Android7.1后對於Toast類型窗口的限制

在處理問題的時候，經常會遇到ANR，停止運行等問題，經過分析和驗證，和如下的一行關鍵log有關

WindowManager: Adding more than one toast window for UID at a time.

這一行log對應的代碼如下：

 // If adding a toast requires a token for this app we always schedule hiding
            // toast windows to make sure they don't stick around longer then necessary.
            // We hide instead of remove such windows as apps aren't prepared to handle
            // windows being removed under them.
            //
            // If the app is older it can add toasts without a token and hence overlay
            // other apps. To be maximally compatible with these apps we will hide the
            // window after the toast timeout only if the focused window is from another
            // UID, otherwise we allow unlimited duration. When a UID looses focus we
            // schedule hiding all of its toast windows.
            if (type == TYPE_TOAST) {
                if (!getDefaultDisplayContentLocked().canAddToastWindowForUid(callingUid)) {
                    Slog.w(TAG_WM, "Adding more than one toast window for UID at a time.");
                    return WindowManagerGlobal.ADD_DUPLICATE_ADD;
                }
                // Make sure this happens before we moved focus as one can make the
                // toast focusable to force it not being hidden after the timeout.
                // Focusable toasts are always timed out to prevent a focused app to
                // show a focusable toasts while it has focus which will be kept on
                // the screen after the activity goes away.
                if (addToastWindowRequiresToken
                        || (attrs.flags & LayoutParams.FLAG_NOT_FOCUSABLE) == 0
                        || mCurrentFocus == null
                        || mCurrentFocus.mOwnerUid != callingUid) {
                    mH.sendMessageDelayed(
                            mH.obtainMessage(H.WINDOW_HIDE_TIMEOUT, win),
                            win.mAttrs.hideTimeoutMilliseconds);
                }
            }

此段代碼在WindowManagerService.java中的addWindow函數，可以對比android源碼，只有android7.1才會有這段代碼，即只有android7.1對toast類型的窗口有限制。

那么這個限制導致到底是什么呢？

可以查看如下修改說明：

https://android.googlesource.com/platform/frameworks/base/+/dc24f93%5E%21/#F6

修改的目的：

Prevent apps to overlay other apps via toast windows

It was possible for apps to put toast type windows that overlay other apps which toast winodws aren't removed after a timeout.