享元模式(FlyweightPattern)--实现对象的复用
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代码
- 享元接口或者抽象类
open abstract class Flyweight {
abstract fun operation(extrinsicState: String)
}
- 享元实体类
/**
* 享元实体类
*
* @constructor
* TODO
*
* @param intrinsicState 内部状态intrinsicState作为成员变量，同一个享元对象其内部状态是一致的
*/
class ConcreteFlyweight(intrinsicState:String) : Flyweight() {
/**
* 外部状态extrinsicState在使用时由外部设置，不保存在享元对象中，
*
* @param extrinsicState
*/
override fun operation(extrinsicState: String) {
println("外部设置数据$extrinsicState")
}
}
- 享元工厂类
/**
* 享元工厂类
*
*/
class FlyweightFactory {
companion object{
//定义一个HashMap用于存储享元对象，实现享元池
val flyweights = HashMap<String, Flyweight>()
fun getFlyweight(key: String): Flyweight? {
//如果对象存在，则直接从享元池获取
return if (flyweights.containsKey(key)){
flyweights[key]
}else{
//如果对象不存在，先创建一个新的对象添加到享元池中，然后返回
val flyweight = ConcreteFlyweight(key)
flyweights[key] = flyweight
flyweights[key]
}
}
}
}
- 测试代码
fun main() {
val flyweight1=FlyweightFactory.getFlyweight("key")
val flyweight2=FlyweightFactory.getFlyweight("key")
println("flyweight1==flyweight2:${flyweight1==flyweight2}")
flyweight1?.operation("外部设置1")
flyweight2?.operation("外部设置2")
}
适用场景
(1) 一个系统有大量相同或者相似的对象，造成内存的大量耗费。
(2) 对象的大部分状态都可以外部化，可以将这些外部状态传入对象中。
(3) 在使用享元模式时需要维护一个存储享元对象的享元池，而这需要耗费一定的系统资源， 因此，应当在需要多次重复使用享元对象时才值得使用享元模式。
优缺点
- 优点：
(1) 可以极大减少内存中对象的数量，使得相同或相似对象在内存中只保存一份，从而可以节 约系统资源，提高系统性能。
(2) 享元模式的外部状态相对独立，而且不会影响其内部状态，从而使得享元对象可以在不同 的环境中被共享。 - 缺点
(1) 享元模式使得系统变得复杂，需要分离出内部状态和外部状态，这使得程序的逻辑复杂 化。
(2) 为了使对象可以共享，享元模式需要将享元对象的部分状态外部化，而读取外部状态将使 得运行时间变长。
Android系统提供对象池实现
Pools
public interface Pool<T> {
@Nullable
T acquire();
boolean release(@NonNull T instance);
}
public static class SimplePool<T> implements Pool<T> {
private final Object[] mPool;
private int mPoolSize;
public SimplePool(int maxPoolSize) {
if (maxPoolSize <= 0) {
throw new IllegalArgumentException("The max pool size must be > 0");
}
mPool = new Object[maxPoolSize];
}
@Override
@SuppressWarnings("unchecked")
public T acquire() {
if (mPoolSize > 0) {
final int lastPooledIndex = mPoolSize - 1;
T instance = (T) mPool[lastPooledIndex];
mPool[lastPooledIndex] = null;
mPoolSize--;
return instance;
}
return null;
}
@Override
public boolean release(@NonNull T instance) {
if (isInPool(instance)) {
throw new IllegalStateException("Already in the pool!");
}
if (mPoolSize < mPool.length) {
mPool[mPoolSize] = instance;
mPoolSize++;
return true;
}
return false;
}
private boolean isInPool(@NonNull T instance) {
for (int i = 0; i < mPoolSize; i++) {
if (mPool[i] == instance) {
return true;
}
}
return false;
}
}
public static class SynchronizedPool<T> extends SimplePool<T> {
private final Object mLock = new Object();
public SynchronizedPool(int maxPoolSize) {
super(maxPoolSize);
}
@Override
public T acquire() {
synchronized (mLock) {
return super.acquire();
}
}
@Override
public boolean release(@NonNull T instance) {
synchronized (mLock) {
return super.release(instance);
}
}
}
Android源码使用享元模式
- RecyclerView#RecyclerViewPool
public static class RecycledViewPool {
private static final int DEFAULT_MAX_SCRAP = 5;
static class ScrapData {
final ArrayList<ViewHolder> mScrapHeap = new ArrayList<>();
int mMaxScrap = DEFAULT_MAX_SCRAP;
long mCreateRunningAverageNs = 0;
long mBindRunningAverageNs = 0;
}
SparseArray<ScrapData> mScrap = new SparseArray<>();
private int mAttachCount = 0;
public void clear() {
for (int i = 0; i < mScrap.size(); i++) {
