Activity、View、Window、ViewRootImpl

Window是什么?

1.先看官方的定义：

/**
 * Abstract base class for a top-level window look and behavior policy.  An
 * instance of this class should be used as the top-level view added to the
 * window manager. It provides standard UI policies such as a background, title
 * area, default key processing, etc.
 *
 * <p>The only existing implementation of this abstract class is
 * android.view.PhoneWindow, which you should instantiate when needing a
 * Window.
 */
 一、作为顶层窗口的抽象类，并规范了窗口的外观与行为，同时也是顶层view的窗口管理器。
 二、提供诸如背景，标题、区域、默认事件处理等等。
 三、只有一个唯一实现类->PhoneWindow

2.文档解释比较简单，其实就是一个抽象的管理类。当我们启动一个Activity，生命周期开始时setContentView()，都知道这个方法要传入即是我们的布局ID：

public void setContentView(View view) {
     getWindow().setContentView(view);
     initWindowDecorActionBar();
 }

3.由于Window是抽象类，而PhoneWindow作为其唯一实现类，很好理解，这里即让Activity与Window(PhoneWindow)关联起来，这是第一步。
4.PhoneWindow中的成员变量DecorView，注释也给出了解释：This is the top-level view of the window, containing the window decor.。DecorView继承自FrameLayout，本质就是一个ViewGroup。为什么需要这个DecorView？往前回忆setContentView()这个方法，在PhoneWindow的重写方法中对DecorView的初始化方法installDecor()，在我们开发过程中，安卓系统默认为我们提供了一些Theme主题，不同的主题对应了不同的资源布局文件。在初始化时，通过addView为DecorView中加入了一个view root并且这个root包含的就是开发者设置的主题信息对应的系统布局。root是一个线性布局，包含了actionbar与一个FrameLayout，这个就是包含我们自身布局的最终容器。到这里Window就和View关联起来了。
5.总结一下：activity中调用的setContenView将window与activity相关联，而phonewindow作为window的唯一实现类，每个activity中包含一个phonewindow，phonewindow中又持有了decorview的实例。decorview作为一个viewgroup，通过addview添加了一个view(root)，最终我们设置的布局会被添加到这个root中的fagmelayout中。这样，activity就与view相关联了起来。window作为管理器的角色。包括view的添加移除，但是这些操作要借助另一个重要的类windowManager。

Window是否多余？

1.经过分析，发现window所包含的一些作用其实view基本都已经涵盖了，那么这个window是否多余呢？答案是否定的，类比MVC,MVP,MVVM等设计模式，window的作用也很类似，使activity与view解耦。view本身代码量很大，处理的事情很多。另外window的添加涉及到IPC的调用处理，再来看看WindowManager这个接口，继承了viewmanager：

/** Interface to let you add and remove child views to an Activity. To get an instance
  * of this class, call {@link android.content.Context#getSystemService(java.lang.String) Context.getSystemService()}.
  */
  //可见windowmanager功能很明确，就是对activity中的view进行添加和移除操作，windowmanager的实现类为WindowManagerImpl
public interface ViewManager
{
    /**
     * Assign the passed LayoutParams to the passed View and add the view to the window.
     * <p>Throws {@link android.view.WindowManager.BadTokenException} for certain programming
     * errors, such as adding a second view to a window without removing the first view.
     * <p>Throws {@link android.view.WindowManager.InvalidDisplayException} if the window is on a
     * secondary {@link Display} and the specified display can't be found
     * (see {@link android.app.Presentation}).
     * @param view The view to be added to this window.
     * @param params The LayoutParams to assign to view.
     */
    public void addView(View view, ViewGroup.LayoutParams params);
    public void updateViewLayout(View view, ViewGroup.LayoutParams params);
    public void removeView(View view);
}

Dialog的context

1.看一段代码：

Dialog dialog = new Dialog(getApplicationContext());
 dialog.show();
 运行之后报出的错误：
 Caused by: android.view.WindowManager$BadTokenException: Unable to add window -- token null is not valid; is your activity running?
 因为我们传入的是getApplicationContext(),那么为什么getApplicationContext不行呢？

2.看看dialog的构造方法：

Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
     if (createContextThemeWrapper) {
         if (themeResId == Resources.ID_NULL) {
             final TypedValue outValue = new TypedValue();
             context.getTheme().resolveAttribute(R.attr.dialogTheme, outValue, true);
             themeResId = outValue.resourceId;
         }
         mContext = new ContextThemeWrapper(context, themeResId);
     } else {
         mContext = context;
     }

