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If PhantomCSS saves several screenshots from a loop but they all show the first page, the loop is probably running faster than the page changes finish. Queue each page change and capture as a separate CasperJS step, wait for a page-specific ready condition, and give every image a unique name. A fixed delay alone is less reliable because page-load time varies.
Why a loop can capture the same page repeatedly
PhantomCSS is a CasperJS module for capturing screenshots and comparing them with baseline images using Resemble.js. A JavaScript for loop runs synchronously: it can issue several page-change operations and screenshot calls before asynchronous navigation, requests, or DOM updates have completed. The captures may therefore happen while the browser still displays the first state.
This symptom has been reported with a loop that changes pages and captures them inside one CasperJS then callback. Moving each iteration into the CasperJS step queue and waiting for the expected page state addresses the timing problem. CasperJS documents ordered steps and wait operations for this purpose: CasperJS API documentation. The reported eight-second delay in that issue is not a general setting; it may be too short for one page and unnecessarily long for another.
Queue one capture step per page
Adapt the page-change function and readiness condition to your application. In this example, moveNext is application-specific, and #page-number is a placeholder selector that should identify the page currently shown.
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var firstPage = 1;
var lastPage = 10;
for (var pageNo = firstPage; pageNo <= lastPage; pageNo++) {
(function (targetPage) {
casper.then(function () {
this.evaluate(function (page) {
moveNext(page); // application-specific page change
}, targetPage);
this.waitFor(function () {
return this.evaluate(function (page) {
var indicator = document.querySelector('#page-number');
return indicator &&
indicator.textContent.trim() === String(page);
}, targetPage);
}, function () {
phantomcss.screenshot('html', 'page-' + targetPage);
}, function () {
this.die('Timed out waiting for page ' + targetPage);
}, 10000);
});
}(pageNo));
}
casper.run();- Capture the loop value. The immediately invoked function expression passes the current number as
targetPage. This avoids older JavaScript environments in which queued callbacks might otherwise all observe the loop’s final value. - Make the page change. Replace
moveNext(page)with the action your application uses to navigate or update its view. If a click or request triggers the transition, ensure it is actually initiated in this step. - Wait for evidence of the new state. The condition checks that the page indicator equals the target number. Use a marker that changes only after the desired content is ready.
- Capture only after readiness. The screenshot is inside the successful wait callback, so a slow or unfinished transition is not silently captured as the previous page.
- Fail visibly on timeout. The timeout callback stops the run with the page number it was waiting for. Adjust the timeout to your application’s expected response time rather than treating the example value as universal.
CasperJS also documents waits for selectors, text, and resources. Choose the signal that most reliably means the specific page is ready. Its wait-family methods are not chainable; when you need them in an ordered flow, wrapping them in casper.then is the documented pattern: CasperJS API documentation.
Choose a readiness signal that matches the page
A useful condition distinguishes the intended page from the previous one; merely checking that the document exists is not enough if that document remains present during the transition.
- Page number or route state: check a visible page indicator, active tab, or route value that changes for each iteration.
- Unique text: wait for text that is specific to the target page, not generic copy shared by all pages.
- Target element: wait for a selector that appears only when the page’s key content is ready.
- Resource completion: if a specific request determines the rendered state, wait for that resource or for a DOM change it causes.
CasperJS’s FAQ notes that intermittent test failures often occur when a test runs before a needed resource or element is available, and recommends waiting for DOM nodes, text, or resources: CasperJS FAQ. If a page can update its marker before its main content arrives, wait for a second condition that confirms the content itself.
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Use unique names and verify the output
Pass a distinct, meaningful name to each PhantomCSS screenshot call, such as page-1, page-2, and so on. PhantomCSS otherwise documents generated names such as screenshot_0.png; explicit names make it easier to identify which iteration produced each file and to match captures with baselines. See the PhantomCSS documentation.
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After a run, check that the expected filenames exist and that each readiness marker matched the intended value. If images still look identical, inspect the page-change handler and the marker itself: the function may not be changing the view, or the marker may be shared across pages and therefore report readiness too early.
Fixed delay or condition-based wait?
| Approach | When it can help | Limitation |
|---|---|---|
| Fixed delay | Useful as a temporary diagnostic when a page has no reliable readiness signal. | A delay that works on one run may be too short under slower conditions or waste time when the page is fast. The eight-second delay in the reported issue is specific to that example, not a general recommendation. |
| Condition-based wait | Preferred when the application exposes a page-specific DOM, text, or resource signal. | The condition must genuinely correspond to the intended rendered state; a stale or generic marker can succeed too early. |
Use a condition-based wait as the default. If you temporarily add a delay to diagnose timing, retain a timeout and replace the delay with a readiness check once you identify a dependable signal.
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Keep visual regression captures stable
Even correctly sequenced screenshots can differ between runs if the page contains mutable content. PhantomCSS recommends predictable UI and suggests static pages or faked data when changing content would make comparisons unstable: PhantomCSS documentation.
- Use controlled test data where practical.
- Make sure animations, rotating content, or asynchronous updates do not change the target state during capture.
- Keep the expected state and screenshot name paired, so a failure can be traced to a specific iteration.
Troubleshoot common failures
Every file shows the first page
The page transitions likely have not completed before capture, or the page-change function is not changing the view. Put each transition and capture in its own queued CasperJS step, then wait for a marker that updates to the target page.
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A callback may be reading the loop variable after the loop has finished. Capture the current value in a closure, as the example does with targetPage, and build the name from that captured value.
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The wait times out
Check that the selector exists on the page, that its text format matches the comparison, and that the page-change action receives the expected target number. If the app updates asynchronously, wait for the completion signal rather than only for the action to be issued. A timeout should reveal a real missing transition, not be removed to force a screenshot.
Some runs pass and others fail
The readiness signal may occur before the dependent content or resource is ready. Choose a later, more specific marker, or wait for the needed element, text, or resource as CasperJS’s FAQ recommends. Also make the test data and UI predictable to reduce unrelated visual changes.
The screenshots have unique names but identical contents
Distinct filenames only distinguish outputs; they do not make the page state distinct. Log or inspect the target page value and verify the page-change handler and readiness marker for each step before changing PhantomCSS configuration.
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Check runtime compatibility before adopting this stack
The documented APIs and reported symptom establish a historical CasperJS and PhantomCSS workflow, but they do not establish whether PhantomCSS, CasperJS, or PhantomJS is currently maintained or suitable for a new project. Check current releases and runtime compatibility for your environment before building a new test system around them.
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Frequently Asked Questions
Does PhantomCSS itself provide the wait for a page change?
In this pattern, CasperJS performs the ordered steps and waits; PhantomCSS is called to capture after the readiness condition succeeds.
Is an eight-second delay the recommended fix?
No. It was used in one reported case. A page-specific readiness condition is a better default.
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