A thread may have multiple Handler instances for processing messages; the Looper ensures that messages are dispatched to the correct Handler. The following code implements what is probably one of the most common use cases. The user presses a button on the screen that could trigger a long operation, e.

To avoid stalling the rendering of the UI, the long operation, represented here by a dummy doLongRunningOperation method, has to be executed on a worker thread. Hence, the setup is merely one producer thread the UI thread and one consumer thread LooperThread. Our code sets up a message queue.

IT handles the button click as usual in the onClick callback, which executes on the UI thread. In our implementation, the callback inserts a dummy message into the message queue. For sake of brevity, layouts and UI components have been left out of the example code. Associate a Looper — and implicitly a MessageQueue — with the thread.

Set up a Handler to be used by the producer for inserting messages in the queue. Here we use the default constructor so it will bind to the Looper of the current thread. Hence, this Handler can created only after Looper. Callback that runs when the message has been dispatched to the worker thread. It checks the what parameter, and then executes the long operation.

Start dispatching messages from the message queue to the consumer thread. This is a blocking call, so the worker thread will not finish. There is race condition between the setup of mHandler on a background thread and this usage on the UI thread. Hence, validate that mHandler is available. Initialize a Message -object with the what argument arbitrarily set to 0. Terminate the background thread. The call to Looper. The message queue is represented by the android. MessageQueue class. It is built with linked messages, constituting an unbound one-directional linked list.

Producer threads insert messages that will later be dispatched to the consumer. The messages are sorted based on timestamps. The pending message with the lowest timestamp value is first in line for dispatch to the consumer. However, a message is dispatched only if the timestamp value is less than the current time.

If not, the dispatch will wait until the current time has passed the timestamp value. Only one message has passed the dispatch barrier, i. Messages eligible for dispatch have a timestamp value less than the current time, i.

If no message has passed the dispatch barrier when the Looper is ready to retrieve the next message, the consumer thread blocks. Execution is resumed as soon as a message passes the dispatch barrier. The producers can insert new messages in the queue at any time and on any position in the queue. The insert position in the queue is based on the timestamp value. If a new message has the lowest timestamp value compared to the pending messages in the queue, it will occupy the first position in the queue, which is next to be dispatched.

Insertions always comply to the timestamp sorting order. Message insertion is discussed further in Handler. If there is no message to process, a consumer thread has some idle time. By default, the consumer thread simply waits for new messages during idle time, but instead of waiting, the thread can be utilized to execute other tasks during these idle slots.

This feature can be utilized to let non-critical tasks postpone their execution until no other messages are competing for execution time. When a pending message has been dispatched, and no other message has passed the dispatch barrier, a time slot occurs where the consumer thread can be utilized for execution of other tasks. An application gets hold of this time slot with the android.

IdleHandler -interface; it is a listener that generates callbacks when the consumer thread is idle. The listener is attached to the MessageQueue and detached from it through the following calls:. When the message queue detects idle time for the consumer thread, it invokes queueIdle on all registered IdleHandler -instances. It is up to the application to implement the callback responsibly.

You should usually avoid long-running tasks, because they will delay pending messages during the time they run. The implementation of queueIdle must return a Boolean value with the following meanings:. All registered IdleHandlers to a MessageQueue are invoked when a thread has idle slots, where it waits for new messages to process. The idle slots can occur before the first message, between messages and after the last message. If multiple content producers should process data sequentially on a consumer thread, the IdleHandler can be used to terminate the consumer thread when all messages are processed, so that the unused thread does not linger in memory.

With the IdleHandler , it is not necessary to keep track of the last inserted message to know when the thread can be terminated. This use case applies only when the producing threads insert messages in the MessageQueue without delay, so that the consumer thread is never idle until the last message is inserted.

Your Answer

The ConsumeAndQuitThread method shows the structure of a consuming thread with Looper and MessageQueue that terminates the thread when there are no more messages to process. Register the IdleHandler on the background thread when it is started and the Looper is prepared, so that the MessageQueue is set up. Let the first queueIdle invocation pass, because it occurs before the first message is received.

Return true on the first invocation so that the IdleHandler still is registered. The message insertion is done from multiple threads concurrently, with a simulated randomness of the insertion time.

Public constructors

Each item on the MessageQueue is of the android. Message class. This is a container object carrying either a data item or a task, never both. Data is processed by the consumer thread, whereas a task is simply executed when it is dequeued and you have no other processing to do. The message knows its recipient processor—i. Handler —and can enqueue itself through Message. As we will see in Handler , the handler is most commonly used for message enqueuing, as it offers more flexibility with regard to message insertion. Simple data values to handle the common use case of handing over integers.

If a maximum of two integer values are to be passed to the consumer, these parameters are more efficient than allocating a Bundle, as explained under the data parameter. Arbitrary object. If the object is handed off to a thread in another process, it has to implement Parcelable. Reference to Handler in some other process.

Enables inter-process message communication, as described in Two-way communication. Task to execute on a thread. This is an internal instance field that holds the Runnable object from the Handler. A MessageQueue can contain any combination of data and task messages. The consumer thread processes them in a sequential manner, independent of the type. If a message is a data message, the consumer processes the data. Task messages are handled by letting the Runnable execute on the consumer thread, but the consumer thread does not receive a message to be processed in Handler. The lifecycle of a message is simple: This description suffices for most use cases, but when a problem arises, a deeper understanding of message handling is invaluable.

The runtime stores message objects in an application-wide pool to enable the reuse of previous messages; this avoids the overhead of creating new instances for every hand-off. The message object execution time is normally very short and many messages are processed per time unit. The state transfers are partly controlled by the application and partly by the platform. Note that the states are not observable, and an application cannot follow the changes from one state to another although there are ways to follow the movement of messages, explained later in Observing the Message Queue.

