The common way to present to the user a list of data elements of undefined length is to start with a small portion at the top of the list - just enough to fill the space on the page. Additional rows are appended to the bottom of the list as the user scrolls down the list.

The problem with this approach is that even though rows at the top of the list become invisible as they scroll out of the view, they are still a part of the page and still consume resources. As the user scrolls down the list grows and the web app slows down.

This becomes a real problem if the html representing a row has event handlers and/or angular watchers attached. A web app of an average complexity can easily introduce 20 watchers per row. Which for a list of 100 rows gives you total of 2000 watchers and a sluggish app.

uiScroll directive

uiScroll directive solves this problem by dynamically destroying elements as they become invisible and recreating them if they become visible again.

Look here for older versions.

###Description

The uiScroll directive is similar to the ngRepeat. Like the ngRepeat, uiScroll directive instantiates a template once per item from a collection. Each template instance gets its own scope, where the given loop variable is set to the current collection item. The collection content is provided by the datasource. The datasource name is specified in the scroll_expression. Starting with v 1.2.0 uiScroll supports animation.

One of the most important use cases for the directive is using it to build scrollable grids. There is no magic in doing it - all what's involved is putting together the HTML to be repeated and there are numerous ways to do it. It is a bit trickier if you want to give your user ability to adjust the grid (column widts, column order, etc.) as well as save/restore the adjustments. uiScrollTd and uiScrollTh companion directives were created to simplify this task.

Dependencies

To use the directive make sure the dist/ui-scroll.js is loaded in your page. You also have to include module name 'ui.scroll' on the list of your application module dependencies. The directive supports angular version 1.2 or later. Earlier versions of angular are no longer supported.

The code in this file relies on a few DOM element methods of jQuery which are currently not implemented in jQlite, namely

• before(elem)
• height() and height(value)
• outerHeight() and outerHeight(true)
• scrollTop() and scrollTop(value)
• offset()

File dist/ui-scroll-jqlite.js houses implementations of the above methods and also has to be loaded in your page. Please note that the methods are implemented in a separate module 'ui.scroll.jqlite' and this name should also be included in the dependency list of the main module. The implementation currently supports missing methods only as necessary for the directive. It is tested on IE8 and up as well as on the Chrome 28 and Firefox 20.

This module is only necessary if you plan to use ui-scroll without jQuery. If full jQuery is loaded, uiScroll will use the jQuery implementatin of the above methods, the ui.scroll.jqlite implementation of them will be ignored. If you plan to use ui-scroll over jQuery feel free to skip ui-scroll-jqlite.

###Usage

<ANY ui-scroll="{scroll_expression}" buffer-size="value" padding="value" start-index="value" adapter="expression">
      ...
</ANY>

Listing ANY for the tag, the directive can be applied to, stretches the truth - a little bit. The directive works well with majority of the 'usual' tags - divs, spans, a, inputs, etc. For all of them the viewport should be a div (unless it is the window). Some other tags require special treatment. If the repeated tag is a li, it is best to use ul or ol as a viewport. For a tr as a repeated tag the viewport has to be the table or tbody. dl as a repeated tag is not supported.

The viewport is an element representing the space where the items from the collection are to be shown. Unless specified explicitly with the uiScrollViewport directive, browser window will be used as the viewport.

Important: viewport height must be constrained. The directive will stop asking the datasource for more elements only when it has enough to fill out the viewport. If the height of the viewport is not constrained (style="height:auto") it will pull the entire content of the datasource and may throw an Error depending on the number of items in the datasource. Even if it does not, using the directive this way does not provide any advantages over using ng-repeat, because item template will be always instantiated for every item in the datasource.

###Directive info

• This directive creates a new scope
• This directive executes at priority level 1000

###Parameters

• uiScroll – scroll expression – The expression indicating how to enumerate a collection. Only one format is currently supported:
  • variable in datasource – where variable is the user defined loop variable and datasource is the name of the data source to enumerate.
• buffer-size - expression, optional - number of items requested from the datasource in a single request. The default is 10 and the minimal value is 3
• padding - expression, optional - extra height added to the visible area for the purpose of determining when the items should be created/destroyed. The value is relative to the visible height of the area, the default is 0.5 and the minimal value is 0.3
• start-index - expression, optional - index of the first item to be requested from the datasource. The default is 1.
• adapter - assignable expression, optional - if provided a reference to the adapter object for the scroller instance will be injected in the appropriate scope. If you have multiple scrollers within the same viewport, make sure that every one of them has its unique adapter name.

