使用Substrate开发区块链存证dApp

jasonruan 2020.08.05

1 前言

前面文章介绍了在Substrate上开发智能合约，包括使用原生的ink!语言开发ERC20智能合约，以及将以太坊的Solidity智能合约跑在Substrate链上，在本文将进一步学习在Substrate链上开发一个自定义的区块链存证dApp。

本文内容参考：https://substrate.dev/docs/en/tutorials/build-a-dapp/

2 前置准备

2.1 rust安装

Substrate是由rust语言开发，首先需要安装rust环境。

2.1.1 软件安装

• Rust的安装比较简单，执行如下一条命令即可，该命令将自动完成软件包的下载、安装、环境变量设置：

$ curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

• 安装成功后，会显示如下日志

  stable installed - rustc 1.45.1 (c367798cf 2020-07-26)
Rust is installed now. Great!

• 安装完成后，在~/.cargo/bin目录可以看到相关命令行工具

.cargo/bin/
├── cargo
├── cargo-clippy
├── cargo-fmt
├── cargo-miri
├── clippy-driver
├── rls
├── rustc
├── rustdoc
├── rustfmt
├── rust-gdb
├── rust-lldb
└── rustup

2.1.2 环变设置

• 执行下面命令，即将export PATH="$HOME/.cargo/bin:$PATH"，追加到~/.bashrc中

$ cat ~/.cargo/env >> ~/.bashrc

• 执行下面命令，使得添加的环境变量生效

$ . ~/.bashrc

• 可执行如下命令查看安装版本

$ rustc --version
rustc 1.45.1 (c367798cf 2020-07-26)

2.1.3 配套安装

（1） Racer安装

Racer是一个由Rust爱好者提供的Rust自动补全和语法分析工具，被用来提供基本的补全功能和自定义跳转功能。本身完全由Rust写成，补全功能比较完善。

• 安装命令

$ cargo install racer
......
    Finished release [optimized] target(s) in 2m 44s
  Installing /home/jason/.cargo/bin/racer
   Installed package `racer v2.1.36` (executable `racer`)

若安装报错：error[E0554]: #![feature] may not be used on the stable release channel

请先执行下面命令，切换到nightly版本后，再进行安装：

$ rustup install nightly
$ rustup default nightly
$ rustc --version
rustc 1.47.0-nightly (6c8927b0c 2020-07-26)

• 查看版本

$ racer -V
racer 2.1.36

（2） 源码下载

为了对Rust标准库进行补全，Racer需要获取Rust源码路径。通过rustup获取源码的好处是rustup update可以随时获取最新代码

• 获取源码

$ rustup component add rust-src
info: downloading component 'rust-src'
info: installing component 'rust-src'

• 更新源码

$ rustup update
info: checking for self-updates

   stable-x86_64-unknown-linux-gnu unchanged - rustc 1.45.2 (d3fb005a3 2020-07-31)
  nightly-x86_64-unknown-linux-gnu unchanged - rustc 1.47.0-nightly (6c8927b0c 2020-07-26)

info: cleaning up downloads & tmp directories

• 环变设置

在.bashrc中添加以下内容：

export RUST_SRC_PATH="$(rustc --print sysroot)/lib/rustlib/src/rust/src"

2.2 yarn安装

Substrate前端模板工程（front-end-template）是使用yarn进行包管理的，在此我们进行安装。

安装步骤参考：https://classic.yarnpkg.com/en/docs/install/#centos-stable

$ curl --silent --location https://dl.yarnpkg.com/rpm/yarn.repo | sudo tee /etc/yum.repos.d/yarn.repo
[yarn]
name=Yarn Repository
baseurl=https://dl.yarnpkg.com/rpm/
enabled=1
gpgcheck=1
gpgkey=https://dl.yarnpkg.com/rpm/pubkey.gpg

