先看看PackageMonitor的基本定义：
package com.android.internal.content;
/**
* Helper class for monitoring the state of packages: adding, removing,
* updating, and disappearing and reappearing on the SD card.
*/
public abstract class PackageMonitor extends android.content.BroadcastReceiver
static {
sPackageFilt.addAction(Intent.ACTION_PACKAGE_ADDED);
sPackageFilt.addAction(Intent.ACTION_PACKAGE_REMOVED);
sPackageFilt.addAction(Intent.ACTION_PACKAGE_CHANGED);
sPackageFilt.addAction(Intent.ACTION_QUERY_PACKAGE_RESTART);
sPackageFilt.addAction(Intent.ACTION_PACKAGE_RESTARTED);
sPackageFilt.addAction(Intent.ACTION_PACKAGE_DATA_CLEARED);
sPackageFilt.addDataScheme("package");
sNonDataFilt.addAction(Intent.ACTION_UID_REMOVED);
sNonDataFilt.addAction(Intent.ACTION_USER_STOPPED);
sNonDataFilt.addAction(Intent.ACTION_PACKAGES_SUSPENDED);
sNonDataFilt.addAction(Intent.ACTION_PACKAGES_UNSUSPENDED);
sExternalFilt.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_AVAILABLE);
sExternalFilt.addAction(Intent.ACTION_EXTERNAL_APPLICATIONS_UNAVAILABLE);
}
可以看到PackageMonitor是一个内部API，并且它实际上是一个BroadcastReceiver，在框架内部注册了接收上述Action的Intent广播。虽然目前没有对第三方App开放，但其设计思想可以借鉴，毕竟注册各种Package相关的Intent比较琐碎，在一个工程中封装一个类似的Monitor作为基本框架模块供复用很有必要。PackageMonitor的主要工作是将注册/解析Intent广播封装，对外暴露成需要实现的有语义的回调。看一下他的注册和有代表性的回调。
（1）注册
PackageMonitor有三个注册方法：
public void register(Context context, Looper thread, boolean externalStorage) public void register(Context context, Looper thread, UserHandle user, boolean externalStorage) public void register(Context context, UserHandle user, boolean externalStorage, Handler handler)
可以看到提供了注册回调执行的线程、注册侦听的用户（支持安卓多用户体系）、选择是否侦听外置存储空间（譬如SD卡）中的package变化等配置项。 （2）有代表性的回调
/**
* Called when a package is really added (and not replaced).
*/
public void onPackageAdded(String packageName, int uid) {
}
/**
* Called when a package is really removed (and not replaced).
*/
public void onPackageRemoved(String packageName, int uid) {
}
/**
* Direct reflection of {@link Intent#ACTION_PACKAGE_CHANGED
* Intent.ACTION_PACKAGE_CHANGED} being received, informing you of
* changes to the enabled/disabled state of components in a package
* and/or of the overall package.
*
* @param packageName The name of the package that is changing.
* @param uid The user ID the package runs under.
* @param components Any components in the package that are changing. If
* the overall package is changing, this will contain an entry of the
* package name itself.
* @return Return true to indicate you care about this change, which will
* result in {@link #onSomePackagesChanged()} being called later. If you
* return false, no further callbacks will happen about this change. The
* default implementation returns true if this is a change to the entire
* package.
*/
public boolean onPackageChanged(String packageName, int uid, String[] components) {
if (components != null) {
for (String name : components) {
if (packageName.equals(name)) {
return true;
}
}
}
return false;
}
public boolean onHandleForceStop(Intent intent, String[] packages, int uid, boolean doit) {
return false;
}
public void onHandleUserStop(Intent intent, int userHandle) {
}
public void onUidRemoved(int uid) {
}
public void onPackagesAvailable(String[] packages) {
}
public void onPackagesUnavailable(String[] packages) {
}
public void onPackagesSuspended(String[] packages) {
}
public void onPackagesUnsuspended(String[] packages) {
}
可以根据自己的需求进行重写方法。针对其中几个研究一下： -onUidRemoved(int uid): 自从android引入对多用户的支持，uid和user就成了容易混淆的概念。参见我的另2篇： Android下uid与多用户释疑（一） Android下uid与多用户释疑（二） 这里的uid是指应用程序。uid removed即指应用程序被删除（卸载）。如果某些业务逻辑中对于应用程序是通过uid标识管理的，可以使用这个回调。 对应的Intent Action是Intent.ACTION_UID_REMOVED。 -onPackagesSuspended()/onPackagesUnsuspended(): 对应Intent.ACTION_PACKAGES_SUSPENDED/ACTION_PACKAGES_UNSUSPENDED。查看PMS、PackageManager的源代码看到是在调用这个api时触发的Intent广播：
/**
* Puts the package in a suspended state, where attempts at starting activities are denied.
*
* <p>It doesn't remove the data or the actual package file. The application notifications
* will be hidden, the application will not show up in recents, will not be able to show
* toasts or dialogs or ring the device.
*
* <p>The package must already be installed. If the package is uninstalled while suspended
* the package will no longer be suspended.
*
* @param packageNames The names of the packages to set the suspended status.
* @param suspended If set to {@code true} than the packages will be suspended, if set to
* {@code false} the packages will be unsuspended.
* @param userId The user id.
*
* @return an array of package names for which the suspended status attemptis not set as requested in
* this method.
*
* @hide
*/
public abstract String[] setPackagesSuspendedAsUser(
String[] packageNames, boolean suspended, @UserIdInt int userId);
关于应用挂起，后续会有文章跟进。
以一个系统服务SearchManagerService为例看下对于PackageMonitor的使用：
/**
* Refreshes the "searchables" list when packages are added/removed.
*/
class MyPackageMonitor extends PackageMonitor {
@Override
public void onSomePackagesChanged() {
updateSearchables();
}
@Override
public void onPackageModified(String pkg) {
updateSearchables();
}
private void updateSearchables() {
final int changingUserId = getChangingUserId();
synchronized (mSearchables) {
// Update list of searchable activities
for (int i = 0; i < mSearchables.size(); i++) {
if (changingUserId == mSearchables.keyAt(i)) {
mSearchables.valueAt(i).updateSearchableList();
break;
}
}
}
// Inform all listeners that the list of searchables has been updated.
Intent intent = new Intent(SearchManager.INTENT_ACTION_SEARCHABLES_CHANGED);
intent.addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING
| Intent.FLAG_RECEIVER_REGISTERED_ONLY_BEFORE_BOOT);
mContext.sendBroadcastAsUser(intent, new UserHandle(changingUserId));
}
}
上述参看代码版本为android7.1。