Android Toast源码分析

前言

    这周去杭州参加了百阿培训,见到了传说中的牛人多隆大神。从多隆大神身上看到了做技术人的纯粹,单纯。除了见到多隆大神,这次培训并没有太多的收获,反而培训过程中遇到了好多产品上的Bug,远程办公快累到死。总结一下跟Toast相关的问题,首先从深入学习Toast的源码实现开始。

Toast源码实现

Toast入口

    我们在应用中使用Toast提示的时候,一般都是一行简单的代码调用,如下所示:

Toast.makeText(context, msg, Toast.LENGTH_SHORT).show();

    makeText就是Toast的入口,我们从makeText的源码来深入理解Toast的实现。源码如下(frameworks/base/core/java/android/widget/Toast.java):

    public static Toast makeText(Context context, CharSequence text, int duration) {
        Toast result = new Toast(context);

        LayoutInflater inflate = (LayoutInflater)
                context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View v = inflate.inflate(com.android.internal.R.layout.transient_notification, null);
        TextView tv = (TextView)v.findViewById(com.android.internal.R.id.message);
        tv.setText(text);
        
        result.mNextView = v;
        result.mDuration = duration;

        return result;
    }

    从makeText的源码里,我们可以看出Toast的布局文件是transient_notification.xml,位于frameworks/base/core/res/res/layout/transient_notification.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:background="?android:attr/toastFrameBackground">

    <TextView
        android:id="@android:id/message"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        android:layout_gravity="center_horizontal"
        android:textAppearance="@style/TextAppearance.Toast"
        android:textColor="@color/bright_foreground_dark"
        android:shadowColor="#BB000000"
        android:shadowRadius="2.75"
        />

</LinearLayout>

    系统Toast的布局文件非常简单,就是在垂直布局的LinearLayout里放置了一个TextView。接下来,我们继续跟到show()方法,研究一下布局形成之后的展示代码实现:

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

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

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

    show方法中有两点是需要我们注意的。(1)TN是什么东东?(2)INotificationManager服务的作用。带着这两个问题,继续我们Toast源码的探索。

TN源码

    很多问题都能通过阅读源码找到答案,关键在与你是否有与之匹配的耐心和坚持。mTN的实现在Toast的构造函数中,源码如下:

    public Toast(Context context) {
        mContext = context;
        mTN = new TN();
        mTN.mY = context.getResources().getDimensionPixelSize(
                com.android.internal.R.dimen.toast_y_offset);
        mTN.mGravity = context.getResources().getInteger(
                com.android.internal.R.integer.config_toastDefaultGravity);
    }

    接下来,我们就从TN类的源码出发,探寻TN的作用。TN源码如下:

    private static class TN extends ITransientNotification.Stub {
        final Runnable mShow = new Runnable() {
            @Override
            public void run() {
                handleShow();
            }
        };

        final Runnable mHide = new Runnable() {
            @Override
            public void run() {
                handleHide();
                // Don't do this in handleHide() because it is also invoked by handleShow()
                mNextView = null;
            }
        };

        private final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams();
        final Handler mHandler = new Handler();    

        int mGravity;
        int mX, mY;
        float mHorizontalMargin;
        float mVerticalMargin;


        View mView;
        View mNextView;

        WindowManager mWM;

        TN() {
            // XXX This should be changed to use a Dialog, with a Theme.Toast
            // defined that sets up the layout params appropriately.
            final WindowManager.LayoutParams params = mParams;
            params.height = WindowManager.LayoutParams.WRAP_CONTENT;
            params.width = WindowManager.LayoutParams.WRAP_CONTENT;
            params.format = PixelFormat.TRANSLUCENT;
            params.windowAnimations = com.android.internal.R.style.Animation_Toast;
            params.type = WindowManager.LayoutParams.TYPE_TOAST;
            params.setTitle("Toast");
            params.flags = WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON
                    | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE;
            /// M: [ALPS00517576] Support multi-user
            params.privateFlags = WindowManager.LayoutParams.PRIVATE_FLAG_SHOW_FOR_ALL_USERS;
        }

