Window和WindowManager(Android开发艺术探索学习笔记)

概述

Window表示一个窗口的概念，它是一个抽象类，它的具体实现类是PhoneWindow。

WindowManager是外界访问Window的入口，Window的具体实现位于WindowManagerService中。

WindowManager和WindowManagerService的交互其实是一个IPC过程。Android中所有视图都是通过Window来呈现的，无论是Activity，Dialog还是Toast，它们的视图都是附加在Window上的，因此Window是View的直接管理者。从事件分发的流程可以知道，触摸事件是由Window传递给DecorView的，Activity的setContentView()在底层也是通过Window来完成的。

WindowManager.LayoutParams

为了分析Window的工作机制，我们先了解WindowManager.LayoutParams，如下：

            mWindowManager = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
            mFloatingButton = new Button(this);
            mFloatingButton.setText("click me");
            mLayoutParams = new WindowManager.LayoutParams(
                    LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT, 0, 0,
                    PixelFormat.TRANSPARENT);
            mLayoutParams.flags = LayoutParams.FLAG_NOT_TOUCH_MODAL
                    | LayoutParams.FLAG_NOT_FOCUSABLE
                    | LayoutParams.FLAG_SHOW_WHEN_LOCKED;
            mLayoutParams.type = LayoutParams.TYPE_SYSTEM_ERROR;
            mLayoutParams.gravity = Gravity.LEFT | Gravity.TOP;
            mLayoutParams.x = 100;
            mLayoutParams.y = 300;
            mFloatingButton.setOnTouchListener(this);
            mWindowManager.addView(mFloatingButton, mLayoutParams);

上述代码将一个Button添加到频幕坐标(100,300)的位置上。其中WindowManager.LayoutParams的两个参数flags和type比较重要。

flag

flag表示Window的属性，可以控制Window的显示特性，下面介绍比较常用的几个：

• FLAG_NOT_FOCUSABLE
  设置之后Window不会获取焦点，也不会接收各种输入事件，最终事件会传递给在其下面的可获取焦点的Window，这个flag同时会启用 FLAG_NOT_TOUCH_MODAL flag。

• FLAG_NOT_TOUCH_MODAL
  这个flag简而言之就是说，当前Window区域以外的点击事件传递给下层Window，当前Window区域以内的点击事件自己处理。

• FLAG_SHOW_WHEN_LOCKED
  一个特殊的flag，使得Window可以在锁屏状态下显示，它使得Window比KeyGuard或其他锁屏界面具有更高的层级。

type

type表示Window的类型，Window有三种类型，分别是应用Window，子Window和系统Window。

应用类Window对应着一个Activity。子Window不能单独存在，它需要附属在特定的父Window中，比如Dialog就是一个子Window。系统Window是需要声明权限才能创建的Window，比如Toast和系统状态栏这些都是系统Window。

Window是分层的，每个Window都有对应的z-ordered，层级大的会覆盖在层级小的Window上。在三类Window中，应用Window的层级范围是1~99，子Window的层级范围是1000~1999，系统Window的层级范围是2000~2999。很显然系统Window的层级是最大的，而且系统层级有很多值，一般我们可以选用TYPE_SYSTEM_ERROR或者TYPE_SYSTEM_OVERLAY，另外重要的是要记得在清单文件中声明权限。

Window的内部机制

Window是一个抽象的概念，每一个Window都对应着一个View和一个ViewRootImpl，Window和View通过ViewRootImpl来建立联系，因此Window并不是实际存在的，它是以View的形式存在，这点从WindowManager的定义可以看出。WindowManager所提供的功能很简单，常用的只有三个方法，即添加View、更新View和删除View，这三个方法定义在ViewManager中，而WindowManager继承了ViewManager。

public interface ViewManager
{
    /**
     * Assign the passed LayoutParams to the passed View and add the view to the window.
     * 
Throws {@link android.view.WindowManager.BadTokenException} for certain programming
     * errors, such as adding a second view to a window without removing the first view.
     * 
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);
}

