BroadcastReceiver
BroadcastReceiver是Android四大組件之一，我們都比較熟悉炎功，稱它為“廣播”
Android中的廣播就類似于電臺(tái)廣播，一個(gè)廣播可以有N個(gè)接收者。這是一個(gè)典型的發(fā)布——訂閱模式缴啡，也可以叫做觀察者模式。
廣播機(jī)制最大的特點(diǎn)就是發(fā)送發(fā)并不關(guān)心接收方是否接收到數(shù)據(jù)瓷们，也不關(guān)心接收方是如何處理數(shù)據(jù)的盟猖，通過(guò)這樣的形式達(dá)到接收雙方的完全解耦讨衣。
BroadcastReceiver是一個(gè)全局的（可跨進(jìn)程）的廣播，也就是說(shuō)所有應(yīng)用都可收到你發(fā)送的BroadcastReceiver式镐。這可能會(huì)帶來(lái)安全隱患反镇，比如其他應(yīng)用惡意模擬發(fā)送廣播、獲取隱私數(shù)據(jù)等等娘汞。
在BroadcastReceiver源碼類注釋中有如下文字：
If you don't need to send broadcasts across applications, consider using
this class with LocalBroadcastManager instead of the more general facilities described below. This will give you a much more efficient implementation (no cross-process communication needed) and allow you to avoid thinking about any security issues related to other applications being able to receive or send your broadcasts.
意思大概是如果你不需要跨進(jìn)程發(fā)送廣播歹茶，請(qǐng)考慮使用LocalBroadcastManager替代BroadcastReceiver，這樣我們就可以避免其他應(yīng)用程序能夠接收或發(fā)送你的應(yīng)用程序的BroadcastReceiver所帶來(lái)的安全問(wèn)題
LocalBroadcastManager的基本使用
- 注冊(cè)廣播
LocalBroadcastManager.getInstance(context).registerReceiver(receiver,intentFilter);
- 反注冊(cè)廣播
LocalBroadcastManager.getInstance(context).unregisterReceiver(receiver);
- 發(fā)送廣播
LocalBroadcastManager.getInstance(context).sendBroadcast(newIntent());
LocalBroadcastManager 源碼解析
代碼量比較少你弦，我先用大白話說(shuō)一遍惊豺，有個(gè)印象之后，看代碼就好理解了
大體框架就是兩個(gè)內(nèi)部類禽作、三個(gè)集合尸昧、一個(gè)Handler、注冊(cè)廣播的方法旷偿、反注冊(cè)廣播的方法烹俗、發(fā)送廣播的方法、通知接收者的方法萍程。
- 兩個(gè)內(nèi)部類用于承載一些廣播數(shù)據(jù)幢妄，如IntentFilter、BroadcastReceiver等
- 三個(gè)集合就是將承載著數(shù)據(jù)的內(nèi)部類存儲(chǔ)起來(lái)茫负，功能：存儲(chǔ)注冊(cè)的接收者蕉鸳、方便在需要的時(shí)候獲取對(duì)象，比如獲取接受者并回調(diào)onReceive方法等
- Handler用于在主線程發(fā)送并接收消息忍法，調(diào)用executePendingBroadcasts()方法為接受者回調(diào)onReceive方法
- 注冊(cè)廣播的方法：用內(nèi)部類封裝數(shù)據(jù)潮尝，并存入集合
- 反注冊(cè)廣播的方法：從集合中刪除對(duì)應(yīng)的數(shù)據(jù)
- 發(fā)送廣播的方法：獲取到Intent中攜帶的數(shù)據(jù)，與集合中保存的數(shù)據(jù)進(jìn)行匹配饿序，將匹配的保存到集合中衍锚，通過(guò)handler發(fā)送消息回調(diào)executePendingBroadcasts()方法為接受者回調(diào)onReceive方法
- 通知接收者的方法：從集合中獲取注冊(cè)的BroadcastReceiver對(duì)象，調(diào)用其onReceive方法
public class LocalBroadcastManager {
/**
* 內(nèi)部類ReceiverRecord 用于保存IntentFilter和BroadcastReceive
*/
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder(128);
builder.append("Receiver{");
builder.append(receiver);
builder.append(" filter=");
builder.append(filter);
builder.append("}");
return builder.toString();
}
}
/**
* 內(nèi)部類BroadcastRecord 承載著ReceiverRecord集合嗤堰，用于發(fā)送廣播時(shí)獲取BroadcastReceiver對(duì)象
*/
private static class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}
private static final String TAG = "LocalBroadcastManager";
private static final boolean DEBUG = false;
private final Context mAppContext;
//以BroadcastReceiver對(duì)象為key戴质，IntentFilter集合為value 在注冊(cè)廣播接收器時(shí)添加 用于以后方便取消注冊(cè)廣播接收器
