javascript - 以同步方式运行异步方法

我有一个简单的for循环，它基本上检查图像是否存储在文件系统中，如果没有，则下载并呈现UI：

for (var t = 0; t < toJSON.length; t++) {
    if (t < 3) {
        var view = Titanium.UI.createImageView({
            width: 320,
            height: 310,
            //top: 10
        });
        image_url = toJSON[t];
        //start
        if (utilities.CheckIfImageExist(utilities.ExtractImageName(image_url))) {
            var parent = Titanium.Filesystem.getApplicationDataDirectory();
            var picture = Titanium.Filesystem.getFile(parent, 'pictures');
            var picturePath = parent + 'pictures/';
            Ti.API.info('picturePath: ' + picturePath);
            var f = Titanium.Filesystem.getFile(picturePath, utilities.ExtractImageName(image_url));
            var blob = f.read();
            // here is saved blog file
            console.log('Image already downloaded');
            var width = blob.width;
            var height = blob.height;
            //crop  so it fits in image view
            if (width > height) {
                view.image = ImageFactory.imageAsCropped(blob, {
                    width: height,
                    height: height,
                    x: 60,
                    y: 0
                });
            } else {
                view.image = ImageFactory.imageAsCropped(blob, {
                    width: (width - 1),
                    height: (width - 1),
                    x: 60,
                    y: 0
                });
            }
        } else {
            //do new loop - async causing problems
            alert('not downloaded');
            // if image is not downloaded we will download it here
            utilities.APIGetRequestImage(image_url, function (e) {
                alert('begin downloaded');
                var status = this.status;
                if (status == 200) {
                    Ti.API.info(this.responseData);
                    //save to directory
                    utilities.SaveImageToDirectory(this.responseData, image_url);
                    //create view
                    var view = Titanium.UI.createImageView({
                        width: 320,
                        height: 310,
                        //top: 10
                    });
                    var width = this.responseData.width;
                    var height = this.responseData.height;
                    //crop  so it fits in image view
                    if (width > height) {
                        var view = Titanium.UI.createImageView({
                            width: 320,
                            height: 310,
                            //top: 10
                        });
                        view.image = ImageFactory.imageAsCropped(this.responseData, {
                            width: height,
                            height: height,
                            x: 60,
                            y: 0
                        });
                        //  $.scrollableView.addView(view);
                        viewArr.push(view);
                    } else {
                        view.image = ImageFactory.imageAsCropped(this.responseData, {
                            width: (width - 1),
                            height: (width - 1),
                            x: 60,
                            y: 0
                        });
                        viewArr.push(view);
                        //when t = 3, all views are put inside array, set image view
                        //if(t==3){
                        //}
                    }
                }
            }, function (err) {
                alert('error downloading image');
            });
        }
    }
}

仅在for循环执行IF语句的前半部分（此处表示“未下载”）之后，代码才说“开始下载”，直到t = 3。

然后for循环执行else语句，我遇到的麻烦是我需要它以同步方式进行操作，因为我依赖于t值来知道要下载哪个图像并将其放置在视图中。

    utilities.APIGetRequestImage(image_url, function(e) {

是一种回调方法，可从服务器获取文件并下载。

如何使两种方法同时运行？

最佳答案

一探究竟：

for (var t = 0; t < toJSON.length; t++) {
    if (t < 3) {
        var view = Titanium.UI.createImageView({
            width: 320,
            height: 310,
            //top: 10
        });
        image_url = toJSON[t];
        //start
        if (utilities.CheckIfImageExist(utilities.ExtractImageName(image_url))) {
            var parent = Titanium.Filesystem.getApplicationDataDirectory();
            var picture = Titanium.Filesystem.getFile(parent, 'pictures');
            var picturePath = parent + 'pictures/';
            Ti.API.info('picturePath: ' + picturePath);
            var f = Titanium.Filesystem.getFile(picturePath, utilities.ExtractImageName(image_url));
            var blob = f.read();
            // here is saved blog file
            console.log('Image already downloaded');
            var width = blob.width;
            var height = blob.height;
            //crop  so it fits in image view
            if (width > height) {
                view.image = ImageFactory.imageAsCropped(blob, {
                    width: height,
                    height: height,
                    x: 60,
                    y: 0
                });
            } else {
                view.image = ImageFactory.imageAsCropped(blob, {
                    width: (width - 1),
                    height: (width - 1),
                    x: 60,
                    y: 0
                });
            }
        } else {
            //do new loop - async causing problems
            alert('not downloaded');
            // if image is not downloaded we will download it here
            utilities.APIGetRequestImage(image_url, (function (t, image_url) {
                return function (e) {                               // <----- wrap callback function
                    alert('begin downloaded');
                    var status = this.status;
                    if (status == 200) {
                        Ti.API.info(this.responseData);
                        //save to directory
                        utilities.SaveImageToDirectory(this.responseData, image_url);
                        //create view
                        var view = Titanium.UI.createImageView({
                            width: 320,
                            height: 310,
                            //top: 10
                        });
                        var width = this.responseData.width;
                        var height = this.responseData.height;
                        //crop  so it fits in image view
                        if (width > height) {
                            var view = Titanium.UI.createImageView({
                                width: 320,
                                height: 310,
                                //top: 10
                            });
                            view.image = ImageFactory.imageAsCropped(this.responseData, {
                                width: height,
                                height: height,
                                x: 60,
                                y: 0
                            });
                            //  $.scrollableView.addView(view);
                            viewArr.push(view);
                        } else {
                            view.image = ImageFactory.imageAsCropped(this.responseData, {
                                width: (width - 1),
                                height: (width - 1),
                                x: 60,
                                y: 0
                            });
                            viewArr.push(view);
                            //when t = 3, all views are put inside array, set image view
                            //if(t==3){
                            //}
                        }
                    }
                };
            })(t, image_url), function (err) {
                alert('error downloading image');
            });
        }
    }
}

通过包装utilities.APIGetRequestImage回调，可以正确传递t和image_url。

关于javascript - 以同步方式运行异步方法，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/23459232/