WindowManagerService relayoutWindow release surface

[Thành Công Vũ]

Hello.

I am working on Android Mashmallow 6.0.0_r3 source code.

I am trying to understand how WindowManagerService (WMS), ActivityManagerService (AMS), and Application (ActivityThread) control the visibility of a window.

As I understand, the AMS will call WMS.setAppVisibility() to change the app's visibility when activity stack changes.

Then WMS call dispatchAppVisibility to its client (ActivityThread) to schedule a traversal.

After a short time, doTraversal is executed and application thread send request to WMS through WMS.relayoutWindow() to create/destroy Surface.

I'd like to ask 2 questions:

1. Is my understand correct ?

2. WMS uses win.mAppToken.clientHidden to control whether should create a new surface or destroy/releasing old surface in relayoutWindow().

http://androidxref.com/6.0.0_r1/xref/frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java#3201

However, the application thread doesn't know about this variable, then there's a case that application request to be visible (visble=View.VISIBLE) but

WMS releases the surface instead of creating (because clientHidden now changed to true for some reason).

For example: AM try to resume an activity right after pausing it -> no surface is drawn.

So why do we need to check for win.mAppToken.clientHidden in relayoutWindow() ?

Thank you!

[Thành Công Vũ]

Hello.

I have fixed a blackscreen issue caused by mismatched states between mAppVisible (Client) and clientHidden (Server).

I post my patch here in case somebody have the same issue.

Issue occurs when many startActivity() are called continuously in short time (< 150 ms) 

 

ActivityThread                        WindowManagerService

     |                                                    |

     |<--------------------------------dispatchAppVisibility(true)

handleAppVisibility(true)                   |

mAppVisible=true                             |

     |                                                    |

     |<--------------------------------dispatchAppVisibility(false)

     |                                                   |

performTraversals()                         |

relayoutWindow(old=8, new=0)---->|   Error: Releasing surface because clientHidden=true!

     |                                                   |

     |<--------------------------------dispatchAppVisibility(true)

     |

handleAppVisibility(false)

mAppVisible=false

     |

handleAppVisibility(true)

mAppVisible=true

     |

performTraversals()

Error: no relayout is called!

This patch is created and tested on MashMallow, I am not sure it should be applied on Nougat.