        /**
         * schedule handleShow into the right thread
         */
        @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);
        }

        public void handleShow() {
            if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
                    + " mNextView=" + mNextView);
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                Context context = mView.getContext().getApplicationContext();
                if (context == null) {
                    context = mView.getContext();
                }
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                // We can resolve the Gravity here by using the Locale for getting
                // the layout direction
                final Configuration config = mView.getContext().getResources().getConfiguration();
                final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection());
                mParams.gravity = gravity;
                if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
                    mParams.horizontalWeight = 1.0f;
                }
                if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
                    mParams.verticalWeight = 1.0f;
                }
                mParams.x = mX;
                mParams.y = mY;
                mParams.verticalMargin = mVerticalMargin;
                mParams.horizontalMargin = mHorizontalMargin;
                if (mView.getParent() != null) {
                    if (localLOGV) Log.v(TAG, "REMOVE! " + mView + " in " + this);
                    mWM.removeView(mView);
                }
                if (localLOGV) Log.v(TAG, "ADD! " + mView + " in " + this);
                mWM.addView(mView, mParams);
                trySendAccessibilityEvent();
            }
        }

        private void trySendAccessibilityEvent() {
            AccessibilityManager accessibilityManager =
                    AccessibilityManager.getInstance(mView.getContext());
            if (!accessibilityManager.isEnabled()) {
                return;
            }
            // treat toasts as notifications since they are used to
            // announce a transient piece of information to the user
            AccessibilityEvent event = AccessibilityEvent.obtain(
                    AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED);
            event.setClassName(getClass().getName());
            event.setPackageName(mView.getContext().getPackageName());
            mView.dispatchPopulateAccessibilityEvent(event);
            accessibilityManager.sendAccessibilityEvent(event);
        }        

        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;
            }
        }
    }

    通过源码,我们能很明显的看到继承关系,TN类继承自ITransientNotification.Stub,用于进程间通信。这里假设读者都有Android进程间通信的基础(不太熟的建议学习罗升阳关于Binder进程通信的一系列博客)。既然TN是用于进程间通信,那么我们很容易想到TN类的具体作用应该是Toast类的回调对象,其他进程通过调用TN类的具体对象来操作Toast的显示和消失。     TN类继承自ITransientNotification.Stub,ITransientNotification.aidl位于frameworks/base/core/java/android/app/ITransientNotification.aidl,源码如下:

package android.app;

/** @hide */
oneway interface ITransientNotification {
    void show();
    void hide();
}

    ITransientNotification定义了两个方法show()和hide(),它们的具体实现就在TN类当中。TN类的实现为:

        /**
         * schedule handleShow into the right thread
         */
        @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);
        }

    这里我们就能知道,Toast的show和hide方法实现是基于Handler机制。而TN类中的Handler实现是:

        final Handler mHandler = new Handler();    

    而且,我们在TN类中没有发现任何Looper.perpare()和Looper.loop()方法。说明,mHandler调用的是当前所在线程的Looper对象。所以,当我们在主线程(也就是UI线程中)可以随意调用Toast.makeText方法,因为Android系统帮我们实现了主线程的Looper初始化。但是,如果你想在子线程中调用Toast.makeText方法,就必须先进行Looper初始化了,不然就会报出
java.lang.RuntimeException: Can’t create handler inside thread that has not called Looper.prepare() 。Handler机制的学习可以参考我之前写过的一篇博客:http://blog.csdn.net/wzy_1988/article/details/38346637。     接下来,继续跟一下mShow和mHide的实现,它俩的类型都是Runnable。

        final Runnable mShow = new Runnable() {
            @Override
            public void run() {
                handleShow();
            }
        };

        final Runnable mHide = new Runnable() {
            @Override
            public void run() {
                handleHide();
                // Don't do this in handleHide() because it is also invoked by handleShow()
                mNextView = null;
            }
        };