Window的添加过程

Window的添加过程通过WindowManager的addView来实现，WindowManager是一个接口，它的真正实现类是WindowManagerImpl。然而WindowManagerImpl并没有直接实现Window的三大操作，而是全部交给WindowManagerGlobal来处理：

    @Override
    public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mDisplay, mParentWindow);
    }

    @Override
    public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.updateViewLayout(view, params);
    }

    @Override
    public void removeView(View view) {
        mGlobal.removeView(view, false);
    }

那WindowManagerGlobal是什么呢？
首先看到WindowManagerImpl中：

    private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();

然后再看WindowManagerGlobal中：

    public static WindowManagerGlobal getInstance() {
        synchronized (WindowManagerGlobal.class) {
            if (sDefaultWindowManager == null) {
                sDefaultWindowManager = new WindowManagerGlobal();
            }
            return sDefaultWindowManager;
        }
    }

可以看到WindowManagerGlobal中获取实例的方法是单例模式，所以其实多个WindowManagerImpl拥有同一个WindowManagerGlobal。

WindowManagerImpl这种工作模式是典型的桥接模式，将所有的操作委托给WindowManagerGlobal来实现。

WindowManagerGlobal的addView()：

    public void addView(View view, ViewGroup.LayoutParams params,
            Display display, Window parentWindow) {
        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;
        if (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;
            }
        }

        ViewRootImpl 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);
            }

            int index = findViewLocked(view, false);
            if (index >= 0) {
                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);
                    }
                }
            }

            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 {
            root.setView(view, wparams, panelParentView);
        } catch (RuntimeException e) {
            // BadTokenException or InvalidDisplayException, clean up.
            synchronized (mLock) {
                final int index = findViewLocked(view, false);
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
            }
            throw e;
        }
    }

其中第一步对参数的合法性进行了校验，如果是子Window还需要调整布局参数。

第二步创建ViewRootImpl并将信息存储到集合中：

    private final ArrayList mViews = new ArrayList();
    private final ArrayList mRoots = new ArrayList();
    private final ArrayList mParams =
            new ArrayList();

我们看到这是WindowManagerGlobal中对三个集合的声明，mViews 存储的是所有Window对应的View，mRoots 存储的是所有Window对应的ViewRootImpl，mParams 存储的是所有Window对应的布局参数。

第三步通过ViewRootImpl来更新界面并完成Window的添加。这个步骤由ViewRootImpl的setView()来完成，setView()内部调用requestLayout()来完成异步刷新请求。接着会通过WindowSession最终来完成Window的添加过程，如下是setView()的部分代码:

                try {
                    mOrigWindowType = mWindowAttributes.type;
                    mAttachInfo.mRecomputeGlobalAttributes = true;
                    collectViewAttributes();
                    res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
                            getHostVisibility(), mDisplay.getDisplayId(),
                            mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
                            mAttachInfo.mOutsets, mInputChannel);
                } catch (RemoteException e) {
                    mAdded = false;
                    mView = null;
                    mAttachInfo.mRootView = null;
                    mInputChannel = null;
                    mFallbackEventHandler.setView(null);
                    unscheduleTraversals();
                    setAccessibilityFocus(null, null);
                    throw new RuntimeException("Adding window failed", e);
                } finally {
                    if (restore) {
                        attrs.restore();
                    }
                }

mWindowSession的类型是IWindowSession，它是一个Binder对象，真正的实现类是Session，也就是Window的添加过程是一次IPC调用。

Session内部通过WindowManagerService来实现Window的添加，代码如下：

    @Override
    public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
            int viewVisibility, int displayId, Rect outContentInsets, Rect outStableInsets,
            Rect outOutsets, InputChannel outInputChannel) {
        return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId,
                outContentInsets, outStableInsets, outOutsets, outInputChannel);
    }

如此一来，Window的添加请求就交给WindowManagerService去处理了，在WindowManagerService内部会为每一个应用保留一个单独的Session。具体Window在WindowManagerService内部怎么添加的，这里就不做分析了。

