Linearlayoutmanager android что это

Содержание
  1. RecyclerView
  2. getItemCount()
  3. onCreateViewHolder
  4. onBindViewHolder()
  5. Горизонтальная прокрутка
  6. Оптимизация
  7. LinearLayoutManager
  8. Class Overview
  9. Summary
  10. XML Attributes
  11. android.support.v7.recyclerview:reverseLayout
  12. android.support.v7.recyclerview:stackFromEnd
  13. android:orientation
  14. Constants
  15. public static final int HORIZONTAL
  16. public static final int INVALID_OFFSET
  17. public static final int VERTICAL
  18. Public Constructors
  19. public LinearLayoutManager (Context context)
  20. public LinearLayoutManager (Context context, int orientation, boolean reverseLayout)
  21. public LinearLayoutManager (Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)
  22. Public Methods
  23. public void assertNotInLayoutOrScroll (String message)
  24. public boolean canScrollHorizontally ()
  25. public boolean canScrollVertically ()
  26. public int computeHorizontalScrollExtent (RecyclerView.State state)
  27. public int computeHorizontalScrollOffset (RecyclerView.State state)
  28. public int computeHorizontalScrollRange (RecyclerView.State state)
  29. public PointF computeScrollVectorForPosition (int targetPosition)
  30. public int computeVerticalScrollExtent (RecyclerView.State state)
  31. public int computeVerticalScrollOffset (RecyclerView.State state)
  32. public int computeVerticalScrollRange (RecyclerView.State state)
  33. public int findFirstCompletelyVisibleItemPosition ()
  34. public int findFirstVisibleItemPosition ()
  35. public int findLastCompletelyVisibleItemPosition ()
  36. public int findLastVisibleItemPosition ()
  37. public View findViewByPosition (int position)
  38. public RecyclerView.LayoutParams generateDefaultLayoutParams ()
  39. public int getOrientation ()
  40. public boolean getRecycleChildrenOnDetach ()
  41. public boolean getReverseLayout ()
  42. public boolean getStackFromEnd ()
  43. public boolean isSmoothScrollbarEnabled ()
  44. public void onDetachedFromWindow (RecyclerView view, RecyclerView.Recycler recycler)
  45. public View onFocusSearchFailed (View focused, int focusDirection, RecyclerView.Recycler recycler, RecyclerView.State state)
  46. public void onInitializeAccessibilityEvent (AccessibilityEvent event)
  47. public void onLayoutChildren (RecyclerView.Recycler recycler, RecyclerView.State state)
  48. public void onRestoreInstanceState (Parcelable state)
  49. public Parcelable onSaveInstanceState ()
  50. public int scrollHorizontallyBy (int dx, RecyclerView.Recycler recycler, RecyclerView.State state)
  51. public void scrollToPosition (int position)
  52. public void scrollToPositionWithOffset (int position, int offset)
  53. public int scrollVerticallyBy (int dy, RecyclerView.Recycler recycler, RecyclerView.State state)
  54. public void setOrientation (int orientation)
  55. public void setRecycleChildrenOnDetach (boolean recycleChildrenOnDetach)
  56. public void setReverseLayout (boolean reverseLayout)
  57. public void setSmoothScrollbarEnabled (boolean enabled)

RecyclerView

Компонент RecyclerView появился в Android 5.0 Lollipop и находится в разделе Containers. Для простоты буду называть его списком, хотя на самом деле это универсальный элемент управления с большими возможностями.

Раньше для отображения прокручиваемого списка использовался ListView. Со временем у него обнаружилось множество недостатков, которые было трудно исправить. Тогда решили создать новый элемент с нуля.

Вначале это был сырой продукт, потом его доработали. На данном этапе можно считать, что он стал полноценной заменой устаревшего ListView.

Схематично работу RecyclerView можно представить следующим образом. На экране отображаются видимые элементы списка. Когда при прокрутке списка верхний элемент уходит за пределы экрана и становится невидимым, его содержимое очищается. При этом сам «чистый» элемент помещается вниз экрана и заполняется новыми данными, иными словами переиспользуется, отсюда название Recycle.

