Android 开发之 App Components

Android的应用程序框架允许您创建丰富和创新应用使用一组可重用的组件。本篇解释如何构建的组件定义应用程序的构建块,并使用意图如何连接在一起。

• Intents and Intent Filters
• Activities
• Services
• Content Providers
• App Widgets
• Processes and Threads

Intent Type可分为显式意图（Explicit intents）和隐式意图（Implicit intents），下图所示。描述一个隐含的意图是通过系统启动另一个活动:[1]活动创建一个意图的行动描述并将其传递给startActivity()。[2]的Android系统搜索所有应用程序意图过滤器匹配的意图。当找到匹配,[3]系统开始匹配的活动(活动B)通过调用itsonCreate()方法并传递它的意图。警告:确保你的应用程序是安全的,开始时总是使用一个显式意图服务,不为你的服务声明意图过滤器。使用隐式意图启动一个服务是一个安全隐患,因为你无法确定服务响应的目的,和用户不能看到哪些服务开始。

在某些时候比如调用系统相册传入action，就可以调出系统相册，当然也可以自定义Action，例如

static final String ACTION_TIMETRAVEL = "com.example.action.TIMETRAVEL";

activity的启动通过startActivityForResult时，需要声明Action:CATEGORY_DEFAULT,用于意图过滤。

<activity android:name="TestActivity">    <intent-filter>        <action android:name="android.intent.action.SEND"/>        <category android:name="android.intent.category.DEFAULT"/>        <data android:mimeType="text/plain"/>    </intent-filter></activity>

一个应用有多少个Context对象？这个问题其实很简单。Application、Activity和Service从继承关系来看，最终父类是Context，我们可以这样认为：每个Activity都是一个Context，每一个Service都是一个Context，而Application也是一个Context。每个应用自启动都会创建一个Application，如果应用内有自定义Application那么在清单文件manifest里面需要声明，Activity和Service同样需要。

在开发中调用系统服务，通过Action过滤，例如调用系统的闹钟，只需要传入ACTION_SET_ALARM即可，实例代码如下：

public void createAlarm(String message, int hour, int minutes) {    Intent intent = new Intent(AlarmClock.ACTION_SET_ALARM)            .putExtra(AlarmClock.EXTRA_MESSAGE, message)            .putExtra(AlarmClock.EXTRA_HOUR, hour)            .putExtra(AlarmClock.EXTRA_MINUTES, minutes);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivity(intent);    }}

调用系统服务需要更具功能请求相应的权限，以设置闹钟为例，权限如下：

<uses-permission android:name="com.android.alarm.permission.SET_ALARM" />

下面是一些列的系统功能调用代码块，先来看倒计时，在（Android 4.4）

public void startTimer(String message, int seconds) {    Intent intent = new Intent(AlarmClock.ACTION_SET_TIMER)            .putExtra(AlarmClock.EXTRA_MESSAGE, message)            .putExtra(AlarmClock.EXTRA_LENGTH, seconds)            .putExtra(AlarmClock.EXTRA_SKIP_UI, true);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivity(intent);    }}

在日历上添加事件，实例和意图过滤如下：

public void addEvent(String title, String location, Calendar begin, Calendar end) {    Intent intent = new Intent(Intent.ACTION_INSERT)            .setData(Events.CONTENT_URI)            .putExtra(Events.TITLE, title)            .putExtra(Events.EVENT_LOCATION, location)            .putExtra(CalendarContract.EXTRA_EVENT_BEGIN_TIME, begin)            .putExtra(CalendarContract.EXTRA_EVENT_END_TIME, end);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivity(intent);    }}

<activity ...>    <intent-filter>        <action android:name="android.intent.action.INSERT" />        <data android:mimeType="vnd.android.cursor.dir/event" />        <category android:name="android.intent.category.DEFAULT" />    </intent-filter></activity>

调用系统Camera

static final int REQUEST_IMAGE_CAPTURE = 1;static final Uri mLocationForPhotos;public void capturePhoto(String targetFilename) {    Intent intent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);    intent.putExtra(MediaStore.EXTRA_OUTPUT,            Uri.withAppendedPath(mLocationForPhotos, targetFilename);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivityForResult(intent, REQUEST_IMAGE_CAPTURE);    }}@Overrideprotected void onActivityResult(int requestCode, int resultCode, Intent data) {    if (requestCode == REQUEST_IMAGE_CAPTURE && resultCode == RESULT_OK) {        Bitmap thumbnail = data.getParcelable("data");        // Do other work with full size photo saved in mLocationForPhotos        ...    }}

读取联系人（READ_CONTACTS）

static final int REQUEST_SELECT_CONTACT = 1;public void selectContact() {    Intent intent = new Intent(Intent.ACTION_PICK);    intent.setType(ContactsContract.Contacts.CONTENT_TYPE);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivityForResult(intent, REQUEST_SELECT_CONTACT);    }}@Overrideprotected void onActivityResult(int requestCode, int resultCode, Intent data) {    if (requestCode == REQUEST_SELECT_CONTACT && resultCode == RESULT_OK) {        Uri contactUri = data.getData();        // Do something with the selected contact at contactUri        ...    }}

查询联系人信息

static final int REQUEST_SELECT_PHONE_NUMBER = 1;public void selectContact() {    // Start an activity for the user to pick a phone number from contacts    Intent intent = new Intent(Intent.ACTION_PICK);    intent.setType(CommonDataKinds.Phone.CONTENT_TYPE);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivityForResult(intent, REQUEST_SELECT_PHONE_NUMBER);    }}@Overrideprotected void onActivityResult(int requestCode, int resultCode, Intent data) {    if (requestCode == REQUEST_SELECT_PHONE_NUMBER && resultCode == RESULT_OK) {        // Get the URI and query the content provider for the phone number        Uri contactUri = data.getData();        String[] projection = new String[]{CommonDataKinds.Phone.NUMBER};        Cursor cursor = getContentResolver().query(contactUri, projection,                null, null, null);        // If the cursor returned is valid, get the phone number        if (cursor != null && cursor.moveToFirst()) {            int numberIndex = cursor.getColumnIndex(CommonDataKinds.Phone.NUMBER);            String number = cursor.getString(numberIndex);            // Do something with the phone number            ...        }    }}

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诸如此类方法块太多就不一一列举。Android系统设置的功能模块的打开方法，除了通过manager实现，也可以通过intent传递意图打开，代码如下：

public void openWifiSettings() {    Intent intent = new Intent(Intent.ACTION_WIFI_SETTINGS);    if (intent.resolveActivity(getPackageManager()) != null) {        startActivity(intent);    }}ACTION:ACTION_SETTINGS       ACTION_WIRELESS_SETTINGS       ACTION_AIRPLANE_MODE_SETTINGS       ACTION_WIFI_SETTINGS       ACTION_APN_SETTINGS       ACTION_BLUETOOTH_SETTINGS       ACTION_DATE_SETTINGS       ACTION_LOCALE_SETTINGS       ACTION_INPUT_METHOD_SETTINGS       ACTION_DISPLAY_SETTINGS       ACTION_SECURITY_SETTINGS       ACTION_LOCATION_SOURCE_SETTINGS       ACTION_INTERNAL_STORAGE_SETTINGS       ACTION_MEMORY_CARD_SETTINGS