读 Beginning Android Games 2nd Edition (六) 创建键盘事件处理类

package com.badlogic.androidgames.framework.impl;

import java.util.ArrayList;
import java.util.List;

import android.view.View;
import android.view.View.OnKeyListener;

import com.badlogic.androidgames.framework.Input.KeyEvent;
import com.badlogic.androidgames.framework.Pool;
import com.badlogic.androidgames.framework.Pool.PoolObjectFactory;

public class KeyboardHandler implements OnKeyListener {
    boolean[] pressedKeys = new boolean[128]; //用于储存按钮的按下状态,按下/非按下
    Pool<KeyEvent> keyEventPool;
    List<KeyEvent> keyEventsBuffer = new ArrayList<KeyEvent>();    
    List<KeyEvent> keyEvents = new ArrayList<KeyEvent>();
    
    public KeyboardHandler(View view) {
        PoolObjectFactory<KeyEvent> factory = new PoolObjectFactory<KeyEvent>() {
            public KeyEvent createObject() {
                return new KeyEvent();
            }
        };
        keyEventPool = new Pool<KeyEvent>(factory, 100);
        view.setOnKeyListener(this);
        view.setFocusableInTouchMode(true);
        view.requestFocus();
    }

    public boolean onKey(View v, int keyCode, android.view.KeyEvent event) {
        if (event.getAction() == android.view.KeyEvent.ACTION_MULTIPLE)
            return false;

        synchronized (this) {
            KeyEvent keyEvent = keyEventPool.newObject();
            keyEvent.keyCode = keyCode;
            keyEvent.keyChar = (char) event.getUnicodeChar();
            if (event.getAction() == android.view.KeyEvent.ACTION_DOWN) {
                keyEvent.type = KeyEvent.KEY_DOWN;
                if(keyCode > 0 && keyCode < 127)
                    pressedKeys[keyCode] = true;
            }
            if (event.getAction() == android.view.KeyEvent.ACTION_UP) {
                keyEvent.type = KeyEvent.KEY_UP;
                if(keyCode > 0 && keyCode < 127)
                    pressedKeys[keyCode] = false;
            }
            keyEventsBuffer.add(keyEvent);
        }
        return false;
    }

    public boolean isKeyPressed(int keyCode) {
        if (keyCode < 0 || keyCode > 127)
            return false;
        return pressedKeys[keyCode];
    }

    public List<KeyEvent> getKeyEvents() {
        synchronized (this) {
            int len = keyEvents.size();
            for (int i = 0; i < len; i++) {
                keyEventPool.free(keyEvents.get(i));
            }
            keyEvents.clear();
            keyEvents.addAll(keyEventsBuffer);
            keyEventsBuffer.clear();
            return keyEvents;
        }
    }
}

此类使用了前一篇中的对象池类,用于缓存KeyEvent实例

类的运行过程可以用图来说明

第一步:我们从UI线程中得到一个新的事件(event).这时池(Pool)里还没有任何东西.所以我们创建一个KeyEvent的实例(KeyEvent1)并插入到keyEventsBuffer list中

UI thread: onKey() ->

           keyEvents = { }, keyEventsBuffer = {KeyEvent1}, pool = { }

第二步:我们调用主线程中的getKeyEvents方法,getKeyEvents方法从keyEventsBuffer list中拿到实例KeyEvent1,并把它插入到keyEvents 
list 并把它返回给调用者
Main thread: getKeyEvents() ->
           keyEvents = {KeyEvent1}, keyEventsBuffer = { }, pool { }

第三步:我们从UI线程中得到另一个事件,在池中仍然没有东西.因此一个新的KeyEvent实例被创建并插入到keyEventsBuffer list中
UI thread: onKey() ->
           keyEvents = {KeyEvent1}, keyEventsBuffer = {KeyEvent2}, pool { }

第四步:主线程再次调用getKeyEvents方法,有趣的事发生了! keyEvents list 仍然保有KeyEvent1实例,介入的循环将把这个事件放入我们的池中,

之后我们清理掉 keyEvents list,然后插入任何的keyEvent实例到keyEventsBuffer,在这种情况下,KeyEvent2就是我们可以反复利用的Key事件

Main thread: getKeyEvents() ->
           keyEvents = {KeyEvent2}, keyEventsBuffer = { }, pool = {KeyEvent1}

第五步:另一个Key事件到达UI线程,这次我们在池中free拿到KeyEvent,重复利用这个对象,避免了垃圾回收

UI thread: onKey() ->
           keyEvents = {KeyEvent2}, keyEventsBuffer = {KeyEvent1}, pool = { }