Question

我正在尝试理解的插件中有一行：

$self.hide().attr("src", $self.data(settings.data_attribute))[settings.effect](settings.effect_speed);

self是一个jquery对象，在这段代码中是一个img dom元素，它隐藏它，然后将这个img对象的src属性设置为html5数据属性。但是现在它访问了.attr返回的jquery对象中的一个属性（在这里是“fadeIn”。但我不明白，jquery对象是否内置了一个效果属性函数内置到jquery对象中？我很困惑这是如何转换为调用.fadeIn（900）。我无法在jquery网站上的任何地方找到这个文档。如果有人可以对此有所了解，谢谢。

抱歉，这是完整的代码，它来自mika tuupola的延迟加载jquery插件：

(function($, window, document, undefined) {
var $window = $jq191(window);

$jq191.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
    };

    function update() {
        var counter = 0;

        elements.each(function() {
            var $this = $jq191(this);
            if (settings.skip_invisible && !$this.is(":visible")) {
                return;
            }
            if ($jq191.abovethetop(this, settings) ||
                $jq191.leftofbegin(this, settings)) {
                    /* Nothing. */
            } else if (!$jq191.belowthefold(this, settings) &&
                !$jq191.rightoffold(this, settings)) {
                    $this.trigger("appear");
                    /* if we found an image we'll load, reset the counter */
                    counter = 0;
            } 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;
        }

        $jq191.extend(settings, options);
    }

    /* Cache container as jQuery as object. */
    $container = (settings.container === undefined ||
                  settings.container === window) ? $window : $jq191(settings.container);

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

    this.each(function() {
        var self = this;
        var $self = $jq191(self);

        self.loaded = false;

        /* When appear is triggered load original image. */
        $self.one("appear", function() {
            if (!this.loaded) {
                if (settings.appear) {
                    var elements_left = elements.length;
                    settings.appear.call(self, elements_left, settings);
                }
                $jq191("<img />")
                    .bind("load", function() {
                        $self
                            .hide()
                            .attr("src", $self.data(settings.data_attribute))
                            [settings.effect](settings.effect_speed);
                        self.loaded = true;

                        /* Remove image from array so it is not looped next time. */
                        var temp = $jq191.grep(elements, function(element) {
                            return !element.loaded;
                        });
                        elements = $jq191(temp);

                        if (settings.load) {
                            var elements_left = elements.length;
                            settings.load.call(self, elements_left, settings);
                        }
                    })
                    .attr("src", $self.data(settings.data_attribute));
            }
        });

        /* When wanted event is triggered load original image */
        /* by triggering appear.                              */
        if (0 !== settings.event.indexOf("scroll")) {
            $self.bind(settings.event, function(event) {
                if (!self.loaded) {
                    $self.trigger("appear");
                }
            });
        }
    });

    /* Check if something appears when window is resized. */
    $window.bind("resize", function(event) {
        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.persisted) {
                elements.each(function() {
                    $jq191(this).trigger("appear");
                });
            }
        });
    }

    /* Force initial check if images should appear. */
    $jq191(window).load(function() {
        update();
    });

    return this;
};

/* Convenience methods in jQuery namespace.           */
/* Use as  $jq191.belowthefold(element, {threshold : 100, container : window}) */

$jq191.belowthefold = function(element, settings) {
    var fold;

    if (settings.container === undefined || settings.container === window) {
        fold = $window.height() + $window.scrollTop();
    } else {
        fold = $jq191(settings.container).offset().top + $jq191(settings.container).height();
    }

    return fold <= $jq191(element).offset().top - settings.threshold;
};

$jq191.rightoffold = function(element, settings) {
    var fold;

    if (settings.container === undefined || settings.container === window) {
        fold = $window.width() + $window.scrollLeft();
    } else {
        fold = $jq191(settings.container).offset().left + $jq191(settings.container).width();
    }

    return fold <= $jq191(element).offset().left - settings.threshold;
};

$jq191.abovethetop = function(element, settings) {
    var fold;

    if (settings.container === undefined || settings.container === window) {
        fold = $window.scrollTop();
    } else {
        fold = $jq191(settings.container).offset().top;
    }

    return fold >= $jq191(element).offset().top + settings.threshold  + $jq191(element).height();
};

$jq191.leftofbegin = function(element, settings) {
    var fold;

    if (settings.container === undefined || settings.container === window) {
        fold = $window.scrollLeft();
    } else {
        fold = $jq191(settings.container).offset().left;
    }

    return fold >= $jq191(element).offset().left + settings.threshold + $jq191(element).width();
};

$jq191.inviewport = function(element, settings) {
     return !$jq191.rightoffold(element, settings) && !$jq191.leftofbegin(element, settings) &&
            !$jq191.belowthefold(element, settings) && !$jq191.abovethetop(element, settings);
 };

/* Custom selectors for your convenience.   */
/* Use as $jq191("img:below-the-fold").something() or */
/* $jq191("img").filter(":below-the-fold").something() which is faster */

$jq191.extend($jq191.expr[':'], {
    "below-the-fold" : function(a) { return $jq191.belowthefold(a, {threshold : 0}); },
    "above-the-top"  : function(a) { return !$jq191.belowthefold(a, {threshold : 0}); },
    "right-of-screen": function(a) { return $jq191.rightoffold(a, {threshold : 0}); },
    "left-of-screen" : function(a) { return !$jq191.rightoffold(a, {threshold : 0}); },
    "in-viewport"    : function(a) { return $jq191.inviewport(a, {threshold : 0}); },
    /* Maintain BC for couple of versions. */
    "above-the-fold" : function(a) { return !$jq191.belowthefold(a, {threshold : 0}); },
    "right-of-fold"  : function(a) { return $jq191.rightoffold(a, {threshold : 0}); },
    "left-of-fold"   : function(a) { return !$jq191.rightoffold(a, {threshold : 0}); }
});

}）（$ jq191，窗口，文档）;

Answer 1

请记住，jQuery只是一个对象。这意味着您可以使用.表示法调用的所有方法，您可以使用[]表示法进行调用。因此，这两行在功能上完全相同：

$("#foo").fadeIn(2000);
$("#foo")['fadeIn'](2000);

其他项settings.effect和settings.effect_speed可能会通过带有插件选项的对象文字输入到插件中。

var settings = {
    effect: 'fadeIn',
    effect_speed: 2000
}

Answer 2

这就是为什么这样做的原因。

首先，jQuery的.hide()方法及其.attr(name,value)方法都以这种jQuery方法的通常方式返回this，以允许链接。但这也意味着我们可以删除这些调用而不会影响问题：

$self[settings.effect](settings.effect_speed);

现在它更简单了。 settings.effect是字符串"fadeIn"，对吧？

然后代码与：

相同

$self["fadeIn"](settings.effect_speed);

在JavaScript中，object["methodName"]与object.methodName完全相同。所以代码实际上与：

相同

$self.fadeIn(settings.effect_speed);

这就是它调用fadeIn方法的原因。

不明白这是如何在Jquery中调用fadeIn方法的

2 个答案: