帶型帶秀專(zhuān)題之 Lazy Load (二)

tigerZH 發(fā)布于2019-08-21 17:23 / 2506人閱讀

摘要：參考文章使用及源碼分析關(guān)于插件的基本介紹和使用請(qǐng)看上一篇文章。顯示方法默認(rèn)為，也可以設(shè)置為，源碼中隱藏了一個(gè)配置屬性，用于設(shè)置動(dòng)畫(huà)運(yùn)行的時(shí)間。是否忽略隱藏的元素設(shè)置為時(shí)會(huì)忽略處理隱藏的元素。在觸發(fā)事件時(shí)執(zhí)行的回調(diào)。

參考文章：jQuery.lazyload使用及源碼分析

關(guān)于 jQuery lazyload 插件的基本介紹和使用請(qǐng)看上一篇文章。（水水一章啦-。-）

Overview

讓我們先從整體上看看代碼的結(jié)構(gòu)：

(function ($, window, document, undefined) {
  var $window = $(window);
  
  /* lazyload 方法主體 */
  $.fn.lazyload = function (options) {
  // ...
  }

  /* 在jQuery命名空間內(nèi)定義了便捷的方法，判斷圖片是否在容器視口范圍內(nèi) */
  $.belowthefold = function (element, settings) {...}
  $.rightoffold = function (element, settings) {...}
  $.abovethetop = function (element, settings) {...}
  $.leftofbegin = function (element, settings) {...}
  $.inviewport = function (element, settings) {...}

  /* 自定義選擇器 */
  $.extend($.expr[":"], {...});
  
})(jQuery, window, document);

可以看一下注釋?zhuān)筒欢嘧鼋忉屃恕?/p> 選項(xiàng)

在上一節(jié)可以看到，使用lazyload最簡(jiǎn)單的方法像這樣：

$("img.lazy").lazyload();

復(fù)雜一點(diǎn)的，我們可以傳入一個(gè)選項(xiàng)對(duì)象自定義lazyload的表現(xiàn)。讓我們從源碼看一下所有可選的選項(xiàng)參數(shù)：

$.fn.lazyload = function (options) {
  var elements = this;
  var $container;
  
  // 默認(rèn)配置
  var settings = {
    threshold       : 0,
    failure_limit   : 0,
    event           : "scroll.lazyload",
    effect          : "show",
    container       : window,
    data_attribute  : "original",
    data_srcset     : "srcset",
    skip_invisible  : false,
    appear          : null,
    load            : null,
    placeholder     : "data:image/gif;base64,R0lGODdhAQABAPAAAMPDwwAAACwAAAAAAQABAAACAkQBADs="
  }
}

分別解釋一下：

threshold : 臨界值

threshold: 0

這個(gè)值是針對(duì)container容器的，即距離container容器視口的臨界值

event : 事件

event: "scroll.lazyload"

container容器默認(rèn)綁定這個(gè)事件，在這個(gè)事件被觸發(fā)時(shí)，會(huì)不斷的判斷img元素是否滿足出發(fā)apper事件（即顯示圖片）的條件。默認(rèn)的事件為scroll，你可以自定義這個(gè)事件。

effect :顯示方法

effect: "show"

默認(rèn)為show，也可以設(shè)置為fadeIn，源碼中隱藏了一個(gè)配置屬性：effectspeed，用于設(shè)置動(dòng)畫(huà)運(yùn)行的時(shí)間。

data_attribute : 圖片地址屬性

data_attribute: "original"

img元素的一個(gè)data屬性，用于存放圖片的真實(shí)地址。

skip_invisible : 是否忽略隱藏的img元素

skip_invisible: false

設(shè)置為true時(shí)會(huì)忽略處理隱藏的img元素。

placeholder : 圖片占位符

placeholder : "data:image/gif;base64,R0lGODdhAQABAPAAAMPDwwAAACwAAAAAAQABAAACAkQBADs="

lazyload會(huì)將該屬性的值設(shè)置為img元素的初始src屬性值。

appear

appear: null

在img觸發(fā)load事件時(shí)執(zhí)行的回調(diào)。

failure_limit

failure_limit: 0

為了便于理解，我們先來(lái)看一段與其有關(guān)的源碼：

var counter = 0;

elements.each(function() {
    if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
        // ...
    } else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
        // ...
    } else {
        if (++counter > settings.failure_limit) {
            return false;
        }
    }
});

