引言
最近，帮同事解决了两个问题，一个问题是 app 接收开机广播的速度太慢，另一个问题是app有时无法接收到广播。同事不知道如何解决这个问题，是因为他们不了解广播发送超时的原理。
很早的时候，我就研究过广播的代码，由于工作比较忙，再加上我这个人比较懒，因此没有写成文章。由于最近这段时间，工作和生活都不是很如意，于是我想静下心来写写东西，因此就有了这篇文章。
在看本文之前，请读者自行了解 普通的广播、粘性(sticky)广播、有序广播 的使用。
注册广播接收器
广播接收器可以分为动态和静态，静态广播接收器就是在 AndroidManifest.xml 中注册的，而动态的广播接收器是在代码中通过 Context#registerReceiver() 注册的。
静态广播接收器，在发送广播时，服务端会从 PKMS 中收集，而动态的广播接收器，需要接收方发送给服务端。因此，下面只分析动态广播接收器的注册过程
// ContextImpl.java
public Intent registerReceiverAsUser(BroadcastReceiver receiver, UserHandle user,
IntentFilter filter, String broadcastPermission, Handler scheduler) {
return registerReceiverInternal(receiver, user.getIdentifier(),
filter, broadcastPermission, scheduler, getOuterContext(), 0);
}
private Intent registerReceiverInternal(BroadcastReceiver receiver, int userId,
IntentFilter filter, String broadcastPermission,
Handler scheduler, Context context, int flags) {
IIntentReceiver rd = null;
if (receiver != null) {
if (mPackageInfo != null && context != null) {
// 默认主线程 Handler
if (scheduler == null) {
scheduler = mMainThread.getHandler();
}
// 1. 获取 IIntentReceiver 对象
// 其实这里获取的就是一个 Binder 对象，用于注册给 AMS，从而接收广播信息的回调
rd = mPackageInfo.getReceiverDispatcher(
receiver, context, scheduler,
mMainThread.getInstrumentation(), true);
} else {
// ...
}
}
try {
// 2. 向 AMS 注册 IIntentReceiver
final Intent intent = ActivityManager.getService().registerReceiverWithFeature(
mMainThread.getApplicationThread(), mBasePackageName, getAttributionTag(),
AppOpsManager.toReceiverId(receiver), rd, filter, broadcastPermission, userId,
flags);
if (intent != null) {
intent.setExtrasClassLoader(getClassLoader());
intent.prepareToEnterProcess(ActivityThread.isProtectedBroadcast(intent),
getAttributionSource());
}
return intent;
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
广播接收方注册接收器的过程如下
- 获取 IIntentReceiver 对象，它是一个 Binder 对象，其实就是一个 Binder 回调。
- 向服务端 AMS 注册 IIntentReceiver 对象，用于接收广播消息的回调。
当接收方收到来自服务端的广播消息后，会通过 IIntentReceiver 对象，调用 BroadcastReceiver#onReceive() 来处理广播。
现在来看下服务端是如何完成广播接收器的注册工作的
// AcitityManagerService.java
public Intent registerReceiverWithFeature(IApplicationThread caller, String callerPackage,
String callerFeatureId, String receiverId, IIntentReceiver receiver,
IntentFilter filter, String permission, int userId, int flags) {
enforceNotIsolatedCaller("registerReceiver");
ArrayList<Intent> stickyIntents = null;
ProcessRecord callerApp = null;
final boolean visibleToInstantApps
= (flags & Context.RECEIVER_VISIBLE_TO_INSTANT_APPS) != 0;
int callingUid;
int callingPid;
boolean instantApp;
synchronized(this) {
// 确保接收方进程存在，并该进程的 uid 和 pid
if (caller != null) {
callerApp = getRecordForAppLOSP(caller);
if (callerApp == null) {
throw new SecurityException(
"Unable to find app for caller " + caller
+ " (pid=" + Binder.getCallingPid()
+ ") when registering receiver " + receiver);
}
if (callerApp.info.uid != SYSTEM_UID
&& !callerApp.getPkgList().containsKey(callerPackage)
&& !"android".equals(callerPackage)) {
throw new SecurityException("Given caller package " + callerPackage
+ " is not running in process " + callerApp);
}
callingUid = callerApp.info.uid;
callingPid = callerApp.getPid();
