交互的基本原理：

打印机通过打印机事件通过广播的方式，通知应用程序，打印机的最新状态。应用程序通过web api 开启打印机扫描、打印以及获取状态信息。

项目需求

应用程序请求扫描/打印任务，把处理扫描/打印任务状态（取消，拒绝，暂停，预览）变为通用逻辑（状态机），把最终结果传递给回调函数

结构图

下面是控制打印机扫描/打印和监听状态的结构图：输入图片说明

代码分析

Scanjob

从状态机的开始请求开始mApplication.getScanJob().scan(requestAttributes)，分析Scanjob类：

public ScanJob(){		this.mHandler = new ScanJobMessageDispatcher();		// sets the listener for the asynchronous communications		this.mHandler.setScanJobListener(new JobListener() {            @Override            public void onChangeJobStatus(ScanJobAttributeSet jobStatus) {                changeJobStatus(jobStatus);            }            @Override            public void setJobId(String jobId) {                setCurrentJobId(jobId);            }            @Override            public String getJobId() {                return getCurrentJobId();            }        });	}public boolean  scan(ScanRequestAttributeSet attributes) throws ScanException {        if( this.mHandler == null ) throw new IllegalStateException("Cannot scan after scanning is completed.");		return mHandler.requestStartScanJob(attributes);	}

从代码中可以看出:扫描调用的是ScanJobMessageDispatcher的requestStartScanJob方法，ScanJob在构造函数中实例化了ScanJobMessageDispatcher，并且向其注册了监听事件

ScanJobMessageDispatcher

接下来我们看实现requestStartScanJob的ScanJobMessageDispatcher类：

public boolean requestStartScanJob(ScanRequestAttributeSet attributes) throws ScanException {        if(this.mJob != null) {            throw new ScanException("Can not start scan(), because running scan process.");        }        // Issues an subscribed ID and registers the asynchronous job event        String subscribedId = FunctionMessageDispatcher.getInstance().addAsyncJobEventListener(this);        if( subscribedId == null ) {            ...        }        ...        Request req = new Request();        ...        try {            Response
    
      resp = mScanner.createJob(req);            ...                mJob = new Job(jobId);                startJobEventDispatcher(jobId);                return true;            }        } catch (IOException e) {           ...        } catch (InvalidResponseException e) {            ...            return false;        }            ...            return false;	}private void startJobEventDispatcher(String jobId) {	    finishJobEventDispatcher();	    mJobEventDispatcher = new JobEventDispatcher(jobId);	    mJobEventDispatcher.start();	}@Override	public void onReceiveJobEvent(GetJobStatusResponseBody event) {	    try {            mJobEventQueue.put(event);        } catch (InterruptedException e) {        }	}private class JobEventDispatcher extends Thread {	    private final String mJobId;	    private volatile boolean mCanceled = false;            ...	    @Override	    public void run() {	        Log.d(SmartSDKApplication.getTagName(), PREFIX + "scan job event dispatcher start (" + mJobId + ")");	        while (!mCanceled) {	            GetJobStatusResponseBody event;	            try {	                event = mJobEventQueue.take();                        ...	                if (listener != null) {	                    listener.onChangeJobStatus(jobStatusResponseToAttributes(event));	                }	            } catch (InterruptedException ignore) {	            }	        }	    }	}

从requestStartScanJob可以看到

1.获取消息接收器的唯一实例，并且向其注册订阅事件，订阅信息是下面的mJob（扫描任务Id）

2.然后通过mScanner调用打印机的web api发起扫描请求

3.最后开始消息读取线程，并将消息传给回调函数。消息处理采用消息队列的目的是：保证接收到的广播事件串行化处理；不影响消息分发线程的效率

FunctionMessageDispatcher

接下来查看FunctionMessageDispatcher类：

private FunctionMessageDispatcher(){	    // Registers an Internal listener to receive asynchronous events from SDKService        mScanAsyncEventReceiver = new ScanEventReceiver();        mScanAsyncEventReceiver.addAsyncEventListener(this);        mScanner = new Scanner();	}public String addAsyncJobEventListener(AsyncJobEventHandler handler){		if(handler == null) {			throw new NullPointerException("handler is null");		}        synchronized (mAsyncEvHandlers) {            if (mSubscribedId == null) {                this.mSubscribedId = mScanAsyncEventReceiver.startReceiveJobEvent();            }            if (mSubscribedId != null) {                mAsyncEvHandlers.add(handler);            }            return this.mSubscribedId;        }	}public String startReceiveJobEvent() {		return setReceiveFunctionJobEvent(RECEIVE_SCAN_JOB_STATE_ACTION, true);}String setReceiveFunctionJobEvent(String action, boolean isReceiveEvent){		...		SDKServiceResultReceiver resultReceiver = new SDKServiceResultReceiver();		sendIntent.putExtra("PRODUCT_ID",SmartSDKApplication.getProductId());                ...		Handler mHandler = new Handler(this.broadcastResultReceiveThread.getLooper());		SmartSDKApplication.getContext().sendOrderedBroadcast(sendIntent, SDK_SERVICE_SEND_CMD_PERMISSION,				resultReceiver, mHandler, Activity.RESULT_OK, null, null);                ...		// synchronous processing		bundle = resultReceiver.getResultExtras();		subscribedId = bundle.getString("SUBSCRIBED_ID");		return subscribedId;}public void onReceive(Context context, Intent intent) {	    postEvent(EventData.EVENT_TYPE_JOB_EVENT, eventData);}void postEvent(int eventType, String eventData) {	this.mMessageThread.post(eventType, eventData, this);}public void onReceiveJobEvent(String eventData) {        if(eventData == null) return;        Map
    
      decoded = GenericJsonDecoder.decodeToMap(eventData);        GetJobStatusResponseBody body = new GetJobStatusResponseBody((Map
     
      ) decoded.get("data"));        String jobId = body.getJobId();        AsyncJobEventHandler[] handlers;        synchronized (mAsyncEvHandlers) {            handlers = mAsyncEvHandlers.toArray(new AsyncJobEventHandler[mAsyncEvHandlers.size()]);        }        for(AsyncJobEventHandler handler : handlers) {            if( handler.getJobId() == null || handler.getJobId().equals(jobId)) {                handler.onReceiveJobEvent(body);              }        }    }MessageDispatchThread.javapublic void run() {		while(isStart) {			try {				EventData ev = mEventQueue.take();				AbstractEventReceiver receiver = ev.getReceiver();				int evType = ev.getEventType();				String data = ev.getEventData();				receiver.notifyAsyncEvent(evType, data);			} catch (InterruptedException e) {	  			}		}	}

从addAsyncJobEventListener方法开始

1.startReceiveJobEvent是向系统底层注册扫描事件监听，系统接收到扫描事件后向ScanEventReceiver发送广播

2.把监听器添加到mAsyncEvHandlers注册列表中

3.ScanEventReceiver.onReceive收到事件后把消息分发给线程mMessageThread,MessageDispatchThread.run()消费消息，把消息发送给receiver，receiver就是this.mMessageThread.post(eventType, eventData, this)传入的FunctionMessageDispatcher实例

4.FunctionMessageDispatcher.onReceiveJobEvent把事件发送给所有的订阅者，发送事件的方法是调用ScanJobMessageDispatcher.onReceiveJobEvent把event放入mJobEventQueue队列中。JobEventDispatcher.run读取事件发送给ScanJob的回调函数

转载于:https://my.oschina.net/leonardtang/blog/1543108

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