由于代码中涉及到UI的布局，不得已看android源码。
源码系android-8即2.2
从调试环境看最初调用NativeStart.java中的main函数，经过底层的若干次调用，途经ActivityThread.java中的main函数调用，代码如下：
public static final void main(String[] args) { SamplingProfilerIntegration.start();
Process.setArgV0(" "); Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
Looper.loop();
if (Process.supportsProcesses()) { throw new RuntimeException("Main thread loop unexpectedly exited"); }
thread.detach();
String name = (thread.mInitialApplication != null)
? thread.mInitialApplication.getPackageName()
: "";
Slog.i(TAG, "Main thread of " + name + " is now exiting");
}
这儿应是程序的起点，Looper.loop()展示的为相关的消息处理机制。使得程序不像普通的main函数一样立刻就结束了。
Looper.java中loop()就是一个消息循环，代码如下：
/**
* Run the message queue in this thread. Be sure to call
* {@link #quit()} to end the loop. */
public static final void loop() { Looper me = myLooper();
MessageQueue queue = me.mQueue;
while (true) { Message msg = queue.next(); // might block
//if (!me.mRun) { // break;
//}
if (msg != null) { if (msg.target == null) { // No target is a magic identifier for the quit message.
return;
}
if (me.mLogging!= null) me.mLogging.println(
">>>>> Dispatching to " + msg.target + " "
+ msg.callback + ": " + msg.what
);
msg.target.dispatchMessage(msg);
if (me.mLogging!= null) me.mLogging.println(
"<<<<< Finished to " + msg.target + " "
+ msg.callback);
msg.recycle();
}
}
}
顺便把怎样结束消息循环的代码也贴出来，方便对照消息开始的地方。
public void quit() { Message msg = Message.obtain();
// NOTE: By enqueueing directly into the message queue, the
// message is left with a null target. This is how we know it is
// a quit message.
mQueue.enqueueMessage(msg, 0);
}
接着看Handler.java
/**
* Handle system messages here.
*/
public void dispatchMessage(Message msg) { if (msg.callback != null) { handleCallback(msg);
} else { if (mCallback != null) { if (mCallback.handleMessage(msg)) { return;
}
}
handleMessage(msg);
}
}
handleMessage（）我们在写消息处理的时候经常用到，看ViewRoot.java中，
@Override
public void handleMessage(Message msg) { switch (msg.what) { case View.AttachInfo.INVALIDATE_MSG:
((View) msg.obj).invalidate();
break;
case View.AttachInfo.INVALIDATE_RECT_MSG:
final View.AttachInfo.InvalidateInfo info = (View.AttachInfo.InvalidateInfo) msg.obj;
info.target.invalidate(info.left, info.top, info.right, info.bottom);
info.release();
break;
case DO_TRAVERSAL:
if (mProfile) { Debug.startMethodTracing("ViewRoot"); }
performTraversals();
if (mProfile) { Debug.stopMethodTracing();
mProfile = false;
}
break;
case FINISHED_EVENT:
handleFinishedEvent(msg.arg1, msg.arg2 != 0);
break;
case DISPATCH_KEY:
if (LOCAL_LOGV) Log.v(
"ViewRoot", "Dispatching key "
+ msg.obj + " to " + mView);
deliverKeyEvent((KeyEvent)msg.obj, true);
break;
case DISPATCH_POINTER: { MotionEvent event = (MotionEvent)msg.obj;
boolean callWhenDone = msg.arg1 != 0;
if (event == null) { try { long timeBeforeGettingEvents;
if (MEASURE_LATENCY) { timeBeforeGettingEvents = System.nanoTime();
}
event = sWindowSession.getPendingPointerMove(mWindow);
if (MEASURE_LATENCY && event != null) { lt.sample("9 Client got events ", System.nanoTime() - event.getEventTimeNano()); lt.sample("8 Client getting events ", timeBeforeGettingEvents - event.getEventTimeNano()); }
} catch (RemoteException e) { }
callWhenDone = false;
}
if (event != null && mTranslator != null) { mTranslator.translateEventInScreenToAppWindow(event);
}
try { boolean handled;
if (mView != null && mAdded && event != null) { // enter touch mode on the down
boolean isDown = event.getAction() == MotionEvent.ACTION_DOWN;
if (isDown) { ensureTouchMode(true);
}
if(Config.LOGV) { captureMotionLog("captureDispatchPointer", event); }
if (mCurScrollY != 0) { event.offsetLocation(0, mCurScrollY);
}
if (MEASURE_LATENCY) { lt.sample("A Dispatching TouchEvents", System.nanoTime() - event.getEventTimeNano()); }
handled = mView.dispatchTouchEvent(event);
if (MEASURE_LATENCY) { lt.sample("B Dispatched TouchEvents ", System.nanoTime() - event.getEventTimeNano()); }
if (!handled && isDown) { int edgeSlop = mViewConfiguration.getScaledEdgeSlop();
final int edgeFlags = event.getEdgeFlags();
int direction = View.FOCUS_UP;
int x = (int)event.getX();
int y = (int)event.getY();
final int[] deltas = new int[2];
if ((edgeFlags & MotionEvent.EDGE_TOP) != 0) { direction = View.FOCUS_DOWN;
if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) { deltas[0] = edgeSlop;
x += edgeSlop;
} else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) { deltas[0] = -edgeSlop;
x -= edgeSlop;
}
} else if ((edgeFlags & MotionEvent.EDGE_BOTTOM) != 0) { direction = View.FOCUS_UP;
if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) { deltas[0] = edgeSlop;
x += edgeSlop;
} else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) { deltas[0] = -edgeSlop;
x -= edgeSlop;
}
} else if ((edgeFlags & MotionEvent.EDGE_LEFT) != 0) { direction = View.FOCUS_RIGHT;
} else if ((edgeFlags & MotionEvent.EDGE_RIGHT) != 0) { direction = View.FOCUS_LEFT;
}
if (edgeFlags != 0 && mView instanceof ViewGroup) { View nearest = FocusFinder.getInstance().findNearestTouchable(
((ViewGroup) mView), x, y, direction, deltas);
if (nearest != null) { event.offsetLocation(deltas[0], deltas[1]);
event.setEdgeFlags(0);
mView.dispatchTouchEvent(event);
}
}
}
}
} finally { if (callWhenDone) { try { sWindowSession.finishKey(mWindow);
} catch (RemoteException e) { }
}
if (event != null) { event.recycle();
}
if (LOCAL_LOGV || WATCH_POINTER) Log.i(TAG, "Done dispatching!");
// Let the exception fall through -- the looper will catch
// it and take care of the bad app for us.
