Activity工作流启动的过程解析

Activity启动流程分为三步：

1. Launcher请求ATMS过程

2. ATMS到ApplicationThread的调用过程

3. ActivityThread启动Activity过程

ActivityThread启动Activity的过程中performLauncherActivity的工作如下：
1.从ActivityClientRecord中获取待启动的Activity的组件信息
2.通过Instrumentation的newActivity方法使用类加载器创建Activity
3.通过LoadedApk的makeApplication方法来尝试创建Applocation对象
4.创建ContextImpl对象并通过Activity的attach方法来完成一些重要的数据的初始化
（此处还会创建PhoneWindow对象）
5.调用Activity的onCreate方法

oncreate前创建了PhoneWindow,PhoneWindow内部有个DecorView，
然后在onresume之前，把DecorView通过wm.addview方法给添加进去

void makeVisible() {
        if (!mWindowAdded) {
            ViewManager wm = getWindowManager();
            wm.addView(mDecor, getWindow().getAttributes());
            mWindowAdded = true;
        }
        mDecor.setVisibility(View.VISIBLE);
    }

此处就涉及了WindowManagerService的知识点；

wm.addview
实际调用的是WindowManagerImpl.addView

@Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
    //设置默认token
        applyDefaultToken(params);
        mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,
                mContext.getUserId());
    }

交给了WindowManagerGolbal

public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow, int userId) {
        //...省略
        ViewRootImpl root;
        View panelParentView = null;

        synchronized (mLock) {
           //...省略
           
          //创建ViewRootImpl
            root = new ViewRootImpl(view.getContext(), display);
            //给DecorView设置LayoutParams信息
            view.setLayoutParams(wparams);
            //保存DecorView
            mViews.add(view);
            //保存ViewRootImpl
            mRoots.add(root);
            //保存WindowManager.LayoutParams
            mParams.add(wparams);

            // do this last because it fires off messages to start doing things
            try {
                //ViewRootImpl#setView
                root.setView(view, wparams, panelParentView, userId);
            } catch (RuntimeException e) {
                // BadTokenException or InvalidDisplayException, clean up.
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
                throw e;
            }
        }
    }

WindowManagerGolbal中创建了ViewRootImpl,调用ViewRootImpl的setVIew方法

public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
            int userId) {
        synchronized (this) {
            if (mView == null) {
                //..省略一堆设置过程
                // any other events from the system.
                requestLayout();
                //...省略
               try {
                    mOrigWinpatibility(mWindowAttributes);
                    //将窗口添加到WMS上面 这里打个标记留意下
                    res = mWindowSession.addToDisplayAsUser(mWindow, mSeq, mWindowAttributes,
                            getHostVisibility(), mDisplay.getDisplayId(), userId, mTmpFrame,
                            mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                            mAttachInfo.mDisplayCutout, inputChannel,
                            mTempInsets, mTempControls);
                    setFrame(mTmpFrame);
                } catch (RemoteException e) {
                    mAdded = false;
                    mView = null;
                    mAttachInfo.mRootView = null;
                    inputChannel = null;
                    mFallbackEventHandler.setView(null);
                    unscheduleTraversals();
                    setAccessibilityFocus(null, null);
                    throw new RuntimeException("Adding window failed", e);
                } finally {
                    if (restore) {
                        attrs.restore();
                    }
                }
                //这里给DecorView设置mParent
                view.assignParent(this);
                //...省略
                
        }
    }
}

requestLayout就是在绘制window中的DecorView和他的子view，
mWindowSession.addToDisplayAsUser就是把window交给wms，而不是直接把view交给wms；

requestLayout就是view绘制的起点。