/**
 * Contains the Cache class, which handles image download requests and caches the already downloaded images.
 * @module "ui/image-cache"
 */ /** */

import * as observable from "../../data/observable";
import * as imageSource from "../../image-source";

/**
 * Represents a single download request.
 */
export interface DownloadRequest {
    /**
     * The url of the image.
     */
    url: string;
    /**
     * The key used to cache the image.
     */
    key: string;
    /**
     * An optional function to be called when the download is complete.
     */
    completed?: (image: any, key: string) => void;
}

/**
 * Represents a class that stores handles image download requests and caches the already downloaded images.
 */
export class Cache extends observable.Observable {
    /**
     * String value used when hooking to downloaded event.
     */
    public static downloadedEvent: string;
    /**
     * The image to be used to notify for a pending download request - e.g. loading indicator.
     */
    placeholder: imageSource.ImageSource;
    /**
     * The maximum number of simultaneous download requests. Defaults to 5.
     */
    maxRequests: number;

    /**
     * Enables previously suspended download requests.
     */
    enableDownload(): void;
    /**
     * Temporary disables download requests.
     */
    disableDownload(): void;

    /**
     * Adds a new download request at the top of the download queue. This will be the next immediate download to start.
     */
    push(request: DownloadRequest);
    /**
     * Adds a new download request at the end of the download queue. This will be the last download to start.
     */
    enqueue(request: DownloadRequest);

    /**
     * Gets the image for the specified key. May be undefined if the key is not present in the cache.
     */
    get(key: string): any;
    /**
     * Sets the image for the specified key.
     */
    set(key: string, image: any): void;
    /**
     * Removes the cache for the specified key.
     */
    remove(key: string): void;
    /**
     * Removes all the previously cached images.
     */
    clear(): void;

    /**
     * A basic method signature to hook an event listener (shortcut alias to the addEventListener method).
     * @param eventNames - String corresponding to events (e.g. "propertyChange"). Optionally could be used more events separated by `,` (e.g. "propertyChange", "change"). 
     * @param callback - Callback function which will be executed when event is raised.
     * @param thisArg - An optional parameter which will be used as `this` context for callback execution.
     */
    on(eventNames: string, callback: (args: observable.EventData) => void , thisArg?: any);

    /**
     * Raised when the image has been downloaded.
     */
    on(event: "downloaded", callback: (args: DownloadedData) => void , thisArg?: any);

    //@private
    /**
     * @private
     */
    _downloadCore(request: DownloadRequest);
    /**
     * @private
     */
    _onDownloadCompleted(key: string, image: any);
    //@endprivate
}

/**
 * Provides data for downloaded event.
 */
export interface DownloadedData extends observable.EventData {
    /**
     * A string indentifier of the cached image.
     */
    key: string;
    /**
     * Gets the cached image.
     */
    image: imageSource.ImageSource;
}
