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本文探查的是，从App冷启动后到MainActivity生命周期切换的系统实现。

调试步骤

1. 在com.android.server.wm.RootWindowContainer#attachApplication 方法下断点，为了attach目标进程
2. 在com.android.server.wm.ActivityTaskSupervisor#realStartActivityLocked 方法下断点，为了进入‘3’
3. 在com.android.server.wm.ClientLifecycleManager#scheduleTransactionAndLifecycleItems(
   • . android.app.IApplicationThread, // 目标进程： IApplicationThread$Stub$Proxy
   • android.app.servertransaction.ClientTransactionItem, // LauncherActivityItem
   • android.app.servertransaction.ActivityLifecycleItem, boolean) // ResumeActivityItem
4. 在ActivityThread$H$handleMessage的EXECUTE_TRANSACTION判断处断点，所有的ClientTransaction都可以从这里查看
   • 可以看到 TopResumedActivityChangeItem，WindowStateResizeItem等ActivityTransactionItem的子类实现
   • 也可看到 LaunchActivityItem ，这是ClientTransactionItem的子类
   • 也可看到 ResumeActivityItem，等生命周期，这些是ActivityLifeCycleItem的子类，ActivityTransactionItem的孙子类
5. 在android.app.servertransaction.TransactionExecutor#executeLifecycleItem 里下断点，看代码：

// Cycle to the state right before the final requested state. 进入最终状态(比如：resume)前，执行的ClientTransatction(即：start)
cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */, transaction);// Execute the final transition with proper parameters. 
lifecycleItem.execute(mTransactionHandler, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, mPendingActions);

6. android.app.servertransaction.TransactionExecutorHelper#getLifecyclePath
   最终在这里，看到生命周期的转换关系。

ClientTransaction相关类图

/aosp15/frameworks/base/core/java/android/app/servertransaction

ClienTransaction

注意这两个属性

/*** List of transaction items that should be executed in order. Including both* {@link ActivityLifecycleItem} and other {@link ClientTransactionItem}.* ClientTransaction列表，顺序执行*/
@Nullable
private List<ClientTransactionItem> mTransactionItems;

/*** Final lifecycle state in which the client activity should be after the transaction is executed.* activity的最终生命周期状态*/ 
@Nullable
private ActivityLifecycleItem mLifecycleStateRequest;