Window的删除过程

Window的删除过程和添加过程一样，通过WindowManagerImpl调用WindowManagerGlobal的removeView()来实现：

    public void removeView(View view, boolean immediate) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }

        synchronized (mLock) {
            int index = findViewLocked(view, true);
            View curView = mRoots.get(index).getView();
            removeViewLocked(index, immediate);
            if (curView == view) {
                return;
            }

            throw new IllegalStateException("Calling with view " + view
                    + " but the ViewAncestor is attached to " + curView);
        }
    }

首先通过findViewLocked()来查找待删除的View的索引，然后再调用removeViewLocked()来做进一步的删除，如下：

    private void removeViewLocked(int index, boolean immediate) {
        ViewRootImpl root = mRoots.get(index);
        View view = root.getView();

        if (view != null) {
            InputMethodManager imm = InputMethodManager.getInstance();
            if (imm != null) {
                imm.windowDismissed(mViews.get(index).getWindowToken());
            }
        }
        boolean deferred = root.die(immediate);
        if (view != null) {
            view.assignParent(null);
            if (deferred) {
                mDyingViews.add(view);
            }
        }
    }

removeViewLocked()是通过ViewRootImpl来完成删除操作的。WindowManager中提供了两种删除接口，removeView()和removeViewImmediate()，他们分表表示异步删除和同步删除。具体的删除操作由ViewRootImpl的die()来完成：

    boolean die(boolean immediate) {
        // Make sure we do execute immediately if we are in the middle of a traversal or the damage
        // done by dispatchDetachedFromWindow will cause havoc on return.
        if (immediate && !mIsInTraversal) {
            doDie();
            return false;
        }

        if (!mIsDrawing) {
            destroyHardwareRenderer();
        } else {
            Log.e(TAG, "Attempting to destroy the window while drawing!\n" +
                    "  window=" + this + ", title=" + mWindowAttributes.getTitle());
        }
        mHandler.sendEmptyMessage(MSG_DIE);
        return true;
    }

在异步删除的情况下，die()只是发送了一个请求删除的消息就返回了，这时候View还没有完成删除操作，所以最后将它添加到mDyingViews中，mDyingViews表示待删除的View的集合。如果是同步删除，不发送消息就直接调用dodie()：

    void doDie() {
        checkThread();
        if (LOCAL_LOGV) Log.v(TAG, "DIE in " + this + " of " + mSurface);
        synchronized (this) {
            if (mRemoved) {
                return;
            }
            mRemoved = true;
            if (mAdded) {
                dispatchDetachedFromWindow();
            }

            if (mAdded && !mFirst) {
                destroyHardwareRenderer();

                if (mView != null) {
                    int viewVisibility = mView.getVisibility();
                    boolean viewVisibilityChanged = mViewVisibility != viewVisibility;
                    if (mWindowAttributesChanged || viewVisibilityChanged) {
                        // If layout params have been changed, first give them
                        // to the window manager to make sure it has the correct
                        // animation info.
                        try {
                            if ((relayoutWindow(mWindowAttributes, viewVisibility, false)
                                    & WindowManagerGlobal.RELAYOUT_RES_FIRST_TIME) != 0) {
                                mWindowSession.finishDrawing(mWindow);
                            }
                        } catch (RemoteException e) {
                        }
                    }

                    mSurface.release();
                }
            }

            mAdded = false;
        }
        WindowManagerGlobal.getInstance().doRemoveView(this);
    }

