我们在开发apk的过程中，经常会用到Toast，它的确很方便，不用开发者编写UI布局，只需要传入字符串就能给用户提示，那么它的显示与消失的流程是怎样的呢，本来就来讲一讲这个流程（所用源码为android 8.0版本）。
1、首先我们来看一下Toast的弹出的写法：
Toast.makeText(ServiceTestActivity.this,"test",Toast.LENGTH_LONG).show();1.1 我们看一下Toast.makeText都做了什么：
/**
* Make a standard toast that just contains a text view.
*
* @param context The context to use. Usually your {@link android.app.Application}
* or {@link android.app.Activity} object.
* @param text The text to show. Can be formatted text.
* @param duration How long to display the message. Either {@link #LENGTH_SHORT} or
* {@link #LENGTH_LONG}
*
*/
public static Toast makeText(Context context, CharSequence text, @Duration int duration) {
return makeText(context, null, text, duration);
}
/**
* Make a standard toast to display using the specified looper.
* If looper is null, Looper.myLooper() is used.
* @hide
*/
public static Toast makeText(@NonNull Context context, @Nullable Looper looper,
@NonNull CharSequence text, @Duration int duration) {
Toast result = new Toast(context, looper);
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;
}我们直接看第二个makeText方法，注意参数Looper对象此时为null，此方法里面，先是new一个Toast对象，然后加载了一个布局，这个布局就是Toast的布局，布局代码如下：
<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>然后就是给Toast对象设置了布局和周期，返回了这个Toast对象。我们需要看一下Toast的构造函数都干了什么：
/**
* Constructs an empty Toast object. If looper is null, Looper.myLooper() is used.
* @hide
*/
public Toast(@NonNull Context context, @Nullable Looper looper) {
mContext = context;
mTN = new TN(context.getPackageName(), looper);
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);
}Toast的构造函数里面又new了一个TN对象，然后给这个对象的两个属性设了值。我们来看一下TN是何方神圣：
private static class TN extends ITransientNotification.StubTN(String packageName, @Nullable Looper looper) {
// 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;
mPackageName = packageName;
if (looper == null) {
// Use Looper.myLooper() if looper is not specified.
looper = Looper.myLooper();
if (looper == null) {
throw new RuntimeException(
"Can't toast on a thread that has not called Looper.prepare()");
}
}
mHandler = new Handler(looper, null) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case SHOW: {
IBinder token = (IBinder) msg.obj;
handleShow(token);
break;
}
case HIDE: {
handleHide();
// Don't do this in handleHide() because it is also invoked by
// handleShow()
mNextView = null;
break;
}
case CANCEL: {
handleHide();
// Don't do this in handleHide() because it is also invoked by
// handleShow()
mNextView = null;
try {
getService().cancelToast(mPackageName, TN.this);
} catch (RemoteException e) {
}
break;
}
}
}
};
}TN继承了一个Stub接口，可知其是一个IBinder对象，用来进程间通信的，其作用是为了和系统服务NOTIFICATION_SERVICE进行相互通信，具体的服务类是WindowManagerImpl.java。我们看一下TN的构造方法，其先检查了looper对象是否为null，若为null，则获取线程的looper对象，如果线程的looper对象仍为空，则会报
java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()
所以，如果想要用Toast，是需要我们的线程有looper的，系统在创建进程和main线程的时候，已经给我们创建好了main线程的looper对象，所以平时我们在Activity、Fragment、Service里面可以直接使用Toast，没有问题。
到这里Toast.makeText方法的关键信息都讲述完了，这里小结一下该方法里面都做了什么：
（1）新建了一个Toast对象，为Toast对象设置了布局和周期。
（2）Toast对象里面有新建了一个TN对象，这个对象是传给远程服务NITIFICATION_SERVICE方便回调的，TN对象里面又创建了一个handler，handler的looper是调用Toast的线程的looper，即handler是在调用Toast的线程处理消息的。
1.2 我们接下来看一下Toast.show()
/**
* Show the view for the specified duration.
