用幻像节点实现waitFor功能

Question

我已经尝试并测试了幻影的例子waitFor，并取得了成功。但是，我很难通过幻影-节点模块实现它，主要是因为page.evaluate在回调中得到了评估。

PhantomJS实现

page.open("http://twitter.com/#!/sencha", function () {
    waitFor(function() {

        // This here is easy to do as the evaluate method returns immediately
        return page.evaluate(function() {
            return $("#signin-dropdown").is(":visible");
        });

    }, function() {
       console.log("The sign-in dialog should be visible now.");
       phantom.exit();
    });
  }
});

但是，使用phantomjs-node，计算函数将在回调中获得返回的数据：

page.evaluate(
    function(){ /* return thing */ },
    function callback(thing) {  /* write code for thing */ }
)

使用phantomjs-node，，如何仅在元素可见后才能在页面上运行函数？

如果上面的链接死了，下面是waitFor函数的实现

/**
* Wait until the test condition is true or a timeout occurs. Useful for waiting
* on a server response or for a ui change (fadeIn, etc.) to occur.
*
* @param testFx javascript condition that evaluates to a boolean,
* it can be passed in as a string (e.g.: "1 == 1" or "$('#bar').is(':visible')" or
* as a callback function.
* @param onReady what to do when testFx condition is fulfilled,
* it can be passed in as a string (e.g.: "1 == 1" or "$('#bar').is(':visible')" or
* as a callback function.
* @param timeOutMillis the max amount of time to wait. If not specified, 3 sec is used.
*/
function waitFor(testFx, onReady, timeOutMillis) {
var maxtimeOutMillis = timeOutMillis ? timeOutMillis : 3000, //< Default Max Timout is 3s
    start = new Date().getTime(),
    condition = false,
    interval = setInterval(function() {
        if ( (new Date().getTime() - start < maxtimeOutMillis) && !condition ) {
            // If not time-out yet and condition not yet fulfilled
            condition = (typeof(testFx) === "string" ? eval(testFx) : testFx()); //<    defensive code
        } else {
            if(!condition) {
                // If condition still not fulfilled (timeout but condition is 'false')
                console.log("'waitFor()' timeout");
                phantom.exit(1);
            } else {
                // Condition fulfilled (timeout and/or condition is 'true')
                console.log("'waitFor()' finished in " + (new Date().getTime() - start) + "ms.");
                typeof(onReady) === "string" ? eval(onReady) : onReady(); //< Do what it's supposed to do once the condition is fulfilled
                clearInterval(interval); //< Stop this interval
            }
        }
    }, 250); //< repeat check every 250ms
};

提前谢谢。

Answer 1

今天遇到了这个问题，我想我可以分享我的解决方案。

  // custom helper function
  function wait(testFx, onReady, maxWait, start) {
    var start = start || new Date().getTime()
    if (new Date().getTime() - start < maxWait) {
      testFx(function(result) {
        if (result) {
          onReady()
        } else {
          setTimeout(function() {
            wait(testFx, onReady, maxWait, start)
          }, 250)
        }
      })
    } else {
      console.error('page timed out')
      ph.exit()
    }
  }

第一步是创建一个新的wait函数。它接受与原始waitFor函数相同的参数，但工作方式略有不同。在触发了来自测试函数wait的回调之后，我们必须递归地运行testFx函数，而不是使用间隔。另外，请注意，您实际上不需要传入start的值，因为它是自动设置的。

  wait(function (cb) {
    return page.evaluate(function () 
      // check if something is on the page (should return true/false)
      return something
    }, cb)
  }, function () { // onReady function
    // code
  }, 5000) // maxWait

在本例中，我将testFx函数的回调设置为对page.evaluate的回调，它根据是否能够在页面上找到某个元素返回一个真/假值。或者，您可以为page.evaluate创建回调，然后从它触发testFx回调，如下所示：

  wait(function (cb) {
    return page.evaluate(function () 
      // check if something is on the page (should return true/false)
      return something
    }, function(result) {
      var newResult = doSomethingCrazy(result)
      cb(newResult)
    }
  }, function () { // onReady function
    // code
  }, 5000) // maxWait

Answer 2

我为幻影-节点编写了一个名为海脊的替代方案。它没有将所有函数调用和赋值转换为异步操作，而是在PhantomJS中执行整个函数。

我认为你的问题可以这样解决：

phridge.spawn()
    .then(function (phantom) {
        return phantom.openPage(url);
    })
    .then(function (page) {
        return page.run(selector, function (selector, resolve, reject) {
            // this function runs inside PhantomJS bound to the webpage instance
            var page = this;
            var intervalId = setInterval(function () {
                var hasBeenFound = page.evaluate(function (selector) {
                    return Boolean(document.querySelector(selector));
                }, selector);

                if (hasBeenFound === false &&
                    /* check if there is still some time left  */) {
                    // wait for next interval
                    return;
                }

                clearInterval(intervalId);

                if (hasBeenFound) {
                    resolve();
                } else {
                    reject(new Error("Wait for " + selector + " timeout"));
                }
            }, 100);
        });
    })
    .then(function () {
        // element has been found
    })
    .catch(function (err) {
        // element has not been found
    });

Answer 3

最近，我创建了一个相当简单的节点模块，将waitFor移植到节点：https://gist.github.com/joseym/1d01edbcc40a7698f55a#file-phantomjs-waitfor-js。

var async = require('async');

module.exports = waitFor;

/**
 * waitFor port used with 
 * @see    {@link https://github.com/ariya/phantomjs/blob/master/examples/waitfor.js}
 * @see    {@link https://github.com/sgentle/phantomjs-node}
 * @callback testFx - Test function, will repeat until true or timeout limit is reached
 * @callback onReady - Fires if/when `testFx` passes.
 * @param {(number|boolean|string)} [timeOut=false] - If defined and falsey or string value of`forever` 
 *                                                    then `waitFor` will run until `testFx` passes without 
 *                                                    timing out, otherwise pass a number in miliseconds.
 */
function waitFor(testFx, onReady, timeOut) {

    var maxtimeOutMillis = typeof timeOut !== 'undefined' ? timeOut : 5000 // Default Max Timout is 5s if not defined
        , start = new Date().getTime()
        , isAsync = testFx.length > 0
        , passing = undefined
    ;

    async.until(
        function Test() { 
            return typeof passing !== 'undefined'; 
        },
        function Action(cb) {
            setTimeout(function(){

                if (!maxtimeOutMillis || maxtimeOutMillis == 'forever' || new Date().getTime() - start < maxtimeOutMillis) {

                    // If a callback is passed to `testFx` we'll handle that.
                    function useCallback(){
                        passing = arguments[0]
                        return cb();
                    };                    

                    passing = (function(){
                        return (typeof(testFx) === "string" ? eval(testFx) : testFx).apply(this, arguments);
                    })(isAsync ? useCallback : undefined);

                    if(!isAsync) cb();

                } else {
                    return cb(new Error('`waitFor` timeout'));
                }

            }, 250);
        },
        function Done(err) {
            return (function(){
                return (typeof(onReady) === "string" ? eval(onReady) : onReady).apply(this, arguments);                  
            })(err, passing);
        }
    );

}