Компонент RecyclerView не является родственником ListView и относится к семейству ViewGroup. Он часто используется как замена ListView, но его возможности шире.

Следует сказать, что при работе с ним приходится писать много кода, который пугает новичков. Если с RecyclerView работать не постоянно, то порой забываются детали и сложно вспомнить необходимые шаги. Многие просто сохраняют отдельные шаблоны и копируют в разные проекты.

Внешний вид можно представить не только в виде обычного списка, но и в виде сетки. При необходимости можно быстро переключиться между разными типами отображения.

Для размещения своих дочерних элементов используется специальный менеджер макетов LayoutManager. Он может быть трёх видов.

  • LinearLayoutManager — дочерние элементы размещаются вертикально (как в ListView) или горизонтально
  • GridLayoutManager — дочерние элементы размещаются в сетке, как в GridView
  • StaggeredGridLayoutManager — неравномерная сетка

Можно создать собственный менеджер на основе RecyclerView.LayoutManager.

RecyclerView.ItemDecoration позволяет работать с дочерними элементами: отступы, внешний вид.

ItemAnimator — отвечает за анимацию элементов при добавлении, удалении и других операций.

RecyclerView.Adapter связывает данные с компонентом и отслеживает изменения.

  • notifyItemInserted(), notifyItemRemoved(), notifyItemChanged() — методы, отслеживающие добавление, удаление или изменение позиции одного элемента
  • notifyItemRangeInserted(), notifyItemRangeRemoved(), notifyItemRangeChanged() — методы, отслеживающие изменение порядка элеметов

Стандартный метод notifyDataSetChanged() поддерживается, но он не приводит к внешнему изменению элементов на экране.

Программисты со стажем знают, что для создания «правильного» ListView нужно было создавать класс ViewHolder. В старых списках его можно было игнорировать. Теперь это стало необходимым условием.

Общая модель работы компонента.

Мы рассмотрим только базовый пример для быстрого знакомства. В реальных проектах примеры будут гораздо сложнее, чтобы обеспечить другие функциональные возможности — обработка жестов, анимация, динамическое удаление и добавление элементов.

Размещаем компонент в макете экрана через панель инструментов. Но сначала добавим зависимость.

Создадим макет для отдельного элемента списка. Варианты могут быть самыми разными — можно использовать один TextView для отображения строк (имена котов), можно использовать связку ImageView и TextView (имена котов и их наглые морды). Мы возьмём для примера две текстовые метки. Создадим новый файл res/layout/recyclerview_item.xml.

Добавим компонент в разметку экрана активности.

Минимальный код для запуска.

Пока ничего сложного, но выводить такой список ничего не будет. Нужен адаптер и данные для отображения. В адаптере описывается способ связи между данными и компонентом.

Начнём по порядку, чтобы запомнить последовательность. Для начала создадим обычный класс и в конструкторе передадим список строк. Список будет содержать имена котов.

Класс MyViewHolder на основе ViewHolder служит для оптимизации ресурсов. Новый класс добавим в состав нашего созданного ранее класса.

В созданном классе нужно просто перечислить используемые компоненты из макета для отдельного элемента списка. В нашем примере задействованы два TextView, инициализируем их через идентификаторы.

Создадим адаптер — наследуем наш класс от класса RecyclerView.Adapter и в качестве параметра указываем созданный нами MyViewHolder. Студия попросит реализовать три метода.

getItemCount()

Как правило данные являются однотипными, например, список или массив строк. Адаптеру нужно знать, сколько элементов нужно предоставить компоненту, чтобы распределить ресурсы и подготовиться к отображению на экране. При работе с коллекциями или массивом мы можем просто вычислить его длину и передать это значение методу адаптера getItemCount(). В простых случаях мы можем записать код в одну строчку.

onCreateViewHolder

В методе onCreateViewHolder нужно указать идентификатор макета для отдельного элемента списка, созданный нами ранее в файле recyclerview_item.xml. А также вернуть наш объект класса ViewHolder.

onBindViewHolder()

В методе адаптера onBindViewHolder() связываем используемые текстовые метки с данными — в одном случае это значения из списка, во втором используется одна и та же строка.