} else {
callerPackage = null;
callingUid = Binder.getCallingUid();
callingPid = Binder.getCallingPid();
}
instantApp = isInstantApp(callerApp, callerPackage, callingUid);
userId = mUserController.handleIncomingUser(callingPid, callingUid, userId, true,
ALLOW_FULL_ONLY, "registerReceiver", callerPackage);
Iterator<String> actions = filter.actionsIterator();
if (actions == null) {
ArrayList<String> noAction = new ArrayList<String>(1);
noAction.add(null);
actions = noAction.iterator();
}
// Collect stickies of users
int[] userIds = { UserHandle.USER_ALL, UserHandle.getUserId(callingUid) };
// 遍历 IntentFilter 保存的所有 action，匹配相应的 sticky 广播，并保存到 stickyIntents
// 从这里可以看出，可以先发送 sticky 广播，然后再注册 sticky 广播接收器
while (actions.hasNext()) {
String action = actions.next();
for (int id : userIds) {
ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(id);
if (stickies != null) {
ArrayList<Intent> intents = stickies.get(action);
if (intents != null) {
if (stickyIntents == null) {
stickyIntents = new ArrayList<Intent>();
}
stickyIntents.addAll(intents);
}
}
}
}
}
// 刚才是用 action 匹配 sticky 广播，现在使用 IntentFilter 再次过滤
// 过滤后的 sticky 广播 ，保存到 allSticky
// 因此 allSticky 保存的才是最终完美匹配到的 sticky 广播
ArrayList<Intent> allSticky = null;
if (stickyIntents != null) {
final ContentResolver resolver = mContext.getContentResolver();
// Look for any matching sticky broadcasts...
for (int i = 0, N = stickyIntents.size(); i < N; i++) {
Intent intent = stickyIntents.get(i);
// Don't provided intents that aren't available to instant apps.
if (instantApp &&
(intent.getFlags() & Intent.FLAG_RECEIVER_VISIBLE_TO_INSTANT_APPS) == 0) {
continue;
}
// If intent has scheme "content", it will need to acccess
// provider that needs to lock mProviderMap in ActivityThread
// and also it may need to wait application response, so we
// cannot lock ActivityManagerService here.
if (filter.match(resolver, intent, true, TAG) >= 0) {
if (allSticky == null) {
allSticky = new ArrayList<Intent>();
}
allSticky.add(intent);
}
}
}
// The first sticky in the list is returned directly back to the client.
// 从这里可以看出，如果注册的广播接收器为 null，那么表示要获取最近一次 sticky 广播的数据
Intent sticky = allSticky != null ? allSticky.get(0) : null;
if (receiver == null) {
return sticky;
}
// ...
synchronized (this) {
IApplicationThread thread;
// 注意学会这里的操作，如何判断原来的进程已经死亡
if (callerApp != null && ((thread = callerApp.getThread()) == null
|| thread.asBinder() != caller.asBinder())) {
// Original caller already died
return null;
}
// 1. mRegisteredReceivers 建立客户端与服务端的广播接收器的映射
// 客户端注册的广播接收器是 IIntentReceiver， 而服务端的是 ReceiverList
ReceiverList rl = mRegisteredReceivers.get(receiver.asBinder());
if (rl == null) {
rl = new ReceiverList(this, callerApp, callingPid, callingUid,
userId, receiver);
if (rl.app != null) {
final int totalReceiversForApp = rl.app.mReceivers.numberOfReceivers();
if (totalReceiversForApp >= MAX_RECEIVERS_ALLOWED_PER_APP) {
throw new IllegalStateException("Too many receivers, total of "
+ totalReceiversForApp + ", registered for pid: "
+ rl.pid + ", callerPackage: " + callerPackage);
}
// ProcessRecord#mReceivers 保存 ReceiverList
rl.app.mReceivers.addReceiver(rl);
} else {
// ...