ScrapData data = mScrap.valueAt(i);
data.mScrapHeap.clear();
}
}
public void setMaxRecycledViews(int viewType, int max) {
ScrapData scrapData = getScrapDataForType(viewType);
scrapData.mMaxScrap = max;
final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
while (scrapHeap.size() > max) {
scrapHeap.remove(scrapHeap.size() - 1);
}
}
public int getRecycledViewCount(int viewType) {
return getScrapDataForType(viewType).mScrapHeap.size();
}
@Nullable
public ViewHolder getRecycledView(int viewType) {
final ScrapData scrapData = mScrap.get(viewType);
if (scrapData != null && !scrapData.mScrapHeap.isEmpty()) {
final ArrayList<ViewHolder> scrapHeap = scrapData.mScrapHeap;
for (int i = scrapHeap.size() - 1; i >= 0; i--) {
if (!scrapHeap.get(i).isAttachedToTransitionOverlay()) {
return scrapHeap.remove(i);
}
}
}
return null;
}
int size() {
int count = 0;
for (int i = 0; i < mScrap.size(); i++) {
ArrayList<ViewHolder> viewHolders = mScrap.valueAt(i).mScrapHeap;
if (viewHolders != null) {
count += viewHolders.size();
}
}
return count;
}
public void putRecycledView(ViewHolder scrap) {
final int viewType = scrap.getItemViewType();
final ArrayList<ViewHolder> scrapHeap = getScrapDataForType(viewType).mScrapHeap;
if (mScrap.get(viewType).mMaxScrap <= scrapHeap.size()) {
return;
}
if (DEBUG && scrapHeap.contains(scrap)) {
throw new IllegalArgumentException("this scrap item already exists");
}
scrap.resetInternal();
scrapHeap.add(scrap);
}
long runningAverage(long oldAverage, long newValue) {
if (oldAverage == 0) {
return newValue;
}
return (oldAverage / 4 * 3) + (newValue / 4);
}
void factorInCreateTime(int viewType, long createTimeNs) {
ScrapData scrapData = getScrapDataForType(viewType);
scrapData.mCreateRunningAverageNs = runningAverage(
scrapData.mCreateRunningAverageNs, createTimeNs);
}
void factorInBindTime(int viewType, long bindTimeNs) {
ScrapData scrapData = getScrapDataForType(viewType);
scrapData.mBindRunningAverageNs = runningAverage(
scrapData.mBindRunningAverageNs, bindTimeNs);
}
boolean willCreateInTime(int viewType, long approxCurrentNs, long deadlineNs) {
long expectedDurationNs = getScrapDataForType(viewType).mCreateRunningAverageNs;
return expectedDurationNs == 0 || (approxCurrentNs + expectedDurationNs < deadlineNs);
}
boolean willBindInTime(int viewType, long approxCurrentNs, long deadlineNs) {
long expectedDurationNs = getScrapDataForType(viewType).mBindRunningAverageNs;
return expectedDurationNs == 0 || (approxCurrentNs + expectedDurationNs < deadlineNs);
}
void attach() {
mAttachCount++;
}
void detach() {
mAttachCount--;
}
void onAdapterChanged(Adapter oldAdapter, Adapter newAdapter,
boolean compatibleWithPrevious) {
if (oldAdapter != null) {
detach();
}
if (!compatibleWithPrevious && mAttachCount == 0) {
clear();
}
if (newAdapter != null) {
attach();
}
}
private ScrapData getScrapDataForType(int viewType) {
ScrapData scrapData = mScrap.get(viewType);
if (scrapData == null) {
scrapData = new ScrapData();
mScrap.put(viewType, scrapData);
}
return scrapData;
}
}
- 事件分发ViewGourp#TouchTarget.obtain(View,pointerIdBits)
private static final class TouchTarget {
private static final int MAX_RECYCLED = 32;
private static final Object sRecycleLock = new Object[0];
private static TouchTarget sRecycleBin;
private static int sRecycledCount;
public static final int ALL_POINTER_IDS = -1; // all ones
// The touched child view.
