问题起源
我们的项目早期开发时，使用Toasty这个开源库实现带状态的Toast，并进行一层封装，通过Handler post到主线程的消息队列执行，达到调用时只要传入字符串这一个参数的效果，使用良好，代码如下：
kotlin
复制代码
fun toastInfo(message: String) {
Handler(Looper.getMainLooper()).post {
Toasty.info(MyApplication.instance, message).show()
}
}
经过若干年的迭代，安卓版本升级，targetSdkVersion也提升到了30+。近期在测试时，猛然发现一些地方的Toast居然没有显示出来。于是开始了问题排除和溯源。
初步探索
首先，并不是所有地方的Toast都不能正常弹出。于是我开始查找代码，看不正常的地方有什么特殊之处。然后发现，不能正常弹出的地方都是这样写的：
scss
复制代码
toastInfo("Toast内容")
finish()
尝试把finish去掉，Toast显示了；把finish移到调用Toast前，Toast也显示了。好吧，把所有这样调用的地方修改一下，问题解决！
但是这样并没有找到问题的根本原因，要改的地方比较多且易漏，而且后续其他的开发者也可能犯同样的错误。加之旧版本的安卓系统上并没有遇到这个问题，且Toasty库常年没有更新，难道是这个库和新的安卓有兼容问题？
那就先做个排除法。把封装的toastInfo方法中Toasty的那行调用直接改成原生Toast： Toast.makeText(MyApplication.instance, message, Toast.LENGTH_SHORT).show(),编译运行，Toast正常显示。看来Toasty这个库有问题。它是怎么做实现的呢？
查看源码
查阅Toasty的实现代码，调用info()之后返回的是一个标准的Toast，这个方法又调用custom方法，进行自定义布局。我把这个方法贴在下面：
ini
复制代码
public static Toast custom(@NonNull Context context, @NonNull CharSequence message, Drawable icon,
@ColorInt int tintColor, @ColorInt int textColor, int duration,
boolean withIcon, boolean shouldTint) {
final Toast currentToast = Toast.makeText(context, "", duration);
final View toastLayout = ((LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE))
.inflate(R.layout.toast_layout, null);
final LinearLayout toastRoot = toastLayout.findViewById(R.id.toast_root);
final ImageView toastIcon = toastLayout.findViewById(R.id.toast_icon);
final TextView toastTextView = toastLayout.findViewById(R.id.toast_text);
Drawable drawableFrame;
if (shouldTint)
drawableFrame = ToastyUtils.tint9PatchDrawableFrame(context, tintColor);
else
drawableFrame = ToastyUtils.getDrawable(context, R.drawable.toast_frame);
ToastyUtils.setBackground(toastLayout, drawableFrame);
if (withIcon) {
if (icon == null)
throw new IllegalArgumentException("Avoid passing 'icon' as null if 'withIcon' is set to true");
if (isRTL && Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1)
toastRoot.setLayoutDirection(View.LAYOUT_DIRECTION_RTL);
ToastyUtils.setBackground(toastIcon, tintIcon ? ToastyUtils.tintIcon(icon, textColor) : icon);
} else {
toastIcon.setVisibility(View.GONE);
}
toastTextView.setText(message);
toastTextView.setTextColor(textColor);
toastTextView.setTypeface(currentTypeface);
toastTextView.setTextSize(TypedValue.COMPLEX_UNIT_SP, textSize);
currentToast.setView(toastLayout);
if (!allowQueue) {
if (lastToast != null)
lastToast.cancel();
lastToast = currentToast;
}
// Make sure to use default values for non-specified ones.
currentToast.setGravity(
toastGravity == -1 ? currentToast.getGravity() : toastGravity,
xOffset == -1 ? currentToast.getXOffset() : xOffset,
yOffset == -1 ? currentToast.getYOffset() : yOffset
);
return currentToast;
}
重点来了：第33行setView是一个Deprecated的方法：
Custom toast views are deprecated. Apps can create a standard text toast with the makeText(Context, CharSequence, int) method, or use a Snackbar when in the foreground. Starting from Android Build.VERSION_CODES.R, apps targeting API level Build.VERSION_CODES.R or higher that are in the background will not have custom toast views displayed.