}
} break;
case DISPATCH_TRACKBALL:
deliverTrackballEvent((MotionEvent)msg.obj, msg.arg1 != 0);
break;
case DISPATCH_APP_VISIBILITY:
handleAppVisibility(msg.arg1 != 0);
break;
case DISPATCH_GET_NEW_SURFACE:
handleGetNewSurface();
break;
case RESIZED:
ResizedInfo ri = (ResizedInfo)msg.obj;
if (mWinFrame.width() == msg.arg1 && mWinFrame.height() == msg.arg2
&& mPendingContentInsets.equals(ri.coveredInsets)
&& mPendingVisibleInsets.equals(ri.visibleInsets)
&& ((ResizedInfo)msg.obj).newConfig == null) { break;
}
// fall through...
case RESIZED_REPORT:
if (mAdded) { Configuration config = ((ResizedInfo)msg.obj).newConfig;
if (config != null) { updateConfiguration(config, false);
}
mWinFrame.left = 0;
mWinFrame.right = msg.arg1;
mWinFrame.top = 0;
mWinFrame.bottom = msg.arg2;
mPendingContentInsets.set(((ResizedInfo)msg.obj).coveredInsets);
mPendingVisibleInsets.set(((ResizedInfo)msg.obj).visibleInsets);
if (msg.what == RESIZED_REPORT) { mReportNextDraw = true;
}
if (mView != null) { forceLayout(mView);
}
requestLayout();
}
break;
case WINDOW_FOCUS_CHANGED: { if (mAdded) { boolean hasWindowFocus = msg.arg1 != 0;
mAttachInfo.mHasWindowFocus = hasWindowFocus;
if (hasWindowFocus) { boolean inTouchMode = msg.arg2 != 0;
ensureTouchModeLocally(inTouchMode);
if (mGlWanted) { checkEglErrors();
// we lost the gl context, so recreate it.
if (mGlWanted && !mUseGL) { initializeGL();
if (mGlCanvas != null) { float appScale = mAttachInfo.mApplicationScale;
mGlCanvas.setViewport(
(int) (mWidth * appScale + 0.5f),
(int) (mHeight * appScale + 0.5f));
}
}
}
}
mLastWasImTarget = WindowManager.LayoutParams
.mayUseInputMethod(mWindowAttributes.flags);
InputMethodManager imm = InputMethodManager.peekInstance();
if (mView != null) { if (hasWindowFocus && imm != null && mLastWasImTarget) { imm.startGettingWindowFocus(mView);
}
mAttachInfo.mKeyDispatchState.reset();
mView.dispatchWindowFocusChanged(hasWindowFocus);
}
// Note: must be done after the focus change callbacks,
// so all of the view state is set up correctly.
if (hasWindowFocus) { if (imm != null && mLastWasImTarget) { imm.onWindowFocus(mView, mView.findFocus(),
mWindowAttributes.softInputMode,
!mHasHadWindowFocus, mWindowAttributes.flags);
}
// Clear the forward bit. We can just do this directly, since
// the window manager doesn't care about it.
mWindowAttributes.softInputMode &=
~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
((WindowManager.LayoutParams)mView.getLayoutParams())
.softInputMode &=
~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
mHasHadWindowFocus = true;
}
if (hasWindowFocus && mView != null) { sendAccessibilityEvents();
}
}
} break;
case DIE:
doDie();
break;
case DISPATCH_KEY_FROM_IME: { if (LOCAL_LOGV) Log.v(
"ViewRoot", "Dispatching key "
+ msg.obj + " from IME to " + mView);
KeyEvent event = (KeyEvent)msg.obj;
if ((event.getFlags()&KeyEvent.FLAG_FROM_SYSTEM) != 0) { // The IME is trying to say this event is from the
// system! Bad bad bad!
event = KeyEvent.changeFlags(event,
event.getFlags()&~KeyEvent.FLAG_FROM_SYSTEM);
}
deliverKeyEventToViewHierarchy((KeyEvent)msg.obj, false);
} break;
case FINISH_INPUT_CONNECTION: { InputMethodManager imm = InputMethodManager.peekInstance();
if (imm != null) { imm.reportFinishInputConnection((InputConnection)msg.obj);
}
} break;
case CHECK_FOCUS: { InputMethodManager imm = InputMethodManager.peekInstance();
if (imm != null) { imm.checkFocus();
}
} break;
case CLOSE_SYSTEM_DIALOGS: { if (mView != null) { mView.onCloseSystemDialogs((String)msg.obj);
}
} break;
}
}