$ curl --silent --location https://rpm.nodesource.com/setup_12.x | sudo bash -

$ sudo yum install yarn

$ yarn --version
1.22.4

3 存证dApp后端节点开发

存证dApp后端节点是基于node-template来开发，它是一个基于FRAME的Substrate后端节点，可以在其基础上，进行修改以便快速搭建属于自己的Substrate网络。

3.1 node-template安装

• 版本

  v2.0.0-rc5

• 下载

[Jason@RUAN:~/Blockchain]$ git clone [email protected]:substrate-developer-hub/substrate-node-template.git

[Jason@RUAN:~/Blockchain/substrate-node-template] (master)$ git checkout -b v2.0.0-rc5 v2.0.0-rc5
切换到一个新分支 'v2.0.0-rc5'
[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ 

• 编译

安装依赖，避免后续编译错误：

[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ rustup target add wasm32-unknown-unknown --toolchain nightly
[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ yum install -y llvm-devel clang-devel

[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ cargo build --release

• 编译错误及处理

• 编译错误1

  • 错误描述

          Finished release [optimized] target(s) in 2m 51s
           Running `/root/Yikuai/substrate-node-template/target/release/wbuild-runner/node-template-runtime3424067592371620269/target/x86_64-unknown-linux-gnu/release/wasm-build-runner-impl`
      Rust WASM toolchain not installed, please install it!
    warning: build failed, waiting for other jobs to finish...
    error: build failed
    

  • 解决办法

    $ rustup target add wasm32-unknown-unknown --toolchain nightly
    info: downloading component 'rust-std' for 'wasm32-unknown-unknown'
    info: installing component 'rust-std' for 'wasm32-unknown-unknown'
    info: Defaulting to 500.0 MiB unpack ram
    

• 编译错误2

  • 错误描述

    warning: couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
    warning: set the LLVM_CONFIG_PATH environment variable to the full path to a valid `llvm-config` executable (including the executable itself)
    
    error: failed to run custom build command for `librocksdb-sys v6.7.4`
    

  • 解决办法

    $ yum install -y llvm-devel
    

• 编译错误3

  • 错误描述

       Compiling librocksdb-sys v6.7.4
    error: failed to run custom build command for `librocksdb-sys v6.7.4`
    
    Caused by:
      process didn't exit successfully: `/root/Yikuai/substrate-node-template/target/release/build/librocksdb-sys-1bb53efdfd682ab6/build-script-build` (exit code: 101)
      --- stdout
      cargo:rerun-if-changed=build.rs
    
      --- stderr
      thread 'main' panicked at 'Unable to find libclang: "couldn\'t find any valid shared libraries matching: [\'libclang.so\', \'libclang-*.so\', \'libclang.so.*\', \'libclang-*.so.*\'], set the `LIBCLANG_PATH` environment variable to a path where one of these files can be found (invalid: [])"', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.53.3/src/lib.rs:1956:31
    

  • 解决办法

    $ yum install -y clang-devel
    

3.2 存证pallet开发

Substrate运行时由FRAME pallets组成。这些pallets可以被认为是定义你的区块链能够做什么的一个个独立的逻辑单元。

Substrate已经提供了许多预置pallets，用于基于FRAME的运行时。如下图所示：

image.png

例如，FRAME中包含一个balances的pallet，这个pallet通过管理系统中所有账户余额来控制你的区块链系统中的基础货币。如果你想向你的区块链系统中添加智能合约功能，你只需要包含合约pallet即可。

本节我们就是要开发一个存证pallet，并将其添加到我们自定义的区块链中。

3.2.1 创建poe pallet工程目录

pos => Proof Of Existence

[Jason@RUAN:~/Blockchain/substrate-node-template/pallets] (v2.0.0-rc5)$ cargo new --lib poe
[Jason@RUAN:~/Blockchain/substrate-node-template/pallets/poe] (v2.0.0-rc5)$ tree
.
├── Cargo.toml
└── src
    └── lib.rs

