CODEWITHSUNDEEP
android
user1730789
user1730789

Reputation: 5177

Android widget, drawing off the UI Thread

Quoting the Android developer guide found here here its says

Additionally, the Andoid UI toolkit is not thread-safe. So, you must not manipulate your UI from a worker thread—you must do all manipulation to your user interface from the UI thread

What does it mean that the widget's is not thread safe ? What causes the application to crash when we change the name of a Button off the UI Thread. I understand there is a event queue for the UI thread, but how does a separate thread cause issues with this queue ? I tried looking around, and everywhere it says you cannot do it, but no reason why ?

Upvotes: 3

Views: 680

Answers (1)

David Wasser
David Wasser

Reputation: 95578

When the documentation says that the UI toolkit is not thread-safe, this means that the UI toolkit is written in such a way that it assumes that all access to UI objects is made from a single thread. By making this assumption, the implementers of the UI toolkit can make unsynchronized access to all the UI objects without fear of data corruption. This makes the UI toolkit easier to implement, easier to test, and improves the performance of the UI toolkit (because it does not need to lock objects before manipulating them).

The UI toolkit is designed to run ONLY on the main thread (otherwise known as the "UI thread"). If you now access UI components from another thread, you run the risk of corrupting the UI toolkit's objects. To ensure that you don't do that, the UI toolkit designers do 2 things:

  • They write in the documentation that you aren't supposed to access the UI toolkit from outside the main thread
  • Some (but not all) methods of the UI toolkit check to see if you are making access from outside of the main thread and throw exceptions in such conditions

However, this doesn't forcibly prevent you from accessing the UI toolkit from another thread. In fact, you can probably change the text on a button from another thread without any bad side-effects. That said, you still shouldn't do it. Because in some cases you will cause crashes, in some cases you will find that the changes you make to the UI just get overwritten or ignored, etc.

I hope this makes sense.

Upvotes: 3

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