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
//以Action字符串為key,以裝載著IntentFilter和BroadcastReceive的內(nèi)部類ReceiverRecord為value，用于發(fā)送廣播后得到廣播接收器
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();
//存儲(chǔ)裝載著ReceiverRecord集合的內(nèi)部類BroadcastRecord的集合 用于發(fā)送廣播時(shí)回調(diào)BroadcastReceiver的onReceive方法
private final ArrayList<BroadcastRecord> mPendingBroadcasts
= new ArrayList<BroadcastRecord>();
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
private final Handler mHandler;
private static final Object mLock = new Object();
private static LocalBroadcastManager mInstance;
//單例模式創(chuàng)建
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
private LocalBroadcastManager(Context context) {
mAppContext = context;
//核心 使用一個(gè)主線程Looper的Handler 用于完成廣播的消息處理
//也就是說(shuō) onReceive接收廣播的時(shí)候 是處于主線程中
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
/**
* 注冊(cè)廣播接收器
*
*/
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
//承載IntentFilter和BroadcastReceiver的內(nèi)部類踢匣，會(huì)將其存儲(chǔ)于mActions集合告匠，方便根據(jù)action查找廣播接收器
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
//將BroadcastReceiver和IntentFilter存入mReceivers集合中，方便取消廣播監(jiān)聽(tīng)器
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
filters.add(filter);
//將action和ReceiverRecord存入mActions集合
for (int i=0; i<filter.countActions(); i++) {
String action = filter.getAction(i);
ArrayList<ReceiverRecord> entries = mActions.get(action);
if (entries == null) {
entries = new ArrayList<ReceiverRecord>(1);
mActions.put(action, entries);
}
entries.add(entry);
}
}
}
/**
* 取消廣播接收器
*/
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
//移除掉mActions集合中的該BroadcastReceiver對(duì)象
ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
//根據(jù)mActions存儲(chǔ)的IntentFilter集合將mActions集合中相關(guān)action對(duì)應(yīng)的ReceiverRecord也刪掉
for (int i=0; i<filters.size(); i++) {
IntentFilter filter = filters.get(i);
for (int j=0; j<filter.countActions(); j++) {
String action = filter.getAction(j);
ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=0; k<receivers.size(); k++) {
if (receivers.get(k).receiver == receiver) {
receivers.remove(k);
k--;
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
/**
* 發(fā)送廣播
* 1.獲取到要發(fā)送的Intent包含的信息
* 2.從注冊(cè)的接收器中找到與之匹配的接收器离唬，存儲(chǔ)到集合中
* 3.通過(guò)Handler在主線程回調(diào)注冊(cè)的接收器的onReceive方法
*/
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
// 獲取Intent中攜帶的相關(guān)信息
final String action = intent.getAction();
final String type = intent.resolveTypeIfNeeded(
mAppContext.getContentResolver());
final Uri data = intent.getData();
final String scheme = intent.getScheme();
final Set<String> categories = intent.getCategories();
final boolean debug = DEBUG ||
((intent.getFlags() & Intent.FLAG_DEBUG_LOG_RESOLUTION) != 0);
if (debug) Log.v(
TAG, "Resolving type " + type + " scheme " + scheme
+ " of intent " + intent);
