讲解 Activity加载View的时机和流程
涉及到的类如下

• frameworks/base/core/java/android/app/Activity.java
• frameworks/base/services/core/java/com/android/server/wm/ActivityTaskSupervisor.java
• frameworks/base/core/java/com/android/internal/policy/PhoneWindow.java
• frameworks/base/core/java/android/view/WindowManagerImpl.java

从之前文章Activity启动流程可知，最终会到 ActivityTaskSupervisor 的 realStartActivityLocked 方法，而 realStartActivityLocked 最终会执行 2个主要步骤 就是 LaunchActivityItem 和 ResumeActivityItem。

1 LaunchActivityItem

1.1 Activity 创建

LaunchActivityItem 的 又会调用 ActivityThread的 handleLaunchActivity 方法

	public void execute(ClientTransactionHandler client, IBinder token,PendingTransactionActions pendingActions) {Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");ActivityClientRecord r = client.getLaunchingActivity(token);client.handleLaunchActivity(r, pendingActions, null /* customIntent */);Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);}

handleLaunchActivity 又会调用 performLaunchActivity 方法

public Activity handleLaunchActivity(ActivityClientRecord r,PendingTransactionActions pendingActions, Intent customIntent) {...final Activity a = performLaunchActivity(r, customIntent);...return a;}

performLaunchActivity 方法中

1. 会通过反射调用对应的 Activity
2. 调用 Activity的 attach 方法创建phoneWindow
3. 通过 callActivityOnCreate 执行 Activity的 onCreate方法
4. 设置 ActivityClientRecord 状态为 ON_CREATE

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {...ContextImpl appContext = createBaseContextForActivity(r);// 进行反射创建对应的 Activity对象Activity activity = null;try {java.lang.ClassLoader cl = appContext.getClassLoader();activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);StrictMode.incrementExpectedActivityCount(activity.getClass());r.intent.setExtrasClassLoader(cl);r.intent.prepareToEnterProcess(isProtectedComponent(r.activityInfo),appContext.getAttributionSource());if (r.state != null) {r.state.setClassLoader(cl);}} catch (Exception e) {if (!mInstrumentation.onException(activity, e)) {throw new RuntimeException("Unable to instantiate activity " + component+ ": " + e.toString(), e);}}try {Application app = r.packageInfo.makeApplication(false, mInstrumentation);if (activity != null) {...// 调用 attach方法创建对应的的phoneView和 windowMangerImplactivity.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.configCallback,r.assistToken, r.shareableActivityToken);...r.activity = activity;// 触发Activity的 onCreate方法if (r.isPersistable()) {mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);} else {mInstrumentation.callActivityOnCreate(activity, r.state);}...}// 设置 ActivityRecord状态为 ON_CREATEr.setState(ON_CREATE);} catch (SuperNotCalledException e) {throw e;} catch (Exception e) {if (!mInstrumentation.onException(activity, e)) {throw new RuntimeException("Unable to start activity " + component+ ": " + e.toString(), e);}}return activity;}

1.2 PhoneWindow 创建

查看上面 Attach 方法

1. 创建了 PhoneWindow
2. 调用 phoneWindow的setWindowManager 方法 创建对应 WindowManagerImpl 对象。
3. 将创建的WindowManagerImpl 赋值给 mWindowManager

final void attach(Context context, ActivityThread aThread...) {...// 初始化 PhoneWindowmWindow = new PhoneWindow(this, window, activityConfigCallback);...// 初始化 并设置WindowManager到phoneWindow对应的变量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());}// 将上一步通过 setWindowManager 创建WindowManagerImpl并赋值给 mWindowManagermWindowManager = mWindow.getWindowManager();
}

1.3 DecorView 创建

执行了 Activity onCreate 方法后 ，我们就会对其调用 setContentView 将要显示的布局ID创建创建视图，setContentView 方法又会调用 phoneWindow的setContentView方法

	public void setContentView(@LayoutRes int layoutResID) {getWindow().setContentView(layoutResID);initWindowDecorActionBar();}

phoneWindow的setContentView主要做了

1. 判断 mContentParent 是否为空（放应用要显示的布局容器），为空的话 则调用 installDecor 方法，创建 DecorView 和 内容容器 mContentParent
2. 调用 inflate 将布局加载到 mContentParent 布局容器中

	public void setContentView(int layoutResID) {// 如果contentview为空则加载decorViewif (mContentParent == null) {installDecor();} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {mContentParent.removeAllViews();}if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,getContext());transitionTo(newScene);} else {// 将对应的布局加载到对应的内容区域mLayoutInflater.inflate(layoutResID, mContentParent);}...}

