思考如下问题：
1、Android9和之前版本startActivity的差异？
2、为什么startActivity是异步的？
3、为什么安卓Looper死循环不会导致UI卡死？
Android从9.0版本开始修改了启动Activity部分流程， 跟以往版本的主要区别在于使用Transaction并删除了ActivityThread内部类H中100~109的code。
从Android8.0开始使用AIDL替换Binder实现系统进程(SystemServer)和应用进程之间的通讯。
Android9.0版本startActivity后会在clientTransaction.addCallback函数传入LaunchActivityItem对象。
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
}
......
clientTransaction = ClientTransaction.obtain(app.thread,
r.appToken);
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
r.persistentState, results, newIntents, mService.isNextTransitionForward(),
profilerInfo));
....
}
Android9.0以前版本直接调用ActivityThread的scheduleLaunchActivity方法发送LAUNCH_ACTIVITY消息。
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
......
app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global and
// override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
r.persistentState, results, newIntents, !andResume,
mService.isNextTransitionForward(), profilerInfo);
}
因为都调用了sendMessage， 所以startActivity是异步操作；
Android9.0以前版本在执行了startActivity后会向主线程MessageQueue中新增一个Message， 如下图所示：
Android9.0及后续版本AndroidQ新增what等于159即EXECUTE_TRANSACTION的消息， 且obj是ClientTransaction类对象。
因为Android消息是个队列， 当启动Activity时会向队列尾部新增一条Message， 如果消息队列中有耗时操作的Message会导致启动Activity时机延后
安卓Looper会循环消费MessageQueue中的Message（注意：并不是真正的先入先出，原因是异步消息可以插队）， 想让某个Message提前执行可以动态调整它在队列中的位置。
由于Linux的epoll机制， 保证了Looper.loop的死循环获取消息时并不会阻塞主线程。 当主线程无消息时进入睡眠状态并阻塞等待nativePollOnce函数， 有消息时唤醒nativePollOnce函数并继续向下执行。
启动优化建议：
1、如果消息队里中有耗时操作的Message， 建议运行时修改启动Activity对应Message在队列中的位置， 从而达到提前启动Activity的目的。
2、App冷启动或启动Activity时将一部分必须运行在主线程的任务调整到IdleHandler中。
调整启动Activity对应Message时序示例代码：
public class MainActivity extends AppCompatActivity {
private TextView mTvInfo;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTvInfo = findViewById(R.id.tv_info);
findViewById(R.id.btn_ok).setOnClickListener(new View.OnClickListener() {
@Override public void onClick(View v) {
//new Handler().post(new Runnable() {
// @Override public void run() {
// try {
// Thread.sleep(200);
// } catch (InterruptedException ex) {
// ex.printStackTrace();
// }
// }
//});
Log.d("brycegao", "------------------queue111111111");
Intent intent = new Intent(MainActivity.this, SecondActivity.class);
startActivity(intent);
MessageQueue queue = Looper.myQueue();
changeMsgSequence(queue);
Log.d("brycegao", "------------------queue22222222");
}
});
}
private void changeMsgSequence(MessageQueue queue) {
Class clz = MessageQueue.class;
try {
Field field = clz.getDeclaredField("mMessages");
field.setAccessible(true);
Object obj = field.get(queue);
if (obj != null && obj instanceof Message) {
Message msg = (Message) obj;
Message newHead; //新的mMessage值
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.P) {
newHead = alterMsgOrderBeforeAndroidP(msg);
} else {
//Android9及以上版本
newHead = alterMsgOrderAfterAndroidP(msg);
}
field.set(queue, newHead);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
private Message getNextMessage(Message msg) {
Class clz = Message.class;
try {
Field field = clz.getDeclaredField("next");
field.setAccessible(true);
Object object = field.get(msg);
if (object != null && object instanceof Message) {
return (Message) object;
}
} catch (Exception ex) {
ex.printStackTrace();
}
return null;
}
/**
* 设置当前Message的下一个为next
* @param cur
* @param next
*/
private void setNextMessage(Message cur, Message next) {
Class clz = Message.class;
try {
Field field = clz.getDeclaredField("next");
field.setAccessible(true);
field.set(cur, next);
} catch (Exception ex) {
ex.printStackTrace();
}
}
/**
* Android9及后续版本Q移动activity启动消息到队列头部
* @param msg， 主线程MessageQueue的mMessages
* @return
*/
private Message alterMsgOrderAfterAndroidP(final Message msg) {
Message head = msg;
Message destMsg = msg;
Message last = msg;
while (head != null) {
if (head.what == 159 && head.getTarget() != null
&& head.getTarget().getClass().toString().contains("android.app.ActivityThread$H")
&& isObjLaunchActivityItem(head.obj)) {
Message nextMsg = getNextMessage(head);
setNextMessage(last, nextMsg);
setNextMessage(head, msg);
return head;
}
last = head;
head = getNextMessage(head);
}
return destMsg;
}
/**
* 判断Message的obj参数是否匹配启动Activity
* @param object, Message的obj参数
* @return
*/
private boolean isObjLaunchActivityItem(Object object) {
if (object == null || !object.getClass().toString()
.contains("android.app.servertransaction.ClientTransaction")) {
return false;
}
try {
Class clz = Class.forName("android.app.servertransaction.ClientTransaction");
Field field = clz.getDeclaredField("mActivityCallbacks");
field.setAccessible(true);
Object data = field.get(object);
if (data != null && data instanceof List) {
List items = (List)data;
if (items.size() > 0) {
Object tranItem = items.get(0);
if (tranItem.getClass().toString().endsWith("LaunchActivityItem")) {
return true;
}
}
}
} catch (Exception ex) {
ex.printStackTrace();
}
return false;
}
private Message alterMsgOrderBeforeAndroidP(final Message msg) {
Message head = msg;
Message destMsg = msg; //新的队列头，即启动activity消息
Message last = msg;
while (head != null) {
if (head.what == 100 && head.getTarget() != null
&& head.getTarget().getClass().toString().contains("android.app.ActivityThread$H")) {
//判断是启动activity的消息
//{ when=-1m0s908ms what=100 obj=ActivityRecord{672a27c token=android.os.BinderProxy@4392705 {com.brycegao.findview/com.brycegao.findview.SecondActivity}} target=android.app.ActivityThread$H }
Message nextMsg = getNextMessage(head);
setNextMessage(last, nextMsg);
setNextMessage(head, msg);
return head;
}
last = head;
head = getNextMessage(head);
}
return destMsg;
}
参考：https://www.jianshu.com/p/aa03c4458b9a