}
mRegisteredReceivers.put(receiver.asBinder(), rl);
} else {
// ...
}
// 2. 创建服务端的广播过滤器 BroadcastFilter，并保存到 mReceiverResolver
BroadcastFilter bf = new BroadcastFilter(filter, rl, callerPackage, callerFeatureId,
receiverId, permission, callingUid, userId, instantApp, visibleToInstantApps);
if (rl.containsFilter(filter)) {
Slog.w(TAG, "Receiver with filter " + filter
+ " already registered for pid " + rl.pid
+ ", callerPackage is " + callerPackage);
} else {
// ReceiverList 是 ArrayList 子类，之所以用一个列表保存 BroadcastFilter
// 是因为在注册广播接收器时，可以为同一个广播接收器匹配多个过滤器
rl.add(bf);
if (!bf.debugCheck()) {
Slog.w(TAG, "==> For Dynamic broadcast");
}
// 解析过滤器的数据，然后用相应的数据结构保存
mReceiverResolver.addFilter(bf);
}
// Enqueue broadcasts for all existing stickies that match
// this filter.
// 注意，这里处理的情况是，注册的 sticky 广播接收器不为 null
// 那么把匹配到的 sticky 广播，发送给这个广播接收器
// 是不是非常有意思，注册 sticky 广播接收器，就能立即收到广播，这得益于 sticky 广播被缓存
if (allSticky != null) {
// 很奇怪，BroadcastFilter 怎么是广播接收器呢?
ArrayList receivers = new ArrayList();
receivers.add(bf);
final int stickyCount = allSticky.size();
for (int i = 0; i < stickyCount; i++) {
Intent intent = allSticky.get(i);
BroadcastQueue queue = broadcastQueueForIntent(intent);
BroadcastRecord r = new BroadcastRecord(queue, intent, null,
null, null, -1, -1, false, null, null, null, OP_NONE, null, receivers,
null, 0, null, null, false, true, true, -1, false, null,
false );
queue.enqueueParallelBroadcastLocked(r);
queue.scheduleBroadcastsLocked();
}
}
return sticky;
}
}
服务端对于动态注册的广播接收器的处理过程如下
- 使用 mRegisteredReceivers 建立客户端与服务端的广播接收器的映射。ReceiverList 代表服务端的广播接收器，IIntentReceiver 代表客户端的广播接收器。
- 使用客户端的 IntentFilter ， 创建服务端的广播过滤器 BroadcastFilter，并保存到 mReceiverResolver。注意，这一步中，ReceiverList 和 BroadcastFilter 互相保存了引用。
这些数据结构都是相互关联的，有何种用意呢？当发送方发送广播到 AMS，AMS 会使用 mReceiverResolver 匹配 BroadcastFilter，BroadcastFilter 找到 ReceiverList，ReceiverList 找到 IIntentReceiver，IIntentReceiver 发送广播给接收方。
另外，由于 sticky 广播是会被缓存的，当注册 sticky 广播的接收器时，有以下两种处理方式
- 如果注册的广播接收器为 null，那么会返回最近的一次广播数据给接收方。
- 如果注册的广播接收器不为null，那么会把匹配到的 sticky 广播发送给接收方的广播接收器，也就是会调用 BroadcastReceiver#onReceive()。
发送广播
发送广播的方法，有多个重载方法，挑选一个最简单的来分析，如下
// ContextImpl.java
public void sendBroadcast(Intent intent) {
warnIfCallingFromSystemProcess();
String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
try {
intent.prepareToLeaveProcess(this);
ActivityManager.getService().broadcastIntentWithFeature(
mMainThread.getApplicationThread(), getAttributionTag(), intent, resolvedType,
null, Activity.RESULT_OK, null, null, null, null ,
null, AppOpsManager.OP_NONE, null, false, false, getUserId());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
// ActivityManagerService.java
public final int broadcastIntentWithFeature(IApplicationThread caller, String callingFeatureId,