其中 installDecor 方法主要逻辑

1. 判断 mDecor 是否为空 ，为空的话调用 generateDecor 方法**（直接new DecorView）,，并赋值**
2. 判断 mContentParent 是否为空，为空的话 调用 generateLayout() 方法获取对应的容器.(里面会通过feature 加载对应的布局，然后获取R.id.content的控件)，并返回。
3. 根据属性 设置对应的标题。

	private void installDecor() {mForceDecorInstall = false;// 为空则创建 DecorViewif (mDecor == null) {mDecor = generateDecor(-1);mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);mDecor.setIsRootNamespace(true);if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);}} else {mDecor.setWindow(this);}// 为空则获取 decorView 用布局加载好里面的id为content的viewgroupif (mContentParent == null) {mContentParent = generateLayout(mDecor);...mTitleView = findViewById(R.id.title);if (mTitleView != null) {if ((getLocalFeatures() & (1 << FEATURE_NO_TITLE)) != 0) {final View titleContainer = findViewById(R.id.title_container);if (titleContainer != null) {titleContainer.setVisibility(View.GONE);} else {mTitleView.setVisibility(View.GONE);}mContentParent.setForeground(null);} else {mTitleView.setText(mTitle);}}}}}

generateLayout 主要

1. 获取一系列属性
2. 调用 getLocalFeatures 生成对应的 features，根据获取到的属性获取对应的布局ID赋值到 layoutResource
3. 调用 decorView 的 onResourcesLoaded 方法 inflage出对应的布局，并通过 addView添加到 decorView，赋值给对应的mContentRoot 属性。
4. 根据mDecorView获取ID为 R.id.content的内容容器，并返回出去

protected ViewGroup generateLayout(DecorView decor) {...// 获取设置一系列属性mIsFloating = a.getBoolean(R.styleable.Window_windowIsFloating, false);int flagsToUpdate = (FLAG_LAYOUT_IN_SCREEN|FLAG_LAYOUT_INSET_DECOR)& (~getForcedWindowFlags());...// Inflate the window decor.// 获取对应的 layout布局文件int layoutResource;int features = getLocalFeatures();// System.out.println("Features: 0x" + Integer.toHexString(features));if ((features & ((1 << FEATURE_LEFT_ICON) | (1 << FEATURE_RIGHT_ICON))) != 0) {if (mIsFloating) {TypedValue res = new TypedValue();getContext().getTheme().resolveAttribute(R.attr.dialogTitleIconsDecorLayout, res, true);layoutResource = res.resourceId;} else {layoutResource = R.layout.screen_title_icons;}// XXX Remove this once action bar supports these features.removeFeature(FEATURE_ACTION_BAR);// System.out.println("Title Icons!");} else if ((features & ((1 << FEATURE_PROGRESS) | (1 << FEATURE_INDETERMINATE_PROGRESS))) != 0&& (features & (1 << FEATURE_ACTION_BAR)) == 0) {// Special case for a window with only a progress bar (and title).// XXX Need to have a no-title version of embedded windows.layoutResource = R.layout.screen_progress;// System.out.println("Progress!");} else if ((features & (1 << FEATURE_CUSTOM_TITLE)) != 0) {// Special case for a window with a custom title.// If the window is floating, we need a dialog layoutif (mIsFloating) {TypedValue res = new TypedValue();getContext().getTheme().resolveAttribute(R.attr.dialogCustomTitleDecorLayout, res, true);layoutResource = res.resourceId;} else {layoutResource = R.layout.screen_custom_title;}// XXX Remove this once action bar supports these features.removeFeature(FEATURE_ACTION_BAR);} else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {// If no other features and not embedded, only need a title.// If the window is floating, we need a dialog layoutif (mIsFloating) {TypedValue res = new TypedValue();getContext().getTheme().resolveAttribute(R.attr.dialogTitleDecorLayout, res, true);layoutResource = res.resourceId;} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {layoutResource = a.getResourceId(R.styleable.Window_windowActionBarFullscreenDecorLayout,R.layout.screen_action_bar);} else {layoutResource = R.layout.screen_title;}// System.out.println("Title!");} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {layoutResource = R.layout.screen_simple_overlay_action_mode;} else {// Embedded, so no decoration is needed.layoutResource = R.layout.screen_simple;// System.out.println("Simple!");}mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);// 根据mDecorView获取ID为 R.id.contentViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);if (contentParent == null) {throw new RuntimeException("Window couldn't find content container view");}...return contentParent;}