    可以看到,show和hide的真正实现分别是调用了handleShow()和handleHide()方法。我们先来看handleShow()的具体实现:

        public void handleShow() {
            if (mView != mNextView) {
                // remove the old view if necessary
                handleHide();
                mView = mNextView;
                Context context = mView.getContext().getApplicationContext();
                if (context == null) {
                    context = mView.getContext();
                }
                mWM = (WindowManager)context.getSystemService(Context.WINDOW_SERVICE);
                // We can resolve the Gravity here by using the Locale for getting
                // the layout direction
                final Configuration config = mView.getContext().getResources().getConfiguration();
                final int gravity = Gravity.getAbsoluteGravity(mGravity, config.getLayoutDirection());
                mParams.gravity = gravity;
                if ((gravity & Gravity.HORIZONTAL_GRAVITY_MASK) == Gravity.FILL_HORIZONTAL) {
                    mParams.horizontalWeight = 1.0f;
                }
                if ((gravity & Gravity.VERTICAL_GRAVITY_MASK) == Gravity.FILL_VERTICAL) {
                    mParams.verticalWeight = 1.0f;
                }
                mParams.x = mX;
                mParams.y = mY;
                mParams.verticalMargin = mVerticalMargin;
                mParams.horizontalMargin = mHorizontalMargin;
                if (mView.getParent() != null) {
                    mWM.removeView(mView);
                }
                mWM.addView(mView, mParams);
                trySendAccessibilityEvent();
            }
        }

    从源码中,我们知道Toast是通过WindowManager调用addView加载进来的。因此,hide方法自然是WindowManager调用removeView方法来将Toast视图移除。     总结一下,通过对TN类的源码分析,我们知道了TN类是回调对象,其他进程调用tn类的show和hide方法来控制这个Toast的显示和消失。

NotificationManagerService

    回到Toast类的show方法中,我们可以看到,这里调用了getService得到INotificationManager服务,源码如下:

    private static INotificationManager sService;

    static private INotificationManager getService() {
        if (sService != null) {
            return sService;
        }
        sService = INotificationManager.Stub.asInterface(ServiceManager.getService("notification"));
        return sService;
    }

    得到INotificationManager服务后,调用了enqueueToast方法将当前的Toast放入到系统的Toast队列中。传的参数分别是pkg、tn和mDuration。也就是说,我们通过Toast.makeText(context, msg, Toast.LENGTH_SHOW).show()去呈现一个Toast,这个Toast并不是立刻显示在当前的window上,而是先进入系统的Toast队列中,然后系统调用回调对象tn的show和hide方法进行Toast的显示和隐藏。     这里INofiticationManager接口的具体实现类是NotificationManagerService类,位于frameworks/base/services/java/com/android/server/NotificationManagerService.java。     首先,我们来分析一下Toast入队的函数实现enqueueToast,源码如下:

    public void enqueueToast(String pkg, ITransientNotification callback, int duration)
    {
    	// packageName为null或者tn类为null,直接返回,不进队列
        if (pkg == null || callback == null) {
            return ;
        }

        // (1) 判断是否为系统Toast
        final boolean isSystemToast = isCallerSystem() || ("android".equals(pkg));

