step1：调用ViewRootImpl的内部类ImeInputStage的成员函数onProcess来判断输入法是否处于激活状态

final class ImeInputStage extends AsyncInputStage
        implements InputMethodManager.FinishedInputEventCallback {
    public ImeInputStage(InputStage next, String traceCounter) {
        super(next, traceCounter);
    }
    @Override
    protected int onProcess(QueuedInputEvent q) {
        if (mLastWasImTarget && !isInLocalFocusMode()) {
  InputMethodManager imm = InputMethodManager.peekInstance();
            if (imm != null) {
                final InputEvent event = q.mEvent;
                if (DEBUG_IMF) Log.v(TAG, "Sending input event to IME: " + event);
                int result = imm.dispatchInputEvent(event, q, this, mHandler);
                if (result == InputMethodManager.DISPATCH_HANDLED) {
                    return FINISH_HANDLED;
                } else if (result == InputMethodManager.DISPATCH_NOT_HANDLED) {
                    return FINISH_NOT_HANDLED;
                } else {return DEFER; // callback will be invoked later
                }
            }
        }
        return FORWARD;
    }

setp2:调用viewRootImpl的内部类ViewPostImeInputStage的成员函数processKeyEvent

final class ViewPostImeInputStage extends InputStage {

    public ViewPostImeInputStage(InputStage next) {
        super(next);
    }

    @Override
    protected int onProcess(QueuedInputEvent q) {
        if (q.mEvent instanceof KeyEvent) {
            return processKeyEvent(q);
        } else {
            // If delivering a new non-key event, make sure the window is
            // now allowed to start updating.
            handleDispatchDoneAnimating();
            final int source = q.mEvent.getSource();
            if ((source & InputDevice.SOURCE_CLASS_POINTER) != 0) {
                return processPointerEvent(q);
            } else if ((source & InputDevice.SOURCE_CLASS_TRACKBALL) != 0) {
                return processTrackballEvent(q);
            } else {
                return processGenericMotionEvent(q);
            }
        }
    }

    private int processKeyEvent(QueuedInputEvent q) {
        final KeyEvent event = (KeyEvent) q.mEvent;

        if (event.getAction() != KeyEvent.ACTION_UP) {
            // If delivering a new key event, make sure the window is
            // now allowed to start updating.
            handleDispatchDoneAnimating();
        }

        // Deliver the key to the view hierarchy.
        if (mView.dispatchKeyEvent(event)) {
            return FINISH_HANDLED;
        }
        ....
    }

setp3:在viewPostImenputStaged的processKeyEvent的函数中调用Decorview的成员函数dispatchKeyEvent
    if (mView.dispatchKeyEvent(event)) {

            return FINISH_HANDLED;
    }


step4:在DecorView的成员函数dispatchKeyEvent中调用Callback对象的dispatchKeyEvent函数
private final class DecorView extends FrameLayout implements RootViewSurfaceTaker {

...    
public DecorView(Context context, int featureId) {
        super(context);
        mFeatureId = featureId;
    }
    @Override
    public boolean dispatchKeyEvent(KeyEvent event) {
        final int keyCode = event.getKeyCode();
        final int action = event.getAction();
        final boolean isDown = action == KeyEvent.ACTION_DOWN;

        if (isDown && (event.getRepeatCount() == 0)) {
            // First handle chording of panel key: if a panel key is held
            // but not released, try to execute a shortcut in it.
            if ((mPanelChordingKey > 0) && (mPanelChordingKey != keyCode)) {
                boolean handled = dispatchKeyShortcutEvent(event);
                if (handled) {
                    return true;
                }
            }
            if ((mPreparedPanel != null) && mPreparedPanel.isOpen) {
                if (performPanelShortcut(mPreparedPanel, keyCode, event, 0)) {
                    return true;
                }
            }
        }
        if (!isDestroyed()) {
            final Callback cb = getCallback();
            final boolean handled = cb != null && mFeatureId < 0 ? cb.dispatchKeyEvent(event)
                    : super.dispatchKeyEvent(event);
            if (handled) {
                return true;
            }
        }

        return isDown ? PhoneWindow.this.onKeyDown(mFeatureId, event.getKeyCode(), event)
                : PhoneWindow.this.onKeyUp(mFeatureId, event.getKeyCode(), event);
    }
...
}

setp5:在Callback对象的dispatchKeyEvent的函数中调用PhoneWindow对象的superDispatchKeyEvent函数
public boolean dispatchKeyEvent(KeyEvent event) {

    onUserInteraction();

    // Let action bars open menus in response to the menu key prioritized over
    // the window handling it
    if (event.getKeyCode() == KeyEvent.KEYCODE_MENU &&
            mActionBar != null && mActionBar.onMenuKeyEvent(event)) {
        return true;
    }
  Window win = getWindow();
    if (win.superDispatchKeyEvent(event)) {
        return true;
    }
    View decor = mDecor;
    if (decor == null) decor = win.getDecorView();
    return event.dispatch(this, decor != null
            ? decor.getKeyDispatcherState() : null, this);
}