2 ResumeActivityItem

经过 onCreate 视图view都创建准备好后，就会进入 resume阶段
ResumeActivityItem 又会执行对应的 handleResumeActivity 方法。

	public void execute(ClientTransactionHandler client, ActivityClientRecord r,PendingTransactionActions pendingActions) {Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityResume");// 执行 ActivityThread 的 handleResumeActivityclient.handleResumeActivity(r, true /* finalStateRequest */, mIsForward,"RESUME_ACTIVITY");Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);}

handleResumeActivity 方法

1. 从 ActivityClientRecord 取出 创建的 Activity,。将 decorView 设置为 INVISIBLE，
2. 将对应的 LayoutParams type 层级设置为 WindowManager.LayoutParams.TYPE_BASE_APPLICATION
3. 调用 phoneWindow 创建的 WindowManagerImpl（attach创建） ,调用 addView 将 decorView添加到 WMS中。

public void handleResumeActivity(ActivityClientRecord r, boolean finalStateRequest,boolean isForward, String reason) {...final Activity a = r.activity;...final int forwardBit = isForward? WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;// If the window hasn't yet been added to the window manager,// and this guy didn't finish itself or start another activity,// then go ahead and add the window.boolean willBeVisible = !a.mStartedActivity;if (!willBeVisible) {willBeVisible = ActivityClient.getInstance().willActivityBeVisible(a.getActivityToken());}if (r.window == null && !a.mFinished && willBeVisible) {// 赋值 phoneWindow 给ActivityClientRecordr.window = r.activity.getWindow();View decor = r.window.getDecorView();decor.setVisibility(View.INVISIBLE);ViewManager wm = a.getWindowManager();WindowManager.LayoutParams l = r.window.getAttributes();// 将 decor赋值给activitya.mDecor = decor;// 设置Activity的层级为applicationl.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;l.softInputMode |= forwardBit;...// 将decorView 通过 wms添加到容器中if (a.mVisibleFromClient) {if (!a.mWindowAdded) {a.mWindowAdded = true;wm.addView(decor, l);} }}}

WindowManagerImpl 添加时候 又会调用 全局单例WindowManagerGlobal 添加

public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {applyTokens(params);mGlobal.addView(view, params, mContext.getDisplayNoVerify(), mParentWindow,mContext.getUserId());}

addView中 会调用

1. 创建对应的 ViewRootImpl
2. 将对应参数添加到各自的数组中
3. 调用 ViewRootImpl 的 setVIew 开始执行添加视图的 3步曲 （addToDisplayAsUser, relayout, finishDraw）

 public void addView(View view, ViewGroup.LayoutParams params,Display display, Window parentWindow, int userId) {if (view == null) {throw new IllegalArgumentException("view must not be null");}if (display == null) {throw new IllegalArgumentException("display must not be null");}if (!(params instanceof WindowManager.LayoutParams)) {throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");}final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;// 如果不为空则认为子窗口添加，通过 adjustLayoutParamsForSubWindow 补全对应的参数 如title 和 tokenif (parentWindow != null) {parentWindow.adjustLayoutParamsForSubWindow(wparams);} else {// If there's no parent, then hardware acceleration for this view is// set from the application's hardware acceleration setting.final Context context = view.getContext();if (context != null&& (context.getApplicationInfo().flags& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;}}// 创建对应的 ViewRootImplViewRootImpl root;View panelParentView = null;synchronized (mLock) {// Start watching for system property changes.if (mSystemPropertyUpdater == null) {mSystemPropertyUpdater = new Runnable() {@Override public void run() {synchronized (mLock) {for (int i = mRoots.size() - 1; i >= 0; --i) {mRoots.get(i).loadSystemProperties();}}}};SystemProperties.addChangeCallback(mSystemPropertyUpdater);}// 从 mViews 获取该view是否已添加int index = findViewLocked(view, false);if (index >= 0) {// 判断该view是否在移除if (mDyingViews.contains(view)) {// Don't wait for MSG_DIE to make it's way through root's queue.mRoots.get(index).doDie();} else {throw new IllegalStateException("View " + view+ " has already been added to the window manager.");}// The previous removeView() had not completed executing. Now it has.}// If this is a panel window, then find the window it is being// attached to for future reference.if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {final int count = mViews.size();for (int i = 0; i < count; i++) {if (mRoots.get(i).mWindow.asBinder() == wparams.token) {panelParentView = mViews.get(i);}}}// 创建viewRootImpl，该方法会通过 WindowManagerGlobal.getWindowSession() 获取sessionroot = new ViewRootImpl(view.getContext(), display);view.setLayoutParams(wparams);// 添加到对应的数组mViews.add(view);mRoots.add(root);mParams.add(wparams);// do this last because it fires off messages to start doing thingstry {root.setView(view, wparams, panelParentView, userId);} catch (RuntimeException e) {// BadTokenException or InvalidDisplayException, clean up.if (index >= 0) {removeViewLocked(index, true);}throw e;}}}

这样完成了Activity View的加载。