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/**

* Copyright 2015 The AMP HTML Authors. All Rights Reserved.

*

* Licensed under the Apache License, Version 2.0 (the "License");

* you may not use this file except in compliance with the License.

* You may obtain a copy of the License at

*

* http://www.apache.org/licenses/LICENSE-2.0

*

* Unless required by applicable law or agreed to in writing, software

* distributed under the License is distributed on an "AS-IS" BASIS,

* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

* See the License for the specific language governing permissions and

* limitations under the License.

*/

import {Observable} from './observable';

import {getService} from './service';

import {getVendorJsPropertyName} from './style';

/**

*/

export class DocumentState {

/**

* @param {!Window} win

*/

constructor(win) {

/** @const {!Window} */

this.win = win;

/** @private @const {!Document} */

this.document_ = win.document;

/** @private @const {string|null} */

this.hiddenProp_ = getVendorJsPropertyName(this.document_, 'hidden', true);

if (this.document_[this.hiddenProp_] === undefined) {

this.hiddenProp_ = null;

}

/** @private @const {string|null} */

this.visibilityStateProp_ = getVendorJsPropertyName(this.document_,

'visibilityState', true);

if (this.document_[this.visibilityStateProp_] === undefined) {

this.visibilityStateProp_ = null;

}

/** @private @const */

this.visibilityObservable_ = new Observable();

/** @private @const {string|null} */

this.visibilityChangeEvent_ = null;

if (this.hiddenProp_) {

this.visibilityChangeEvent_ = 'visibilitychange';

const vendorStop = this.hiddenProp_.indexOf('Hidden');

if (vendorStop != -1) {

this.visibilityChangeEvent_ =

this.hiddenProp_.substring(0, vendorStop) + 'Visibilitychange';

}

}

/** @private @const {!Function} */

this.boundOnVisibilityChanged_ = this.onVisibilityChanged_.bind(this);

if (this.visibilityChangeEvent_) {

this.document_.addEventListener(this.visibilityChangeEvent_,

this.boundOnVisibilityChanged_);

}

}

/** @private */

cleanup_() {

if (this.visibilityChangeEvent_) {

this.document_.removeEventListener(this.visibilityChangeEvent_,

this.boundOnVisibilityChanged_);

}

}

/**

* Returns the value of "document.hidden" property. The reasons why it may

* not be visible include document in a non-active tab or when the document

* is being pre-rendered via link with rel="prerender".

* @return {boolean}

*/

isHidden() {

if (!this.hiddenProp_) {

return false;

}

return this.document_[this.hiddenProp_];

}

/**

* Returns the value of "document.visibilityState" property. Possible values

* are: 'hidden', 'visible', 'prerender', and 'unloaded'.

* @return {string}

*/

getVisibilityState() {

if (!this.visibilityStateProp_) {

return this.isHidden() ? 'hidden' : 'visible';

}

return this.document_[this.visibilityStateProp_];

}

/**

* @param {function()} handler

* @return {!UnlistenDef}

*/

onVisibilityChanged(handler) {

return this.visibilityObservable_.add(handler);

}

/** @private */

onVisibilityChanged_() {

this.visibilityObservable_.fire();

}

}

/**

* @param {!Window} window

* @return {!DocumentState}

*/

export function documentStateFor(window) {

return getService(window, 'documentState', () => {

return new DocumentState(window);

});

};