*/
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
}
}1.2.1 show()方法里面会调用getService方法获取服务，我们看一下获取什么服务：
static private INotificationManager getService() {
if (sService != null) {
return sService;
}
sService = INotificationManager.Stub.asInterface(ServiceManager.getService("notification"));
return sService;
}可知其实是通过Binder获取到名称为“notification”的系统服务的代理对象。“notification”服务的启动是在SystemServer.startOtherService里面启动的
mSystemServiceManager.startService(NotificationManagerService.class);NotificationManagerService的onStart里面回把服务注册到ServiceManager类里面。系统服务的启动和注册有关流程见我的另两篇博客《SystemServer类解析》和《getSystemService追根溯源》。
1.2.2 返回到show方法，接下来会调用INotificationManager的enqueueToast方法,参数是本应用的包名、TN对象、周期。根据Binde机制的原理，我们可知在NotificationManagerSrevice里面会有一个INotificationManager.Stub类的实现类，该实现类会实现enqueueToast方法：
private final IBinder mService = new INotificationManager.Stub() {
// Toasts
// ============================================================================
@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<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;
}
}
}
}
Binder token = new Binder();
mWindowManagerInternal.addWindowToken(token,
WindowManager.LayoutParams.TYPE_TOAST);
record = new ToastRecord(callingPid, pkg, callback, duration, token);
mToastQueue.add(record);
index = mToastQueue.size() - 1;
keepProcessAliveIfNeededLocked(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);
}
}
}该方法显得略长，无关主流程的逻辑不在赘述，我们只捡重要的说。直接进到synchronized关键字修饰的同步代码段，先调用了indexOfToastLocked方法，该方法是返回相同包名并且TN对象相同的的ToastRecord对象在队列中的索引，找不到返回-1，根据我们99%的惯用写法，返回值为-1，所以我们直接看index为-1的逻辑。然后会判断是否为系统弹出的toast，若不是系统Toast，则会对每个应用所能在队列中存留的Toast的数量不能超过MAX_PACKAGE_NOTIFICATIONS（值为50），即一个应用连续发送Toast，超过50个Toast未处理就会报异常。接下来就是新建一个ToastRecord对象，入队列，添加到队列末尾。最后有一个判断，如果index为0，则会调用showNextToastLocked方法，看到这里可能有读者可能会有疑问，这里为什么要加一个index==0的判断，如果index不为0，那么如何显示Toast呢，别着急，其实这些逻辑就在showNextToastLocked里面。
1.3 showNextToastLocked方法
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(record.token);
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);
}
keepProcessAliveIfNeededLocked(record.pid);
if (mToastQueue.size() > 0) {
record = mToastQueue.get(0);
} else {
record = null;
}
}
}
}进入到该方法，第一就代码就是获取到队列的首部的ToastRecord对象，然后进入了一个while循环。我们来看一下循环体的逻辑。
1.3.1 while循环体里面调用了record.callback.show(record.token)，我们来分一下这句代码的逻辑。record.callback其实就是前面讲到TN对象，我们来看一下TN.show方法：
/**
* schedule handleShow into the right thread
*/
@Override
public void show(IBinder windowToken) {
if (localLOGV) Log.v(TAG, "SHOW: " + this);
mHandler.obtainMessage(SHOW, windowToken).sendToTarget();
}可知是调用mHandler发送了一个SHOW的message，该handler创建请见TN的构造函数。消息处理是在TN.handleShow方法里面：
public void handleShow(IBinder windowToken) {
if (localLOGV) Log.v(TAG, "HANDLE SHOW: " + this + " mView=" + mView
+ " mNextView=" + mNextView);
// If a cancel/hide is pending - no need to show - at this point
// the window token is already invalid and no need to do any work.
