rxjava是一個在java虛擬機上的響應式擴展，通過使用可觀察的序列將異步和基于事件的程序組合起來的一個庫。
它擴展了觀察者模式來支持數據/事件序列，并且添加了操作符，這些操作符允許你聲明性地組合序列，同時抽象出要關注的問題：比如低級線程、同步、線程安全和并發數據結構等。
rxjava相信大家都非常了解吧，今天分享一下rxjava的消息發送和線程源碼的分析。最后并分享一個相關demo,讓大家更加熟悉我們天天都在用的框架。
消息訂閱發送
首先讓我們看看消息訂閱發送最基本的代碼組成：
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observable observable = observable.create(new observableonsubscribe<string>() { @override public void subscribe(observableemitter<string> emitter) throws exception { emitter.onnext("jack1"); emitter.onnext("jack2"); emitter.onnext("jack3"); emitter.oncomplete(); } }); observer<string> observer = new observer<string>() { @override public void onsubscribe(disposable d) { log.d(tag, "onsubscribe"); } @override public void onnext(string s) { log.d(tag, "onnext : " + s); } @override public void onerror(throwable e) { log.d(tag, "onerror : " + e.tostring()); } @override public void oncomplete() { log.d(tag, "oncomplete"); } }; observable.subscribe(observer); |
代碼很簡單,observable為被觀察者,observer為觀察者,然后通過observable.subscribe(observer),把觀察者和被觀察者關聯起來。被觀察者發送消息(emitter.onnext("內容")),觀察者就可以在onnext()方法里回調出來。
我們先來看observable，創建是用observable.create()方法進行創建，源碼如下：
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public static <t> observable<t> create(observableonsubscribe<t> source) { objecthelper.requirenonnull(source, "source is null"); return rxjavaplugins.onassembly(new observablecreate<t>(source));}public static <t> t requirenonnull(t object, string message) { if (object == null) { throw new nullpointerexception(message); } return object; }public static <t> observable<t> onassembly(@nonnull observable<t> source) { function<? super observable, ? extends observable> f = onobservableassembly; if (f != null) { return apply(f, source); } return source;} |
可以看出，create()方法里最主要的還是創建用observableonsubscribe傳入創建了一個observablecreate對象并且保存而已。
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public final class observablecreate<t> extends observable<t> { final observableonsubscribe<t> source; public observablecreate(observableonsubscribe<t> source) { this.source = source; }} |
接著是創建observer，這比較簡單只是單純創建一個接口對象而已
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public interface observer<t> { void onsubscribe(@nonnull disposable d); void onnext(@nonnull t t); void onerror(@nonnull throwable e); void oncomplete();} |
訂閱發送消息
observable.subscribe(observer)的subscribe方法如下：
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public final void subscribe(observer<? super t> observer) { objecthelper.requirenonnull(observer, "observer is null"); try { observer = rxjavaplugins.onsubscribe(this, observer); objecthelper.requirenonnull(observer, "plugin returned null observer"); subscribeactual(observer); } catch (nullpointerexception e) { // nopmd throw e; } catch (throwable e) { exceptions.throwiffatal(e); rxjavaplugins.onerror(e); nullpointerexception npe = new nullpointerexception("actually not, but can't throw other exceptions due to rs"); npe.initcause(e); throw npe; }}//objecthelper.requirenonnull()方法public static <t> t requirenonnull(t object, string message) { if (object == null) { throw new nullpointerexception(message); } return object;}//rxjavaplugins.onsubscribe()方法public static <t> observer<? super t> onsubscribe(@nonnull observable<t> source, @nonnull observer<? super t> observer) { bifunction<? super observable, ? super observer, ? extends observer> f = onobservablesubscribe; if (f != null) { return apply(f, source, observer); } return observer;} |
從上面源碼可以看出requirenonnull()只是做非空判斷而已，而rxjavaplugins.onsubscribe()也只是返回最終的觀察者而已。所以關鍵代碼是抽象方法subscribeactual(observer);那么subscribeactual對應哪個代碼段呢？
還記得observable.create()創建的observablecreate類嗎，這就是subscribeactual()具體實現類，源碼如下：
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protected void subscribeactual(observer<? super t> observer) { createemitter<t> parent = new createemitter<t>(observer); observer.onsubscribe(parent); try { source.subscribe(parent); } catch (throwable ex) { exceptions.throwiffatal(ex); parent.onerror(ex); }} |