Intent intent, String resolvedType, IIntentReceiver resultTo,
int resultCode, String resultData, Bundle resultExtras,
String[] requiredPermissions, String[] excludedPermissions,
String[] excludedPackages, int appOp, Bundle bOptions,
boolean serialized, boolean sticky, int userId) {
enforceNotIsolatedCaller("broadcastIntent");
synchronized(this) {
intent = verifyBroadcastLocked(intent);
final ProcessRecord callerApp = getRecordForAppLOSP(caller);
final int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
try {
return broadcastIntentLocked(callerApp,
callerApp != null ? callerApp.info.packageName : null, callingFeatureId,
intent, resolvedType, resultTo, resultCode, resultData, resultExtras,
requiredPermissions, excludedPermissions, excludedPackages, appOp, bOptions,
serialized, sticky, callingPid, callingUid, callingUid, callingPid, userId);
} finally {
Binder.restoreCallingIdentity(origId);
}
}
}
final int broadcastIntentLocked(ProcessRecord callerApp,
String callerPackage, String callerFeatureId, Intent intent, String resolvedType,
IIntentReceiver resultTo, int resultCode, String resultData,
Bundle resultExtras, String[] requiredPermissions, String[] excludedPermissions,
String[] excludedPackages, int appOp, Bundle bOptions, boolean ordered,
boolean sticky, int callingPid,
int callingUid, int realCallingUid, int realCallingPid, int userId) {
return broadcastIntentLocked(callerApp, callerPackage, callerFeatureId, intent,
resolvedType, resultTo, resultCode, resultData, resultExtras, requiredPermissions,
excludedPermissions, excludedPackages, appOp, bOptions, ordered, sticky, callingPid,
callingUid, realCallingUid, realCallingPid, userId,
false ,
null , null );
}
final int broadcastIntentLocked(ProcessRecord callerApp, String callerPackage,
@Nullable String callerFeatureId, Intent intent, String resolvedType,
IIntentReceiver resultTo, int resultCode, String resultData,
Bundle resultExtras, String[] requiredPermissions,
String[] excludedPermissions, String[] excludedPackages, int appOp, Bundle bOptions,
boolean ordered, boolean sticky, int callingPid, int callingUid,
int realCallingUid, int realCallingPid, int userId,
boolean allowBackgroundActivityStarts,
@Nullable IBinder backgroundActivityStartsToken,
@Nullable int[] broadcastAllowList) {
// 克隆一个 Intent，防止原始 intent 数据被修改
intent = new Intent(intent);
// ...
// If we have not finished booting, don't allow this to launch new processes.
// AMS 还没有启动完成前，广播只能发送给动态注册的广播接收器，这样可以防止拉起新的进程
if (!mProcessesReady && (intent.getFlags()&Intent.FLAG_RECEIVER_BOOT_UPGRADE) == 0) {
intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY);
}
// ...
final String action = intent.getAction();
BroadcastOptions brOptions = null;
// bOptions 可以解决发送广播的一些限制，例如从后台启动Activity，只有系统 app 才能用到的 API
if (bOptions != null) {
// ...