if (mHandler.hasMessages(CANCEL) || mHandler.hasMessages(HIDE)) {
return;
}
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.hideTimeoutMilliseconds = mDuration ==
Toast.LENGTH_LONG ? LONG_DURATION_TIMEOUT : SHORT_DURATION_TIMEOUT;
mParams.token = windowToken;
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);
// Since the notification manager service cancels the token right
// after it notifies us to cancel the toast there is an inherent
// race and we may attempt to add a window after the token has been
// invalidated. Let us hedge against that.
try {
mWM.addView(mView, mParams);
trySendAccessibilityEvent();
} catch (WindowManager.BadTokenException e) {
/* ignore */
}
}
}该方法核心逻辑是获取WINDOWS_SERVICE服务，该服务类是WindowManagerImpl，该服务是在SystemReServiceRegistry.java的静态代码块里面注册到的，调用WindowManagerImpl.addView方法来显示Toast。至于WindowsManagerImpl是如何加载布局的，本文这里就不在分析了，感兴趣的朋友可以去研究一下。到这里的朋友可能会注意到，Toast的布局的显示利用了WINDOW_SERVICE服务，该处代码是运行在handler所在的进程，如果handler是子线程的，那么该处代码是运行在子线程的，android的子线程不是不能操作UI吗，这里不会有问题吗，该问题请见我的另一篇博客《子线程能弹Toast吗》。
1.3.2 再回到NoificationManagerService的showNextToastLocked方法，接下来我们看一下scheduleTimeroutLocked方法：
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);
}该方法就是利用Handler发送一个延时消息MESSAGE_TIMEOUT，延迟时间为LONG_DELAY或者SHORT_DELAY，这个延迟时间是根据我们创建Toast的传的参数LENGTH_LONG或者LENGTH_SHORT来决定的，我们来看看分别到时代表多长时间：
static final int LONG_DELAY = PhoneWindowManager.TOAST_WINDOW_TIMEOUT;
static final int SHORT_DELAY = 2000; // 2 seconds/**PhoneWindowManager
Amount of time (in milliseconds) a toast window can be shown. */
public static final int TOAST_WINDOW_TIMEOUT = 3500; // 3.5 seconds可知LENGHT_LONG对应的时间是3.5秒，LENGHT_SHORT对应的时间为2秒。需要注意的是，现在的handler不是TOAST里面的handler了，是NotificationManagerService里面的handler，我们来看一下其是如何处理MESSAGE_TIMEOUT消息的：
case MESSAGE_TIMEOUT:
handleTimeout((ToastRecord)msg.obj);private void handleTimeout(ToastRecord record)
{
if (DBG) Slog.d(TAG, "Timeout pkg=" + record.pkg + " callback=" + record.callback);
synchronized (mToastQueue) {
int index = indexOfToastLocked(record.pkg, record.callback);
if (index >= 0) {
cancelToastLocked(index);
}
}
}在handleTimeout方法里面，会对mToastQueue加锁，如果队列里面知道到了对应的ToastRecord（即当前正在显示的Toast），调用cancelToastLocked方法，看一下该方法：
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
}
ToastRecord lastToast = mToastQueue.remove(index);
mWindowManagerInternal.removeWindowToken(lastToast.token, true);
keepProcessAliveIfNeededLocked(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();
}
}1.3.2.1 我们来看一下record.callback.hide()方法都干了什么，其调用的是Toast里面的TN的hide方法，该方法也就是发送了一个HIDE消息，我们来看一下handler是如何处理该消息的，直接来看handleHide方法：
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.removeViewImmediate(mView);
}
mView = null;
}
}该方法就是调用WidowManageImpl的removeViewImmediate方法使Toast消失。
1.3.2.2 回到cancelToastLocked方法，我们看一下后面的逻辑，后面的逻辑就是mToastQueue删除掉刚才的ToastRecord对象，因为该Toast的显示与消失工作已经做完了。最后就是判断mToastQueue里面还有没有ToastRecord，如果有，调用showNextToastLocked方法继续处理剩余的ToastRecord。