Должно получиться следующее.

Подключаем в активности. Создадим пока бессмысленный список строк, который передадим в адаптер.

Запускаем ещё раз.

Вариант с числами нам не интересен, поэтому добавим котов. Имена котов и кошек разместим в ресурсах в виде массива в файле res/values/strings.xml.

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Создадим новую функцию для получения списка котов из ресурсов и передадим его адаптеру.

Горизонтальная прокрутка

Можем указать горизонтальный вариант прокрутки. Остальной код менять не придётся.

А можно вообще обойтись только XML-декларацией.

Оптимизация

При прокрутке под капотом выполняется сложная работа по обновлению контента. Поэтому не следует перегружать элементы списка сложной структурой и не перебарщивайте с вложенными элементами, а также нужно следить за оптимизацией, чтобы избежать лагов.

При работе с изображениями старайтесь использовать готовые библиотеки Picasso, Glide, Fresco и т.д.

Если картинки загружаются с сервера, неплохо заранее вычислять их размеры и пропорции. В некоторых случаях желательно позаботиться, чтобы картинки были одного размера (если это возможно).

Не перегружайте лишним кодом метод onBindViewHolder(). Только самое необходимое.

Источник

LinearLayoutManager

java.lang.Object
android.support.v7.widget.RecyclerView.LayoutManager
android.support.v7.widget.LinearLayoutManager
Known Direct Subclasses
GridLayoutManager A RecyclerView.LayoutManager implementations that lays out items in a grid.

Class Overview

A RecyclerView.LayoutManager implementation which provides similar functionality to ListView .

Summary

Nested Classes
LinearLayoutManager.LayoutChunkResult
XML Attributes
android.support.v7.recyclerview:reverseLayout LinearLayoutManager(Context,AttributeSet,int,int)
android.support.v7.recyclerview:stackFromEnd LinearLayoutManager(Context,AttributeSet,int,int)
android:orientation LinearLayoutManager(Context,AttributeSet,int,int)
[Expand]
Constants
int HORIZONTAL
int INVALID_OFFSET
int VERTICAL
Public Constructors

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Called when the LayoutManager should save its state.

Scroll the RecyclerView to make the position visible.

Smooth scroll to the specified adapter position.

Returns the amount of extra space that should be laid out by LayoutManager.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Override this method if you want to support scroll bars.

Called when the LayoutManager should save its state.

Smooth scroll to the specified adapter position.

Starts a smooth scroll using the provided SmoothScroller.

XML Attributes

android.support.v7.recyclerview:reverseLayout

Related Methods

android.support.v7.recyclerview:stackFromEnd

Related Methods

android:orientation

Related Methods

Constants

public static final int HORIZONTAL

public static final int INVALID_OFFSET

public static final int VERTICAL

Public Constructors

public LinearLayoutManager (Context context)

Creates a vertical LinearLayoutManager

Parameters
context Current context, will be used to access resources.

public LinearLayoutManager (Context context, int orientation, boolean reverseLayout)

Parameters
context Current context, will be used to access resources.
orientation Layout orientation. Should be HORIZONTAL or VERTICAL .
reverseLayout When set to true, layouts from end to start.

public LinearLayoutManager (Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes)

Constructor used when layout manager is set in XML by RecyclerView attribute «layoutManager». Defaults to vertical orientation.

Related XML Attributes

Public Methods

public void assertNotInLayoutOrScroll (String message)

Checks if RecyclerView is in the middle of a layout or scroll and throws an IllegalStateException if it is.

Parameters
message The message for the exception. Can be null.

public boolean canScrollHorizontally ()

Query if horizontal scrolling is currently supported. The default implementation returns false.

Returns

public boolean canScrollVertically ()

Query if vertical scrolling is currently supported. The default implementation returns false.