在dodie()内部会调用dispatchDetachedFromWindow()，真正删除View的逻辑就在dispatchDetachedFromWindow()中：

    void dispatchDetachedFromWindow() {
        if (mView != null && mView.mAttachInfo != null) {
            mAttachInfo.mTreeObserver.dispatchOnWindowAttachedChange(false);
            mView.dispatchDetachedFromWindow();
        }

        mAccessibilityInteractionConnectionManager.ensureNoConnection();
        mAccessibilityManager.removeAccessibilityStateChangeListener(
                mAccessibilityInteractionConnectionManager);
        mAccessibilityManager.removeHighTextContrastStateChangeListener(
                mHighContrastTextManager);
        removeSendWindowContentChangedCallback();

        destroyHardwareRenderer();

        setAccessibilityFocus(null, null);

        mView.assignParent(null);
        mView = null;
        mAttachInfo.mRootView = null;

        mSurface.release();

        if (mInputQueueCallback != null && mInputQueue != null) {
            mInputQueueCallback.onInputQueueDestroyed(mInputQueue);
            mInputQueue.dispose();
            mInputQueueCallback = null;
            mInputQueue = null;
        }
        if (mInputEventReceiver != null) {
            mInputEventReceiver.dispose();
            mInputEventReceiver = null;
        }
        try {
            mWindowSession.remove(mWindow);
        } catch (RemoteException e) {
        }

        // Dispose the input channel after removing the window so the Window Manager
        // doesn't interpret the input channel being closed as an abnormal termination.
        if (mInputChannel != null) {
            mInputChannel.dispose();
            mInputChannel = null;
        }

        mDisplayManager.unregisterDisplayListener(mDisplayListener);

        unscheduleTraversals();
    }

dispatchDetachedFromWindow()主要做几件事情：

• 垃圾回收相关操作，比如清除数据和消息，移除回调。
• 通过Session的remove()删除Window，这同样是一个IPC过程，最终会调用WindowManagerService的removeWindow()。
• 调用View的dispatchDetachedFromWindow()，进而调用View的onDetachedFromWindow()，onDetachedFromWindowInternal()。
  onDetachedFromWindow()对于大家来说一定不陌生，我们可以在这个方法内部做一些资源回收工作，比如终止动画、停止线程等。

最后再调用WindowManagerGlobal的doRemoveView()方法刷新数据，包括mRoots、mParams、mViews和mDyingViews，将当前Window所关联的对象从集合中删除。

Window的更新过程

首先看WindowManagerGlobal的updateViewLayout()：

    public void updateViewLayout(View view, ViewGroup.LayoutParams params) {
        if (view == null) {
            throw new IllegalArgumentException("view must not be null");
        }
        if (!(params instanceof WindowManager.LayoutParams)) {
            throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
        }

        final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams)params;

        view.setLayoutParams(wparams);

        synchronized (mLock) {
            int index = findViewLocked(view, true);
            ViewRootImpl root = mRoots.get(index);
            mParams.remove(index);
            mParams.add(index, wparams);
            root.setLayoutParams(wparams, false);
        }
    }

updateViewLayout()做的事情就比较简单了，首先先更新View的LayoutParams，接着更新ViewRootImpl的LayoutParams。ViewRootImpl会在setLayoutParams()中调用scheduleTraversals()来对View重新布局重绘。除此之外ViewRootImpl还会通过WindowSession来更新Window的视图，这个过程最终由WindowManagerService的relayoutWindow()来具体实现，这同样是一个IPC过程。

Window的创建过程

Activity的Window创建过程

见Activity的Window创建及DecorView的添加(Android开发艺术探索学习笔记)

Dialog的Window创建过程

Dialog的Window创建过程和Activity的类似。
首先创建Window，通过Dialog的构造方法：

    Dialog(@NonNull Context context, @StyleRes int themeResId, boolean createContextThemeWrapper) {
        if (createContextThemeWrapper) {
            if (themeResId == 0) {
                final TypedValue outValue = new TypedValue();
                context.getTheme().resolveAttribute(R.attr.dialogTheme, outValue, true);
                themeResId = outValue.resourceId;
            }
            mContext = new ContextThemeWrapper(context, themeResId);
        } else {
            mContext = context;
        }

        mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);

        final Window w = new PhoneWindow(mContext);
        mWindow = w;
        w.setCallback(this);
        w.setOnWindowDismissedCallback(this);
        w.setWindowManager(mWindowManager, null, null);
        w.setGravity(Gravity.CENTER);

        mListenersHandler = new ListenersHandler(this);
    }

然后初始化DecorView，并将Dialog的视图添加到DecorView中：

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

最后将DecorView添加到Window中并显示：

    public void show() {
            ……
            mWindowManager.addView(mDecor, l);
            mShowing = true;
            ……     
    }