}
// 限制受保护的广播，只能由系统代码发送
final boolean isProtectedBroadcast;
try {
isProtectedBroadcast = AppGlobals.getPackageManager().isProtectedBroadcast(action);
} catch (RemoteException e) {
Slog.w(TAG, "Remote exception", e);
return ActivityManager.BROADCAST_SUCCESS;
}
final boolean isCallerSystem;
switch (UserHandle.getAppId(callingUid)) {
case ROOT_UID:
case SYSTEM_UID:
case PHONE_UID:
case BLUETOOTH_UID:
case NFC_UID:
case SE_UID:
case NETWORK_STACK_UID:
isCallerSystem = true;
break;
default:
isCallerSystem = (callerApp != null) && callerApp.isPersistent();
break;
}
if (!isCallerSystem) {
if (isProtectedBroadcast) {
String msg = "Permission Denial: not allowed to send broadcast "
+ action + " from pid="
+ callingPid + ", uid=" + callingUid;
Slog.w(TAG, msg);
throw new SecurityException(msg);
} else if (AppWidgetManager.ACTION_APPWIDGET_CONFIGURE.equals(action)
|| AppWidgetManager.ACTION_APPWIDGET_UPDATE.equals(action)) {
// ...
}
}
boolean timeoutExempt = false;
if (action != null) {
// 如果系统配置文件中允许发送这个后台广播，那么添加 Intent.FLAG_RECEIVER_INCLUDE_BACKGROUND 标志位
// 例如 frameworks/base/data/etc/framework-sysconfig.xml 允许发送如下后台广播
// <allow-implicit-broadcast action="android.intent.action.SIM_STATE_CHANGED" />
if (getBackgroundLaunchBroadcasts().contains(action)) {
if (DEBUG_BACKGROUND_CHECK) {
Slog.i(TAG, "Broadcast action " + action + " forcing include-background");
}
intent.addFlags(Intent.FLAG_RECEIVER_INCLUDE_BACKGROUND);
}
switch (action) {
// 对一些特殊的广播进行处理...
}
}
// 1. 缓存 sticky 广播到 mStickyBroadcasts
if (sticky) {
// 检查权限
if (checkPermission(android.Manifest.permission.BROADCAST_STICKY,
callingPid, callingUid)
!= PackageManager.PERMISSION_GRANTED) {
String msg = "Permission Denial: broadcastIntent() requesting a sticky broadcast from pid="
+ callingPid + ", uid=" + callingUid
+ " requires " + android.Manifest.permission.BROADCAST_STICKY;
Slog.w(TAG, msg);
throw new SecurityException(msg);
}
if (requiredPermissions != null && requiredPermissions.length > 0) {
Slog.w(TAG, "Can't broadcast sticky intent " + intent
+ " and enforce permissions " + Arrays.toString(requiredPermissions));
return ActivityManager.BROADCAST_STICKY_CANT_HAVE_PERMISSION;
}
if (intent.getComponent() != null) {
throw new SecurityException(
"Sticky broadcasts can't target a specific component");
}
// 确保使用 userId 发送的 sticky 广播，不会与使用 USER_ALL 发送的 sticky 广播有冲突
if (userId != UserHandle.USER_ALL) {
ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(
UserHandle.USER_ALL);
if (stickies != null) {
ArrayList<Intent> list = stickies.get(intent.getAction());
if (list != null) {
int N = list.size();
int i;
for (i=0; i<N; i++) {
if (intent.filterEquals(list.get(i))) {
throw new IllegalArgumentException(
"Sticky broadcast " + intent + " for user "
+ userId + " conflicts with existing global broadcast");
}
}
}
}
}
// 获取 userId 对应的 sticky 广播缓存，并把这个stikcy广播添加/替换到缓存中
ArrayMap<String, ArrayList<Intent>> stickies = mStickyBroadcasts.get(userId);
if (stickies == null) {
stickies = new ArrayMap<>();
mStickyBroadcasts.put(userId, stickies);
}
ArrayList<Intent> list = stickies.get(intent.getAction());
if (list == null) {
list = new ArrayList<>();
stickies.put(intent.getAction(), list);
}
final int stickiesCount = list.size();
int i;
// 存在就替换
for (i = 0; i < stickiesCount; i++) {
if (intent.filterEquals(list.get(i))) {
// This sticky already exists, replace it.
list.set(i, new Intent(intent));
break;
}
}
// 不存在就添加
if (i >= stickiesCount) {
list.add(new Intent(intent));
}
}
int[] users;
if (userId == UserHandle.USER_ALL) {
// 如果以 USER_ALL 名字发送，那么获取所有的启动的 user
users = mUserController.getStartedUserArray();
} else {
users = new int[] {userId};
}
// Figure out who all will receive this broadcast.
