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60 lines
1.9 KiB
Markdown
60 lines
1.9 KiB
Markdown
---
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title: "C11. Use the Progress Callback"
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order: 11
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status: "draft"
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---
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To display download or upload progress, pass a `DownloadProgress` or `UploadProgress` callback. Both take two arguments: `(current, total)`.
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## Download progress
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```cpp
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httplib::Client cli("http://localhost:8080");
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auto res = cli.Get("/large-file",
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[](size_t current, size_t total) {
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auto percent = (total > 0) ? (current * 100 / total) : 0;
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std::cout << "\rDownloading: " << percent << "% ("
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<< current << "/" << total << ")" << std::flush;
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return true; // return false to abort
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});
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std::cout << std::endl;
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```
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The callback fires each time data arrives. `total` comes from the Content-Length header — if the server doesn't send one, it may be `0`. In that case, you can't compute a percentage, so just display bytes received.
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## Upload progress
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Uploads work the same way. Pass an `UploadProgress` as the last argument to `Post()` or `Put()`.
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```cpp
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httplib::Client cli("http://localhost:8080");
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std::string body = load_large_body();
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auto res = cli.Post("/upload", body, "application/octet-stream",
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[](size_t current, size_t total) {
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auto percent = current * 100 / total;
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std::cout << "\rUploading: " << percent << "%" << std::flush;
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return true;
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});
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std::cout << std::endl;
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```
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## Cancel mid-transfer
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Return `false` from the callback to abort the transfer. This is how you wire up a "Cancel" button in a UI — flip a flag, and the next progress tick stops the transfer.
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```cpp
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std::atomic<bool> cancelled{false};
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auto res = cli.Get("/large-file",
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[&](size_t current, size_t total) {
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return !cancelled.load();
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});
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```
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> **Note:** `ContentReceiver` and the progress callback can be used together. When you want to stream to a file and show progress at the same time, pass both.
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> For a concrete example of saving to a file, see [C01. Get the response body / save to a file](c01-get-response-body).
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