casper.waitForSelector，超时和错误处理

问题描述

我制作了一个快速脚本来从Reddit帖子列表中捕获屏幕快照。该脚本从json文件中获取reddit网址（如下所示的示例），然后访问每个页面以捕获屏幕截图。

I made a quick script to capture a screen shot from a list of Reddit posts. The script fetches the reddit urls from a json file(example shown below) and then visits each page to capture a screenshot.

该脚本最适合大多数情况。但是，如果不符合 casper.waitForSelector 条件，它将不时地挂起/继续运行。最终，它将使整个服务器停止运行。我以为我做了足够的错误处理。如何修改脚本以忽略任何不符合 casper.waitForSelector 标准的页面，然后转到下一个屏幕捕获项目？

The script works well for the most. However, every once in a while it will hang/continue to run if casper.waitForSelector criteria is not met. Eventually it will stall the whole server. I thought I was doing sufficient error handling. How can I modify the script to ignore any page that doesn't fit the casper.waitForSelector criteria and move on to the next item for screen capture?

JS

var casper = require('casper').create({
    verbose: true,
    viewportSize: {
        width: 1280,
        height: 720
    },
    logLevel: 'error',
    pageSettings: {
        ignoreSslErrors: true,
        loadImages: true, // load images
        loadPlugins: true, // do not load NPAPI plugins (Flash, Silverlight, ...)
        webSecurityEnabled: false, // ajax
        userAgent: 'Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.2 Safari/537.36'
    }
});

//Create random names for files
function randString(x) {
    var s = "";
    while (s.length < x && x > 0) {
        var r = Math.random();
        s += (r < 0.1 ? Math.floor(r * 100) : String.fromCharCode(Math.floor(r * 26) + (r > 0.5 ? 97 : 65)));
    }
    return s;
}


function getReddit(reddit, filename) {
    casper.thenOpen(reddit, function() {
        if (casper.exists("#thing_" + filename)) {
            casper.waitForSelector("#thing_" + filename, function() {
                casper.captureSelector(randString(10) + '.png', "#thing_" + filename, {
                    quality: 100
                });
            });
        } else {
            console.log("Reddit Error: " + filename);
        }
    });
}

var link = 'http://localhost/test.json';

casper.start(link);

casper.on("resource.received", function(resource) {
    var results = this.evaluate(function(url) {
        return __utils__.sendAJAX(url, "GET");
    }, resource.url);
    var x = JSON.parse(results);
    //this.echo(x.length);
    for (var i = 0; i < x.length; ++i) {
        var reddit = x[i].post_url;
        var filename = x[i].id;
        getReddit(reddit, filename);
    }
});

casper.run();

JSON

{
post_url: "https://www.reddit.com/r/todayilearned/comments/4marhg/til_that_in_the_16th_century_christians_called/",
bit_id: "l6KE0vzMmgQ",
id: "t3_4marhg"
},
{
post_url: "https://www.reddit.com/r/videos/comments/4mbbab/man_ignores_museum_rules_touches_priceless_clock/",
bit_id: "2GK22rGYWKx",
id: "t3_4mbbab"
},
{
post_url: "https://www.reddit.com/r/space/comments/4mc1av/an_alien_world_67p_as_seen_by_rosetta_two_days_ago/",
id: "t3_4mc1av"
},
{
post_url: "https://www.reddit.com/r/worldnews/comments/4mc8uv/young_fish_become_hooked_on_eating_plastic_in_the/",
id: "t3_4mc8uv"
},
{
post_url: "https://www.reddit.com/r/todayilearned/comments/4mcda3/til_that_the_giant_tortoise_did_not_receive_a/",
id: "t3_4mcda3"
},
{
post_url: "https://www.reddit.com/r/science/comments/4mcl0y/a_new_study_has_shown_that_mothers_who_are/",
id: "t3_4mcl0y"
},
{
post_url: "https://www.reddit.com/r/news/comments/4mcveg/bp_agrees_to_pay_175_million_to_settle_claims_by/",
id: "t3_4mcveg"
},
{
post_url: "https://www.reddit.com/r/todayilearned/comments/4mdddw/til_that_when_a_british_captive_officer/",
id: "t3_4mdddw"
}

推荐答案

如果将 onTimeout 回调函数提供给 waitForSelector （第3个参数），则该函数将被执行而不是错误时的默认行为（停止脚本）。

If you provide the onTimeout callback function to waitForSelector (3rd argument), then it will be executed instead of the default behavior (stop script) on error.

您可以传入空函数或带有某些日志记录的函数：

You can pass in an empty function or a function with some logging:

casper.waitForSelector("#thing_" + filename, function _then() {
    this.captureSelector(randString(10) + '.png', "#thing_" + filename, {
        quality: 100
    });
}, function _onTimeout(){
    this.echo("#thing_" + filename + " not found", "WARNING");
});

您还可以更改选项 casper.options.silentErrors 到 true ，如果您希望每个功能都使用此行为。

You can also change the option casper.options.silentErrors to true if you want this behavior for every function.

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