// 现在开始找出谁需要接收这个广播
List receivers = null;
List<BroadcastFilter> registeredReceivers = null;
// 2. 收集动态和静态广播接收器
// Intent.FLAG_RECEIVER_REGISTERED_ONLY 表示广播只能发送给动态注册的广播接收器
// 没有指定这个标志位，那么就需要收集静态注册的广播接收器
if ((intent.getFlags()&Intent.FLAG_RECEIVER_REGISTERED_ONLY)
== 0) {
// receivers 此时保存的是静态广播接收器
receivers = collectReceiverComponents(
intent, resolvedType, callingUid, users, broadcastAllowList);
}
if (intent.getComponent() == null) {
if (userId == UserHandle.USER_ALL && callingUid == SHELL_UID) {
// ...以 USER_ALL 身份，从 shell 发送的广播，那么需要获取所有用户注册的广播接收器
} else {
// 收集单个用户注册的动态广播接收器
registeredReceivers = mReceiverResolver.queryIntent(intent,
resolvedType, false , userId);
}
}
// 是否替换即将发送的广播
final boolean replacePending =
(intent.getFlags()&Intent.FLAG_RECEIVER_REPLACE_PENDING) != 0;
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Enqueueing broadcast: " + intent.getAction()
+ " replacePending=" + replacePending);
// broadcastAllowList 是能接收广播 app 白名单
// 如果收集的动态注册的广播接收器，不属于白名单中的 app，那么移除它
if (registeredReceivers != null && broadcastAllowList != null) {
for (int i = registeredReceivers.size() - 1; i >= 0; i--) {
final int owningAppId = UserHandle.getAppId(registeredReceivers.get(i).owningUid);
if (owningAppId >= Process.FIRST_APPLICATION_UID
&& Arrays.binarySearch(broadcastAllowList, owningAppId) < 0) {
registeredReceivers.remove(i);
}
}
}
// 3. 对于非有序广播(包括sticky广播)，先"并行"地发送给动态广播接收器
int NR = registeredReceivers != null ? registeredReceivers.size() : 0;
if (!ordered && NR > 0) {
if (isCallerSystem) {
checkBroadcastFromSystem(intent, callerApp, callerPackage, callingUid,
isProtectedBroadcast, registeredReceivers);
}
final BroadcastQueue queue = broadcastQueueForIntent(intent);
BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage,
callerFeatureId, callingPid, callingUid, callerInstantApp, resolvedType,
requiredPermissions, excludedPermissions, excludedPackages, appOp, brOptions,
registeredReceivers, resultTo, resultCode, resultData, resultExtras, ordered,
sticky, false, userId, allowBackgroundActivityStarts,
backgroundActivityStartsToken, timeoutExempt);
if (DEBUG_BROADCAST) Slog.v(TAG_BROADCAST, "Enqueueing parallel broadcast " + r);
final boolean replaced = replacePending
&& (queue.replaceParallelBroadcastLocked(r) != null);
// Note: We assume resultTo is null for non-ordered broadcasts.
if (!replaced) {
queue.enqueueParallelBroadcastLocked(r);
queue.scheduleBroadcastsLocked();
}
registeredReceivers = null;
// 注意，对于发送非有序广播，当把广播发送给动态接收器后， NR 重置为 0
NR = 0;
}
// 4. 对于有序广播，按照优先级从高到低的顺序，合并静态和动态广播接收器到 receivers
int ir = 0;
// receivers 收集的是静态注册的广播接收器
if (receivers != null) {
// ...
