jQuery.lazyload使用及源码分析

朱雀 2021-06-26 16:06 626阅读 0赞

## 前言： ##

貌似以前自己也写过图片懒加载插件，但是新公司使用的是`jQuery.lazyload`插件，为了更好的运用，自己还是把源码看了遍，分别记录了如何使用，

插件原理，各个配置属性的完整解释，demo实例，源码分析(较简短)，源码分析可以配合使用，配置属性，原理进行阅读

## 一、如何使用 ##

// 最简单的使用，不带参数
    $('img').lazyload();
    
    
    // 带参数(配置对象)，下面配置对象中的各个属性值都是默认的
    $('img').lazyload({ 
        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"
    });

## 二、内部原理 ##

首先选中的`img`元素都绑定了一个`appear`事件(处理`img`显示真实的图片地址)，方便以后满足条件时触发该事件；  
在配置对象中有一个`container`属性配置，默认为`window`，如果`img`元素在该`container`容器视口中，则触发`appear`事件；  
为了判断`img`元素是否在`container`容器视口范围中，造了如下四个轮子：

$.belowthefold = function(element, settings) { };    // 在视口下方
    $.rightoffold = function(element, settings) { };        // 在视口右方
    $.abovethetop = function(element, settings) { };        // 在视口上方
    $.leftofbegin = function(element, settings) { };        // 在视口左方

看看源码中是如何利用这四个轮子：

if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { 
        /* Nothing. */
    }
    // 不满足在上方，左方；也不满足在下方，右方； 则触发appear事件
    else if (!$.belowthefold(this, settings) && !$.rightoffold(this, settings)) { 
        $this.trigger("appear");
    }

## 三、配置对象中的其他属性 ##

临界值，这个值是针对`container`容器的，即距离`container`容器视口的临界值

{ 
        threshold: 0
    }

事件，`container`容器默认绑定这个事件，在这个事件被触发时，会不断的判断`img`元素是否满足触发`appear`的条件，因此当浏览器不停的滚动下来时，如果满足条件，则显示图片；

另外还有一点，如果这个事件不是`scroll`事件，则选中的`img`元素都会绑定这个事件，绑定的这个事件中同样会触发内部`appear`事件；

{ 
        event: 'scroll'
    }

显示方法，默认为`show`，也可以设置为`fadeIn`，`API`中隐藏了一个配置属性`effectspeed`，动画运行的时间

{ 
        effect: "show"
    }

`img`元素的一个`data`属性，用于存放图片的真实地址

{ 
        data_attribute: "original",
    }

忽略隐藏的`img`元素

{ 
        skip_invisible: true
    }

图片占位符，`img`元素默认`src`属性为`1*1`像素的透明图片

{ 
        placeholder: "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC"
    }

在`img`触发`appear`事件时执行的回调

{ 
        appear: null
    }

在`img`触发`load`事件时执行的回调

{ 
        load: null
    }

最后一个配置属性`failure_limit`

{ 
        failure_limit: 0
    }

为了便于理解，我们先来看一段与其有关的源码：

什么意思呢，如果找到的是第 `failure_limit` 个`img`元素，且不在`container`视口上方，左方及视口内（可以允许在视口下方，右方），则中断循环

## 三、`demo` ##

看完原理和配置属性，是否觉得很简单呢，来看看几个`demo`吧  
demo1 下拉滚动: [http://jsfiddle.net/ddEPL/][http_jsfiddle.net_ddEPL]  
demo2 Tab切换: [http://jsfiddle.net/ddEPL/1/][http_jsfiddle.net_ddEPL_1]  
[http://jsfiddle.net/ddEPL/8/][http_jsfiddle.net_ddEPL_8]

## 四、源码分析 ##

/* * 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 = $(this);
                    // 如果图片隐藏，且忽略隐藏，则中断循环
                    if (settings.skip_invisible && !$this.is(":visible")) { 
                        return;
                    }
                    if ($.abovethetop(this, settings) || $.leftofbegin(this, settings)) { 
                            /* Nothing. */
                    } 
                    // img满足在container视口中，则显示
                    else if (!$.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）个img元素，且不在container视口上方，左方及视口内（可以允许在视口下方，右方），
                    // 则中断循环
                    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. */
            // 为container绑定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. */
                // 设置占位符
                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绑定appear，触发后则移除该事件
                $self.one("appear", function() { 
                    if (!this.loaded) { 
                        // 存在回调则触发
                        if (settings.appear) { 
                            var elements_left = elements.length;
                            settings.appear.call(self, elements_left, settings);
                        }
                        $("<img />")
                            .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，过滤掉已经加载的img元素，避免下次在update中轮循
                                var temp = $.grep(elements, function(element) { 
                                    return !element.loaded;
                                });
                                elements = $(temp);
                                
                                // 存在回调则触发
                                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. */
                // 绑定不是scroll的事件，用于触发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);

[http_jsfiddle.net_ddEPL]: http://jsfiddle.net/ddEPL/
[http_jsfiddle.net_ddEPL_1]: http://jsfiddle.net/ddEPL/1/
[http_jsfiddle.net_ddEPL_8]: http://jsfiddle.net/ddEPL/8/