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Reactive Extensions for Java
/**
* Copyright (c) 2016-present, RxJava Contributors.
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License is
* distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
* the License for the specific language governing permissions and limitations under the License.
*/
package io.reactivex.internal.operators.observable;
import java.util.concurrent.atomic.*;
import io.reactivex.*;
import io.reactivex.disposables.Disposable;
/**
* An observable which auto-connects to another observable, caches the elements
* from that observable but allows terminating the connection and completing the cache.
*
* @param the source element type
*/
public final class ObservableCache extends AbstractObservableWithUpstream
implements Observer {
/**
* The subscription to the source should happen at most once.
*/
final AtomicBoolean once;
/**
* The number of items per cached nodes.
*/
final int capacityHint;
/**
* The current known array of observer state to notify.
*/
final AtomicReference[]> observers;
/**
* A shared instance of an empty array of observers to avoid creating
* a new empty array when all observers dispose.
*/
@SuppressWarnings("rawtypes")
static final CacheDisposable[] EMPTY = new CacheDisposable[0];
/**
* A shared instance indicating the source has no more events and there
* is no need to remember observers anymore.
*/
@SuppressWarnings("rawtypes")
static final CacheDisposable[] TERMINATED = new CacheDisposable[0];
/**
* The total number of elements in the list available for reads.
*/
volatile long size;
/**
* The starting point of the cached items.
*/
final Node head;
/**
* The current tail of the linked structure holding the items.
*/
Node tail;
/**
* How many items have been put into the tail node so far.
*/
int tailOffset;
/**
* If {@link #observers} is {@link #TERMINATED}, this holds the terminal error if not null.
*/
Throwable error;
/**
* True if the source has terminated.
*/
volatile boolean done;
/**
* Constructs an empty, non-connected cache.
* @param source the source to subscribe to for the first incoming observer
* @param capacityHint the number of items expected (reduce allocation frequency)
*/
@SuppressWarnings("unchecked")
public ObservableCache(Observable source, int capacityHint) {
super(source);
this.capacityHint = capacityHint;
this.once = new AtomicBoolean();
Node n = new Node(capacityHint);
this.head = n;
this.tail = n;
this.observers = new AtomicReference[]>(EMPTY);
}
@Override
protected void subscribeActual(Observer t) {
CacheDisposable consumer = new CacheDisposable(t, this);
t.onSubscribe(consumer);
add(consumer);
if (!once.get() && once.compareAndSet(false, true)) {
source.subscribe(this);
} else {
replay(consumer);
}
}
/**
* Check if this cached observable is connected to its source.
* @return true if already connected
*/
/* public */boolean isConnected() {
return once.get();
}
/**
* Returns true if there are observers subscribed to this observable.
* @return true if the cache has observers
*/
/* public */ boolean hasObservers() {
return observers.get().length != 0;
}
/**
* Returns the number of events currently cached.
* @return the number of currently cached event count
*/
/* public */ long cachedEventCount() {
return size;
}
/**
* Atomically adds the consumer to the {@link #observers} copy-on-write array
* if the source has not yet terminated.
* @param consumer the consumer to add
*/
void add(CacheDisposable consumer) {
for (;;) {
CacheDisposable[] current = observers.get();
if (current == TERMINATED) {
return;
}
int n = current.length;
@SuppressWarnings("unchecked")
CacheDisposable[] next = new CacheDisposable[n + 1];
System.arraycopy(current, 0, next, 0, n);
next[n] = consumer;
if (observers.compareAndSet(current, next)) {
return;
}
}
}
/**
* Atomically removes the consumer from the {@link #observers} copy-on-write array.
* @param consumer the consumer to remove
*/
@SuppressWarnings("unchecked")
void remove(CacheDisposable consumer) {
for (;;) {
CacheDisposable[] current = observers.get();
int n = current.length;
if (n == 0) {
return;
}
int j = -1;
for (int i = 0; i < n; i++) {
if (current[i] == consumer) {
j = i;
break;
}
}
if (j < 0) {
return;
}
CacheDisposable[] next;
if (n == 1) {
next = EMPTY;
} else {
next = new CacheDisposable[n - 1];
System.arraycopy(current, 0, next, 0, j);
System.arraycopy(current, j + 1, next, j, n - j - 1);
}
if (observers.compareAndSet(current, next)) {
return;
}
}
}
/**
* Replays the contents of this cache to the given consumer based on its
* current state and number of items requested by it.