從上面的代碼可以看出，首先創建了一個createemitter對象并傳入observer，然后回到observer的onsubscribe()方法，而source就是我們之前創建observablecreate傳入的observableonsubscribe對象。
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class createemitter<t> extends atomicreference<disposable> implements observableemitter<t>, disposable { } |
而createemitter又繼承observableemitter接口，又回調observableonsubscribe的subscribe方法，對應著我們的：
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observable observable = observable.create(new observableonsubscribe<string>() { @override public void subscribe(observableemitter<string> emitter) throws exception { emitter.onnext("jack1"); emitter.onnext("jack2"); emitter.onnext("jack3"); emitter.oncomplete(); }}); |
當它發送消息既調用emitter.onnext()方法時，既調用了createemitter的onnext()方法：
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public void onnext(t t) { if (t == null) { onerror(new nullpointerexception("onnext called with null. null values are generally not allowed in 2.x operators and sources.")); return; } if (!isdisposed()) { observer.onnext(t); }} |
可以看到最終又回調了觀察者的onnext()方法，把被觀察者的數據傳輸給了觀察者。有人會問
isdisposed()是什么意思，是判斷要不要終止傳遞的，我們看emitter.oncomplete()源碼：
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public void oncomplete() { if (!isdisposed()) { try { observer.oncomplete(); } finally { dispose(); } }}public static boolean dispose(atomicreference<disposable> field) { disposable current = field.get(); disposable d = disposed; if (current != d) { current = field.getandset(d); if (current != d) { if (current != null) { current.dispose(); } return true; } } return false; }public static boolean isdisposed(disposable d) { return d == disposed;} |
dispose()方法是終止消息傳遞，也就付了個disposed常量，而isdisposed()方法就是判斷這個常量而已。這就是整個消息訂閱發送的過程，用的是觀察者模式。
線程切換
在上面模板代碼的基礎上，線程切換只是改變了如下代碼：
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observable.subscribeon(schedulers.io()) .observeon(androidschedulers.mainthread()) .subscribe(observer); |
下面我們對線程切換的源碼進行一下分析，分為兩部分：subscribeon()和observeon()
subscribeon()
首先是subscribeon()源碼如下：
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public final observable<t> subscribeon(scheduler scheduler) { objecthelper.requirenonnull(scheduler, "scheduler is null"); return rxjavaplugins.onassembly(new observablesubscribeon<t>(this, scheduler));} |
我們傳進去了一個scheduler類，scheduler是一個調度類，能夠延時或周期性地去執行一個任務。
scheduler有如下類型：
| 類型 | 使用方式 | 含義 | 使用場景 |
|---|---|---|---|
| ioscheduler | schedulers.io() | io操作線程 | 讀寫sd卡文件，查詢數據庫，訪問網絡等io密集型操作 |
| newthreadscheduler | schedulers.newthread() | 創建新線程 | 耗時操作等 |
| singlescheduler | schedulers.single() | 單例線程 | 只需一個單例線程時 |
| computationscheduler | schedulers.computation() | cpu計算操作線程 | 圖片壓縮取樣、xml,json解析等cpu密集型計算 |
| trampolinescheduler | schedulers.trampoline() | 當前線程 | 需要在當前線程立即執行任務時 |
| handlerscheduler | androidschedulers.mainthread() | android主線程 | 更新ui等 |
接著就沒什么了，只是返回一個observablesubscribeon對象而已。
observeon()
首先看源碼如下：
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public final observable<t> observeon(scheduler scheduler) { return observeon(scheduler, false, buffersize());}public final observable<t> observeon(scheduler scheduler, boolean delayerror, int buffersize) { objecthelper.requirenonnull(scheduler, "scheduler is null"); objecthelper.verifypositive(buffersize, "buffersize"); return rxjavaplugins.onassembly(new observableobserveon<t>(this, scheduler, delayerror, buffersize));} |
這里也是沒什么，只是最終返回一個observableobserveon對象而已。
接著還是像原來那樣調用subscribe()方法進行訂閱，看起來好像整體變化不大，就是封裝了一些對象而已，不過著恰恰是rxjava源碼的精華，當他再次調用subscribeactual()方法時，已經不是之前的observablecreate()里subscribeactual方法了，而是最先調用observableobserveon的subscribeactual()方法，對應源碼如下：
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protected void subscribeactual(observer<? super t> observer) { if (scheduler instanceof trampolinescheduler) { source.subscribe(observer); } else { scheduler.worker w = scheduler.createworker(); source.subscribe(new observeonobserver<t>(observer, w, delayerror, buffersize)); }} |
