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Use a completion-driven queue, not a tight for loop. iclems/iOS-htmltopdf renders each URL or HTML string asynchronously in a UIWebView. Start one conversion, retain its NDHTMLtoPDF object, and begin the next file only from the previous job’s success or error callback. Give every input and output a unique path, and pass the page size and margins explicitly.
This approach avoids the overlapping-work crash reported for loop-based implementations, keeps failures from stalling the batch, and makes pagination problems reproducible.
What the library converts
iclems/iOS-htmltopdf provides URL-based PDF creation and HTML-string overloads. A URL conversion receives a source URL, destination path, paper size, margins, and either delegate or block callbacks. In-memory HTML uses createPDFWithHTML; its baseURL: variants provide the origin needed to resolve relative CSS, images, and fonts.
The documented A4 example is CGSizeMake(595.2,841.8). Margins are supplied as UIEdgeInsets. Keep both values explicit for every item in a batch.
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Prepare a safe batch
Use unique names
Derive each PDF path from an identifier rather than reusing one temporary filename. For example, invoice-0001.pdf, invoice-0002.pdf, and so on. Decide whether an existing file should be overwritten or skipped, and log that decision.
Retain the converter
The README warns that NDHTMLtoPDF works asynchronously and should be retained in a property such as PDFCreator. A local variable that goes out of scope can leave the operation without a live owner.
Keep the queue serial
A serial queue is the most predictable pattern for UIWebView-backed conversion. It limits memory pressure, prevents two jobs from competing for shared state, and gives each completion callback a clear next action. The Stack Overflow report associated with this workflow describes crashes when a tight loop starts overlapping block work; treat that as a concurrency warning, not as a drop-in implementation.
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The following structure assumes pendingHTMLFiles contains file-system paths and that documentsDirectory is already set. Adapt the exact initializer spelling to the library version you have installed; the project documents both delegate and block signatures.
- (void)convertNextHTML {
if (self.pendingHTMLFiles.count == 0) {
NSLog(@"Batch complete. Successes: %lu, failures: %lu",
(unsigned long)self.completedFiles.count,
(unsigned long)self.failedFiles.count);
return;
}
NSString *htmlPath = self.pendingHTMLFiles.firstObject;
[self.pendingHTMLFiles removeObjectAtIndex:0];
NSURL *htmlURL = [NSURL fileURLWithPath:htmlPath];
NSString *stem = htmlPath.lastPathComponent.stringByDeletingPathExtension;
NSString *pdfName = [stem stringByAppendingPathExtension:@"pdf"];
NSString *pdfPath = [self.documentsDirectory
stringByAppendingPathComponent:pdfName];
CGSize paper = CGSizeMake(595.2, 841.8); // documented A4 example
UIEdgeInsets margins = UIEdgeInsetsMake(10, 5, 10, 5);
self.PDFCreator = [NDHTMLtoPDF createPDFWithURL:htmlURL
pathForPDF:pdfPath
pageSize:paper
margins:margins
successBlock:^(NDHTMLtoPDF *job) {
[self.completedFiles addObject:htmlPath];
[self convertNextHTML];
}
errorBlock:^(NDHTMLtoPDF *job) {
[self.failedFiles addObject:htmlPath];
NSLog(@"PDF failed for %@", htmlPath);
[self convertNextHTML];
}];
}Initialize mutable arrays, the pending list, and the documents directory before calling convertNextHTML. Call the method on the main thread if required by the version of the library, because the renderer is based on UIWebView. In production, store an error description and the selected paper and margin values with each failed item.
Why both callbacks must advance the queue
If only the success block starts the next item, one malformed document can halt the entire batch. The error block should record the failure and call the same queue-advancing method. This gives you a complete result list instead of an apparently frozen export.
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Convert HTML held in memory
When your app generates HTML strings rather than reading files, use the documented createPDFWithHTML overload. Pass baseURL: whenever markup contains relative references such as css/print.css, images/logo.png, or web fonts.
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NSURL *baseURL = [NSURL fileURLWithPath:self.templateDirectory
isDirectory:YES];
NSString *pdfPath = [self.documentsDirectory
stringByAppendingPathComponent:@"invoice-0001.pdf"];
self.PDFCreator = [NDHTMLtoPDF createPDFWithHTML:html
baseURL:baseURL
pathForPDF:pdfPath
pageSize:CGSizeMake(595.2, 841.8)
margins:UIEdgeInsetsMake(10, 5, 10, 5)
successBlock:^(NDHTMLtoPDF *job) {
// Persist success, then dequeue the next HTML string.
}
errorBlock:^(NDHTMLtoPDF *job) {
// Persist failure, then dequeue the next HTML string.
