消息处理管道

TODO: 待写

消息处理管道

一个App看作是系统，外部输入消息需要经过一系列处理，涉及不同接收者。消息处理的跟踪（Trace）和结果的保存。
比如设计一个用来接收服务器推送消息的处理框架？

以Android中处理InputEvent的设计作为借鉴。

案例

android.view.ViewRootImpl.deliverInputEvent()分发消息
InputStage处理阶段

使用了什么模式？
解决了哪些问题？
什么时候使用？

处理的阶段：InputStage

InputStage mFirstInputStage;

private void deliverInputEvent(QueuedInputEvent q) {        
try {            

            InputStage stage = q.shouldSkipIme() ? mFirstPostImeInputStage : mFirstInputStage;
if (stage != null) {
                stage.deliver(q);
            } else {
finishInputEvent(q);
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
}

/**
 * Base class for implementing a stage in the chain of responsibility
 * for processing input events.
 * <p>
 * Events are delivered to the stage by the {@link #deliver} method. The stage
 * then has the choice of finishing the event or forwarding it to the next stage.
 * </p>
 */
 abstract class InputStage {
 private final InputStage mNext;

 protected static final int FORWARD = 0;
 protected static final int FINISH_HANDLED = 1;
 protected static final int FINISH_NOT_HANDLED = 2;

 /**
 * Creates an input stage.
 * @param next The next stage to which events should be forwarded.
 */
 public InputStage(InputStage next) {
 mNext = next;
 }

 /**
 * Delivers an event to be processed.
 */
 public final void deliver(QueuedInputEvent q) {
 if ((q.mFlags & QueuedInputEvent.FLAG_FINISHED) != 0) {
 forward(q);
 } else if (shouldDropInputEvent(q)) {
 finish(q, false);
 } else {
 apply(q, onProcess(q));
 }
 }

 /**
 * Marks the the input event as finished then forwards it to the next stage.
 */
 protected void finish(QueuedInputEvent q, boolean handled) {
 q.mFlags |= QueuedInputEvent.FLAG_FINISHED;
 if (handled) {
 q.mFlags |= QueuedInputEvent.FLAG_FINISHED_HANDLED;
 }
 forward(q);
 }

 /**
 * Forwards the event to the next stage.
 */
 protected void forward(QueuedInputEvent q) {
 onDeliverToNext(q);
 }

 /**
 * Applies a result code from {@link #onProcess} to the specified event.
 */
 protected void apply(QueuedInputEvent q, int result) {
 if (result == FORWARD) {
 forward(q);
 } else if (result == FINISH_HANDLED) {
 finish(q, true);
 } else if (result == FINISH_NOT_HANDLED) {
 finish(q, false);
 } else {
 throw new IllegalArgumentException("Invalid result: " + result);
 }
 }

 /**
 * Called when an event is ready to be processed.
 * @return A result code indicating how the event was handled.
 */
 protected int onProcess(QueuedInputEvent q) {
 return FORWARD;
 }

 /**
 * Called when an event is being delivered to the next stage.
 */
 protected void onDeliverToNext(QueuedInputEvent q) {
 if (mNext != null) {
 mNext.deliver(q);
 } else {
 finishInputEvent(q);
 }
 }

 protected boolean shouldDropInputEvent(QueuedInputEvent q) {
 if (mView == null || !mAdded) {
 Slog.w(TAG, "Dropping event due to root view being removed: " + q.mEvent);
 return true;
 } else if (!mAttachInfo.mHasWindowFocus &&
 !q.mEvent.isFromSource(InputDevice.SOURCE_CLASS_POINTER) &&
 !isTerminalInputEvent(q.mEvent)) {
 // If this is a focused event and the window doesn't currently have input focus,
 // then drop this event. This could be an event that came back from the previous
 // stage but the window has lost focus in the meantime.
 Slog.w(TAG, "Dropping event due to no window focus: " + q.mEvent);
 return true;
 }
 return false;
 }
 ...
 }