// NT 表示静态广播接收器的数量
int NT = receivers != null ? receivers.size() : 0;
int it = 0;
ResolveInfo curt = null;
BroadcastFilter curr = null;
// 注意 NR 的值，前面发送非有序广播给动态接收器时，NR 重置为 0
// 如果此时 NR 还不为 0， 那么表示发送的是有序广播
// 那么根据广播的优先级，按照从高到低的顺序，把动态广播接收器和静态广播接收器，合并到 receivers
while (it < NT && ir < NR) {
if (curt == null) {
curt = (ResolveInfo)receivers.get(it);
}
if (curr == null) {
curr = registeredReceivers.get(ir);
}
if (curr.getPriority() >= curt.priority) {
// Insert this broadcast record into the final list.
receivers.add(it, curr);
ir++;
curr = null;
it++;
NT++;
} else {
// Skip to the next ResolveInfo in the final list.
it++;
curt = null;
}
}
}
while (ir < NR) {
if (receivers == null) {
receivers = new ArrayList();
}
receivers.add(registeredReceivers.get(ir));
ir++;
}
if (isCallerSystem) {
checkBroadcastFromSystem(intent, callerApp, callerPackage, callingUid,
isProtectedBroadcast, receivers);
}
// 5. “序列化”地发送广播
// 注意，这里分两种情况
// 如果发送的非有序广播，那么 receivers 只保存了静态注册的广播接收器
// 如果发送的是有序广播，那么 receivers 保存了静态和动态注册的广播接收器
if ((receivers != null && receivers.size() > 0)
|| resultTo != null) {
BroadcastQueue queue = broadcastQueueForIntent(intent);
BroadcastRecord r = new BroadcastRecord(queue, intent, callerApp, callerPackage,
callerFeatureId, callingPid, callingUid, callerInstantApp, resolvedType,
requiredPermissions, excludedPermissions, excludedPackages, appOp, brOptions,
receivers, resultTo, resultCode, resultData, resultExtras,
ordered, sticky, false, userId, allowBackgroundActivityStarts,
backgroundActivityStartsToken, timeoutExempt);
final BroadcastRecord oldRecord =
replacePending ? queue.replaceOrderedBroadcastLocked(r) : null;
if (oldRecord != null) {
// 处理替换广播的情况 ....
} else {
queue.enqueueOrderedBroadcastLocked(r);
queue.scheduleBroadcastsLocked();
}
} else {
// 没有找到接收广播的接收器，简单记录下这个发送广播的操作 ...
}
return ActivityManager.BROADCAST_SUCCESS;
}
这段代码设计的并不是很好，很多细节都杂糅到一个方法中。对于一些细节，我进行了详细的注释，有兴趣的读者可以自行研究，而本文只关心发送广播的主要流程。
我不打算按照代码的逻辑来讲解流程，我自己总结了一套流程。
对于非有序广播(包括 sticky 广播)，发送流程如下
- 把广播“并行”地发送给动态广播接收器。
- 把广播“串行”地发送给静态广播接收器。
为何要把非有序广播(包括 sticky 广播)优先发送给动态接收器？最简单的理由就是，不需要先拉起进程！因为"快”，所以先发送。
对于有序广播，发送的流程如下
- 按照广播的优先级，从高到低，把动态和静态广播接收器，合并到一起。
- “串行”发送广播给所有的接收器。
有序广播，为何不先发送给动态接收器呢？因为它强调一个“有序”，所以要根据优先级来发送。
对于 sticky 广播，由于它的特性，是需要对它的广播 Intent 进行缓存的。根据前面注册广播接收器的分析，当注册的广播接收器匹配到缓存的 sticky 广播 Intent，那么会立即返回数据给接收方，无论是通过函数的返回值，还是直接调用 BroadcastReceiver#onReceive()。
结束
本文从整体上介绍了广播接收器的注册流程、发送广播的流程。而发送广播有“并行”的方式，也有“串行”的方式，限于篇幅原因，留到下一篇文章分析，更多关于ActivityManagerService广播注册发送的资料请关注编程网其它相关文章！
