path: root/doc/src/howtos/scalabilityintro.qdoc

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/*!
    \title Scalability
    \page scalability.html
    \preliminary

    \omit preliminary docs for next SDK release \endomit
    \omit Somewhere I need to mention applications with more than
          one page (top-level layouts). \endomit

    A scalable application is an application that can run on more than
    one form factor. In particular, it can cope with different screen
    sizes, DPI, and aspect ratios. You need to consider scalability
    when:

    \list
        \o your application will be deployed to more than one device
           handset, or more than one device form factor.
        \o your application will be deployed for a long period of time,
           so that new device handsets might appear on the market after
           your initial deployment.
    \endlist

    This document discusses how scalable applications can be created.

    \section1 Developing Scalable UIs

    This section shows the basics of how we advice scalable
    applications to be implemented using QML. We recommend that you
    follow these techniques:

    \list
        \o Create separate top-level layout
           definitions for each form factor.
        \o Keep the layouts small and let components
           scale relative to their immediate parent.
        \o Define device independent measurements, such as dp
           (device independent pixels), and use
           these to scale components and for layout measurement.
        \o Define layouts in a
           proportional way using the built-in layout features of QML.
    \endlist

    Using small top-level layouts makes your codebase smaller and
    easier to maintain. Also, components that scales relative to their
    parent are more reusable. The layouts should be children of the
    application's root item. You can change between them by, for
    instance, using the opacity property of Item; that is, if your
    application has more tham one top-level layout. Such a top-level
    layout is also often referred to as a page, i.e., a layout that
    uses the entire screen. For instance, an organizer application
    will typically have separate pages for showing the calender and
    editing todo items.

    You should define the measurements separate from the UI, for
    instance by using a JavaScript object that you fill in with a
    script on application start up.

    QML provides several ways of laying out components, e.g, using
    anchor based layout, the more classic Grid; Column; and Row
    elements, and by setting the dimensions of Items directly. When
    laying out components in scalable applications, you should
    generally prefer using anchors and set width and height based on
    parent size where possible. Layouts are not only relevant to
    top-level layouts; components often contain child Items.

    The following sections describe in more detail the different
    aspects of scalability that should be considered in order to
    achieve the desired level of flexibility within your application.

    \section1 Implementing the Top-Level Layouts

    As mentioned, each application should use separate top-level
    layout QML definitions to support separate layout configurations /
    form factors.

    Consider an application that has to be deployed to at least two
    devices, which both have very different screen sizes and DPI
    values. The two form factors of the application will share many
    common components and attributes, and will most likely connect to
    the same data model.

    Therefore, the top-level definitions should be quite
    straightforward and short, with the majority of the functionality
    refactored into contained Components. It is important to try to
    avoid unnecessary duplication between these top-level definitions,
    in order to improve maintainability.

    There are some patterns that you might consider when designing
    your top level layouts:

    \list
    \o In some cases, the contents of an entire page in a smaller
     handset could form a component element of a layout in a
     larger device. Therefore, consider making that a separate
     component (i.e. defined in a separate QML file), and in the
     smaller handset, the Page will simply contain an instance of
     that component. On the larger device, there may be enough
     space to show two separate items. For example, in an email
     viewer, if the screen is large enough, it may be possible to
     show the email list view, and the email reader view side by
     side.
    \o In some cases, the contents of a view might be quite similar
     on all screen sizes, but with an expanded content area. In
     this case, it may be possible to re-use the same layout
     definition, if defined appropriately using anchors.
    \endlist

    The \l{Loader} component can be used to load separate QML files
    based on some criteria, such as Device Profile (configuration of
    screen pixel resolution and DPI density). In the case of form
    factor, this information will not change during the application's
    lifetime, therefore there is no issue with memory usage or
    performance.

    \section1 Defining Measurements

    When you are defining the measurements within an application or
    component layout, there are a number aspects to consider:

    \list
    \o The layout structure, the high-level relationship between
     items. Which item is the parent? How are the items arranged
     relatively on the screen? Are they in a grid or column?
    \o The layout measurements. How big is an item, or a margin
     inside the edge of an item, or an anchor between items?
    \o The implicit size of contained items. Some child items will
     require a certain amount of space, such as a button
     containing a text. That may also depend on the current
     platform and style. How do you ensure that you leave enough
     space, and what happens if your children change size?
    \endlist

    These aspects combine together to resolve the final layout for a
    given Device Profile. However, although there are dependencies
    between them, it is important to manage and control the different
    aspects separately.

    It is strongly recommended that Layout measurements should be
    stored in a separate place from the component layout structure
    definition files. The reason for this is that layout structure,
    for a given form factor, can be re-used for different Device
    Profiles. However, measurements will almost always vary between
    Device Profiles or Device Categories.

    If the opposite approach (complete duplication of entire QML
    files) was taken, then all of the layout states and structure
    definitions would be duplicated between the copied QML files, and
    only the measurement values would change.