在這里有兩點要講，一點是observeonobserver是執行觀察者的線程，后面還會詳解，然后就是source.subscribe，這個source.subscribe調的是observablesubscribeon的subscribe方法，而subscribe方法因為繼承的也是observable，是observable里的方法，所以和上面的observablecreate一樣的方法，所以會調用observablesubscribeon里的subscribeactual()方法，對應的代碼如下：
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public void subscribeactual(final observer<? super t> s) { final subscribeonobserver<t> parent = new subscribeonobserver<t>(s); s.onsubscribe(parent); parent.setdisposable(scheduler.scheduledirect(new subscribetask(parent)));} |
上面代碼中，首先把observeonobserver返回給來的用subscribeonobserver“包裝”起來，然后在回調observer的onsubscribe()，就是對應模板代碼的onsubscribe()方法。
接著看subscribetask類的源碼：
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final class subscribetask implements runnable { private final subscribeonobserver<t> parent; subscribetask(subscribeonobserver<t> parent) { this.parent = parent; } @override public void run() { source.subscribe(parent); }} |
其中的source.subscribe(parent)，就是我們執行子線程的回調方法，對應我們模板代碼里的被觀察者的subscribe()方法。它放在run()方法里，并且繼承runnable,說明這個類主要是線程運行。接著看scheduler.scheduledirect()方法對應的源碼如下：
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public disposable scheduledirect(@nonnull runnable run) { return scheduledirect(run, 0l, timeunit.nanoseconds);}public disposable scheduledirect(@nonnull runnable run, long delay, @nonnull timeunit unit) { final worker w = createworker(); final runnable decoratedrun = rxjavaplugins.onschedule(run); disposetask task = new disposetask(decoratedrun, w); w.schedule(task, delay, unit); return task;} |
在這里，createworker()也是一個抽象方法，調用的是我們的調度類對應的schedulers類里面的方法，這里是ioscheduler類，
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public final class ioscheduler extends scheduler{ final atomicreference<cachedworkerpool> pool; //省略.... public worker createworker() { return new eventloopworker(pool.get()); } static final class eventloopworker extends scheduler.worker { private final compositedisposable tasks; private final cachedworkerpool pool; private final threadworker threadworker; final atomicboolean once = new atomicboolean(); eventloopworker(cachedworkerpool pool) { this.pool = pool; this.tasks = new compositedisposable(); this.threadworker = pool.get(); } //省略.... @nonnull @override public disposable schedule(@nonnull runnable action, long delaytime, @nonnull timeunit unit) { if (tasks.isdisposed()) { // don't schedule, we are unsubscribed return emptydisposable.instance; } return threadworker.scheduleactual(action, delaytime, unit, tasks); } }} static final class cachedworkerpool implements runnable { //省略.... threadworker get() { if (allworkers.isdisposed()) { return shutdown_thread_worker; } while (!expiringworkerqueue.isempty()) { threadworker threadworker = expiringworkerqueue.poll(); if (threadworker != null) { return threadworker; } } threadworker w = new threadworker(threadfactory); allworkers.add(w); return w; } //省略....} |
這就是ioscheduler的createworker()的方法，其實最主要的意思就是獲取線程池，以便于生成子線程，讓subscribetask()可以運行。然后直接調用 w.schedule(task, delay, unit)方法讓它在線程池里執行。上面中那threadworker的源碼如下：
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static final class threadworker extends newthreadworker { private long expirationtime; threadworker(threadfactory threadfactory) { super(threadfactory); this.expirationtime = 0l; } //省略代碼.... }public class newthreadworker extends scheduler.worker implements disposable { private final scheduledexecutorservice executor; public newthreadworker(threadfactory threadfactory) { executor = schedulerpoolfactory.create(threadfactory); } public scheduledrunnable scheduleactual(final runnable run, long delaytime, @nonnull timeunit unit, @nullable disposablecontainer parent) { runnable decoratedrun = rxjavaplugins.onschedule(run); scheduledrunnable sr = new scheduledrunnable(decoratedrun, parent); if (parent != null) { if (!parent.add(sr)) { return sr; } } future<?> f; try { if (delaytime <= 0) { f = executor.submit((callable<object>)sr); } else { f = executor.schedule((callable<object>)sr, delaytime, unit); } sr.setfuture(f); } catch (rejectedexecutionexception ex) { if (parent != null) { parent.remove(sr); } rxjavaplugins.onerror(ex); } return sr; }} |
