Chapter 18. Working with Scrolling Content

Table of Contents

Scroll Synchronization

Pixel based scrolling tables and scrolling layers provide the ability to scroll content in a smooth and uniform fashion. Configuring table's for scrolling is covered in the chapter Chapter 17, Creating Lists and Tables. Scrolling layers are based entirely on the bounds of their child group and control content and require a simple property enablement Enable layer scrolling behaviour within the Properties View.

When scrolling is enabled the object that is being scrolled, be it a layer or a table, is being moved as if it were positioned virtually and the object bounds provide the viewport into that virtual position. To track and measure this virtual positioning there are two internal Storyboard variables grd_xoffset and grd_yoffset that can be used to determine the virtual object position relative to the it's actual position. Considering the y offset value, when it is at 0 the content is aligned with the top of the control. When the y offset value is negative the content appears to be moving down.

When scrolling is enabled in the properties for an object the Storyboard Engine will start automatically tracking any press, release and motion events targeted at the object. While a press is active, the object will track exactly to the user motion. Once the press turns to a release, then the object will scroll automatically based on the scrolling properties configured.

Orientation Vertical or Horizontal scrolling
DecayMaximum time in milliseconds the scrolling layer will scroll after a flick
BounceNumber of pixels for the scrolling layer to bounce when reaching the edge of the contents within the layer
Scroll FrictionA number from 0 to 100 to determine the friction level of the scrolling layer. With higher friction it becomes more resistant to moving when swiped and more inclined to slowing down when released

Scroll Synchronization

When we have scrolling content, we frequently want some sort of visual indication of where we are within the context of that content, like a scrollbar. Storyboard doesn't dictate what type of visual presentation is used for this tracking, but we'll describe here how you might make a scrollbar using a simple fill render extension. This technique can be readily applied to any other visual representation and is demonstrated in the Scrollbar > Storyboard Sample as well as the Address Book Sample

The first thing that we want to consider is what information we want to convey to the user. For a simple scrollbar we will use a proportional measurement that represents the percentage that our y offset

    positionPercent = (-1 * objectYOffset) / (contentTotalHeight - objectHeight)                   

Here, we'll assumed a vertically scrolling list, but the same principle applies for horizontally scrolling. With this positionPercent we have a representation of where our viewport is with respect to the total content available.

In order to apply this formula to synchronize a scrollbar representation with the content as it scrolls, we need to have a notification of the scroll change. This can be accomplished by making an event association with a variable as described in the section called “Triggering Events on Variable Changes”. The variable that we want to bind an event to will be the internal variable grd_yoffset on the model object that we are scrolling.

With an event bound to an action, we might structure a synchronization function that might look something like the following:

-- Assume we are synchronizing a table named Layer.MyTable
function CBSyncScrollIndicator()
    -- These values could be cached, only yoffset would be changing normally
    local tableInfo = gre.get_table_attrs
    ("Layer.MyTable", "height", "yoffset", "rows")
    local cellInfo = gre.get_table_cell_attrs("Layer.MyTable", 1, 1, "height")
    local totalHeight = tableInfo.rows * cellInfo.height
    local positionPercent = (-1 * tablInfo.yoffset) / 
    (totalHeight - tableInfo.height)
    -- Now apply the position_percent to your scrollbar object