Queue full android studio

Объект многопоточной очереди модуля queue.

Объекты очереди создаются в момент инициализации классов queue.Queue() , queue.LifoQueue() или queue.PriorityQueue() и имеют открытые методы, описанные ниже.

Содержание:

• Размер очереди Queue.qsize() ,
• True , если очередь пуста Queue.empty() ,
• True , если очередь заполнена Queue.full() ,
• Положить элемент в очередь Queue.put() ,
• Разместить элемент в очереди без ожидания Queue.put_nowait() ,
• Получить элемент из очереди Queue.get() ,
• Эквивалентен вызову Queue.get(False) Queue.get_nowait() ,
• Сообщить очереди, что задача завершена Queue.task_done() ,
• Заблокировать очередь Queue.join() ,
• Пример работы с очередью (ожидания выполнения задач очереди).

Queue.qsize() :

Метод Queue.qsize() возвращает примерный размер очереди.

Обратите внимание, что SimpleQueue.qsize() > 0 не гарантирует, что последующий метод Queue.get() не будет блокироваться.

Queue.empty() :

Метод Queue.empty() возвращает True , если очередь пуста, иначе False .

Если SimpleQueue.empty() возвращает False , это не гарантирует, что последующий вызов метода Queue.get() не будет блокирован.

Queue.full() :

Метод Queue.full() возвращает True , если очередь заполнена и False в противном случае.

• Если Queue.full() возвращает True , это не гарантирует, что последующий вызов Queue.get() не будет блокирован.
• Аналогично, если Queue.full() возвращает False , это не гарантирует, что последующий вызов Queue.put() не будет блокирован.

Queue.put(item, block=True, timeout=None) :

Метод Queue.put() размещает элемент item в очереди.

Если необязательный аргумент block имеет значение True , а время ожидания равно None (по умолчанию), то при необходимости добавление элемента в очереди блокируется, пока не освободится свободный слот.

Если тайм-аут timeout является положительным числом, то добавление элемента в очередь блокируется на максимальное время ожидания timeout и если в течение этого времени не было свободного места, то выдается исключение queue.Full . В противном случае, когда block=False и если свободный слот сразу же доступен, то элемент item добавляется в очередь, иначе вызывается исключение queue.Full . В этом случае аргумент тайм-аут timeout игнорируется.

Queue.put_nowait(item) :

Метод Queue.put_nowait() эквивалентен вызову метода Queue.put(item, False) с указанными параметрами.

Queue.get(block=True, timeout=None) :

Метод Queue.get() возвращает, а затем удаляет элемент из очереди.

Если необязательный аргумент block имеет значение True , а время ожидания timeout=None (по умолчанию), то при необходимости происходит блокировка, на время пока элемент не станет доступен.

Если тайм-аут timeout является положительным числом, то извлечение элемента из очереди блокируется на максимальное время ожидания timeout и если в течение этого времени ни один элемент не был доступен, то вызывается исключение queue.Empty . В противном случае, когда block=False и если элемент сразу доступен, то элемент возвращается, иначе вызывается исключение queue.Empty ,в этом случае timeout игнорируется.

Для всех версий в Windows, если block=True и время ожидания timeout=None , то эта операция переходит в непрерывное ожидание базовой блокировки. Это означает, что исключений может не быть, и в частности, SIGINT не будет вызывать KeyboardInterrupt .

Queue.get_nowait() :

Метод Queue.get_nowait() эквивалентен вызову метода Queue.get(False) .

Для поддержки отслеживания того, были ли поставленные в очередь задачи полностью обработаны потоками-потребителями демонов, объекты очереди предлагают два метода.

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Android Queue vs Stack

Why does java.util.Stack allows me to create a new Stack in an android activity with a simple constructor like:

and i cannot do the same with java.util.Queue ? Shouldn’t a queue have a similar constructor? Strange enough on http://developer.android.com/reference/java/util/Stack.html it says the Stack has a public constructor and on http://developer.android.com/reference/java/util/Queue.html i don’t see a similar constructor for a queue.. why is that? what is the way to have a Queue of ImageView elements for example? Thanks.

