# Preview Actions

The preview action is an action that allows the application to generate a preview of the action before the action is applied. The preview is controlled by the HUD UI.

PreviewActions contain one or more parameters which form the preview parameters of the action.

The PreviewAction has additional extra requirements compared to other Actions:

- - It can have multiple parameters whose values can be seen and changed over the bus
- - The parameters' ordering can be specified by client code
- - It adds new signals to control the preview mode: started, cancelled, reset

The ordering of the parameters is specified by the order in which the actions are defined in the parameters list.

PreviewAction does not have a value of it's own. Instead the values of the parameters provide the information on which the preview is generated.

HUD UI showing Color Balance action with four slider parameters

`\part`Control Signals Diagram describing when the different signals are emitted:

started signal informs the application that it must set up a preview view and start updating the preview based on the values of the defined preview parameters.

resetted signal informs the application that it has to reset the parameter values to their original values.

cancelled signal informs the application that the user cancelled the PreviewAction. At this point the application must return the the state it was in at the time the started() signal was triggered without applying any changes.

triggered signal informs the application the user wants the changes to be applied.

`\part`Example

    import QtQuick 2.0
    import Lomiri.Action 1.0

    Item {
        ActionManager {

            PreviewAction {
                text: "Exposure"

                PreviewRangeParameter {
                    id: exposureParameter
                    text: "Exposure Amount"
                    minimumValue: -50
                    maximumValue: 50
                    value: 25

                    onValueChanged: {
                        // gets the values from the HUD UI when user interacts with the slider
                        // Application then updates the preview based on this value
                        console.debug("range value: " + value);
                    }
                }

                onStarted: {
                    // action was activated in the HUD UI.
                    // do the necessary initialization and start providing preview
                    // based on the values of the parameters.
                }

                onResetted: {
                     // reset the parameter values
                }

                onCancelled: {
                    // action was cancelled
                }

                onTriggered: {
                    // action was completed.
                    // apply the action with the values of the parameters
                }
            }

        }
    }