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
简书地址：http://www.jianshu.com/p/4e3f80691e70
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<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;
}
}
}
}
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<ToastRecord> mToastQueue = new ArrayList<ToastRecord>();
...
// 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;
}
}
}
}
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。