// 根據(jù)action從mActions集合中獲取到ReceiverRecord集合
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
if (debug) Log.v(TAG, "Action list: " + entries);
ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {
ReceiverRecord receiver = entries.get(i);
if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
//broadcasting是內(nèi)部類ReceiverRecord的一個(gè)屬性后专，用于防止重復(fù)加入receiver到receivers集合
//被加入receivers集合時(shí)賦值為true,遍歷結(jié)束后將改屬性全部賦值為false
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
//用斷定filter是否匹配
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
if (match >= 0) { //匹配
if (debug) Log.v(TAG, " Filter matched! match=0x" +
Integer.toHexString(match));
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
//將匹配的receiver加入receivers集合
receivers.add(receiver);
receiver.broadcasting = true;
} else { //不匹配
if (debug) {
String reason;
switch (match) {
case IntentFilter.NO_MATCH_ACTION: reason = "action"; break;
case IntentFilter.NO_MATCH_CATEGORY: reason = "category"; break;
case IntentFilter.NO_MATCH_DATA: reason = "data"; break;
case IntentFilter.NO_MATCH_TYPE: reason = "type"; break;
default: reason = "unknown reason"; break;
}
Log.v(TAG, " Filter did not match: " + reason);
}
}
}
if (receivers != null) {
//添加結(jié)束，將所有receiver對(duì)象的broadcasting屬性賦值為false
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
//封裝進(jìn)BroadcastRecord中输莺，并添加到mPendingBroadcasts集合
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
//若主線程隊(duì)列中沒(méi)有MSG_EXEC_PENDING_BROADCASTS戚哎，則發(fā)送該消息進(jìn)行onReceive回調(diào)
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
/**
* 發(fā)送同步廣播
* 指的是在一次廣播的發(fā)送完成之前裸诽，不會(huì)同時(shí)發(fā)送其他廣播
*/
public void sendBroadcastSync(Intent intent) {
if (sendBroadcast(intent)) {
executePendingBroadcasts();
}
}
/**
* 回調(diào)注冊(cè)的BroadcastReceiver的onReceive方法
*/
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
//將所有匹配的集合這保存到BroadcastRecord數(shù)組中
mPendingBroadcasts.toArray(brs);
//將該集合清除 如果本次循環(huán)結(jié)束前未添加新的對(duì)象到該集合，那么下次循環(huán)將被return
mPendingBroadcasts.clear();
}
//遍歷回調(diào)匹配的BroadcastReceiver的onReceive方法
for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
}
看完源碼型凳，我們會(huì)發(fā)現(xiàn)丈冬，BroadcastReceiver的跨進(jìn)程通信是依賴Binder機(jī)制來(lái)完成的，而LocalBroadcastManager這個(gè)不跨進(jìn)程的廣播并沒(méi)有那么復(fù)雜甘畅，說(shuō)白了就是單例模式的LocalBroadcastManager就是普通觀察這模式埂蕊，將廣播接收者（即觀察者）存儲(chǔ)到集合中，發(fā)送廣播時(shí)去集合找到對(duì)應(yīng)的接受者（即當(dāng)可觀察者發(fā)生變更時(shí)）疏唾，配合了Handler的使用回調(diào)onReceive對(duì)象罷了（即通知觀察者做出相應(yīng)的處理）蓄氧，此模式可以看看Observable類的源碼，加深理解槐脏。
利與弊
它的好處是：
- 僅在本應(yīng)用內(nèi)廣播喉童，不用擔(dān)心數(shù)據(jù)泄露與其他應(yīng)用模擬發(fā)送廣播，比BroadcastReceiver更安全
- 效率比使用Binder的BroadcastReceiver高
相對(duì)而言顿天，缺點(diǎn)就是：
- 不能跨進(jìn)程通信堂氯。這就是它的雙刃劍啊
- 不能用于系統(tǒng)Action，例如電量露氮、Wifi狀態(tài)等
總而言之祖灰，就是根據(jù)自己的實(shí)際情況去選擇適合的工具钟沛，這樣才能讓它們都“工作”在自己真正應(yīng)該在的崗位上畔规，才能發(fā)揮出最大的價(jià)值！