3.2.2 代码框架

在新生成lib.rs文件中，填写以下代码框架，这也是从宏观角度来讲，Substrate pallet可以拆分成的6个部分：

// 1. Imports
use frame_support::{decl_module, decl_storage, decl_event, decl_error, dispatch};
use frame_system::{self as system, ensure_signed};

// 2. Pallet Configuration
pub trait Trait: system::Trait { /* --snip-- */ }

// 3. Pallet Storage Items
decl_storage! { /* --snip-- */ }

// 4. Pallet Events
decl_event! { /* --snip-- */ }

// 5. Pallet Errors
decl_error! { /* --snip-- */ }

// 6. Callable Pallet Functions
decl_module! { /* --snip-- */ }

3.2.3 添加依赖

（1）完善引用

#![cfg_attr(not(feature = "std"), no_std)]

use frame_support::{
    decl_module, decl_storage, decl_event, decl_error, ensure, StorageMap
};
use frame_system::{self as system, ensure_signed};
use sp_std::vec::Vec;

（2）完善Cargo.toml文件

将pallets/template/Cargo.toml拷贝至pallets/poe目录，并增加以下内容：

# 增加段
[dependencies.sp-std]
git = 'https://github.com/paritytech/substrate.git'
default-features = false
tag = 'v2.0.0-rc5'
version = '2.0.0-rc5'

[features]
default = ['std']
std = [
    'codec/std',
    'frame-support/std',
    'frame-system/std',
    'sp-std/std',                      # <-- 增加行
]

3.2.4 配置pallet

每一个pallet都有一个配置trait

// 2. Pallet Configuration
pub trait Trait: system::Trait {
    /// The overarching event type.
    type Event: From> + Into<::Event>;
} 

3.2.5 定义事件

事件：可以展示pallet成功被调用的时间和信息。

// 4. Pallet Events 
decl_event! {       
    pub enum Event where AccountId = ::AccountId {
        /// Event emitted when a proof has been claimed.
        ClaimCreated(AccountId, Vec),
        /// Event emitted when a claim is revoked by the owner.
        ClaimRevoked(AccountId, Vec),
    }
}

我们的存证palet，包含了以下事件：

• ClaimCreated：存证创建
• ClaimRevoked：存证撤销

事件可以包含一些附加数据，例如：

• AccountId：谁触发了事件
• Vec：存储或撤销的存证数据

3.2.6 定义错误

错误：可以展示pallet调用失败的时间，及失败原因。

// 5. Pallet Errors
decl_error! {
    pub enum Error for Module {
        /// This proof has already been claimed
        ProofAlreadyClaimed,
        /// The proof does not exist, so it cannot be revoked
        NoSuchProof,                                                                                                    
        /// The proof is claimed by another account, so caller can't revoke it
        NotProofOwner,
    }
}

3.2.7 定义存储

要添加一个新的存证到我们的区块链上，就是要将其存储到我们的pallet的存储里面。在这里创建我们的存储结构。

// 3. Pallet Storage Items
decl_storage! { 
    trait Store for Module as TemplateModule {
        /// The storage item for our proofs.
        /// It maps a proof to the user who made the claim and when they made it.
        Proofs: map hasher(blake2_128_concat) Vec => (T::AccountId, T::BlockNumber);                                                                                          
    }   
}  

3.2.8 实现接口

// 6. Callable Pallet Functions
decl_module! {
    /// The module declaration.
    pub struct Module for enum Call where origin: T::Origin {
        // Initializing errors
        // this includes information about your errors in the node's metadata.
        // it is needed only if you are using errors in your pallet
        type Error = Error;

        // A default function for depositing events
        fn deposit_event() = default;

        /// Allow a user to claim ownership of an unclaimed proof
        #[weight = 10_000]
        fn create_claim(origin, proof: Vec) {
            // Verify that the incoming transaction is signed and store who the
            // caller of this function is.
            let sender = ensure_signed(origin)?;