Some of the properties offered by the adapter can also be accessed directly from the directive by using matching attributes. In the same way as for the adapter attribute, syntax for such attributes allows for providing a reference expression to be used to access the corresponding value. Below is a list of such attributes:

• is-loading - assignable expression, optional - a boolean value indicating whether there are any pending load requests will be injected in the appropriate scope. See also isLoading adapter property.
• top-visible - assignable expression, optional - a reference to the item currently in the topmost visible position will be injected in the appropriate scope. See also topVisible adapter property.
• top-visible-element - assignable expression, optional - a reference to the DOM element currently in the topmost visible position will be injected in the appropriate scope. See also topVisibleElement adapter property.
• top-visible-scope - assignable expression, optional - a reference to the scope created for the item currently in the topmost visible position will be injected in the appropriate scope. See also topVisibleScope adapter property.

The expression can be any angular expression (assignable expression where so specified). All expressions are evaluated once at the time when the scroller is initalized. Changes in the expresion value after scroller intialization will have no impact on the scroller behavior.

The assignable expressions will be used by scroller to inject the requested value into the target scope. The scope associated with the viewport (the element marked with the uiScrollViewport directive) will be used as the target scope. If the viewport is not defined (window viewport), the $rootScope will be used as the target scope.

The second format expression on controller can be used to explicitly target the scope associated with the specified controller as the target scope for the injection. In this format expression is any angular assignable expression, and controller is the name of controller constructor function as specified in the ng-controller directive. The scroller will traverse its parents to locate the target scope associated with the specified controller.

###Data Source Data source is an object to be used by the uiScroll directive to access the data.

The directive will locate the object using the provided data source name. It will first look for a property with the given name on its $scope. If none found it will try to get an angular service with the provided name.

The data source object implements methods and properties to be used by the directive to access the data:

• Method get

    get(descriptor, success)
  

  or

    get(index, count, success)
  

  Description

  this is a mandatory method used by the directive to retrieve the data.

Parameters

* **descriptor** is an object defining the portion of the dataset requested. The object will have 3 properties. Two of them named  `index` and `count`. They have the same meaning as in the alternative signature when the parameters passed explicitly (see below). The third one will be named either `append` if the items will be appended to the last item in the buffer, or `prepend` if they are to be prepended to the first item in the buffer. The value of the property in either case will be the item the new items will be appended/prepended to. This is useful if it is easier to identify the items to be added based on the previously requested items rather than on the index. Keep in mind that in certain use cases (i.e. on initial load) the value of the append/prepend property can be undefined.
* **index** indicates the first data row requested
* **count** indicates number of data rows requested
* **success** function to call when the data are retrieved. The implementation of the data source has to call this function when the data are retrieved and pass it an array of the items retrieved. If no items are retrieved, an empty array has to be passed.

Important: Make sure to respect the index and count parameters of the request. The array passed to the success method should have exactly count elements unless it hit eof/bof

• Properties minIndex and maxIndex

  Description

  As the scroller recieves the items requested by the get method, the value of minimum and maximum values of the item index are placed in the minIndex and maxIndex properties respectively. The values of the properties are cumulative - the value of the minIndex will never increase, and the value of the maxIndex will never decrease - except the values are reset in response to a call to the adapter reload method. The values of the properties are used to maintain the appearance of the scroller scrollBar.

  Values of the properties can be assigned programmatically. If the range of the index values is known in advance, assigneing them programmatically would improve the usability of the scrollBar.

###Adapter The adapter object is an internal object created for every instance of the scroller. Properties and methods of the adapter can be used to manipulate and assess the scroller the adapter was created for. Adapter based API replaces old (undocumented) event based API introduced earlier for this purpose. The event based API is now deprecated and no longer supported.

Adapter object implements the following properties:

• isLoading - a boolean value indicating whether there are any pending load requests.
• topVisible - a reference to the item currently in the topmost visible position.
• topVisibleElement - a reference to the DOM element currently in the topmost visible position.
• topVisibleScope - a reference to the scope created for the item currently in the topmost visible position.
• disabled - setting disabled to true disables scroller's scroll/resize events handlers. This can be useful if you have multiple scrollers within the same scrollViewport and you want to prevent some of them from responding to the events.

