Android 13 源代码分析] onCreate、onStart、onResume 的活动生命周期-2-1. onCreate

onCreate 是一个新 Activity 启动的第一个生命周期。而且根据前面的铺垫，也知道了他是在 onStart 和 onResume 执行的。
对应的事务为 LaunchActivityItem

# LaunchActivityItem
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
            // trace
            Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
            ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
                    mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
                    mPendingResults, mPendingNewIntents, mActivityOptions, mIsForward, mProfilerInfo,
                    client, mAssistToken, mShareableActivityToken, mLaunchedFromBubble,
                    mTaskFragmentToken);
            // 应用段处理
            client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
            Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
    @Override
    public void postExecute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        
            client.countLaunchingActivities(-1);
    }

完整的调用链如下：

LaunchActivityItem::execute
    ActivityThread::handleLaunchActivity
        ActivityThread::performLaunchActivity
            Instrumentation::newActivity      --- 创建Activity
            Activity::attach                  --- 创建Window
                Window::init
                Window::setWindowManager
            Instrumentation::callActivityOnCreate  
                Activity::performCreate
                    Activity::onCreate        --- onCreate
            ActivityClientRecord::setState    --- 设置状态为 ON_CREATE（1）

这部分的代码其实在【Activity启动流程-3】的末尾有解释，单独拎出来再看一遍。

# ActivityThread

    public Activity handleLaunchActivity(ActivityClientRecord r,
                PendingTransactionActions pendingActions, Intent customIntent) {
                    ......
                    final Activity a = performLaunchActivity(r, customIntent);
                    ......
                }
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ......
                Activity activity = null;
                try {
                    // 重点* 1. 通过Instrumentation 反射创建Activity
                    java.lang.ClassLoader cl = appContext.getClassLoader();
                    activity = mInstrumentation.newActivity(
                            cl, component.getClassName(), r.intent);
                    ......
                }
                try {
                    ......
                    // 重点* 2. 执行 attach 流程
                    activity.attach(appContext, this, getInstrumentation(), r.token,
                            r.ident, app, r.intent, r.activityInfo, title, r.parent,
                            r.embeddedID, r.lastNonConfigurationInstances, config,
                            r.referrer, r.voiceInteractor, window, r.activityConfigCallback,
                            r.assistToken, r.shareableActivityToken);
                    if (r.isPersistable()) {
                        mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                    } else {
                        重点* 3. OnCreate流程
                        mInstrumentation.callActivityOnCreate(activity, r.state);
                    }
                    // 设置状态为 ON_CREATE（1）
                    r.setState(ON_CREATE);
                } ......
        ......
    }

# Instrumentation
    public void callActivityOnCreate(Activity activity, Bundle icicle) {
        prePerformCreate(activity);
        // onCreate流程
        activity.performCreate(icicle);
        postPerformCreate(activity);
    }
# Activity
    final void performCreate(Bundle icicle) {
        performCreate(icicle, null);
    }
    final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        ......
        if (persistentState != null) {
            onCreate(icicle, persistentState);
        } else {
            // 执行onCreate
            onCreate(icicle);
        }
        // wm_on_create_called
        EventLogTags.writeWmOnCreateCalled(mIdent, getComponentName().getClassName(),
                "performCreate");
        ......
    }

onCreate 的流程还是比较简单的，在 ActivityThread::performLaunchActivity 做了四件事：

• 1. 反射创建 Activity 对象
• 2. 执行 activity.attach，这里会创建Window
• 3. 执行到 onCreate 生命周期
• 4. 设置 ActivityClientRecord 中的状态为 ON_CREATE（1）

这里需要留意下会先执行 Activity::attach 来创建Window，然后才是后面的 Activity的onCreate。
创建 Window 的时候会创建 DecorView，有了这个 DecorView 我们在 onCreate 的时候才可以把对应的XML布局设置进去。