        // 判断当前toast所属的pkg是否为系统不允许发生Toast的pkg.NotificationManagerService有一个HashSet数据结构,存储了不允许发生Toast的包名
        if (ENABLE_BLOCKED_TOASTS && !noteNotificationOp(pkg, Binder.getCallingUid()) && !areNotificationsEnabledForPackageInt(pkg)) {
            if (!isSystemToast) {
                return;
            }
        }

        synchronized (mToastQueue) {
            int callingPid = Binder.getCallingPid();
            long callingId = Binder.clearCallingIdentity();
            try {
                ToastRecord record;
                // (2) 查看该Toast是否已经在队列当中
                int index = indexOfToastLocked(pkg, callback);
                // 如果Toast已经在队列中,我们只需要更新显示时间即可
                if (index >= 0) {
                    record = mToastQueue.get(index);
                    record.update(duration);
                } else {
                	// 非系统Toast,每个pkg在当前mToastQueue中Toast有总数限制,不能超过MAX_PACKAGE_NOTIFICATIONS
                    if (!isSystemToast) {
                        int count = 0;
                        final int N = mToastQueue.size();
                        for (int i=0; i<N; i++) {
                             final 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;
                                 }
                             }
                        }
                    }

                    // 将Toast封装成ToastRecord对象,放入mToastQueue中
                    record = new ToastRecord(callingPid, pkg, callback, duration);
                    mToastQueue.add(record);
                    index = mToastQueue.size() - 1;
                    // (3) 将当前Toast所在的进程设置为前台进程
                    keepProcessAliveLocked(callingPid);
                }
                // (4) 如果index为0,说明当前入队的Toast在队头,需要调用showNextToastLocked方法直接显示
                if (index == 0) {
                    showNextToastLocked();
                }
            } finally {
                Binder.restoreCallingIdentity(callingId);
            }
        }
    }

    可以看到,我对上述代码做了简要的注释。代码相对简单,但是还有4点标注代码需要我们来进一步探讨。     (1) 判断是否为系统Toast。如果当前Toast所属的进程的包名为“android”,则为系统Toast,否则还可以调用isCallerSystem()方法来判断。该方法的实现源码为:

    boolean isUidSystem(int uid) {
        final int appid = UserHandle.getAppId(uid);
        return (appid == Process.SYSTEM_UID || appid == Process.PHONE_UID || uid == 0);
    }
    boolean isCallerSystem() {
        return isUidSystem(Binder.getCallingUid());
    }

    isCallerSystem的源码也比较简单,就是判断当前Toast所属进程的uid是否为SYSTEM_UID、0、PHONE_UID中的一个,如果是,则为系统Toast;如果不是,则不为系统Toast。     是否为系统Toast,通过下面的源码阅读可知,主要有两点优势:

  1. 系统Toast一定可以进入到系统Toast队列中,不会被黑名单阻止。
  2. 系统Toast在系统Toast队列中没有数量限制,而普通pkg所发送的Toast在系统Toast队列中有数量限制。

    (2) 查看将要入队的Toast是否已经在系统Toast队列中。这是通过比对pkg和callback来实现的,具体源码如下所示:

    private int indexOfToastLocked(String pkg, ITransientNotification callback)
    {
        IBinder cbak = callback.asBinder();
        ArrayList<ToastRecord> list = mToastQueue;
        int len = list.size();
        for (int i=0; i<len; i++) {
            ToastRecord r = list.get(i);
            if (r.pkg.equals(pkg) && r.callback.asBinder() == cbak) {
                return i;
            }
        }
        return -1;
    }

    通过上述代码,我们可以得出一个结论,只要Toast的pkg名称和tn对象是一致的,则系统把这些Toast认为是同一个Toast。     (3) 将当前Toast所在进程设置为前台进程。源码如下所示:

    private void keepProcessAliveLocked(int pid)
    {
        int toastCount = 0; // toasts from this pid
        ArrayList<ToastRecord> list = mToastQueue;
        int N = list.size();
        for (int i=0; i<N; i++) {
            ToastRecord r = list.get(i);
            if (r.pid == pid) {
                toastCount++;
            }
        }
        try {
            mAm.setProcessForeground(mForegroundToken, pid, toastCount > 0);
        } catch (RemoteException e) {
            // Shouldn't happen.
        }
    }

