(AUTHORITY, "category/#", CATEGORY_ITEM);
        return uriMatcher;
    }

    static {
        uriMatcher = getUriMatcher();
    }

    @Override
    public boolean onCreate() {
        dbHelper = new MyDatabaseHelper(getContext(), "mydb", null, 1);
        return true;
    }

    @Override
    public Cursor query(@NonNull Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        SQLiteDatabase db = dbHelper.getReadableDatabase();
        Cursor cursor = null;
        switch (uriMatcher.match(uri)) {
            case BOOK_DIR:
                cursor = db.query("book", projection, selection, selectionArgs, null, null, sortOrder);
                break;
            case BOOK_ITEM:
                String bookId = uri.getPathSegments().get(1);
                cursor = db.query("book", projection, "id = ?", new String[]{bookId}, null, null, sortOrder);
                break;
            case CATEGORY_DIR:
                cursor = db.query("category", projection, selection, selectionArgs, null, null, sortOrder);
                break;
            case CATEGORY_ITEM:
                String categoryId = uri.getPathSegments().get(1);
                cursor = db.query("category", projection, "id = ?", new String[]{categoryId}, null, null, sortOrder);
                break;
            default:
                break;
        }
        return cursor;
    }

    @Override
    public String getType(@NonNull Uri uri) {
        switch (uriMatcher.match(uri)) {
            case BOOK_DIR:
                return "vnd.android.cursor.dir/vnd.com.example.myprovider.book";
            case BOOK_ITEM:
                return "vnd.android.cursor.item/vnd.com.example.myprovider.book";
            case CATEGORY_DIR:
                return "vnd.android.cursor.dir/vnd.com.example.myprovider.category";
            case CATEGORY_ITEM:
                return "vnd.android.cursor.item/vnd.com.example.myprovider.category";
        }
        return null;
    }

    @Override
    public Uri insert(@NonNull Uri uri, ContentValues values) {
        SQLiteDatabase db = dbHelper.getWritableDatabase();
        Uri uriReturn = null;
        switch (uriMatcher.match(uri)) {
            case BOOK_DIR:
            case BOOK_ITEM:
                long newBookId = db.insert("book", null, values);
                uriReturn = Uri.parse("content://" + AUTHORITY + "/book" + newBookId);
                break;
            case CATEGORY_DIR:
            case CATEGORY_ITEM:
                long newCategoryId = db.insert("category", null, values);
                uriReturn = Uri.parse("content://" + AUTHORITY + "/category" + newCategoryId);
                break;
            default:
                break;
        }
        return uriReturn;
    }

    @Override
    public int delete(@NonNull Uri uri, String selection, String[] selectionArgs) {
        SQLiteDatabase db = dbHelper.getWritableDatabase();
        int deletedRows = 0;
        switch (uriMatcher.match(uri)) {
            case BOOK_DIR:
                deletedRows = db.delete("book", selection, selectionArgs);
                break;
            case BOOK_ITEM:
                String bookId = uri.getPathSegments().get(1);
                deletedRows = db.delete("book", "id = ?", new String[]{bookId});
                break;
            case CATEGORY_DIR:
                deletedRows = db.delete("category", selection, selectionArgs);
                break;
            case CATEGORY_ITEM:
                String categoryId = uri.getPathSegments().get(1);
                deletedRows = db.delete("category", "id = ?", new String[]{categoryId});
                break;
            default:
                break;
        }
        return deletedRows;
    }

    @Override
    public int update(@NonNull Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        SQLiteDatabase db = dbHelper.getWritableDatabase();
        int updatedRows = 0;
        switch (uriMatcher.match(uri)) {
            case BOOK_DIR:
                updatedRows = db.update("book", values, selection, selectionArgs);
                break;
            case BOOK_ITEM:
                String bookId = uri.getPathSegments().get(1);
                updatedRows = db.update("book", values, "id = ?", new String[]{bookId});
                break;
            case CATEGORY_DIR:
                updatedRows = db.update("category", values, selection, selectionArgs);
                break;
            case CATEGORY_ITEM:
                String categoryId = uri.getPathSegments().get(1);
                updatedRows = db.update("category", values, "id = ?", new String[]{categoryId});
                break;
            default:
                break;
        }
        return updatedRows;
    }
}

BroadcastReceiver（广播接收器）

BroadcastReceiver（广播接收器）是Android中用于接收广播消息的组件，可以在指定广播消息到达时执行相应的操作。

示例：以下是一个简单的Android广播接收器的示例代码：

public class MyBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (intent.getAction().equals(Intent.ACTION_BOOT_COMPLETED)) {
            Intent i = new Intent(context, MainActivity.class);
            i.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
            context.startActivity(i);
        }
    }
}

以上就是四大应用程序组件的详细攻略。

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