Returns

public int computeHorizontalScrollExtent (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current state of RecyclerView
Returns
  • The horizontal extent of the scrollbar’s thumb

public int computeHorizontalScrollOffset (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current State of RecyclerView where you can find total item count
Returns
  • The horizontal offset of the scrollbar’s thumb

public int computeHorizontalScrollRange (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current State of RecyclerView where you can find total item count
Returns
  • The total horizontal range represented by the vertical scrollbar

public PointF computeScrollVectorForPosition (int targetPosition)

public int computeVerticalScrollExtent (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current state of RecyclerView
Returns
  • The vertical extent of the scrollbar’s thumb

public int computeVerticalScrollOffset (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current State of RecyclerView where you can find total item count
Returns
  • The vertical offset of the scrollbar’s thumb

public int computeVerticalScrollRange (RecyclerView.State state)

Override this method if you want to support scroll bars.

Default implementation returns 0.

Parameters
state Current State of RecyclerView where you can find total item count
Returns
  • The total vertical range represented by the vertical scrollbar

public int findFirstCompletelyVisibleItemPosition ()

Returns the adapter position of the first fully visible view. This position does not include adapter changes that were dispatched after the last layout pass.

Note that bounds check is only performed in the current orientation. That means, if LayoutManager is horizontal, it will only check the view’s left and right edges.

Returns
  • The adapter position of the first fully visible item or NO_POSITION if there aren’t any visible items.
See Also

public int findFirstVisibleItemPosition ()

Returns the adapter position of the first visible view. This position does not include adapter changes that were dispatched after the last layout pass.

Note that, this value is not affected by layout orientation or item order traversal. ( setReverseLayout(boolean) ). Views are sorted by their positions in the adapter, not in the layout.

If RecyclerView has item decorators, they will be considered in calculations as well.

LayoutManager may pre-cache some views that are not necessarily visible. Those views are ignored in this method.

Returns
  • The adapter position of the first visible item or NO_POSITION if there aren’t any visible items.
See Also

public int findLastCompletelyVisibleItemPosition ()

Returns the adapter position of the last fully visible view. This position does not include adapter changes that were dispatched after the last layout pass.

Note that bounds check is only performed in the current orientation. That means, if LayoutManager is horizontal, it will only check the view’s left and right edges.

Returns
  • The adapter position of the last fully visible view or NO_POSITION if there aren’t any visible items.
See Also

public int findLastVisibleItemPosition ()

Returns the adapter position of the last visible view. This position does not include adapter changes that were dispatched after the last layout pass.

Note that, this value is not affected by layout orientation or item order traversal. ( setReverseLayout(boolean) ). Views are sorted by their positions in the adapter, not in the layout.

If RecyclerView has item decorators, they will be considered in calculations as well.

LayoutManager may pre-cache some views that are not necessarily visible. Those views are ignored in this method.

Returns
  • The adapter position of the last visible view or NO_POSITION if there aren’t any visible items.
See Also

public View findViewByPosition (int position)

Finds the view which represents the given adapter position.

This method traverses each child since it has no information about child order. Override this method to improve performance if your LayoutManager keeps data about child views.

If a view is ignored via ignoreView(View) , it is also ignored by this method.

Parameters
position Position of the item in adapter
Returns
  • The child view that represents the given position or null if the position is not laid out

public RecyclerView.LayoutParams generateDefaultLayoutParams ()

Create a default LayoutParams object for a child of the RecyclerView.

LayoutManagers will often want to use a custom LayoutParams type to store extra information specific to the layout. Client code should subclass RecyclerView.LayoutParams for this purpose.

Returns
  • A new LayoutParams for a child view

public int getOrientation ()

Returns the current orientaion of the layout.

Returns
See Also

public boolean getRecycleChildrenOnDetach ()

Returns whether LayoutManager will recycle its children when it is detached from RecyclerView.

Returns
  • true if LayoutManager will recycle its children when it is detached from RecyclerView.

public boolean getReverseLayout ()

Returns if views are laid out from the opposite direction of the layout.

Returns
See Also

public boolean getStackFromEnd ()

public boolean isSmoothScrollbarEnabled ()

Returns the current state of the smooth scrollbar feature. It is enabled by default.