* @param consumer the consumer to continue replaying items to
*/
void replay(CacheDisposable consumer) {
// make sure there is only one replay going on at a time
if (consumer.getAndIncrement() != 0) {
return;
}
// see if there were more replay request in the meantime
int missed = 1;
// read out state into locals upfront to avoid being re-read due to volatile reads
long index = consumer.index;
int offset = consumer.offset;
Node node = consumer.node;
Observer downstream = consumer.downstream;
int capacity = capacityHint;
for (;;) {
// if the consumer got disposed, clear the node and quit
if (consumer.disposed) {
consumer.node = null;
return;
}
// first see if the source has terminated, read order matters!
boolean sourceDone = done;
// and if the number of items is the same as this consumer has received
boolean empty = size == index;
// if the source is done and we have all items so far, terminate the consumer
if (sourceDone && empty) {
// release the node object to avoid leaks through retained consumers
consumer.node = null;
// if error is not null then the source failed
Throwable ex = error;
if (ex != null) {
downstream.onError(ex);
} else {
downstream.onComplete();
}
return;
}
// there are still items not sent to the consumer
if (!empty) {
// if the offset in the current node has reached the node capacity
if (offset == capacity) {
// switch to the subsequent node
node = node.next;
// reset the in-node offset
offset = 0;
}
// emit the cached item
downstream.onNext(node.values[offset]);
// move the node offset forward
offset++;
// move the total consumed item count forward
index++;
// retry for the next item/terminal event if any
continue;
}
// commit the changed references back
consumer.index = index;
consumer.offset = offset;
consumer.node = node;
// release the changes and see if there were more replay request in the meantime
missed = consumer.addAndGet(-missed);
if (missed == 0) {
break;
}
}
}
@Override
public void onSubscribe(Disposable d) {
// we can't do much with the upstream disposable
}
@Override
public void onNext(T t) {
int tailOffset = this.tailOffset;
// if the current tail node is full, create a fresh node
if (tailOffset == capacityHint) {
Node n = new Node(tailOffset);
n.values[0] = t;
this.tailOffset = 1;
tail.next = n;
tail = n;
} else {
tail.values[tailOffset] = t;
this.tailOffset = tailOffset + 1;
}
size++;
for (CacheDisposable consumer : observers.get()) {
replay(consumer);
}
}
@SuppressWarnings("unchecked")
@Override
public void onError(Throwable t) {
error = t;
done = true;
for (CacheDisposable consumer : observers.getAndSet(TERMINATED)) {
replay(consumer);
}
}
@SuppressWarnings("unchecked")
@Override
public void onComplete() {
done = true;
for (CacheDisposable consumer : observers.getAndSet(TERMINATED)) {
replay(consumer);
}
}
/**
* Hosts the downstream consumer and its current requested and replay states.
* {@code this} holds the work-in-progress counter for the serialized replay.
* @param the value type
*/
static final class CacheDisposable extends AtomicInteger
implements Disposable {
private static final long serialVersionUID = 6770240836423125754L;
final Observer downstream;
final ObservableCache parent;
Node node;
int offset;
long index;
volatile boolean disposed;
/**
* Constructs a new instance with the actual downstream consumer and
* the parent cache object.
* @param downstream the actual consumer
* @param parent the parent that holds onto the cached items
*/
CacheDisposable(Observer downstream, ObservableCache parent) {
this.downstream = downstream;
this.parent = parent;
this.node = parent.head;
}
@Override
public void dispose() {
if (!disposed) {
disposed = true;
parent.remove(this);
}
}
@Override
public boolean isDisposed() {
return disposed;
}
}
/**
* Represents a segment of the cached item list as
* part of a linked-node-list structure.
* @param the element type
*/
static final class Node {
/**
* The array of values held by this node.
*/
final T[] values;
/**
* The next node if not null.
*/
volatile Node next;
@SuppressWarnings("unchecked")
Node(int capacityHint) {
this.values = (T[])new Object[capacityHint];
}
}
}
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