}];Use a distinct base URL for each template set if relative assets live in different directories. If all resources are embedded as data URLs or absolute file URLs, a base URL may not be necessary, but supplying the correct context is safer.
Pagination: when one document becomes two pages
A project issue reports an HTML document expected to fit on one page producing two pages, even when the PDF appears to have unused space. Check the layout before treating that result as a converter defect.
- Remove or account for the browser’s default
bodymargin. - Inspect fixed heights and widths on wrappers, tables, and image containers.
- Check image dimensions; a natural-size image can extend the effective content box.
- Look for line wrapping caused by the selected paper width, font availability, or long unbreakable strings.
- Subtract the supplied
UIEdgeInsetsmargins from the paper dimensions when estimating printable height. - Confirm that CSS used for screen layout is not adding extra blocks, floats, or page-breaking behavior.
For every item, log the HTML identifier, paper CGSize, margins, and destination path. That record lets you reproduce a second-page result instead of guessing which document used different settings.
Batch reliability and operational choices
Serial versus overlapping jobs
Serial processing is the recommended default for this library because each conversion owns a web view and asynchronous state. Launching many jobs at once can increase memory use and recreate the reported loop crash. The available material does not establish a throughput number or timing benchmark, so size a queue from your own documents and device tests.
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Failure handling
Never delete the source before its success callback. Write failures to a retry list with the source path and error context. A retry should receive a new destination policy: either remove a known partial file first or deliberately overwrite it.
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Offline assets
Local HTML and local resources can be rendered without a hosted conversion service, provided the web view can resolve every reference. Remote assets introduce network availability and authentication concerns; use absolute URLs, local copies, or a correct baseURL: as appropriate.
Common errors and fixes
The app crashes when processing a list
Cause: a tight loop starts multiple asynchronous conversions, or the converter is not retained. Fix: retain the active NDHTMLtoPDF, remove one item from the queue, and invoke the next conversion only inside both completion paths.
Images, CSS, or fonts are missing
Cause: relative URLs have no resolvable origin. Fix: use the HTML overload with an appropriate baseURL:, verify the directory exists, and check filename case on case-sensitive storage.
The wrong PDF is overwritten
Cause: every job uses the same output path or an unexpanded template name. Fix: create a collision-resistant name from the item identifier, verify the expanded path in logs, and choose overwrite or skip behavior explicitly.
The queue stops partway through
Cause: the next call exists only in the success block. Fix: record the error and call convertNextHTML from the error block as well.
A supposedly single-page document spills onto page two
Cause: CSS box sizes, default margins, image dimensions, wrapping, or the explicit PDF margins exceed the printable area. Fix: inspect those values, reduce unintended spacing, and keep paper and margin settings identical while debugging.
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Do-it-yourself checklist
- Collect source paths or HTML strings and assign a stable identifier to each.
- Choose a paper size and
UIEdgeInsetsmargins. - Generate a unique destination path for every item.
- Provide
baseURL:for relative resources. - Retain the
NDHTMLtoPDFinstance. - Start one job and advance only from success or error.
- Record successes, failures, settings, and partial-file cleanup decisions.
- Review page count and asset loading on representative documents before a large export.
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cURL:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpPython:
import requests
r = requests.get("https://api.screenshotneo.com/v1/shot", params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"}, timeout=90)
open("shot.webp", "wb").write(r.content)Node.js:
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When on-device conversion is the better fit
Keep iOS-htmltopdf when documents must render from local data, work offline, or remain inside your app’s storage boundary. Choose a hosted endpoint when you need URL capture without maintaining web-view setup, want automated cleanup of consent UI, or need an MCP workflow for AI agents. For either approach, validate pagination and asset loading with your own HTML; no independent throughput or compatibility benchmark is established for this exact multi-file workflow.
Frequently Asked Questions
Can I start all conversions with a dispatch group?
A dispatch group can coordinate asynchronous work, but launching many UIWebView conversions concurrently is the pattern associated with the reported crash. A serial completion-driven queue is easier to control; use a group only if you have separately verified safe concurrency for your library version and device targets.
Do I need a baseURL for absolute image URLs?
Not usually. The base URL is specifically useful for relative CSS, images, and fonts. Supplying the correct base URL remains a safe default for template-based HTML.
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Does the documented A4 size use points?
The README expresses A4 as CGSizeMake(595.2,841.8). Pass that CGSize directly, and keep the same units when reasoning about margins and printable height.
Are performance timings available for this batch pattern?
No published independent timing or throughput statistic is established for this exact workflow. Measure representative documents on the iOS devices and HTML versions you support.
Quick Recap
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