JavaScript简易缩放程序

一、前言：

上一篇随笔中已经把拖动程序完成了，这篇主要把缩放程序完成，后面合并后可以做成一个图片裁剪的功能

简易缩放程序DEMO：http://jsfiddle.net/UN99R/
限制缩放程序DEMO：http://jsfiddle.net/kHWQZ/

二、设计思路：

1、在一个可以缩放元素中，共有8个触发点(上、下、左、右、左上、右上、左下、右下)，首先分别设置好它们的CSS属性cursor值。

2、同拖放程序一样，在触发点上绑定鼠标按下事件，在文档(document)上绑定鼠标移动、弹起事件处理程序，在移动事件中，完成拖放元素css的操作，
在鼠标弹起事件中，解绑移动事件处理程序。

主要难点：在于如何处理拖放元素的CSS属性

三、源码实现细节：

在8个触发点的事件处理程序中，diffX, diffY 分别获取的是鼠标在移动与按下过程时指针的差值

var diffX = e.clientX - clientX, // 负数代表向左移动，反之向右
    diffY = e.clientY - clientY; // 负数代表想上移动，反之向下

在鼠标按下事件处理中，w, h, l, t 分别记录的是拖放元素的width, height, left, top 属性值，下面讲一下鼠标移动处理程序
举一个示例：

up事件程序代码：

up = function(e){
    var diffY = e.clientY - clientY;
    $target.css({
        height: h - diffY + 'px',
        top: t + Math.min(diffY, h) + 'px'
    });
    e.preventDefault();
};

1、在up事件程序中，拖放元素变动的是height, top属性；

2、如果鼠标向上移动，diffY为负数，而height应增，因此 height = h - diffY, top则相反，top = t + diffY;

3、反之，diffY为正数，而height应减，因此 height = h - diffY, top = t + diffY;

4、为了让top值不为负数，top = t + Math.min(diffY, h);

最后阻止默认浏览器事件，避免鼠标拖动过程中出现禁止icon

处理完up事件后，其他down, left, right事件处理同理，而后 upLeft, upRight, downLeft, downRight 这些事件处理都是在叠加前四个事件处理就行了

主程序代码：

jQuery.fn.extend({
    /**
     *   Autor: 博客园华子yjh 2014/02/26
     */
    resize: function(options) {
        var controlSelector = '.cursor-north, .cursor-south, .cursor-west, .cursor-east,' +
                              '.cursor-north-west, .cursor-north-east, .cursor-south-west, .cursor-south-east';
        $(this).each(function(){
            var 
                // 上、下、左、右、左上、右上、左下、右下 八个点的鼠标拖动时的事件处理程序
                up, down, left, right, upLeft, upRight, downLeft, downRight,

                // 鼠标拖动时的事件处理程序
                handleEvent,

                // 记录鼠标按下时鼠标的位置
                clientX, clientY,

                // 记录鼠标按下时元素的大小及位置
                w, h, l, t,

                // 目标元素
                $target = $(this), self = this;

            up = function(e){
                var diffY = e.clientY - clientY;
                $target.css({
                    height: h - diffY + 'px',
                    top: t + Math.min(diffY, h) + 'px'
                });
                e.preventDefault();
            };
            down = function(e){
                var diffY = e.clientY - clientY;
                $target.css({
                    height: h + diffY + 'px'
                });
                e.preventDefault();
            };
            left = function(e){
                var diffX = e.clientX - clientX;
                $target.css({
                     w - diffX + 'px',
                    left: l + Math.min(diffX, w) + 'px'
                });
                e.preventDefault();
            };
            right = function(e){
                var diffX = e.clientX - clientX;
                $target.css({
                     w + diffX + 'px'
                });
                e.preventDefault();
            };
            upLeft = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                    height: h - diffY + 'px',
                    top: t + Math.min(diffY, h) + 'px',
                     w - diffX + 'px',
                    left: l + Math.min(diffX, w) + 'px'
                });
                e.preventDefault();
            };
            upRight = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                    height: h - diffY + 'px',
                    top: t + Math.min(diffY, h) + 'px',
                     w + diffX + 'px'
                });
                e.preventDefault();
            };
            downLeft = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                     w - diffX + 'px',
                    left: l + Math.min(diffX, w) + 'px',
                    height: h + diffY + 'px'
                });
                e.preventDefault();
            };
            downRight = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                     w + diffX + 'px',
                    height: h + diffY + 'px'
                });
                e.preventDefault();
            };            

            $target.delegate(controlSelector, 'mousedown', function(e){
                var className = $(this).attr('class');
                clientX = e.clientX;
                clientY = e.clientY;

                w = $target.width(),
                h = $target.height(),
                l = $target.get(0).offsetLeft,
                t = $target.get(0).offsetTop;