说自定义的Toast在安卓11+，targetAPI30+，且APP在background状态时，则不会显示。可是我的APP明明在前台啊，只不过正好在finish Activity；但和标准的Toast的主要区别也就这一处了。好吧，那就去看看谷歌是怎么改动这里的代码的吧。
上cs.android.com看看谷歌的实现。Toast类的show方法的代码是这样的：
java
复制代码
public void show() {
if (Compatibility.isChangeEnabled(CHANGE_TEXT_TOASTS_IN_THE_SYSTEM)) {
checkState(mNextView != null || mText != null, "You must either set a text or a view");
} else {
if (mNextView == null) {
throw new RuntimeException("setView must have been called");
}
}
INotificationManager service = getService();
String pkg = mContext.getOpPackageName();
TN tn = mTN;
tn.mNextView = mNextView;
final int displayId = mContext.getDisplayId();
try {
if (Compatibility.isChangeEnabled(CHANGE_TEXT_TOASTS_IN_THE_SYSTEM)) {
if (mNextView != null) {
// It's a custom toast
service.enqueueToast(pkg, mToken, tn, mDuration, displayId);
} else {
// It's a text toast
ITransientNotificationCallback callback =
new CallbackBinder(mCallbacks, mHandler);
service.enqueueTextToast(pkg, mToken, mText, mDuration, displayId, callback);
}
} else {
service.enqueueToast(pkg, mToken, tn, mDuration, displayId);
}
} catch (RemoteException e) {
// Empty
}
}
看来enqueueToast是重点，跟进去看一下：
java
复制代码
@Override
public void enqueueToast(String pkg, IBinder token, ITransientNotification callback,
int duration, boolean isUiContext, int displayId) {
enqueueToast(pkg, token, /* text= */ null, callback, duration, isUiContext, displayId,
/* textCallback= */ null);
}
private void enqueueToast(String pkg, IBinder token, @Nullable CharSequence text,
@Nullable ITransientNotification callback, int duration, boolean isUiContext,
int displayId, @Nullable ITransientNotificationCallback textCallback) {
if (DBG) {
Slog.i(TAG, "enqueueToast pkg=" + pkg + " token=" + token + " duration=" + duration
+ " isUiContext=" + isUiContext + " displayId=" + displayId);
}
if (pkg == null || (text == null && callback == null)
|| (text != null && callback != null) || token == null) {
Slog.e(TAG, "Not enqueuing toast. pkg=" + pkg + " text=" + text + " callback="
+ " token=" + token);
return;
}
final int callingUid = Binder.getCallingUid();
if (!isUiContext && displayId == Display.DEFAULT_DISPLAY
&& mUm.isVisibleBackgroundUsersSupported()) {
// When the caller is a visible background user using a non-UI context (like the
// application context), the Toast must be displayed in the display the user was
// started visible on.
int userId = UserHandle.getUserId(callingUid);
int userDisplayId = mUmInternal.getMainDisplayAssignedToUser(userId);
if (displayId != userDisplayId) {
if (DBG) {
Slogf.d(TAG, "Changing display id from %d to %d on user %d", displayId,
userDisplayId, userId);
}
displayId = userDisplayId;
}
}
checkCallerIsSameApp(pkg);
final boolean isSystemToast = isCallerIsSystemOrSystemUi()
|| PackageManagerService.PLATFORM_PACKAGE_NAME.equals(pkg);
boolean isAppRenderedToast = (callback != null);
if (!checkCanEnqueueToast(pkg, callingUid, displayId, isAppRenderedToast,
isSystemToast)) {
return;
}
synchronized (mToastQueue) {
int callingPid = Binder.getCallingPid();
final long callingId = Binder.clearCallingIdentity();
try {
ToastRecord record;
int index = indexOfToastLocked(pkg, token);
// 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 can enqueue.
// Prevents DOS attacks and deals with leaks.
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_TOASTS) {
Slog.e(TAG, "Package has already queued " + count
+ " toasts. Not showing more. Package=" + pkg);
return;
}
}
}
Binder windowToken = new Binder();
mWindowManagerInternal.addWindowToken(windowToken, TYPE_TOAST, displayId,
null /* options */);
record = getToastRecord(callingUid, callingPid, pkg, isSystemToast, token,
text, callback, duration, windowToken, displayId, textCallback);
mToastQueue.add(record);
index = mToastQueue.size() - 1;
keepProcessAliveForToastIfNeededLocked(callingPid);
}
// If it's at index 0, it's the current toast. It doesn't matter if it's
// new or just been updated, show it.
// 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(false);
}
} finally {
Binder.restoreCallingIdentity(callingId);
}
}
}
44行的checkCanEnqueueToast进去看一下：
arduino
复制代码
private boolean checkCanEnqueueToast(String pkg, int callingUid, int displayId,
boolean isAppRenderedToast, boolean isSystemToast) {
final boolean isPackageSuspended = isPackagePaused(pkg);
final boolean notificationsDisabledForPackage = !areNotificationsEnabledForPackage(pkg,
callingUid);
final boolean appIsForeground;
final long callingIdentity = Binder.clearCallingIdentity();
try {
appIsForeground = mActivityManager.getUidImportance(callingUid)
== IMPORTANCE_FOREGROUND;
} finally {
Binder.restoreCallingIdentity(callingIdentity);
}
if (!isSystemToast && ((notificationsDisabledForPackage && !appIsForeground)
|| isPackageSuspended)) {
Slog.e(TAG, "Suppressing toast from package " + pkg
+ (isPackageSuspended ? " due to package suspended."
: " by user request."));
return false;
}
if (blockToast(callingUid, isSystemToast, isAppRenderedToast,
isPackageInForegroundForToast(callingUid))) {
Slog.w(TAG, "Blocking custom toast from package " + pkg
+ " due to package not in the foreground at time the toast was posted");
return false;
}
int userId = UserHandle.getUserId(callingUid);
if (!isSystemToast && !mUmInternal.isUserVisible(userId, displayId)) {
Slog.e(TAG, "Suppressing toast from package " + pkg + "/" + callingUid + " as user "
+ userId + " is not visible on display " + displayId);
return false;
}
return true;
}
这些情况都会导致Toast不显示，怎么查看系统输出的日志呢？上网查找后得到答案：adb logcat -b system。终于捕获到了踪迹：W NotificationService: Blocking custom toast from package com.my.package.name due to package not in the foreground at time the toast was posted
好吧，还真是说不在foreground导致的。25行isPackageInForegroundForToast进去看一下：
arduino
复制代码
/**
* Implementation note: Our definition of foreground for toasts is an implementation matter
* and should strike a balance between functionality and anti-abuse effectiveness. We
* currently worry about the following cases:
*
- App with fullscreen activity: Allow toasts
*
- App behind translucent activity from other app: Block toasts
*
- App in multi-window: Allow toasts
*
- App with expanded bubble: Allow toasts
*
- App posting toasts on onCreate(), onStart(), onResume(): Allow toasts
*
- App posting toasts on onPause(), onStop(), onDestroy(): Block toasts
*
*
* Checking if the UID has any resumed activities satisfy use-cases above.
*
*
Checking if {@code mActivityManager.getUidImportance(callingUid) ==
* IMPORTANCE_FOREGROUND} does not work because it considers the app in foreground if it has
* any visible activities, failing case 2 in list above.
*/
private boolean isPackageInForegroundForToast(int callingUid) {
return mAtm.hasResumedActivity(callingUid);
}
看来问题找到了。谷歌在这里所说的background并不是我想当然的以为APP不在前台，而是存在ResumedActivity。而我调用Toast同时finish的代码正好命中最后一条App posting toasts on onPause(), onStop(), onDestroy(): Block toasts，导致了Toast不展示。
解决问题
问题找到，解决的思路也就有了。
方案1：用纯文字Toast，不使用setView出来的Toast。对显示效果影响太大，不采用。
方案2：改变调用次序。如前所述缺点，不采用。
方案3：换用其他活跃的Toast框架。查询Github后，发现现在支持自定义布局的Toast框架大多数是自定义的布局甚至悬浮窗实现（这里的框架代码没有细看，可能有不准确），前者不能跨Activity展示，那我调用的同时finish Activity也是不能正常展示的；后者还要请求悬浮窗权限，颇有杀鸡用牛刀的感觉，因此还是不采用。
最后我使用的方案4：将post改成postDelayed，设置一个小延时，这样等到Activity结束后Toast出来（应该不会有Activity在100ms都结束不了的吧，那怕是性能有问题了），避免了被系统判中，改动小，对用户感知的影响不明显。改动后的代码如下：
kotlin
复制代码
fun toastInfo(message: String) {
Handler(Looper.getMainLooper()).postDelayed({
Toasty.info(MyApplication.instance, message).show()
}, 100)
}
后话
这是在已有的老项目中，为了最小的改动而提出的解决方案。如果是新的APP，那就干脆别用Toast的setView去搞什么自定义Toast了，毕竟都已经被谷歌Deprecated了，别再去踩坑了。
文章仅为个人理解，如果有不准确之处或者更好的办法，欢迎提出。