如果找到的是第failure_limit個(gè)img元素，且不在container視口上方，左方及視口內(nèi)（可以允許在視口下方，右方），則中斷循環(huán)。

兩個(gè)示例：

demo1 下拉滾動(dòng): http://jsfiddle.net/ddEPL/
demo2 Tab切換: http://jsfiddle.net/ddEPL/1/>...

源碼分析

/*
 * Lazy Load - jQuery plugin for lazy loading images
 *
 * Copyright (c) 2007-2013 Mika Tuupola
 *
 * Licensed under the MIT license:
 *   http://www.opensource.org/licenses/mit-license.php
 *
 * Project home:
 *   http://www.appelsiini.net/projects/lazyload
 *
 * Version:  1.9.3
 *
 */

(function($, window, document, undefined) {
    var $window = $(window);

    $.fn.lazyload = function(options) {
        var elements = this;
        var $container;
        var settings = {
            threshold       : 0,
            failure_limit   : 0,
            event           : "scroll",
            effect          : "show",
            container       : window,
            data_attribute  : "original",
            skip_invisible  : true,
            appear          : null,
            load            : null,
            placeholder     : "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
        };

        function update() {
            var counter = 0;

            elements.each(function() {
                var $this = $(this);
                // 如果圖片隱藏，且忽略隱藏，則中斷循環(huán)
                if (settings.skip_invisible && !$this.is(":visible")) {
                    return;
                }
                if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) {
                        /* Nothing. */
                } 
                // img滿足在container視口中，則顯示
                else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) {
                        $this.trigger("appear");
                        /* if we found an image we"ll load, reset the counter */
                        counter = 0;
                }
                // 如果找到的是第（failure_limit + 1）個(gè)img元素，且不在container視口上方，左方及視口內(nèi)（可以允許在視口下方，右方），
                // 則中斷循環(huán)
                else {
                    if (++counter > settings.failure_limit) {
                        return false;
                    }
                }
            });

        }

        if(options) {
            /* Maintain BC for a couple of versions. */
            if (undefined !== options.failurelimit) {
                options.failure_limit = options.failurelimit;
                delete options.failurelimit;
            }
            if (undefined !== options.effectspeed) {
                options.effect_speed = options.effectspeed;
                delete options.effectspeed;
            }

            $.extend(settings, options);
        }

        /* Cache container as jQuery as object. */
        $container = (settings.container === undefined ||
                      settings.container === window) ? $window : $(settings.container);

        /* Fire one scroll event per scroll. Not one scroll event per image. */
        // 為container綁定scroll事件
        if (0 === settings.event.indexOf("scroll")) {
            $container.bind(settings.event, function() {
                return update();
            });
        }

        this.each(function() {
            var self = this;
            var $self = $(self);

            self.loaded = false;

            /* If no src attribute given use data:uri. */
            // 設(shè)置占位符
            if ($self.attr("src") === undefined || $self.attr("src") === false) {
                if ($self.is("img")) {
                    $self.attr("src", settings.placeholder);
                }
            }

            /* When appear is triggered load original image. */
            // one綁定appear，觸發(fā)后則移除該事件
            $self.one("appear", function() {
                if (!this.loaded) {
                    // 存在回調(diào)則觸發(fā)
                    if (settings.appear) {
                        var elements_left = elements.length;
                        settings.appear.call(self, elements_left, settings);
                    }
                    $("")
                        .bind("load", function() {

