78 lines
2.2 KiB
Markdown
78 lines
2.2 KiB
Markdown
# Threads (and Services)
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Activities are regularly transitioned to the background
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* i.e. when they are stopped or destroyed
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* As the user continues with the *task*
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### Thread of Execution
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Android applications use a **single thread model**
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* A single thread of execution called *main*
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* Started when a *process* is created
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* Handles and dispatches user interface events
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* Drawing the interface, responding to `onClick()`
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* Handles activity life cycle events
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* `onCreate()`, `onDestroy()`
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* For all components in an application
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### Looper and Handler
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`HandlerThread` - extension of thread with support for looper
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**Looper**
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* Each `HandlerThread` can have one looper
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* A Java thread dies when the run method returns
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* Maintains a message queue
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* `looper.loop()` - loops through the message queue and processes waiting messages
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**Message**
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* A task to be completed
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* Might contain data or a reference to a `Runnable` object
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**Handler**
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* Attached to a looper
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* Enqueues messages in the looper message queue
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* Handles messages from the message queue
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* **Thread-safe**
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* One looper can have many handlers associated with it
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If we run code that takes >5 s, the application will appear to hang.
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* Put longer-running / non-instantaneous code in a separate thread of execution
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* Network access, file access
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* 2 rules
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1. Do not block the UI thread
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2. Do not access the UI thread from outside the UI thread
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- Android view objects are not thread-safe
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- *Post* messages via the Handler to the UI thread's looper
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We can programmatically interact with UI components
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- e.g. `textField.setText(string)`
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- But cannot call this method from outside the UI thread (rule 2)
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Instead, split code into two parts
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1. Long(ish)-running code that *does not* involve the UI
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2. Instantaneous code that *does* involve the UI
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- Which needs to be posted to the UI thread responsible for the view
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We need to remember the activity life-cycle; otherwise, we risk having **orphaned threads**.
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#### AsyncTask and others
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###### Accessing the UI thread from another thread
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```java
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Activity.runOnUiThread(Runnable);
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View.post(Runnable);
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View.postDelayed(Runnable, long);
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MutableLiveData.postValue;
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``` |