Загадочная проблема "прогрессивного замедления" в цикле выполнения / drawRect - PullRequest
7 голосов
/ 24 января 2011

Вот настоящая загадка.

Представьте, что вы выполняете обычный процесс рисования сложного изображения вне экрана (CGLayer), и вы периодически обновляете изображение на экране.

Как вы знаете, способ сделать это: запустить большой процесс рисования в фоновом режиме и при необходимости вызвать передний план для setNeedsDisplays, вызывая обновление изображения. Это тривиально и достигается с помощью двух строк кода.

Однако, когда вы делаете это, возникает загадочная проблема: время, затрачиваемое на каждый цикл рисования, увеличивается с каждым разом, становясь непригодным для использования . Кроме того, время может изменяться беспорядочно.

Это известная ошибка iOS или? Кто-нибудь видел это раньше?

Вот очень простое и понятное приложение, которое демонстрирует проблему:

http://www.fileswap.com/dl/p8lU3gAi/stepwiseDrawingV2.zip.html

Пример вывода ниже.

Кто-нибудь знает, почему это происходит? Это очень странное поведение со стороны ОС.

Позже ... FELZ разработал поразительный обходной путь. Felz копирует CGLayer один раз в каждом раунде. Это полностью останавливает странное поведение.

Однако до сих пор нет четкого понимания того, что именно происходит, черт возьми: отсюда, буквально, где используется время, когда проявляется странная проблема?

А вот длинный пример вывода ...

Обратите внимание, что иногда вы получаете «вдвойне странный» результат, подобный следующему: он демонстрирует «нормальное» причудливое поведение: время увеличивается с каждым обходом. Однако время от времени время снижается до «невероятно быстрого» в течение пары раундов, а затем возвращается. Странно, да? Кроме того, если вы работаете на симуляторе, быстро переходите вперед и назад к несвязанным приложениям на вашем Mac для получения «еще более странных» результатов.

Хотя Фельц дал прекрасный рабочий ответ, фактический механизм до сих пор остается загадкой.

:26:56.697 stepwiseDrawing[5334:1a03] time difference was     0
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:26:59.215 stepwiseDrawing[5334:1a03] time difference was     6
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:26:59.768 stepwiseDrawing[5334:1a03] time difference was   100
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:26:59.857 stepwiseDrawing[5334:1a03] time difference was     1
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Ответы [ 3 ]

10 голосов
/ 25 января 2011

Сначала позвольте мне сказать, что это была интересная головоломка.Мне было очень весело работать над этим.Хороший вопрос и хороший пример кода.

Вот мой вариант возможного ответа:

Я вижу следующую проблему.CGLayer отлично подходит для многократного рисования, потому что они кэшируют их рендеринг в плоское изображение.Кэш становится недействительным, когда к контексту добавляются дополнительные операции.Когда это происходит, CGLayer отображается снова.Замедление является причиной, потому что код поддерживает один CGLayer ( offscreenPrefabCGL ), контекст которого изменяется на каждой итерации цикла в paintActualGutsOfHugeImage .Поскольку последовательность операций в контексте offscreenPrefabCGL растет с каждым цветком, каждый рендеринг занимает больше времени.Это объясняет увеличение времени для каждого дисплея.

Я вижу два решения:

  1. Обрезать новый рендеринг в частности в PaintView.Вместо того, чтобы аннулировать полное представление, аннулируйте только тот CGRect, который действительно нуждается в перерисовке.Это, как я понимаю, решение, предложенное @ v01d.
  2. Свести слой.Вместо того, чтобы держать один CGLayer, копируйте CGLayer на каждую итерацию.Это сохраняет плоское изображение в слое, которое очень быстро рендерится.

Я реализовал (2) с хорошими результатами.Первые 100 цветов быстрее, а затем он стабилизируется через 120 мс (в устройстве).Я тестировал мою модифицированную версию с максимум 2000 цветами.

Моя модифицированная версия находится в:

http://dl.dropbox.com/u/9866261/felz_mod_stepwiseDrawingV2.zip

3 голосов
/ 25 января 2011

EDIT2:

Вот рабочий (без замедления) проект отладки: http://bit.ly/eukSgR

Этот проект ни в коем случае не является оптимальным.

Что интересно, так это необходимость растрового контекста.CGLayer кеширует ваше изображение и вызывает ваше прогрессивное замедление.С растровым контекстом это вынуждено сделать;и затем вы просто помещаете эти пиксели в CALayer UIView в drawRect:.

Я думаю, чтобы получить большую скорость, вы можете использовать setNeedsDisplayInRect: чтобы минимизировать область для перерисовки.

Но, кроме, может быть, оптимизации или двух здесь и там, проталкивание пикселей через UIView всегда будет медленнее, чем полное выполнение этого в графическом процессоре через, скажем, GL.

0 голосов
/ 25 января 2011

В качестве одного эксперимента я предлагаю вам обновить анимированное представление прогресса, не пытаясь рисовать или читать из незавершенного большого растрового изображения или в процессе рисования слоя.Считывание или рисование с незаконченным наложением в другой контекст рисования может делать интересные вещи для конвейера GPU или состояния рисования.Попробуйте нарисовать что-нибудь еще в своем перцовом методе.Простой счетчик или что-то.

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