                // 根据触发元素的className选择要触发的事件处理程序
                switch (className) {
                    case 'cursor-north':
                        handleEvent = up;
                        break;

                    case 'cursor-south':
                        handleEvent = down;
                        break;

                    case 'cursor-west':
                        handleEvent = left;
                        break;

                    case 'cursor-east':
                        handleEvent = right;
                        break;

                    case 'cursor-north-west':
                        handleEvent = upLeft;
                        break;

                    case 'cursor-north-east':
                        handleEvent = upRight;
                        break;

                    case 'cursor-south-west':
                        handleEvent = downLeft;
                        break;

                    case 'cursor-south-east':
                        handleEvent = downRight;
                        break;

                    default: 
                        break;
                }

                $(document)
                .bind('mousemove', handleEvent)
                .bind('mouseup', function(){
                    $(document).unbind('mousemove', handleEvent);
                    $target.removeData('data-resize'); // 移除标志
                });
                $target.data('data-resize', true); // 缓存标志
            });
        });
        return this;
    }
});

带限制范围缩放代码：

jQuery.fn.extend({
    /**
     *   Autor: 博客园华子yjh 2014/02/26
     */
    resize: function(options) {
        var defaultOptions, boundaryElem, limitObj, controlSelector;
        controlSelector = '.cursor-north, .cursor-south, .cursor-west, .cursor-east, .cursor-north-west, .cursor-north-east, .cursor-south-west, .cursor-south-east';    
        defaultOptions = { // 默认配置项
            boundaryElem: 'body' // 边界容器
        };
        options = jQuery.extend( defaultOptions, options || {} );
        boundaryElem = $(options.boundaryElem).get(0);
        limitObj = {
            _left: boundaryElem.offsetLeft,
            _top: boundaryElem.offsetTop,
            _right: boundaryElem.offsetLeft + boundaryElem.clientWidth,
            _bottom: boundaryElem.offsetTop + boundaryElem.clientHeight
        };

        $(this).each(function(){
            var up, down, left, right, upLeft, upRight, downLeft, downRight,
                clientX, clientY, w, h, l, t,
                $target = $(this), self = this,
                handleEvent = function(){};

            up = function(e){
                var diffY = e.clientY - clientY;
                $target.css({
                    height: h + Math.min(0 - diffY, t) + 'px',
                    top: Math.max(t + Math.min(diffY, h), 0) + 'px'
                });
                e.preventDefault();
            };
            down = function(e){
                var diffY = e.clientY - clientY;
                $target.css({
                    height: h + Math.min(diffY, limitObj._bottom - limitObj._top - t - h) + 'px'
                });
                e.preventDefault();
            };
            left = function(e){
                var diffX = e.clientX - clientX;
                $target.css({
                     w  + Math.min(0- diffX, l) + 'px',
                    left: Math.max(l + Math.min(diffX, w), 0) + 'px'
                });
                e.preventDefault();
            };
            right = function(e){
                var diffX = e.clientX - clientX;
                $target.css({
                     w + Math.min(diffX, limitObj._right - limitObj._left - l - w) + 'px'
                });
                e.preventDefault();
            };
            upLeft = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                    height: h + Math.min(0 - diffY, t) + 'px',
                    top: Math.max(t + Math.min(diffY, h), 0) + 'px',
                     w  + Math.min(0- diffX, l) + 'px',
                    left: Math.max(l + Math.min(diffX, w), 0) + 'px'
                });
                e.preventDefault();
            };
            upRight = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                    height: h + Math.min(0 - diffY, t) + 'px',
                    top: Math.max(t + Math.min(diffY, h), 0) + 'px',
                     w + Math.min(diffX, limitObj._right - limitObj._left - l - w) + 'px'
                });
                e.preventDefault();
            };
            downLeft = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                     w  + Math.min(0- diffX, l) + 'px',
                    left: Math.max(l + Math.min(diffX, w), 0) + 'px',
                    height: h + Math.min(diffY, limitObj._bottom - limitObj._top - t - h) + 'px'
                });
                e.preventDefault();
            };
            downRight = function(e){
                var diffX = e.clientX - clientX,
                    diffY = e.clientY - clientY;
                $target.css({
                     w + Math.min(diffX, limitObj._right - limitObj._left - l - w) + 'px',
                    height: h + Math.min(diffY, limitObj._bottom - limitObj._top - t - h) + 'px'
                });
                e.preventDefault();
            };            
            
            $target.delegate(controlSelector, 'mousedown', function(e){
                var className = $(this).attr('class');
                clientX = e.clientX;
                clientY = e.clientY;

                w = $target.width(),
                h = $target.height(),
                l = $target.get(0).offsetLeft,
                t = $target.get(0).offsetTop;

                switch (className) {
                    case 'cursor-north':
                        handleEvent = up;
                        break;

                    case 'cursor-south':
                        handleEvent = down;
                        break;

                    case 'cursor-west':
                        handleEvent = left;
                        break;

                    case 'cursor-east':
                        handleEvent = right;
                        break;

                    case 'cursor-north-west':
                        handleEvent = upLeft;
                        break;

                    case 'cursor-north-east':
                        handleEvent = upRight;
                        break;

                    case 'cursor-south-west':
                        handleEvent = downLeft;
                        break;

                    case 'cursor-south-east':
                        handleEvent = downRight;
                        break;

                    default: 
                        break;
                }
                $(document)
                .bind('mousemove', handleEvent)
                .bind('mouseup', function(){
                    $(document).unbind('mousemove', handleEvent);
                    $target.removeData('data-resize'); // 移除标志
                });
                $target.data('data-resize', true); // 缓存标志
            })            
        });
        return this;
    }
});

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