2 Answers 2

Because Queue is an interface, you should initial it with a LinkedList :

It is because Queue is just an interface. To create a Queue object you need a class, what implements the methods of a Queue.

Some actual implementations of a Queue: link

Not the answer you’re looking for? Browse other questions tagged java android or ask your own question.

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Full queue, lose event in Android Emulator

I’m using the Android emulator running API version 15 from the SDK tools and in the console I’m getting this error:

I can’t click on anything obviously. I googled it and the only solution is to delete the emulator and recreate it. This works for a while but then I have the same problem again.

Does anyone know what causes this error and how to fix it?

7 Answers 7

You can try the following to clear the old data and applications (your emulator will return to its default settings and data)

Tools —> Android —> AVD Manager

Then select your Emulator

From Actions column hit the arrow and select Wipe Data

I was facing same problem, There is Nothing RAM problem.

Solution is here’s

Go to Tools -> AVD Manager ->Find the emulator which you are having this error -> Right click on your emulator and click on COLD BOOT NOW.

i probably figured it out. try this:

While creating the android virtual device, you must have given its size in KIBs or upto 32 mibs as shown in one of the tutorials on web.

now delete the previous device and make a new device giving its size as 512 MIB. It solved my problem. Hope it’ll work for you also, just give it a try.

I have ran into the same problem and I realized that I had dedicated only 256 MB of RAM for the virtual device. It had simply run out of memory.

Here’s the solution:

-Go to Tools -> Android -> AVD Manager

-Find the emulator you are having this error on.

-Click to the «edit» button next to it. (In 3.0, this appears as a green pencil)

-Click «Show advanced settings»

-Scroll down and assign some more RAM to the device, 1GB should cut it, 2GB is definitely enough if you are running a simple app. I would also recommend assigning 512 MB of heap.

-Hit finish. Exit the emulator and restart it.

You should be good now, I hope it’s been helpful. Happy coding!

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Coroutines On Android (part III): real work

This is part of a multi-part series about using Coroutines on Android. This post focuses on solving practical problems using coroutines by implementing one shot requests.

Other articles in this series:

Coroutines on Android (part I): Getting the background

What problems do coroutines solve?

Coroutines on Android (part II): Getting started

Learn about CoroutineScope and how it can help you avoid leaking work on Android.

Solving real-world problems with coroutines

Part one and two of this series focused on how coroutines can be used to simplify code, provide main-safety on Android, and avoid leaking work. With that background, they look like a great solution to both background processing and a way to simplify callback based code on Android.

So far, we’ve focused on what coroutines are and how to manage them. In this post we’ll look at how to use them to accomplish some real tasks. Coroutines are a general purpose programming language feature at the same level as functions — so you can use them to implement anything that you could with functions and objects. However, there are two types of tasks that come up all the time in real code that coroutines are a great solution for:

1. One shot requests are requests that are run each time they are called — they always complete after the result is ready.
2. Streaming requests are requests that continue to observe changes and report them to caller — they don’t complete when the first result is ready.

Coroutines are a great solution to both of these tasks. In this post, we’ll look deeply into one shot requests and explore how to implement them using coroutines on Android.

One shot requests

A one shot request is performed once each time it’s called and completes as soon as a result is ready. This pattern is the same as a regular function call — it gets called, does some work, then returns. Due to the similarity to function calls they tend to be easier to understand than streaming requests.

A one shot request is performed each time it’s called. It stops executing as soon as a result is ready.

For an example of a one shot request, consider how your browser loaded this page. When you clicked the link to this post your browser sent a network request to the server to load the page. Once the page was transferred to your browser it stopped talking to the backend — it had all the data it needed. If the server modified the post, the new changes would not be shown in your browser — you would have to refresh the page.

So, while they lack the live-push of streaming requests, one shot requests are pretty powerful. There’s a whole lot of things you can do in an Android app that can be solved by one shot requests like fetching, storing, or updating data. It’s also a good pattern for things like sorting a list.