     //关键代码，activity的context获得的是activity中windowmanager，而application获得的是一个新对象-->WindowManagerImpl
     mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);

     final Window w = new PhoneWindow(mContext);
     mWindow = w;
     w.setCallback(this);
     w.setOnWindowDismissedCallback(this);
     w.setOnWindowSwipeDismissedCallback(() -> {
         if (mCancelable) {
             cancel();
         }
     });
     w.setWindowManager(mWindowManager, null, null);
     w.setGravity(Gravity.CENTER);

     mListenersHandler = new ListenersHandler(this);
 }

3.mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);假设我们传入的是activity的context，那么得到的是activity的windowManager：

@Override
 public Object getSystemService(@ServiceName @NonNull String name) {
     if (getBaseContext() == null) {
         throw new IllegalStateException(
                 "System services not available to Activities before onCreate()");
     }

     if (WINDOW_SERVICE.equals(name)) {
         return mWindowManager;
     } else if (SEARCH_SERVICE.equals(name)) {
         ensureSearchManager();
         return mSearchManager;
     }
     return super.getSystemService(name);
 }

看windowManager的实现类WindowManagerImpl，其中包含成员变量private IBinder mDefaultToken;很明显这个token指向的就是我们的activity，因此传入activity而实例化的dialog自然不会报token null is not valid这个错误。
4.现在看dialog中的show方法，解释为什么application类型的context会报错：

public void show() {
        if (mShowing) {
            if (mDecor != null) {
                if (mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
                    mWindow.invalidatePanelMenu(Window.FEATURE_ACTION_BAR);
                }
                mDecor.setVisibility(View.VISIBLE);
            }
            return;
        }

        mCanceled = false;

        if (!mCreated) {
            dispatchOnCreate(null);
        } else {
            // Fill the DecorView in on any configuration changes that
            // may have occured while it was removed from the WindowManager.
            final Configuration config = mContext.getResources().getConfiguration();
            mWindow.getDecorView().dispatchConfigurationChanged(config);
        }

        onStart();
        mDecor = mWindow.getDecorView();

        if (mActionBar == null && mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
            final ApplicationInfo info = mContext.getApplicationInfo();
            mWindow.setDefaultIcon(info.icon);
            mWindow.setDefaultLogo(info.logo);
            mActionBar = new WindowDecorActionBar(this);
        }

        WindowManager.LayoutParams l = mWindow.getAttributes();
        boolean restoreSoftInputMode = false;
        if ((l.softInputMode
                & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) == 0) {
            l.softInputMode |=
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
            restoreSoftInputMode = true;
        }

        //上面经过分析得出，如果是application或者service的context，获得了新对象windowmanagerimpl，此时mDefaultToken为null
        mWindowManager.addView(mDecor, l);
        if (restoreSoftInputMode) {
            l.softInputMode &=
                    ~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
        }
        mShowing = true;
        sendShowMessage();
    }

5.看windowmanagerimpl中两个关键方法：

private IBinder mDefaultToken;

/**
 * Sets the window token to assign when none is specified by the client or
 * available from the parent window.
 *
 * @param token The default token to assign.
 */
public void setDefaultToken(IBinder token) {
    mDefaultToken = token;
}

@Override
 public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
   //为token赋值，由于基于application的context创建的，所以token还是null，也就是为什么报了viewrootimpl中错误
   //Unable to add window --tokenis not valid; is your activity running?"
     applyDefaultToken(params);
     mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
 }

 private void applyDefaultToken(@NonNull ViewGroup.LayoutParams params) {
       // Only use the default token if we don't have a parent window.
       if (mDefaultToken != null && mParentWindow == null) {
           if (!(params instanceof WindowManager.LayoutParams)) {
               throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
           }
           // Only use the default token if we don't already have a token.
           final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
           if (wparams.token == null) {
               wparams.token = mDefaultToken;
           }
       }
   }

系统电量低“弹窗”？

1.作为一个系统的全局“弹窗”，如果真的是dialog，那么上述分析就是错误，因为这个“弹窗”不依赖任何activity。下面是构造方法：

@Inject
 public PowerNotificationWarnings(Context context, ActivityStarter activityStarter) {
   //原来是个通知
     mContext = context;
     mNoMan = mContext.getSystemService(NotificationManager.class);
     mPowerMan = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
     mKeyguard = mContext.getSystemService(KeyguardManager.class);
     mReceiver.init();
     mActivityStarter = activityStarter;
 }