@UnsupportedAppUsage
public View child;
// The combined bit mask of pointer ids for all pointers captured by the target.
public int pointerIdBits;
// The next target in the target list.
public TouchTarget next;
@UnsupportedAppUsage
private TouchTarget() {
}
public static TouchTarget obtain(@NonNull View child, int pointerIdBits) {
if (child == null) {
throw new IllegalArgumentException("child must be non-null");
}
final TouchTarget target;
synchronized (sRecycleLock) {
if (sRecycleBin == null) {
target = new TouchTarget();
} else {
target = sRecycleBin;
sRecycleBin = target.next;
sRecycledCount--;
target.next = null;
}
}
target.child = child;
target.pointerIdBits = pointerIdBits;
return target;
}
public void recycle() {
if (child == null) {
throw new IllegalStateException("already recycled once");
}
synchronized (sRecycleLock) {
if (sRecycledCount < MAX_RECYCLED) {
next = sRecycleBin;
sRecycleBin = this;
sRecycledCount += 1;
} else {
next = null;
}
child = null;
}
}
}
- Message
public final class Message {
public int what;
public int arg1;
public int arg2;
public Object obj;
public Messenger replyTo;
public static final int UID_NONE = -1;
public int sendingUid = UID_NONE;
public int workSourceUid = UID_NONE;
@UnsupportedAppUsage
@VisibleForTesting(visibility = VisibleForTesting.Visibility.PACKAGE)
public long when;
/*package*/ Bundle data;
@UnsupportedAppUsage
/*package*/ Handler target;
@UnsupportedAppUsage
/*package*/ Runnable callback;
// sometimes we store linked lists of these things
@UnsupportedAppUsage
/*package*/ Message next;
/** @hide */
public static final Object sPoolSync = new Object();
private static Message sPool;
private static int sPoolSize = 0;
private static final int MAX_POOL_SIZE = 50;
private static boolean gCheckRecycle = true;
public static Message obtain() {
synchronized (sPoolSync) {
if (sPool != null) {
Message m = sPool;
sPool = m.next;
m.next = null;
m.flags = 0; // clear in-use flag
sPoolSize--;
return m;
}
}
return new Message();
}
public static Message obtain(Message orig) {
Message m = obtain();
m.what = orig.what;
m.arg1 = orig.arg1;
m.arg2 = orig.arg2;
m.obj = orig.obj;
m.replyTo = orig.replyTo;
m.sendingUid = orig.sendingUid;
m.workSourceUid = orig.workSourceUid;
if (orig.data != null) {
m.data = new Bundle(orig.data);
}
m.target = orig.target;
m.callback = orig.callback;
return m;
}
public static Message obtain(Handler h) {
Message m = obtain();
m.target = h;
return m;
}
public static Message obtain(Handler h, Runnable callback) {
Message m = obtain();
m.target = h;
m.callback = callback;
return m;
}
public static Message obtain(Handler h, int what) {
Message m = obtain();
m.target = h;
m.what = what;
return m;
}
public static Message obtain(Handler h, int what, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.obj = obj;
return m;
}
public static Message obtain(Handler h, int what, int arg1, int arg2) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
return m;
}
public static Message obtain(Handler h, int what,
int arg1, int arg2, Object obj) {
Message m = obtain();
m.target = h;
m.what = what;
m.arg1 = arg1;
m.arg2 = arg2;
m.obj = obj;
return m;
}
public void recycle() {
if (isInUse()) {
if (gCheckRecycle) {
throw new IllegalStateException("This message cannot be recycled because it "
+ "is still in use.");
}
return;
}
recycleUnchecked();
}
@UnsupportedAppUsage
void recycleUnchecked() {
// Mark the message as in use while it remains in the recycled object pool.
// Clear out all other details.
flags = FLAG_IN_USE;
what = 0;
arg1 = 0;
arg2 = 0;
obj = null;
replyTo = null;
sendingUid = UID_NONE;
workSourceUid = UID_NONE;
when = 0;
target = null;
callback = null;
data = null;
synchronized (sPoolSync) {
if (sPoolSize < MAX_POOL_SIZE) {
next = sPool;
sPool = this;
sPoolSize++;
}
}
}
/*package*/ boolean isInUse() {
return ((flags & FLAG_IN_USE) == FLAG_IN_USE);
}
public Message() {
}
}
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