Problem: Displaying a sorted list

Let’s explore one-shot requests by looking at how you might display a sorted list. To make the example concrete, let’s build an inventory app for use by an employee at a store. It will be used to lookup products based on when they were last stocked — they’ll want to be able to sort the list both ascending and descending. It has so many products that sorting it may take almost a second — so we’ll use coroutines to avoid blocking the main thread!

In this app all of the products are stored in a Room database. This is a good use case to explore since it doesn’t need to involve a network request so we can focus on the pattern. Even though the example is simpler because it doesn’t use the network, it exposes the patterns needed to implement one shot requests.

To implement this request using coroutines, you will introduce coroutines to the ViewModel , Repository , and Dao . Lets walk through each one at a time and see how to integrate them with coroutines.

ProductsViewModel is responsible for receiving events from the UI layer, then asking the repository for the updated data. It uses LiveData to hold the currently sorted list to for display by the UI. When a new event comes in sortProductsBy starts a new coroutine to sort the list and updates the LiveData when the result is ready. The ViewModel is typically the right place to start most coroutines in this architecture, since it can cancel the coroutine in onCleared . If the user leaves the screen they usually have no use for outstanding work.

If you haven’t used LiveData much, check out this great post by @CeruleanOtter introducing how they work to store data for UIs.

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android — what is message queue native poll once in android?

I know that threads have a message queue and handlers are able to push runnables or messages to them, but when I profile my android application using Android Studio tools, there is a strange process:

It uses the CPU more than all the other processes. What is it and how can I reduce the time that the CPU spends on it? You can find the profiler result below.

1 Answer 1

Short answer:

The nativePollOnce method is used to «wait» till the next Message becomes available. If the time spent during this call is long, your main (UI) thread has no real work to do and waits for next events to process. There’s no need to worry about that.

Explanation:

Because the «main» thread is responsible for drawing UI and handling various events, it’s Runnable has a loop which processes all these events. The loop is managed by a Looper and its job is quite straightforward: it processes all Messages in the MessageQueue .

A Message is added to the queue for example in response to input events, as frame rendering callback or even your own Handler.post calls. Sometimes the main thread has no work to do (that is, no messages in the queue), which may happen e.g. just after finishing rendering single frame (the thread has just drawn one frame and is ready for the next one, just waits for a proper time). Two Java methods in the MessageQueue class are interesting to us: Message next() and boolean enqueueMessage(Message, long) . Message next() , as its name suggest, takes and returns the next Message from the queue. If the queue is empty (and there’s nothing to return), the method calls native void nativePollOnce(long, int) which blocks until a new message is added. At this point you might ask how does nativePollOnce know when to wake up. That’s a very good question. When a Message is added to the queue, the framework calls the enqueueMessage method, which not only inserts the message into the queue, but also calls native static void nativeWake(long) , if there’s need to wake up the queue. The core magic of nativePollOnce and nativeWake happens in the native (actually, C++) code. Native MessageQueue utilizes a Linux system call named epoll , which allows to monitor a file descriptor for IO events. nativePollOnce calls epoll_wait on a certain file descriptor, whereas nativeWake writes to the descriptor, which is one of the IO operations, epoll_wait waits for. The kernel then takes out the epoll-waiting thread from the waiting state and the thread proceeds with handling the new message. If you’re familiar with Java’s Object.wait() and Object.notify() methods, you can imagine that nativePollOnce is a rough equivalent for Object.wait() and nativeWake for Object.notify() , except they’re implemented completely differently: nativePollOnce uses epoll and Object.wait() uses futex Linux call. It’s worth noticing that neither nativePollOnce nor Object.wait() waste CPU cycles, as when a thread enters either method, it becomes disabled for thread scheduling purposes (quoting the javadoc for the Object class). However, some profilers may mistakenly recognize epoll-waiting (or even Object-waiting) threads as running and consuming CPU time, which is incorrect. If those methods actually wasted CPU cycles, all idle apps would use 100% of the CPU, heating and slowing down the device.

Conclusion:

You shouldn’t worry about nativePollOnce . It just indicates that processing of all Messages has been finished and the thread waits for the next one. Well, that simply means you don’t give too much work to your main thread 😉

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