                            var original = $self.attr("data-" + settings.data_attribute);
                            $self.hide();
                            if ($self.is("img")) {
                                $self.attr("src", original);
                            } else {
                                $self.css("background-image", "url("" + original + "")");
                            }
                            $self[settings.effect](settings.effect_speed);

                            self.loaded = true;

                            /* Remove image from array so it is not looped next time. */
                            // 更新elements，過(guò)濾掉已經(jīng)加載的img元素，避免下次在update中輪循
                            var temp = $.grep(elements, function(element) {
                                return !element.loaded;
                            });
                            elements = $(temp);
                            
                            // 存在回調(diào)則觸發(fā)
                            if (settings.load) {
                                var elements_left = elements.length;
                                settings.load.call(self, elements_left, settings);
                            }
                        })
                        .attr("src", $self.attr("data-" + settings.data_attribute));
                }
            });

            /* When wanted event is triggered load original image */
            /* by triggering appear.                              */
            // 綁定不是scroll的事件，用于觸發(fā)appear事件
            if (0 !== settings.event.indexOf("scroll")) {
                $self.bind(settings.event, function() {
                    if (!self.loaded) {
                        $self.trigger("appear");
                    }
                });
            }
        });

        /* Check if something appears when window is resized. */
        $window.bind("resize", function() {
            update();
        });

        /* With IOS5 force loading images when navigating with back button. */
        /* Non optimal workaround. */
        if ((/(?:iphone|ipod|ipad).*os 5/gi).test(navigator.appVersion)) {
            $window.bind("pageshow", function(event) {
                if (event.originalEvent && event.originalEvent.persisted) {
                    elements.each(function() {
                        $(this).trigger("appear");
                    });
                }
            });
        }

        /* Force initial check if images should appear. */
        $(document).ready(function() {
            update();
        });

        return this;
    };

    /* Convenience methods in jQuery namespace.           */
    /* Use as  $.belowthefold(element, {threshold : 100, container : window}) */

    $.belowthefold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = (window.innerHeight ? window.innerHeight : $window.height()) + $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top + $(settings.container).height();
        }

        return fold <= $(element).offset().top - settings.threshold;
    };

    $.rightoffold = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.width() + $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left + $(settings.container).width();
        }

        return fold <= $(element).offset().left - settings.threshold;
    };

    $.abovethetop = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollTop();
        } else {
            fold = $(settings.container).offset().top;
        }

        return fold >= $(element).offset().top + settings.threshold  + $(element).height();
    };

    $.leftofbegin = function(element, settings) {
        var fold;

        if (settings.container === undefined || settings.container === window) {
            fold = $window.scrollLeft();
        } else {
            fold = $(settings.container).offset().left;
        }

        return fold >= $(element).offset().left + settings.threshold + $(element).width();
    };

    $.inviewport = function(element, settings) {
         return !$.rightoffold(element, settings) && !$.leftofbegin(element, settings) &&
                !$.belowthefold(element, settings) && !$.abovethetop(element, settings);
     };

    /* Custom selectors for your convenience.   */
    /* Use as $("img:below-the-fold").something() or */
    /* $("img").filter(":below-the-fold").something() which is faster */

    $.extend($.expr[":"], {
        "below-the-fold" : function(a) { return $.belowthefold(a, {threshold : 0}); },
        "above-the-top"  : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-screen": function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-screen" : function(a) { return !$.rightoffold(a, {threshold : 0}); },
        "in-viewport"    : function(a) { return $.inviewport(a, {threshold : 0}); },
        /* Maintain BC for couple of versions. */
        "above-the-fold" : function(a) { return !$.belowthefold(a, {threshold : 0}); },
        "right-of-fold"  : function(a) { return $.rightoffold(a, {threshold : 0}); },
        "left-of-fold"   : function(a) { return !$.rightoffold(a, {threshold : 0}); }
    });

})(jQuery, window, document);