《Android开发艺术探索》读书笔记——Toast调用流程的源码分析

本文将介绍Android中Toast显示时的调用过程,细节逻辑我们不去分析,而是从整体上来看Toast内部的调用过程

首先看Toast的show()方法

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
        }
}

注意这句INotificationManager service = getService(); 很明显,INotificationManager是一个Binder接口,他的具体实现是NotificationManagerService里的一个成员变量:mService,看一下它的enqueueToast方法:

@Override
 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));
            final boolean isPackageSuspended =
                    isPackageSuspendedForUser(pkg, Binder.getCallingUid());

            if (ENABLE_BLOCKED_TOASTS && (!noteNotificationOp(pkg, Binder.getCallingUid())
                    || isPackageSuspended)) {
                if (!isSystemToast) {
                    Slog.e(TAG, "Suppressing toast from package " + pkg
                            + (isPackageSuspended
                                    ? " due to package suspended by administrator."
                                    : " 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; i= 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);
                }
            }
        }

这里有一个细节,注意这段代码:

static final int MAX_PACKAGE_NOTIFICATIONS = 50;
final ArrayList mToastQueue = new ArrayList();
...
 // 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; i= MAX_PACKAGE_NOTIFICATIONS) {
                                         Slog.e(TAG, "Package has already posted " + count
                                                + " toasts. Not showing more. Package=" + pkg);
                                         return;
                                     }
                                 }
                            }
                        }

为了防止DOS(Denial of Service,拒绝服务)攻击,系统做了一个限制,非系统应用的toast队列只让放50个,如果大于50个直接return,否则record = new ToastRecord(callingPid, pkg, callback, duration);创建一个新的ToastRecord放入队列。

然后调用了showNextToastLocked()方法,留意scheduleTimeoutLocked(record);在后文会分析这个方法的作用。

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;
                }
            }
        }
}

注意这句:

record.callback.show();

这里的callback是什么呢?回到前面看看ToastRecord创建代码:

record = new ToastRecord(callingPid, pkg, callback, duration);

这里传入了一个callback,这个callback是

public void enqueueToast(String pkg, ITransientNotification callback, int duration)

里的参数,从前文的分析中,这个方法是Toast里的show方法里面调用的

service.enqueueToast(pkg, tn, mDuration);

所以record.callback.show();里的show实际上是回调了TN中的show方法,这里是一个跨进程调用。

TN中show()方法直接用mHandler post了一个Runnable

/**
* schedule handleShow into the right thread
*/
@Override
public void show() {
    if (localLOGV) Log.v(TAG, "SHOW: " + this);
    mHandler.post(mShow);
}

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

最终执行了handleShow()方法。

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();
                String packageName = mView.getContext().getOpPackageName();
                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;
                mParams.packageName = packageName;
                mParams.removeTimeoutMilliseconds = mDuration ==
                    Toast.LENGTH_LONG ? LONG_DURATION_TIMEOUT : SHORT_DURATION_TIMEOUT;
                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();
            }
}

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;
            }
}

handleShow()方法中,首先调用了handleHide()将Toast的视图从WindowManager里移除,然后再添加进去。至此Toast显示过程就结束了。

现在来分析showNextToastLocked方法中的scheduleTimeoutLocked(record)方法,这个方法的作用是延时一段时间让Toast消失,具体时间根据创建Toast时duration字段值来判断。

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

所以我们设置Toast.LENGTH_LONG其实是3.5秒,Toast.LENGTH_SHORT是2秒。

然后通过mHandler发送了一个MESSAGE_TIMEOUT消息,看下mHandler怎么处理这个消息,在NotificationManagerService里的mHandler是一个WorkerHandler对象

private final class WorkerHandler extends Handler
    {
        @Override
        public void handleMessage(Message msg)
        {
            switch (msg.what)
            {
                case MESSAGE_TIMEOUT:
                    handleTimeout((ToastRecord)msg.obj);
                    break;
                case MESSAGE_SAVE_POLICY_FILE:
                    handleSavePolicyFile();
                    break;
                case MESSAGE_SEND_RANKING_UPDATE:
                    handleSendRankingUpdate();
                    break;
                case MESSAGE_LISTENER_HINTS_CHANGED:
                    handleListenerHintsChanged(msg.arg1);
                    break;
                case MESSAGE_LISTENER_NOTIFICATION_FILTER_CHANGED:
                    handleListenerInterruptionFilterChanged(msg.arg1);
                    break;
            }
        }
    }

mHandler处理MESSAGE_TIMEOUT调用了handleTimeout((ToastRecord)msg.obj),handleTimeout有调用了cancelToastLocked(index),

void cancelToastLocked(int index) {
        ToastRecord record = mToastQueue.get(index);
        try {
            record.callback.hide();
        } catch (RemoteException e) {
            Slog.w(TAG, "Object died trying to hide notification " + record.callback
                    + " in package " + record.pkg);
            // 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();
        }
}

首先会调用record.callback.hide(),实际上是最终调用了TN里的handleHide()方法隐藏当前显示的Toast,这个过程和record.callback.show()一样,这里就不分析了。然后从mToastQueue中移除已经显示了的Toast,最后判断mToastQueue中还有没有Toast要显示,有就继续执行showNextToastLocked()显示Toast。

至此Toast的显示和消失流程就分析完啦,希望能帮到你。

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