            // Verify that the specified proof has not been claimed yet or error with the message
            ensure!(!Proofs::::contains_key(&proof), Error::::ProofAlreadyClaimed);

            // Call the `system` pallet to get the current block number
            let current_block = >::block_number();

            // Store the proof with the sender and the current block number
            Proofs::::insert(&proof, (&sender, current_block));

            // Emit an event that the claim was created
            Self::deposit_event(RawEvent::ClaimCreated(sender, proof));
        }

        /// Allow the owner to revoke their claim
        #[weight = 10_000]
        fn revoke_claim(origin, proof: Vec) {
            // Determine who is calling the function
            let sender = ensure_signed(origin)?;

            // Verify that the specified proof has been claimed
            ensure!(Proofs::::contains_key(&proof), Error::::NoSuchProof);

            // Get owner of the claim
            let (owner, _) = Proofs::::get(&proof);

            // Verify that sender of the current call is the claim owner
            ensure!(sender == owner, Error::::NotProofOwner);

            // Remove claim from storage
            Proofs::::remove(&proof);

            // Emit an event that the claim was erased
            Self::deposit_event(RawEvent::ClaimRevoked(sender, proof));
        }
    }
}

3.2.9 完善runtime配置

• 修改runtime/Cargo.toml

# 增加段
[dependencies.poe]  
default-features = false
package = 'pallet-poe'
path = '../pallets/poe'                                                                                                                                                           
version = '2.0.0-rc5'

[features]         
default = ['std']  
std = [    
    'aura/std',    
    'balances/std',
    'codec/std',   
    'frame-executive/std',
    'frame-support/std',
    'grandpa/std', 
    'randomness-collective-flip/std',
    'serde',       
    'sp-api/std',  
    'sp-block-builder/std',
    'sp-consensus-aura/std',
    'sp-core/std', 
    'sp-inherents/std',
    'sp-io/std',   
    'sp-offchain/std',
    'sp-runtime/std',
    'sp-session/std',
    'sp-std/std',  
    'sp-transaction-pool/std',
    'sp-version/std',
    'sudo/std',
    'system/std',
    'timestamp/std',
    'transaction-payment/std',
    'template/std',
    'poe/std',                                                                           # <-- 增加行                                                                                          
]

• 修改runtime/src/lib.rs

//  增加代码块
impl poe::Trait for Runtime {
        type Event = Event;
}           
   
construct_runtime!(
        pub enum Runtime where
                Block = Block,
                NodeBlock = opaque::Block,
                UncheckedExtrinsic = UncheckedExtrinsic
        {   
                System: system::{Module, Call, Config, Storage, Event},
                RandomnessCollectiveFlip: randomness_collective_flip::{Module, Call, Storage},
                Timestamp: timestamp::{Module, Call, Storage, Inherent},
                Aura: aura::{Module, Config, Inherent},
                Grandpa: grandpa::{Module, Call, Storage, Config, Event},
                Balances: balances::{Module, Call, Storage, Config, Event},
                TransactionPayment: transaction_payment::{Module, Storage},
                Sudo: sudo::{Module, Call, Config, Storage, Event},
                TemplateModule: template::{Module, Call, Storage, Event},
                PoeModule: poe::{Module, Call, Storage, Event},                                        // <-- 增加代码行
        }                                                                                                                                                                         
); 

3.3 node-template节点编译

完成存证pallet的开发后，需要重新编译节点。

[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ cargo build --release
   Compiling node-template-runtime v2.0.0-rc5 (/root/Blockchain/substrate-node-template/runtime)
   Compiling pallet-poe v2.0.0-rc5 (/root/Blockchain/substrate-node-template/pallets/poe)
   Compiling node-template v2.0.0-rc5 (/root/Blockchain/substrate-node-template/node)
    Finished release [optimized] target(s) in 12m 18s

3.4 node-template节点启动

[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ ./target/release/node-template purge-chain --dev
Are you sure to remove "/root/.local/share/node-template/chains/dev/db"? [y/N]: y
"/root/.local/share/node-template/chains/dev/db" removed.