    这里的mAm=ActivityManagerNative.getDefault(),调用了setProcessForeground方法将当前pid的进程置为前台进程,保证不会系统杀死。这也就解释了为什么当我们finish当前Activity时,Toast还可以显示,因为当前进程还在执行。     (4) index为0时,对队列头的Toast进行显示。源码如下:

    private void showNextToastLocked() {
    	// 获取队列头的ToastRecord
        ToastRecord record = mToastQueue.get(0);
        while (record != null) {
            try {
            	// 调用Toast的回调对象中的show方法对Toast进行展示
                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;
                }
            }
        }
    }

    这里Toast的回调对象callback就是tn对象。接下来,我们看一下,为什么系统Toast的显示时间只能是2s或者3.5s,关键在于scheduleTimeoutLocked方法的实现。原理是,调用tn的show方法展示完Toast之后,需要调用scheduleTimeoutLocked方法来将Toast消失。(
如果大家有疑问:不是说tn对象的hide方法来将Toast消失,为什么要在这里调用scheduleTimeoutLocked方法将Toast消失呢?是因为tn类的hide方法一执行,Toast立刻就消失了,而平时我们所使用的Toast都会在当前Activity停留几秒。如何实现停留几秒呢?原理就是scheduleTimeoutLocked发送MESSAGE_TIMEOUT消息去调用tn对象的hide方法,但是这个消息会有一个delay延迟,这里也是用了Handler消息机制)。

    private static final int LONG_DELAY = 3500; // 3.5 seconds
    private static final int SHORT_DELAY = 2000; // 2 seconds
    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);
    }

    首先,我们看到这里并不是直接发送了MESSAGE_TIMEOUT消息,而是有个delay的延迟。
而delay的时间从代码中“long delay = r.duration == Toast.LENGTH_LONG ? LONG_DELAY : SHORT_DELAY;”看出只能为2s或者3.5s,这也就解释了为什么系统Toast的呈现时间只能是2s或者3.5s。自己在Toast.makeText方法中随意传入一个duration是无作用的。     接下来,我们来看一下WorkerHandler中是如何处理MESSAGE_TIMEOUT消息的。mHandler对象的类型为WorkerHandler,源码如下:

    private final class WorkerHandler extends Handler
    {
        @Override
        public void handleMessage(Message msg)
        {
            switch (msg.what)
            {
                case MESSAGE_TIMEOUT:
                    handleTimeout((ToastRecord)msg.obj);
                    break;
            }
        }
    }

    可以看到,WorkerHandler对MESSAGE_TIMEOUT类型的消息处理是调用了handlerTimeout方法,那我们继续跟踪handleTimeout源码:

    private void handleTimeout(ToastRecord record)
    {
        synchronized (mToastQueue) {
            int index = indexOfToastLocked(record.pkg, record.callback);
            if (index >= 0) {
                cancelToastLocked(index);
            }
        }
    }

    handleTimeout代码中,首先判断当前需要消失的Toast所属ToastRecord对象是否在队列中,如果在队列中,则调用cancelToastLocked(index)方法。真相就要浮现在我们眼前了,继续跟踪源码:

    private void cancelToastLocked(int index) {
        ToastRecord record = mToastQueue.get(index);
        try {
            record.callback.hide();
        } catch (RemoteException e) {
            // don't worry about this, we're about to remove it from
            // the list anyway
        }
        mToastQueue.remove(index);
        keepProcessAliveLocked(record.pid);
        if (mToastQueue.size() > 0) {
            // Show the next one. If the callback fails, this will remove
            // it from the list, so don't assume that the list hasn't changed
            // after this point.
            showNextToastLocked();
        }
    }

    哈哈,看到这里,我们回调对象的hide方法也被调用了,同时也将该ToastRecord对象从mToastQueue中移除了。到这里,一个Toast的完整显示和消失就讲解结束了。

    原文作者:Android源码分析
    原文地址: https://blog.csdn.net/wzy_1988/article/details/43341761
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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