Adapater object implements the following methods

• Method isBOF

    isBOF()
  

  Description

  returns true if the first item of the dataset is already in the buffer. No further requests for preceding items will be issued, returns false otherwise.

• Method isEOF

    isEOF()
  

  Description

  returns true if the last item of the dataset is already in the buffer. No further requests for tailing items will be issued, returns false otherwise,

• Method reload

    reload()
  

  or

    reload(startIndex)
  

  Description

  Calling this method reinitializes and reloads the scroller content. startIndex is an integer indicating what item index the scroller will use to start the load process. The value of the argument replaces the value provided with the start-index attribute. Calling reload() is equivalent to calling reload method with current value of the start-index attribute .

  important: startIndex should fall within underlying datset boundaries The scroller will request two batches of items one starting from the startIndex and another one preceding the first one (starting from startIndex - bufferSize). If both requests come back empty, the scroller will consider the dataset to be empty and will place no further data requests.

• Method applyUpdates

        applyUpdates(index, newItems)
  

  Description

  Replaces the item in the buffer at the given index with the new items.

Parameters

* **index** provides position of the item to be affected in the dataset (not in the buffer). If the item with the given index currently is not in the buffer no updates will be applied. `$index` property of the item $scope can be used to access the index value for a given item
* **newItems** is an array of items to replace the affected item. If the array is empty (`[]`) the item will be deleted, otherwise the items in the array replace the item. If the newItem array contains the old item, the old item stays in place.

        applyUpdates(updater)

#### Description
Updates scroller content as determined by the updater function

Parameters

* **updater** is a function to be applied to every item currently in the buffer. The function will receive 3 parameters: `item`, `scope`, and `element`. Here `item` is the item to be affected, `scope` is the item $scope, and `element` is the html element for the item. The return value of the function should be an array of items. Similarly to the `newItem` parameter (see above), if the array is empty(`[]`), the item is deleted, otherwise the item is replaced by the items in the array. If the return value is not an array, the item remains unaffected, unless some updates were made to the item in the updater function. This can be thought of as in place update.

• Method append

        append(newItems)
  

  Description

  Adds new items after the last item in the buffer.

Parameters

* **newItems** provides an array of items to be appended.

• Method prepend

        prepend(newItems)
  

  Description

  Adds new items before the first item in the buffer.

Parameters

* **newItems** provides an array of items to be prepended.

####Manipulating the scroller content with adapter methods

Adapter methods applyUpdates, append and prepend provide a way to update the scroller content without full reload of the content from the datasource. The updates are performed by changing the items in the scroller internal buffer after they are loaded from the datasource. Items in the buffer can be deleted or replaced with one or more items.

Important: update datasource to match the scroller buffer content: Keep in mind that the modifications made by the adapter methods are only applied to the content of the buffer. As the items in response to scrolling are pushed out of the buffer, the modifications are lost. It is your responsibility to ensure that as the scroller is scrolled back and a modified item is requested from the datasource again the values returned by the datasource would reflect the updated state. In other words you have to make sure that in addition to manipulating the scroller content you also apply the modifications to the dataset underlying the datasource.

###Animations In the fashion similar to ngRepeat the following animations are supported:

• .enter - when a new item is added to the list
• .leave - when an item is removed from the list

Animations are only supported for the updates made via applyUpdates method. Updates caused by scrolling are not going through animation transitions. Usual rules of working with Angular animations apply. Look here for an example of animations in the scroller

uiScrollViewport directive

###Description

The uiScrollViewport directive marks a particular element as viewport for the uiScroll directive. If no parent of the uiScroll directive is marked with uiScrollViewport directive, the browser window object will be used as viewport

###Usage

<ANY ui-scroll-viewport>
      ...
</ANY>

uiScrollTh and uiScrollTd directives

###Description

The uiScrollTh and uiScrollTd directives provide a way to build flexible dynamic grids. Handling of grid rows is done by the uiScroll directive itself. In addition to this uiScrollTh and uiScrollTd directive provide tools to programmatically change grid layout, inclduing applying styles to columns, changing column size and order, as well as saving the modifications to the layout and applying previosly saved layouts. At this point the above functionality is supported only for table based scrollable grids.