Therefore, an application should not make any assumptions about the current state when handling a message. In the initialized state, a message object with mutable state has been created and, if it is a data message, populated with data. The application is responsible for creating the message object using one of the following calls. They take an object from the object pool. The message has been inserted into the queue by the producer thread, and it is waiting to be dispatched to the consumer thread.

In this state, the Looper has retrieved and removed the message from the queue. The message has been dispatched to the consumer thread and is currently being processed. There is no application API for this operation because the dispatch is controlled by the Looper, without the influence of the application. When the looper dispatches a message, it checks the delivery information of the message, and delivers the message to the correct recipient. Once dispatched, the message is executed on the consumer thread.

At this point in the lifecycle, the message state is cleared and the instance is returned to the message pool.

• Android: Looper, Handler, HandlerThread. Part II.?
• Android: Looper, Handler, HandlerThread. Part II.;
• 4. Thread Communication - Efficient Android Threading [Book].

The Looper handles the recycling of the message when it has finished executing on the consumer thread. Recycling of messages is handler by the runtime and should not be done explicitly by the application. Once a message is inserted in the queue, the content should not be altered. In theory, it is valid to change the content before the message is dispatched. However, because the state is not observable, the message may be processed by the consumer thread while the producer tries to change the data, raising thread safety concerns.

It would be even worse if the message has been recycled, because it then has been returned to the message pool and possibly used by another producer to pass data in another queue. The android. Looper class handles the dispatch of messages in the queue to the associated handler. As long as the queue has messages eligible for dispatch, the Looper will ensure that the consumer thread receives the messages. When no messages have passed the dispatch barrier, the consumer thread will block until a message has passed the dispatch barrier.

The consumer thread does not interact with the message queue directly to retrieve the messages. Instead, a message queue is added to the thread when the looper has been attached. The looper manages the message queue and facilitates the dispatch of messages to the consumer thread. By default, only the UI thread has a Looper; threads created in the application need to get a Looper associated explicitly. When the Looper is created for a thread, it is connected to a message queue. The Looper acts as the intermediator between the queue and the thread.

The Looper setup is done in the run method of the thread:. The first step is to create the Looper, which is done with the static prepare method; it will create a message queue and associate it with the current thread. At this point, the message queue is ready for insertion of messages, but they are not dispatched to the consumer thread.

Start handling messages in the message queue. This is a blocking method that ensures the run method is not finished; while run blocks, the Looper dispatches messages to the consumer thread for processing. A thread can have only one associated Looper; a runtime error will occur if the application tries to set up a second one. Consequently, a thread can have only one message queue, meaning that messages sent by multiple producer threads are processed sequentially on the consumer thread. Hence, the currently executing message will postpone subsequent messages until it has been processed.

Messages with long execution times shall not be used if they can delay other important tasks in the queue.

The Looper is requested to stop processing messages with either quit or quitSafely: Pending messages that are eligable for dispatch will be processed before the Looper is terminated. Previous API levels only support quit. Terminating a Looper does not terminate the thread; it merely exits Looper. But you cannot start the old looper or a new one, so the thread can no longer enqueue or handle messages. If you call Looper. The UI thread is the only thread with an associated Looper by default.

There are a few practical differences between the UI thread Looper and other application thread Loopers:. So far, the focus has been on the internals of Android thread communication, but an application mostly interacts with the android. Handler class. It is a two-sided API that both handles the insertion of messages into the queue and the message processing.

While carrying out its responsibilities, the Handler interacts with the Looper, message queue, and message. Without a Looper, a Handler can not function; it cannot couple with a queue to insert messages and consequently it will not receive any messages to process. Hence, a Handler instance is bound to a Looper instance already at construction time:. Constructors without an explicit Looper bind to the Looper of the current thread. Constructors with an explicit Looper bind to that Looper.

If the constructors without an explicit Looper are called on a thread without a Looper i. Once a handler is bound to a Looper, the binding is final.

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Multiple handlers will not enable concurrent execution. The messages are still in the same queue and are processed sequentially. For simplicity, the Handler class offers wrapper functions for the factory methods shown in Initialized to create objects of the Message class. The message obtained from a Handler is retrieved from the message pool and implicitly connected to the Handler instance that requested it. This connection enables the looper to dispatch each message to the correct handler. I believe I've seen NPE before on some phones when doing this but it's been awhile. I've had some issues with removeCallbacksAndMessages null wouldn't remove some of my callbacks.

When I would want to stop receiving Callbacks, I'd call handler.

java - How to remove all callbacks from a Handler? - Stack Overflow

Snaker Have you solved your issue yet? I'm having same issue where Handler. Callback is being called even after removing callbacks and messages by setting null. Devunwired Devunwired Thanks, I know that. But I have a lot of Runnable in many sub-classes, and manage them all is a epic work! Is there anyway to remove them all, in the onStop event? Understood, I updated the answer with a bit more information.

Handler In Android

Short version is you can't call a method to broadly clear a Handler's message queue Define a new handler and runnable: False way: True way: JamesRobert JamesRobert 21 4. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. This is how a Runnable object would look like:. A Handler in Android is a construct that can be used to communicate between UI thread and separate background threads.

A Handler belongs to the thread that is creating it. For example, if you create a handler in the onCreate method of an Activity , it belongs to the UI thread, and this is usually what you want to do because you can then use this handler to post feedback to the UI thread from background threads. Let's look in concrete code and appropriate layout.

Here, we used the postDelayed Runnable, time method to send a message with a delayed time. In this case, the message is a runnable object that represents a command than can be executed. Hire me About Read Contacts Search.