[Jason@RUAN:~/Blockchain/substrate-node-template] (v2.0.0-rc5)$ ./target/release/node-template --dev --ws-external --rpc-external --rpc-cors=all
2020-08-04 22:23:44 Substrate Node
2020-08-04 22:23:44 ✌️  version 2.0.0-rc5-8f769db-x86_64-linux-gnu
2020-08-04 22:23:44 ❤️  by Substrate DevHub , 2017-2020
2020-08-04 22:23:44  Chain specification: Development
2020-08-04 22:23:44   Node name: gray-island-3707
2020-08-04 22:23:44  Role: AUTHORITY
2020-08-04 22:23:44  Database: RocksDb at /root/.local/share/node-template/chains/dev/db
2020-08-04 22:23:44 ⛓  Native runtime: node-template-1 (node-template-1.tx1.au1)
2020-08-04 22:23:44  Initializing Genesis block/state (state: 0x5ea9…1904, header-hash: 0x6dac…f18d)
2020-08-04 22:23:44  Loading GRANDPA authority set from genesis on what appears to be first startup.
2020-08-04 22:23:44 ⏱  Loaded block-time = 6000 milliseconds from genesis on first-launch
2020-08-04 22:23:44  Highest known block at #0
2020-08-04 22:23:44 Using default protocol ID "sup" because none is configured in the chain specs
2020-08-04 22:23:44   Local node identity is: 12D3KooWBSKitzNNzfSszWXRggcMe44bv6WfyKy9kyM2DwjcjJNr (legacy representation: QmX77kaM8ydN99qjyRTRznRqkHahzi5jX286MnQTqUp3UR)
2020-08-04 22:23:44 〽 Prometheus server started at 127.0.0.1:9615
2020-08-04 22:23:48  Starting consensus session on top of parent 0x6dac7f7bfbd9cbc4e91be19069d230c9b044ef6080d781e6717a9c99e442f18d
2020-08-04 22:23:48  Prepared block for proposing at 1 [hash: 0xd129eea95a079183db3dd87947194add22643588a1fec10e778b93a867d0f161; parent_hash: 0x6dac…f18d; extrinsics (1): [0xc502…67b6]]
2020-08-04 22:23:48  Pre-sealed block for proposal at 1. Hash now 0x2305ab1c8aee785cb991c993e849b79d7231ad8206b0e7e9b75ef17c3ee90b64, previously 0xd129eea95a079183db3dd87947194add22643588a1fec10e778b93a867d0f161.
2020-08-04 22:23:48 ✨ Imported #1 (0x2305…0b64)
2020-08-04 22:23:49  Idle (0 peers), best: #1 (0x2305…0b64), finalized #0 (0x6dac…f18d), ⬇ 0 ⬆ 0
2020-08-04 22:23:54  Starting consensus session on top of parent 0x2305ab1c8aee785cb991c993e849b79d7231ad8206b0e7e9b75ef17c3ee90b64
2020-08-04 22:23:54  Prepared block for proposing at 2 [hash: 0xf68d2a78c2da715618cfeb4488db17ac925386f06f0b7afa0946f006bef4a770; parent_hash: 0x2305…0b64; extrinsics (1): [0xef36…63d4]]
2020-08-04 22:23:54  Pre-sealed block for proposal at 2. Hash now 0x2acd73ad04ea2e9a5cbd4e3eb4273c56e7a55eb2a3e15d45aed422a682bc5a72, previously 0xf68d2a78c2da715618cfeb4488db17ac925386f06f0b7afa0946f006bef4a770.
2020-08-04 22:23:54 ✨ Imported #2 (0x2acd…5a72)
2020-08-04 22:23:54  Idle (0 peers), best: #2 (0x2acd…5a72), finalized #0 (0x6dac…f18d), ⬇ 0 ⬆ 0
2020-08-04 22:23:59  Idle (0 peers), best: #2 (0x2acd…5a72), finalized #0 (0x6dac…f18d), ⬇ 0 ⬆ 0
2020-08-04 22:24:00  Starting consensus session on top of parent 0x2acd73ad04ea2e9a5cbd4e3eb4273c56e7a55eb2a3e15d45aed422a682bc5a72
2020-08-04 22:24:00  Prepared block for proposing at 3 [hash: 0x3a954db6a3e82636161ce4cbc60e5d12617caafad52912512a5bd3a77c2d5eca; parent_hash: 0x2acd…5a72; extrinsics (1): [0x0cd5…444d]]
2020-08-04 22:24:00  Pre-sealed block for proposal at 3. Hash now 0x7e2616b9f3d0b18caf611bf3c4109609c1fa5edb4391e813d15f2f0e957903ba, previously 0x3a954db6a3e82636161ce4cbc60e5d12617caafad52912512a5bd3a77c2d5eca.
2020-08-04 22:24:00 ✨ Imported #3 (0x7e26…03ba)
2020-08-04 22:24:04  Idle (0 peers), best: #3 (0x7e26…03ba), finalized #1 (0x2305…0b64), ⬇ 0 ⬆ 0