###Usage

Here is the basic html template for scrollable grid using the uiScrollTh and uiScrollTd directives. Keep in mind that the height of the scroll viewport (in this case the <TABLE> tag) should be constrained. Also make sure that the initial column widths are applied uniformly to both headers (<TH>) and cells (<TD>)

<TABLE ui-scroll-viewport class="grid">
    <THEAD style="display:block">
       <TR>
         <TH ui-scroll-th class="col1">header 1...</TH>
         <TH ui-scroll-th class="col2">header 2...</TH>
         ...
       </TR>
    </THEAD>
    <TBODY>
       <TR ui-scroll="item in datasource" adapter="adapter">
         <TD ui-scroll-td class="col1">...</TD>
         <TD ui-scroll-td class="col2">...</TD>
         ...
       </TR>
    </TBODY>
</TABLE>

Dependencies

The grid directives have the same dependency requirements as the uiScroll directive itself. To use the directives make sure the ui.scroll.grid module is on the list of the module dependencies. Also you have to load the dist/ui-scroll-grid.js file in your page.

Controlling scrollable grid with the GridAdapter

GridAdapter object (along with ColumnAdapter objects) provides methods and properties to be used to change the scrollable grid layout. A reference to this object is injected as a property named gridAdapterin the scroller adapter.

GridAdapter object implements the following properties:

• Property columns - returns an array of ColumnAdapter objects to be used to control the scrollable grid layout. The columns are listed in the same order as they appear in the browser.

GridAdapter object implements the following methods:

• Method getLayout() - returns an object describing current scrollable grid layout.
• Method applyLayout(layout) - restores scrollabel grid layout to the state as defined by the object passed as the parameter
• Method columnFromPoint(x,y) - if the coordinates belong to a scrollable grid column returns the appropriate ColumnAdapter object. Otherwise returns undefined.

ColumnAdapter object implements the following methods:

• Method css(name, value) - sets the css property name to value for the column header as well as for the column cells.
• Method moveBefore(column) - moves the column in front of the column refrenced by the parameter. If the parameter is null the column will become the rightmost column.

###Examples

Examples (look here for sources) consist of several pages (.html files) showing various ways to use the ui-scroll directive. Each page relays on its own datasource service (called datasource) defined in the javascript file with the same name and .js extension.

I intentionally broke every rule of proper html/css structure (i.e. embedded styles). This is done to keep the html as bare bones as possible and leave it to you to do it properly - whatever properly means in your book.

To run the examples use this link

###History

###v1.5.1

• Refactored adjustments during elements append/prepend.
• Bof/eof information is accessible out of the scroller.

###v1.5.0

• Implemented uiScrollTh and uiScrollTd directives; uiScroll major refactoring.
• Added "disabled" property to the adapter object to ignore user's scroll events.
• Implemented "on controller" syntax to specify the scope where an adapter object has to be injected.

###v1.4.1

• Developed a new complex approach of paddings elements height calculation (see details).
• Added startIndex attribute.
• Changed clipTop/clipBottom methods logic.
• Some new demos, tests, cleanup and other minor refactoring.

###v1.4.0

• Migrated sources from CoffeeScript to ES6.
• Optimized scroll events handling, removed odd $digest cycles.
• Examples (demo) refactoring.

###v1.3.3

• Implemented new signature of the Datasource get(descriptor, success) method.
• Implemented new signature of the Adapter reload(startIndex) method.
• Changed the logic of scroll bar adjustment (minIndex, maxIndex).

###v1.3.2

• Implemented the logic for adjustBuffer triggering during invisible items became visible.

###v1.3.1

• Changed the logic of viewport scroll/padding recalculation (to solve the problem #8).
• Splitted test specifications.
• Updated dev-dependencies (package.json).
• Implemented append/prepend methods on the adapter.

###v1.3.0

• Reorganized the repository structure.

####v1.2.1

• Dismiss pending requests on applyUpdates().

####v1.2.0

• Changed the algorithm of list items building.
• Integration with angular $animation.
• Insert/update/delete events are no longer supported.

####v1.1.2

• Fixed inserting elements via applyUpdates error.

####v1.1.1

• Fixed jqlite on $destroy error.

####v1.1.0

• Introduced API to dynamically update scroller content.
• Deep 'name' properties access via dot-notation in template.
• Fixed the problem occurring if the scroller is $destroyed while there are requests pending: #64.