Returns
  • True if smooth scrollbar is enabled, false otherwise.
See Also

public void onDetachedFromWindow (RecyclerView view, RecyclerView.Recycler recycler)

Called when this LayoutManager is detached from its parent RecyclerView or when its parent RecyclerView is detached from its window.

Subclass implementations should always call through to the superclass implementation.

Parameters
view The RecyclerView this LayoutManager is bound to
recycler The recycler to use if you prefer to recycle your children instead of keeping them around.

public View onFocusSearchFailed (View focused, int focusDirection, RecyclerView.Recycler recycler, RecyclerView.State state)

Called when searching for a focusable view in the given direction has failed for the current content of the RecyclerView.

This is the LayoutManager’s opportunity to populate views in the given direction to fulfill the request if it can. The LayoutManager should attach and return the view to be focused. The default implementation returns null.

Parameters
focused The currently focused view
focusDirection One of FOCUS_UP , FOCUS_DOWN , FOCUS_LEFT , FOCUS_RIGHT , FOCUS_BACKWARD , FOCUS_FORWARD or 0 for not applicable
recycler The recycler to use for obtaining views for currently offscreen items
state Transient state of RecyclerView
Returns

public void onInitializeAccessibilityEvent (AccessibilityEvent event)

public void onLayoutChildren (RecyclerView.Recycler recycler, RecyclerView.State state)

Lay out all relevant child views from the given adapter. The LayoutManager is in charge of the behavior of item animations. By default, RecyclerView has a non-null ItemAnimator , and simple item animations are enabled. This means that add/remove operations on the adapter will result in animations to add new or appearing items, removed or disappearing items, and moved items. If a LayoutManager returns false from supportsPredictiveItemAnimations() , which is the default, and runs a normal layout operation during onLayoutChildren(Recycler, State) , the RecyclerView will have enough information to run those animations in a simple way. For example, the default ItemAnimator, DefaultItemAnimator , will simply fade views in and out, whether they are actually added/removed or whether they are moved on or off the screen due to other add/remove operations.

A LayoutManager wanting a better item animation experience, where items can be animated onto and off of the screen according to where the items exist when they are not on screen, then the LayoutManager should return true from supportsPredictiveItemAnimations() and add additional logic to onLayoutChildren(Recycler, State) . Supporting predictive animations means that onLayoutChildren(Recycler, State) will be called twice; once as a «pre» layout step to determine where items would have been prior to a real layout, and again to do the «real» layout. In the pre-layout phase, items will remember their pre-layout positions to allow them to be laid out appropriately. Also, removed items will be returned from the scrap to help determine correct placement of other items. These removed items should not be added to the child list, but should be used to help calculate correct positioning of other views, including views that were not previously onscreen (referred to as APPEARING views), but whose pre-layout offscreen position can be determined given the extra information about the pre-layout removed views.

The second layout pass is the real layout in which only non-removed views will be used. The only additional requirement during this pass is, if supportsPredictiveItemAnimations() returns true, to note which views exist in the child list prior to layout and which are not there after layout (referred to as DISAPPEARING views), and to position/layout those views appropriately, without regard to the actual bounds of the RecyclerView. This allows the animation system to know the location to which to animate these disappearing views.

The default LayoutManager implementations for RecyclerView handle all of these requirements for animations already. Clients of RecyclerView can either use one of these layout managers directly or look at their implementations of onLayoutChildren() to see how they account for the APPEARING and DISAPPEARING views.

Parameters
recycler Recycler to use for fetching potentially cached views for a position
state Transient state of RecyclerView

public void onRestoreInstanceState (Parcelable state)

public Parcelable onSaveInstanceState ()

Called when the LayoutManager should save its state. This is a good time to save your scroll position, configuration and anything else that may be required to restore the same layout state if the LayoutManager is recreated.

RecyclerView does NOT verify if the LayoutManager has changed between state save and restore. This will let you share information between your LayoutManagers but it is also your responsibility to make sure they use the same parcelable class.

