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：
- 系统Toast一定可以进入到系统Toast队列中，不会被黑名单阻止。
- 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。