    The main benefit of using separate measurement definition files
    are:

    \list
    \o To reduce the amount of duplication, and hence increase
      maintainability.
    \o It becomes much easier to change the layout structure,
     perhaps due to subsequent specification changes. In that
     case, the layout structure can be modified once, and many or
     all of the layout measurements would remain unchanged.
    \o It becomes much easier to add support for additional Device
     Profiles, simply by adding another measurement definition
     file.
    \endlist

    \section1 Using QML's Layout Features

    For a given form factor, top-level Layouts structure definitions,
    or component layout structure definitions, should in general be
    defined in a proportional way using a combination of

    \list
    \o \l{Item::anchors.top}{anchors} within an Item
    \o \l{Row} / \l{Column} / \l{Grid}
    \o simple javascript expressions such as width: Math.round(parent.width / 3.0).
    \endlist

    These basic building blocks, along with the powerful evaluation
    capabilities of javascript expressions within every QML binding,
    are designed to allow the majority of the layout structure
    definition to be defined in a Device Profile independent way.

    There are some limitations of the basic grid type layouts. They
    are designed to accommodate a number of Items, but use the current
    sizes of those items. There is a similar issue with the basic
    anchor type layout. In particular, it can be difficult to spread a
    number of child items proportionately across an area of their
    container.

    By combining the features of the layout managers with simple
    javascript expressions, a richer variety of designs can be
    expressed, without having to resort to additional layout
    measurement parameters or measurement values.

    Here are some things not to do with layouts:

    \list
    \o Don't define complex javascript functions that are regularly
     evaluated. This will cause poor performance, particularly
     during animated transitions.
    \o Don't define all of your layouts using x, y, width and
     height. Reserve this for items that cannot easily be defined
     using anchors (anchors are evaluated in a more efficient
     way).
    \o Don't make assumptions about the container size, or about
     the size of child items. Try to make flexible layout
     definitions that can absorb changes in the available space.
    \endlist

    \section1 Orientation Switches

    Application top-level page definitions, and reusable component
    definitions, should use one QML layout definition for the layout
    structure. This single definition should include the layout design
    for separate Device Orientations and Aspect Ratios. The reason for
    this is that performance during an orientation switch is critical,
    and it is therefore a good idea to ensure that all of the
    components needed by both orientations are loaded when the
    orientation changes.

    On the contrary, you should perform thorough tests if you choose
    to use a \l{Loader} to load additional QML that is needed in separate
    orientations, as this will affect the performance of the
    orientation change.

    In order to enable layout animations between the orientations, the
    anchor definitions must reside within the same containing
    component. Therefore the structure of a page or a component
    should consist of a common set of child components, a common set
    of anchor definitions, and a collection of states (defined in a
    StateGroup) representing the different aspect ratios supported by
    the component. (However note that orientation change animations
    are not possible on Symbian due to compatibility support for S60
    applications).

    If a component contained within a page needs to be
    hosted in numerous different form factor definitions, then the
    layout states of the view should depend on the aspect ratio of the
    page (its immediate container). Similarly, different instances of
    a component might be situated within numerous different containers
    in a UI, and so its layout states should be determined by the
    aspect ratio of its parent. The conclusion is that layout states
    should always follow the aspect ratio of the direct container (not
    the "orientation" of the current device screen).

    Within each layout \l{State}, you should define the relationships
    between items using native QML layout definitions. See below for
    more information. During transitions between the states (triggered
    by the top level orientation change), in the case of anchor
    layouts, AnchorAnimation elements can be used to control the
    transitions. In some cases, you can also use a NumberAnimation on
    e.g. the width of an item. Remember to avoid complex javascript
    calculations during each frame of animation. Using simple anchor
    definitions and anchor animations can help with this in the
    majority of cases.

    There are a few additional cases to consider:

    \list
    \o What if you have a single page that looks completely
     different between landscape and portrait, i.e. all of the
     child items are different? For each page, have two child
     components, with separate layout definitions, and make one
     or other of the items have zero opacity in each state. You
     can use a cross-fade animation by simply applying a
     NumberAnimation transition to the opacity.
    \o What if you have a single page that shares 30% or more of
     the same layout contents between portrait and landscape? In
     that case, consider having one component with landscape and
     portrait states, and a collection of separate child items
     whose opacity (or position) depends on the orientation
     state. This will enable you to use layout animations for the
     items that are shared between the orientations, whilst the
     other items are either faded in/out, or animated on/off
     screen.
    \o What if you have two pages on a handheld device that need to
     be on screen at the same time, for example on a larger form
     factor device? In this case, notice that your view component
     will no longer be occupying the full screen. Therefore it's
     important to remember in all components (in particular, list
     delegate items) should depend on the size of the containing
     component width, not on the screen width. It may be
     necessary to set the width in a Component.onCompleted()
     handler in this case, to ensure that the list item delegate
     has been constructed before the value is set.
    \o What if the two orientations take up too much memory to have
     them both in memory at once? Use a \l{Loader} if necessary, if
     you cannot keep both versions of the view in memory at once,
     but beware performance on the cross-fade animation during
     layout switch. One solution could be to have two "splash
     screen" items that are children of the Page, then you cross
     fade between those during rotation. Then you can use a
     \l{Loader} to load another child component that loads the actual
     model data to another child Item, and cross-fade to that
     when the \l{Loader} has completed.
    \endlist
   */