ios微信网页浏览器，微信jssdk的图片下载接口 wx.downloadImage，多个调用间隔较短时会有部分无法调起，没有任何回调。

大致代码如下：

for(var i=0; i<arr.lenght; i++){

    renderImg(arr[i].dom,arr[i].serverId)

}

function renderImg(dom,serverId){

    renderCount++

    alert("startRender") //此处加上alert也不会出现问题

    setTimeout(function(){//加入较长延迟就没问题了

        wx.downloadImage({

            serverId: serverId, // 需要下载的图片的服务器端ID，由uploadImage接口获得

            isShowProgressTips: 0, // 默认为1，显示进度提示

            success: function (res) {

                    var message = '<img style="max-width:150px" class="chat-preview-img" src="'+ res.localId + '">'

                    dom.append(message)

                    alreadyRenderCount++;

                    showMessages();

                    dom.children('.chat-preview-img').on('click',function () {

                        var curImgSrc = $(this).attr("src");

                        var imgsSrc = [];

                        imgsSrc.push(curImgSrc)

                        wx.previewImage({

                            current: curImgSrc, // 当前显示图片的http链接

                            urls: imgsSrc // 需要预览的图片http链接列表

                        });

                });

            },

        fail: function(){

            var message = '加载失败';

            dom.append(message);

            alreadyRenderCount++;

            showMessages();

        },

        complete: function(res){

        }

    });

    },renderCount*1000)

}