4 存证dApp前端界面开发

存证dApp前端界面是基于front-end-template开发，它是Substrate前端应用开发模板，可以通过其连接Substrate后端节点。

4.1 front-end-template安装

• 版本

  v2.0.0-rc5

• 下载

[Jason@RUAN:~/Blockchain]$ git clone [email protected]:substrate-developer-hub/substrate-front-end-template.git

[Jason@RUAN:~/Blockchain/substrate-front-end-template] (master)$ git checkout -b v2.0.0-rc5 v2.0.0-rc5
切换到一个新分支 'v2.0.0-rc5'
[Jason@RUAN:~/Blockchain/substrate-front-end-template] (v2.0.0-rc5)$

• 安装

安装依赖，避免后续编译错误：

$ yum instally -y libusbx-devel libusb-devel libudev-devel

[Jason@RUAN:~/Blockchain/substrate-front-end-template] (v2.0.0-rc5)$ yarn install
yarn install v1.22.4

• 安装错误及处理

• 安装错误1

  • 错误描述

    Package libusb-1.0 was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libusb-1.0.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libusb-1.0' found
    

  • 解决办法

    $ yum install libusbx-devel libusb-devel -y
    

• 安装错误2

  • 错误描述

    libudev.h：没有那个文件或目录
    

  • 解决办法

    $ yum install libudev-devel
    

• 启动命令

[Jason@RUAN:~/Blockchain/substrate-front-end-template] (v2.0.0-rc5)$ yarn start
Compiled successfully!

You can now view substrate-front-end-template in the browser.

  Local:            http://localhost:8000/
  On Your Network:  http://172.29.0.4:8000/

Note that the development build is not optimized.
To create a production build, use yarn build.

4.2 存证React组件开发

4.2.1 实现存证组件

在substrate-front-end-template/src目录下创建PoeModule.js，代码如下：

React and Semantic UI elements.
import React, { useState, useEffect } from 'react';
import { Form, Input, Grid, Message } from 'semantic-ui-react';
// Pre-built Substrate front-end utilities for connecting to a node
// and making a transaction.
import { useSubstrate } from './substrate-lib';
import { TxButton } from './substrate-lib/components';
// Polkadot-JS utilities for hashing data.
import { blake2AsHex } from '@polkadot/util-crypto';

// Our main Proof Of Existence Component which is exported.
export function Main (props) {
    // Establish an API to talk to our Substrate node.
    const { api } = useSubstrate();
    // Get the 'selected user' from the `AccountSelector` component.
    const { accountPair } = props;
    // React hooks for all the state variables we track.
    // Learn more at: https://reactjs.org/docs/hooks-intro.html
    const [status, setStatus] = useState('');
    const [digest, setDigest] = useState('');
    const [owner, setOwner] = useState('');
    const [block, setBlock] = useState(0);

