Merge pull request #594 from ulan/ulan/qrcode

Add a QR code generator example

rust/qrcode/.gitignore

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+# Various IDEs and Editors
+.vscode/
+.idea/
+**/*~
+
+# Mac OSX temporary files
+.DS_Store
+**/.DS_Store
+
+# dfx temporary files
+.dfx/
+
+# generated files
+src/declarations/
+
+# rust
+target/
+
+# frontend code
+node_modules/
+dist/
+
+# environment variables
+.env

rust/qrcode/Cargo.toml

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+[workspace]
+members = [
+    "src/qrcode_backend",
+]

rust/qrcode/README.md

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+# QR code generator on the Internet Computer
+
+## Overview
+
+This example shows that an Internet Computer dapp can perform a long-running computation, like image processing, in a single message execution.
+This is possible due to a unique feature called Deterministic Time Slicing (DTS), which automatically divides long computations into smaller slices executed across multiple blocks.
+Developers can write long-running code as usual and don't require anything special to take advantage of DTS, as demonstrated in this example.
+
+You try the live version of the dapp running on the Internet Computer mainnet here: [https://khpe2-4qaaa-aaaao-a2fnq-cai.icp0.io/](https://khpe2-4qaaa-aaaao-a2fnq-cai.icp0.io/).
+
+## Prerequisites
+This example requires an installation of:
+
+- [x] Install the [IC SDK](https://internetcomputer.org/docs/current/developer-docs/setup/install/index.mdx).
+- [x] Install `node.js` to build the web frontend.
+- [x] Clone this project to a local directory.
+
+## Running locally
+
+Start a local replica of the Internet Computer by running:
+
+```
+dfx start --background
+```
+
+You can omit the `--background` argument if you want to see log messages of the dapp.
+
+Now you can build and deploy the dapp with a single command
+```
+dfx deploy
+```
+
+If you see any error, it might be worthwhile to consult the [developer forum](https://forum.dfinity.org/).
+In case of successful deployment, you will see an output with local URLs:
+
+```
+Deployed canisters.
+URLs:
+  Frontend canister via browser
+    qrcode_frontend: ...
+  Backend canister via Candid interface:
+    qrcode_backend: ...
+```
+
+Navigate to the frontend URL in your browser and you'll be able to interact with the dapp.
+
+![Screenshot of the frontend UI](screenshot.png)
+
+
+## How it works
+
+The initial code of the dapp was autogenerated by `dfx` using the standard frontend/backend template.
+
+The frontend consists of an HTML page with a form where users can enter text for the QR code and choose various options.
+When the user clicks the "Generate!" button, a JavaScript handler initiates a call to the backend canister.
+The heavy-lifting of this call is managed by `candid`, `js-agent`, and `dfx`, which automatically generates a JavaScript object from the backend's Candid interface.
+That object contains `async` functions for each of the backend's endpoints, and the button handler uses them to make the calls.
+
+The backend, written in Rust, uses the `qrcode-generator`` and `image`` crates to create a QR code from user text.
+It also performs some image processing, to add the Internet Computer logo and a color gradient to the final result.
+Note the amount of computational work may be significant for large images.
+
+For educational purposes the backend offers two public endpoint for QR code generation: one for updates and another for queries.
+Currently, DTS is supported for updates, but not for queries.
+As a result, the update endpoint has larger instruction limit compared to the query endpoint and thus can handle larger images.
+

rust/qrcode/dfx.json

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+{
+  "canisters": {
+    "qrcode_backend": {
+      "candid": "src/qrcode_backend/qrcode_backend.did",
+      "package": "qrcode_backend",
+      "type": "rust"
+    },
+    "qrcode_frontend": {
+      "dependencies": [
+        "qrcode_backend"
+      ],
+      "frontend": {
+        "entrypoint": "src/qrcode_frontend/src/index.html"
+      },
+      "source": [
+        "src/qrcode_frontend/assets",
+        "dist/qrcode_frontend/"
+      ],
+      "type": "assets"
+    }
+  },
+  "defaults": {
+    "build": {
+      "args": "",
+      "packtool": ""
+    }
+  },
+  "output_env_file": ".env",
+  "version": 1
+}