从上面三个步骤，可以看出Dialog的Window创建过程和Activity的基本一样。

普通Dialog有一个特殊之处，那就是构造方法必须传入Activity的Context，如果采用Application的就会报错，因为需要应用的token。

除此之外，系统的Window比较特殊，可以不需要应用的token。所以要将Dialog对应的Window指定为系统层级：

        dialog.getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ERROR);

并且在清单文件中声明权限：

    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>

Toast的Window创建过程

Toast和Dialog不同，它的工作过程稍显复杂。首先Toast也是基于Window来实现的，但是由于Toast具有定时取消这一功能，所以系统采用了Handler。在Toast内部有两个IPC过程，一个是Toast访问NotificationManagerService，第二个是NotificationManagerService回调Toast里的TN接口。
Toast提供了show()和cancel()分别用来显示和隐藏Toast，它们内部是一个IPC过程：

    public void show() {
        if (mNextView == null) {
            throw new RuntimeException("setView must have been called");
        }

        INotificationManager service = getService();
        String pkg = mContext.getOpPackageName();
        TN tn = mTN;
        tn.mNextView = mNextView;

        try {
            service.enqueueToast(pkg, tn, mDuration);
        } catch (RemoteException e) {
            // Empty
        }
    }

    public void cancel() {
        mTN.hide();

        try {
            getService().cancelToast(mContext.getPackageName(), mTN);
        } catch (RemoteException e) {
            // Empty
        }
    }

从上面的代码来看，显示和隐藏Toast都要通过NMS来实现，由于NMS运行在系统进程中，所以只能通过远程调用的方式来显示和隐藏Toast。TN是一个Binder类，在Toast和NMS进行IPC的过程中，当MNS处理Toast的请求时会跨进程回调TN中的方法，这个时候由于TN运行在Binder线程池当中，所以需要通过Handler将其切换到当前线程中。注意由于使用了Handler，这意味着Toast无法在没有Looper的线程中弹出。

Toast的show()中调用了NMS的enqueueToast()，第一参数代表当前应用的包名，第二个参数代表远程回调，第三个参数代表Toast显示的时长：

public void enqueueToast(String pkg, ITransientNotification callback, int duration)
        {
            if (DBG) {
                Slog.i(TAG, "enqueueToast pkg=" + pkg + " callback=" + callback
                        + " duration=" + duration);
            }

            if (pkg == null || callback == null) {
                Slog.e(TAG, "Not doing toast. pkg=" + pkg + " callback=" + callback);
                return ;
            }

            final boolean isSystemToast = isCallerSystem() || ("android".equals(pkg));

            if (ENABLE_BLOCKED_TOASTS && !noteNotificationOp(pkg, Binder.getCallingUid())) {
                if (!isSystemToast) {
                    Slog.e(TAG, "Suppressing toast from package " + pkg + " by user request.");
                    return;
                }
            }

            synchronized (mToastQueue) {
                int callingPid = Binder.getCallingPid();
                long callingId = Binder.clearCallingIdentity();
                try {
                    ToastRecord record;
                    int index = indexOfToastLocked(pkg, callback);
                    // If it's already in the queue, we update it in place, we don't
                    // move it to the end of the queue.
                    if (index >= 0) {
                        record = mToastQueue.get(index);
                        record.update(duration);
                    } else {
                        // Limit the number of toasts that any given package except the android
                        // package can enqueue.  Prevents DOS attacks and deals with leaks.
                        if (!isSystemToast) {
                            int count = 0;
                            final int N = mToastQueue.size();
                            for (int i=0; ifinal ToastRecord r = mToastQueue.get(i);
                                 if (r.pkg.equals(pkg)) {
                                     count++;
                                     if (count >= MAX_PACKAGE_NOTIFICATIONS) {
                                         Slog.e(TAG, "Package has already posted " + count
                                                + " toasts. Not showing more. Package=" + pkg);
                                         return;
                                     }
                                 }
                            }
                        }