    // Our `FileReader()` which is accessible from our functions below.
    let fileReader;

    // Takes our file, and creates a digest using the Blake2 256 hash function.
    const bufferToDigest = () => {
        // Turns the file content to a hexadecimal representation.
        const content = Array.from(new Uint8Array(fileReader.result))
            .map((b) => b.toString(16).padStart(2, '0'))
            .join('');

        const hash = blake2AsHex(content, 256);
        setDigest(hash);
    };

    // Callback function for when a new file is selected.
    const handleFileChosen = (file) => {
        fileReader = new FileReader();
        fileReader.onloadend = bufferToDigest;
        fileReader.readAsArrayBuffer(file);
    };

    // React hook to update the 'Owner' and 'Block Number' information for a file.
    useEffect(() => {
        let unsubscribe;

        // Polkadot-JS API query to the `proofs` storage item in our pallet.
        // This is a subscription, so it will always get the latest value,
        // even if it changes.
        api.query.poeModule
            .proofs(digest, (result) => {
                // Our storage item returns a tuple, which is represented as an array.
                setOwner(result[0].toString());
                setBlock(result[1].toNumber());
            })
            .then((unsub) => {
                unsubscribe = unsub;
            });

        return () => unsubscribe && unsubscribe();
        // This tells the React hook to update whenever the file digest changes
        // (when a new file is chosen), or when the storage subscription says the
        // value of the storage item has updated.
    }, [digest, api.query.poeModule]);

    // We can say a file digest is claimed if the stored block number is not 0.
    function isClaimed () {
        return block !== 0;
    }

    // The actual UI elements which are returned from our component.
    return (
        
        
Proof Of Existence
        {/* Show warning or success message if the file is or is not claimed. */}
        

        
        {/* File selector with a callback to `handleFileChosen`. */}
         handleFileChosen(e.target.files[0])}
        />
        {/* Show this message if the file is available to be claimed */}
        
        {/* Show this message if the file is already claimed. */}
        
        
        {/* Buttons for interacting with the component. */}
        
        {/* Button to create a claim. Only active if a file is selected,
          and not already claimed. Updates the `status`. */}
        
        {/* Button to revoke a claim. Only active if a file is selected,
          and is already claimed. Updates the `status`. */}
        
        
        {/* Status message about the transaction. */}
        
{status}
        
        
    );
}

export default function PoeModule (props) {
    const { api } = useSubstrate();
    return (api.query.poeModule && api.query.poeModule.proofs
        ? 
 : null);
}

4.2.2 添加存证组件

在substrate-front-end-template/src/App.js中添加存证组件，代码如下：

import PoeModule from './PoeModule';

  return (   
    

      
        
      
      
        
          ......
          
                                                                                                                                           
          
        
        
      
    
   
  );         

4.2.3 修改连接配置

配置文件：substrate-front-end-template/src/config/development.json

    "PROVIDER_SOCKET": "ws://119.28.233.229:9944"

4.3 front-end-template启动

$ yarn start
Compiled successfully!

You can now view substrate-front-end-template in the browser.

  Local:            http://localhost:8000/
  On Your Network:  http://172.29.0.6:8000/

Note that the development build is not optimized.
To create a production build, use yarn build.

4.4 存证界面展示

image.png

5 存证dApp使用展示

5.1 提交存证

• 选择文件

image.png

• 提交存证

  • 存证入块

image.png

• 存证入块确认

image.png

5.2 撤销存证

撤销存证的按钮，只对创建对应存证的用户可见，例如Alice创建的存证，在切换到Bob账号后，撤销存证的按钮会灰掉：

image.png

image.png

5.3 事件查看

可以查看到提交存证和撤销存证接口调用后触发的事件。

image.png

6 参考资料

https://substrate.dev/docs/en/tutorials/build-a-dapp/