Returns
  • Necessary information for LayoutManager to be able to restore its state

public int scrollHorizontallyBy (int dx, RecyclerView.Recycler recycler, RecyclerView.State state)

Scroll horizontally by dx pixels in screen coordinates and return the distance traveled. The default implementation does nothing and returns 0.

Parameters
dx distance to scroll by in pixels. X increases as scroll position approaches the right.
recycler Recycler to use for fetching potentially cached views for a position
state Transient state of RecyclerView
Returns
  • The actual distance scrolled. The return value will be negative if dx was negative and scrolling proceeeded in that direction. Math.abs(result) may be less than dx if a boundary was reached.

public void scrollToPosition (int position)

Scroll the RecyclerView to make the position visible.

RecyclerView will scroll the minimum amount that is necessary to make the target position visible. If you are looking for a similar behavior to setSelection(int) or setSelectionFromTop(int, int) , use scrollToPositionWithOffset(int, int) .

Note that scroll position change will not be reflected until the next layout call.

Parameters
position Scroll to this adapter position
See Also

public void scrollToPositionWithOffset (int position, int offset)

Scroll to the specified adapter position with the given offset from resolved layout start. Resolved layout start depends on getReverseLayout() , getLayoutDirection(android.view.View) and getStackFromEnd() .

For example, if layout is VERTICAL and getStackFromEnd() is true, calling scrollToPositionWithOffset(10, 20) will layout such that item[10] ‘s bottom is 20 pixels above the RecyclerView’s bottom.

Note that scroll position change will not be reflected until the next layout call.

If you are just trying to make a position visible, use scrollToPosition(int) .

Parameters
position Index (starting at 0) of the reference item.
offset The distance (in pixels) between the start edge of the item view and start edge of the RecyclerView.
See Also

public int scrollVerticallyBy (int dy, RecyclerView.Recycler recycler, RecyclerView.State state)

Scroll vertically by dy pixels in screen coordinates and return the distance traveled. The default implementation does nothing and returns 0.

Parameters
dy distance to scroll in pixels. Y increases as scroll position approaches the bottom.
recycler Recycler to use for fetching potentially cached views for a position
state Transient state of RecyclerView
Returns
  • The actual distance scrolled. The return value will be negative if dy was negative and scrolling proceeeded in that direction. Math.abs(result) may be less than dy if a boundary was reached.

public void setOrientation (int orientation)

Sets the orientation of the layout. LinearLayoutManager will do its best to keep scroll position.

Parameters

public void setRecycleChildrenOnDetach (boolean recycleChildrenOnDetach)

Set whether LayoutManager will recycle its children when it is detached from RecyclerView.

If you are using a RecyclerView.RecycledViewPool , it might be a good idea to set this flag to true so that views will be avilable to other RecyclerViews immediately.

Note that, setting this flag will result in a performance drop if RecyclerView is restored.

Parameters
recycleChildrenOnDetach Whether children should be recycled in detach or not.

public void setReverseLayout (boolean reverseLayout)

Used to reverse item traversal and layout order. This behaves similar to the layout change for RTL views. When set to true, first item is laid out at the end of the UI, second item is laid out before it etc. For horizontal layouts, it depends on the layout direction. When set to true, If RecyclerView is LTR, than it will layout from RTL, if RecyclerView > is RTL, it will layout from LTR. If you are looking for the exact same behavior of setStackFromBottom(boolean) , use setStackFromEnd(boolean)

public void setSmoothScrollbarEnabled (boolean enabled)

When smooth scrollbar is enabled, the position and size of the scrollbar thumb is computed based on the number of visible pixels in the visible items. This however assumes that all list items have similar or equal widths or heights (depending on list orientation). If you use a list in which items have different dimensions, the scrollbar will change appearance as the user scrolls through the list. To avoid this issue, you need to disable this property. When smooth scrollbar is disabled, the position and size of the scrollbar thumb is based solely on the number of items in the adapter and the position of the visible items inside the adapter. This provides a stable scrollbar as the user navigates through a list of items with varying widths / heights.

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