                        record = new ToastRecord(callingPid, pkg, callback, duration);
                        mToastQueue.add(record);
                        index = mToastQueue.size() - 1;
                        keepProcessAliveLocked(callingPid);
                    }
                    // If it's at index 0, it's the current toast.  It doesn't matter if it's
                    // new or just been updated.  Call back and tell it to show itself.
                    // If the callback fails, this will remove it from the list, so don't
                    // assume that it's valid after this.
                    if (index == 0) {
                        showNextToastLocked();
                    }
                } finally {
                    Binder.restoreCallingIdentity(callingId);
                }
            }
        }

在enqueueToast()中，Toast请求被封装成ToastRecord对象并被添加到一个名为mToastQueue的队列中。并且mToastQueue最多能存下50个ToastRecord，这是由MAX_PACKAGE_NOTIFICATIONS决定的，防止破坏性的连续请求。之后通过showNextToastLocked()来显示当前的Toast：

    void showNextToastLocked() {
        ToastRecord record = mToastQueue.get(0);
        while (record != null) {
            if (DBG) Slog.d(TAG, "Show pkg=" + record.pkg + " callback=" + record.callback);
            try {
                record.callback.show();
                scheduleTimeoutLocked(record);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Object died trying to show notification " + record.callback
                        + " in package " + record.pkg);
                // remove it from the list and let the process die
                int index = mToastQueue.indexOf(record);
                if (index >= 0) {
                    mToastQueue.remove(index);
                }
                keepProcessAliveLocked(record.pid);
                if (mToastQueue.size() > 0) {
                    record = mToastQueue.get(0);
                } else {
                    record = null;
                }
            }
        }
    }

从以上代码可以看到，Toast的显示是由ToastRecord的callback来完成的，这个callback实际上就是Toast中的TN对象的远程Binder，通过callback访问TN中的方法需要跨进程来完成。Toast显示以后，NMS还会通过scheduleTimeoutLocked()来发送一个延时消息，具体的延时取决于Toast的时长：

    private void scheduleTimeoutLocked(ToastRecord r)
    {
        mHandler.removeCallbacksAndMessages(r);
        Message m = Message.obtain(mHandler, MESSAGE_TIMEOUT, r);
        long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;
        mHandler.sendMessageDelayed(m, delay);
    }

NMS会通过cancelToastLocked()方法来隐藏Toast并将对应的ToastRecord从mToastQueue中移除。Toast的隐藏也是通过ToastRecord的callback来完成的，同样和Toast的显示一样也是一次IPC过程。

通过上面的分析大家知道，Toast的显示和隐藏实际上是通过Toast中的TN这个类来实现的，它有两个方法show()和hide()：

        @Override
        public void show() {
            if (localLOGV) Log.v(TAG, "SHOW: " + this);
            mHandler.post(mShow);
        }

        /**
         * schedule handleHide into the right thread
         */
        @Override
        public void hide() {
            if (localLOGV) Log.v(TAG, "HIDE: " + this);
            mHandler.post(mHide);
        }

由于这两个方法被NMS以跨进程的方式调用，因此它们运行在Binder线程池中，为了执行切换到Toast发起请求的线程中，在它们的内部使用了Handler。

以上代码中mShow和mHide是两个Runnable，内部分别调用了handleShow()和handleHide()：

        public void handleShow() {
                ……
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                ……
                mWM.addView(mView, mParams);
                ……
        }

        public void handleHide() {
            if (localLOGV) Log.v(TAG, "HANDLE HIDE: " + this + " mView=" + mView);
            if (mView != null) {
                // note: checking parent() just to make sure the view has
                // been added...  i have seen cases where we get here when
                // the view isn't yet added, so let's try not to crash.
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }

                mView = null;
            }
        }

以上两个方法就是Toast正真完成显示和隐藏的地方。