思考如下問題：
1、Android9和之前版本startActivity的差異？
2、爲什麼startActivity是異步的？
3、爲什麼安卓Looper死循環不會導致UI卡死？
Android從9.0版本開始修改了啓動Activity部分流程， 跟以往版本的主要區別在於使用Transaction並刪除了ActivityThread內部類H中100~109的code。
從Android8.0開始使用AIDL替換Binder實現系統進程(SystemServer)和應用進程之間的通訊。
Android9.0版本startActivity後會在clientTransaction.addCallback函數傳入LaunchActivityItem對象。
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以前版本直接調用ActivityThread的scheduleLaunchActivity方法發送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);
}
因爲都調用了sendMessage， 所以startActivity是異步操作；
Android9.0以前版本在執行了startActivity後會向主線程MessageQueue中新增一個Message， 如下圖所示：
Android9.0及後續版本AndroidQ新增what等於159即EXECUTE_TRANSACTION的消息， 且obj是ClientTransaction類對象。
因爲Android消息是個隊列， 當啓動Activity時會向隊列尾部新增一條Message， 如果消息隊列中有耗時操作的Message會導致啓動Activity時機延後
安卓Looper會循環消費MessageQueue中的Message（注意：並不是真正的先入先出，原因是異步消息可以插隊）， 想讓某個Message提前執行可以動態調整它在隊列中的位置。
由於Linux的epoll機制， 保證了Looper.loop的死循環獲取消息時並不會阻塞主線程。 當主線程無消息時進入睡眠狀態並阻塞等待nativePollOnce函數， 有消息時喚醒nativePollOnce函數並繼續向下執行。
啓動優化建議：
1、如果消息隊裏中有耗時操作的Message， 建議運行時修改啓動Activity對應Message在隊列中的位置， 從而達到提前啓動Activity的目的。
2、App冷啓動或啓動Activity時將一部分必須運行在主線程的任務調整到IdleHandler中。
調整啓動Activity對應Message時序示例代碼：
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;
}
/**
* 設置當前Message的下一個爲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及後續版本Q移動activity啓動消息到隊列頭部
* @param msg， 主線程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$H")
&& isObjLaunchActivityItem(head.obj)) {
Message nextMsg = getNextMessage(head);
setNextMessage(last, nextMsg);
setNextMessage(head, msg);
return head;
}
last = head;
head = getNextMessage(head);
}
return destMsg;
}
/**
* 判斷Message的obj參數是否匹配啓動Activity
* @param object, Message的obj參數
* @return
*/
private boolean isObjLaunchActivityItem(Object object) {
if (object == null || !object.getClass().toString()
.contains("android.app.servertransaction.ClientTransaction")) {
return false;
}
try {
Class clz = Class.forName("android.app.servertransaction.ClientTransaction");
Field field = clz.getDeclaredField("mActivityCallbacks");
field.setAccessible(true);
Object data = field.get(object);
if (data != null && data instanceof List) {
List items = (List)data;
if (items.size() > 0) {
Object tranItem = items.get(0);
if (tranItem.getClass().toString().endsWith("LaunchActivityItem")) {
return true;
}
}
}
} catch (Exception ex) {
ex.printStackTrace();
}
return false;
}
private Message alterMsgOrderBeforeAndroidP(final Message msg) {
Message head = msg;
Message destMsg = msg; //新的隊列頭，即啓動activity消息
Message last = msg;
while (head != null) {
if (head.what == 100 && head.getTarget() != null
&& head.getTarget().getClass().toString().contains("android.app.ActivityThread$H")) {
//判斷是啓動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;
}
參考：https://www.jianshu.com/p/aa03c4458b9a