Activity启动过程分析

原文： Android中Activity启动过程探究 https://www.cnblogs.com/kross/p/4025075.html

这篇文章是对上面文章的总结

1.因为Android也是 Java程序，所以必然有一个Main方法入口，这个入口，就是在ActivityThread中，因此首先调用android.app.ActivityThread.main()方法

2.在ActivityThread中有一个继承于Handler的内部类，名叫H，这个H结合Looper和MessageQuene等用于ActivityThread这个主线程的消息处理机制，在H的handleMessage()方法中，有个LAUNCH_ACTIVITY的what值，从字面意思来看就是启动Activity的意思，这个分支下又调用了android.app.ActitityThread.handleLaunchActivity()，如下图

Activity启动过程分析_第1张图片

3.handleLaunchActivity()方法中又先后调用了preformLaunchActivity() 和 handleResumeActivity()，并且preformLaunchActivity()方法生成了Activity对象，在判断Activity对象不等于null的情况下，接着调用了 handleResumeActivity()

Activity启动过程分析_第2张图片

3.1 preformLaunchActivity()方法：其中mInstrumentation.callActivityOnCreate(activity, r.state)则是调用了Activity的onCreate方法，Activity的生命周期方法都是由Instrumentation对象来调用的。

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {        ...    Activity activity = null;    try {        java.lang.ClassLoader cl = r.packageInfo.getClassLoader();        //生成一个Activity对象        activity = mInstrumentation.newActivity(                cl, component.getClassName(), r.intent);        ...    } catch (Exception e) {        //抛出异常  Unable to instantiate activity    }    try {        ...        if (activity != null) {            ...            //            activity.attach(appContext, this, getInstrumentation(), r.token,                    r.ident, app, r.intent, r.activityInfo, title, r.parent,                    r.embeddedID, r.lastNonConfigurationInstances, config);            ...            //调用Activity的onCreate方法，Activity的生命周期方法都是由Instrumentation对象来调用的。            mInstrumentation.callActivityOnCreate(activity, r.state);        }    }    ...}

3.2 android.app.activity.attach()方法

final void attach(Context context, ActivityThread aThread,            Instrumentation instr, IBinder token, int ident,            Application application, Intent intent, ActivityInfo info,            CharSequence title, Activity parent, String id,            NonConfigurationInstances lastNonConfigurationInstances,            Configuration config) {    attachBaseContext(context);    mFragments.attachActivity(this, mContainer, null);    //生成一个Window对象    mWindow = PolicyManager.makeNewWindow(this);    //设置WindowManager    mWindow.setWindowManager(            (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),            mToken, mComponent.flattenToString(),            (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);    if (mParent != null) {        mWindow.setContainer(mParent.getWindow());    }    mWindowManager = mWindow.getWindowManager();    mCurrentConfig = config;}

3.3 接下来mInstrumentation.callActivityOnCreate(activity, r.state)，就执行了Activity的setContentView方法了，可以参考另外一片文章：https://blog.csdn.net/jinchen_boke/article/details/88351987

4.android.app.ActivityThread.handleResumeActivity()方法，这个方法在ViewRootImpl的分析中也有，如下

final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward,        boolean reallyResume) {    ...    //生成了一个ActivityClientRecord对象    //方法内部其实是  通过Instrumentation调用Activity的onResume()方法。    ActivityClientRecord r = performResumeActivity(token, clearHide);    if (r != null) {        final Activity a = r.activity;        ...        if (r.window == null && !a.mFinished && willBeVisible) {            r.window = r.activity.getWindow();            View decor = r.window.getDecorView();            decor.setVisibility(View.INVISIBLE);                        //父类引用指向子类对象，WindowManager是继承于ViewManager的            //这个实现其实WindowManagerImpl,而WindowManagerImpl的实现里            //又调用了WindowManagerGlocal这个类的方法，如增删改View这些方法            ViewManager wm = a.getWindowManager();            WindowManager.LayoutParams l = r.window.getAttributes();            a.mDecor = decor;            l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;            l.softInputMode |= forwardBit;            if (a.mVisibleFromClient) {                a.mWindowAdded = true;                //WindowManager添加decorView                //这个地方底层其实就是调用了WindowManagerGlocal的addView了。                wm.addView(decor, l);            }            ...        }    ...    }}

WindowManagerGlocal的addView，如下，可以看出WindowManagerGlocal的addView其实是调用了ViewRootImpl的setView

public void addView(View view, ViewGroup.LayoutParams params,        Display display, Window parentWindow) {        ...        ViewRootImpl root;        View panelParentView = null;        ...        //获取ViewRootImpl对象        root = new ViewRootImpl(view.getContext(), display);        view.setLayoutParams(wparams);        mViews.add(view);        mRoots.add(root);        mParams.add(wparams);         // do this last because it fires off messages to start doing things    try {        //ViewRootImpl的setView        root.setView(view, wparams, panelParentView);    } catch (RuntimeException e) {        ...        throw e;    }}

ViewRootImpl 的setView

/** * We have one child */public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {    synchronized (this) {        if (mView == null) {            mView = view;            ...            // Schedule the first layout -before- adding to the window            // manager, to make sure we do the relayout before receiving            // any other events from the system.            requestLayout();             ...            view.assignParent(this);            ...        }    }}

android.view.ViewRootImpl.requestLayout()方法如下图

Activity启动过程分析_第3张图片

android.view.ViewRootImpl.scheduleTraversals(）方法，如下图

Activity启动过程分析_第4张图片

在mTraversalRunnable 的 run方法中，调用了doTraversal()方法。而doTraversal()方法又调用了performTraversals()方法，这个方法非常长，依次调用了performMeasure()，performLayout()，performDraw()三个方法，进行的View的绘制。

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