可以看到，這就調了原始的javaapi來進行線程池操作。
然后最后一環在子線程調用source.subscribe(parent)方法，然后回調剛開始創建的observablecreate的subscribeactual(),既：
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protected void subscribeactual(observer<? super t> observer) { createemitter<t> parent = new createemitter<t>(observer); observer.onsubscribe(parent); try { source.subscribe(parent); } catch (throwable ex) { exceptions.throwiffatal(ex); parent.onerror(ex); }} |
進行消息的訂閱綁定。
當我們在調用 emitter.onnext(內容)時，是在io線程里的，那回調的onnext()又是什么時候切換的？那就是前面為了整個流程流暢性沒講的在observeon()里的observeonobserver是執行觀察者的線程的過程。
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class observeonobserver<t> extends basicintqueuedisposable<t> implements observer<t>, runnable { //省略代碼.... observeonobserver(observer<? super t> actual, scheduler.worker worker, boolean delayerror, int buffersize) { this.actual = actual; this.worker = worker; this.delayerror = delayerror; this.buffersize = buffersize; } @override public void onsubscribe(disposable s) { if (disposablehelper.validate(this.s, s)) { this.s = s; if (s instanceof queuedisposable) { @suppresswarnings("unchecked") queuedisposable<t> qd = (queuedisposable<t>) s; int m = qd.requestfusion(queuedisposable.any | queuedisposable.boundary); if (m == queuedisposable.sync) { sourcemode = m; queue = qd; done = true; actual.onsubscribe(this); schedule(); return; } if (m == queuedisposable.async) { sourcemode = m; queue = qd; actual.onsubscribe(this); return; } } queue = new spsclinkedarrayqueue<t>(buffersize); actual.onsubscribe(this); } } @override public void onnext(t t) { if (done) { return; } if (sourcemode != queuedisposable.async) { queue.offer(t); } schedule(); } void schedule() { if (getandincrement() == 0) { worker.schedule(this); } } //省略代碼.... } |
當調用emitter.onnext(內容)方法，會調用上面的onnext()方法，然后在這個方法里會把數據壓入一個隊列，然后執行worker.schedule(this)方法，work是什么呢，還記得androidschedulers.mainthread()嗎，這個對應這個handlerscheduler這個類，所以createworker()對應著：
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private static final class mainholder { static final scheduler default = new handlerscheduler(new handler(looper.getmainlooper()));}public worker createworker() { return new handlerworker(handler);}private static final class handlerworker extends worker { private final handler handler; private volatile boolean disposed; handlerworker(handler handler) { this.handler = handler; } @override public disposable schedule(runnable run, long delay, timeunit unit) { if (run == null) throw new nullpointerexception("run == null"); if (unit == null) throw new nullpointerexception("unit == null"); if (disposed) { return disposables.disposed(); } run = rxjavaplugins.onschedule(run); scheduledrunnable scheduled = new scheduledrunnable(handler, run); message message = message.obtain(handler, scheduled); message.obj = this; // used as token for batch disposal of this worker's runnables. handler.sendmessagedelayed(message, unit.tomillis(delay)); if (disposed) { handler.removecallbacks(scheduled); return disposables.disposed(); } return scheduled; }} |
在next()方法里，運用android自帶的handler消息機制，通過把方法包裹在message里，同通過handler.sendmessagedelayed()發送消息，就會在ui線程里回調next()方法，從而實現從子線程切換到android主線程的操作。我們在主線程拿到數據就可以進行各種在主線程的操作了。
總結一下：
observablecreate 一> observablesubscribeon 一> observableobserveon為初始化順序
當調用observable.subscribe(observer)時的執行順序
observableobserveon 一> observablesubscribeon 一> observablecreate
當發送消息的執行順序
observablecreate 一> observablesubscribeon 一> observableobserveon
以上就是消息訂閱和線程切換的源碼的所有講解了。
為了讓你們理解更清楚，我仿照rxjava寫了大概的消息訂閱和線程切換的最基本代碼和基本功能，以幫助你們理解
https://github.com/jack921/rxjava2demo
以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持服務器之家。
原文鏈接：https://www.jianshu.com/p/264b68fd96fa
