Detailed explanation of React controlled components and uncontrolled components

This article mainly explains to you in detail the controlled components and uncontrolled components of React. There is not much information about controlled components and uncontrolled components on the official website and domestic websites. Some people think that they are available or not, and they don't care. This just shows the power of React to meet the needs of projects of different sizes. For example, if you just do a simple data display like ListView and capture the data, then a for loop and {} are enough, but there are a large number of reports in the background system, different forms are linked, and there is a lack of controlled components. Really can not.

Controlled components and uncontrolled components are the entry points for React to process forms. From the perspective of React, the author must let the data control everything, or simply understand that the generation and update of the page must faithfully execute the JSX instructions.

But form elements have their own special features. Users can change the display of the interface through keyboard input and mouse selection. The change of the interface also means that some data has been changed, the more obvious ones are value of input, innerHTML of textarea, checked of radio/checkbox, not so much. The obvious ones are selected and selectedIndex of option, which are passively modified.

<input value={this.state.value} />

When the input.value is taken from the component's state.value, when the user makes an input modification, and then JSX refreshes the view again, at this time, the input.value takes the user's new value or the state's new value. value? Based on this disagreement, React gave a compromise solution that was supported by both, and today's topic was born.

React believes that value/checked cannot exist alone and needs to be used together with onInput/onChange/disabed/readOnly and other properties or events that control value/checked. Together they form controlled components, which are controlled by JSX. If the user does not write these additional properties and events, the framework will add some events to it internally, such as onClick, onInput, and onChange, preventing you from inputting or selecting, and preventing you from modifying its value. Inside the framework, there is a stubborn variable, which I call persistValue, that keeps the value last assigned to it by JSX and can only be modified by internal events.

So we can assert that the controlled component is the control of value that can be completed through event.

In controlled components, persistValue can always be refreshed.

Let’s look at the uncontrolled components again. Since value/checked has been occupied, React enables another set of ignored attributes defaultValue/defaultChecked in HTML. It is generally believed that they are similar to value/checked, that is, when value does not exist, the value of defaultValue is regarded as value.

We have said above that the display of form elements is controlled by the internal persistValue, so defaultXXX will also synchronize persistValue, and then persistValue will synchronize the DOM. However, the starting point of uncontrolled components is to be loyal to user operations. If the user

input.value = "xxxx"

and later

<input defaultValue={this.state.value} />

in the code will no longer take effect, it will always be xxxx.

How does it do this, and how to identify whether the modification comes from inside or outside the framework? I looked through the source code of React, and it turns out that it has a thing called valueTracker to track the user's input

var tracker = {
    getValue: function () {
      return currentValue;
    },
    setValue: function (value) {
      currentValue = '' + value;
    },
    stopTracking: function () {
      detachTracker(node);
      delete node[valueField];
    }
  };
  return tracker;
}

This thing is entered into the value/checked of the element through Object.defineProperty, so it knows the user The assignment operation to its value.

But value/checked are still two very core attributes, involving too many internal mechanisms (for example, value and oninput, onchange, input method events oncompositionstart,
compositionchange, oncompositionend, onpaste, oncut), In order to modify value/checked smoothly,
also use Object.getOwnPropertyDescriptor. If I want to be compatible with IE8, there is no such advanced gadget. I take another safer way,
only use Object.defineProperty to modify defaultValue/defaultChecked.

First I add an _uncontrolled attribute to the element to indicate that I have hijacked defaultXXX. Then add a switch, _observing, to the set method of the description object (the third parameter of Object.defineProperty). When the view is updated inside the frame, this value is false. After the update, it is set to true.

This way you will know whether input.defaultValue = "xxx" was modified by the user or the framework.

f (!dom._uncontrolled) {
    dom._uncontrolled = true;
    inputMonitor.observe(dom, name); //重写defaultXXX的setter/getter
}
dom._observing = false;//此时是框架在修改视图，因此需要关闭开关
dom[name] = val;
dom._observing = true;//打开开关，来监听用户的修改行为

The implementation of inputMonitor is as follows

export var inputMonitor = {};
var rcheck = /checked|radio/;
var describe = {
    set: function(value) {
        var controllProp = rcheck.test(this.type) ? "checked" : "value";
        if (this.type === "textarea") {
            this.innerHTML = value;
        }
        if (!this._observing) {
            if (!this._setValue) {
                //defaultXXX只会同步一次_persistValue
                var parsedValue = (this[controllProp] = value);
                this._persistValue = Array.isArray(value) ? value : parsedValue;
                this._setValue = true;
            }
        } else {
            //如果用户私下改变defaultValue，那么_setValue会被抺掉
            this._setValue = value == null ? false : true;
        }
        this._defaultValue = value;
    },
    get: function() {
        return this._defaultValue;
    },
    configurable: true
};

inputMonitor.observe = function(dom, name) {
    try {
        if ("_persistValue" in dom) {
            dom._setValue = true;
        }
        Object.defineProperty(dom, name, describe);
    } catch (e) {}
};

I accidentally posted such brain-burning code. This is a bad habit of coders. However, at this point, everyone understands that both official react and anu/qreact control user input through Object.defineProperty.

So we can understand the behavior of the following code

    var a =  ReactDOM.render(<textarea defaultValue="foo" />, container);
    ReactDOM.render(<textarea defaultValue="bar" />, container);
    ReactDOM.render(<textarea defaultValue="noise" />, container);
    expect(a.defaultValue).toBe("noise");
    expect(a.value).toBe("foo");
    expect(a.textContent).toBe("noise");
    expect(a.innerHTML).toBe("noise");

Since the user has not manually modified the defaultValue, dom._setValue has always been false/undefined, so _persistValue can always be modified.

Another example:

var renderTextarea = function(component, container) {
    if (!container) {
        container = document.createElement("p");
    }
    const node = ReactDOM.render(component, container);
    node.defaultValue = node.innerHTML.replace(/^\n/, "");
    return node;
};

const container = document.createElement("p");
//注意这个方法，用户在renderTextarea中手动改变了defaultValue，_setValue就变成true
const node = renderTextarea(<textarea defaultValue="giraffe" />, container);

expect(node.value).toBe("giraffe");

// _setValue后，gorilla就不能同步到_persistValue，因此还是giraffe
renderTextarea(<textarea defaultValue="gorilla" />, container);
//  expect(node.value).toEqual("giraffe");

node.value = "cat";
// 这个又是什么回事了呢，因此非监控属性是在diffProps中批量处理的，在监控属性，则是在更后的方法中处理
// 检测到node.value !== _persistValue，于是重写 _persistValue = node.value，于是输出cat
renderTextarea(<textarea defaultValue="monkey" />, container);
expect(node.value).toEqual("cat");

Of course, there are many types of form elements, and each form element also has its default behavior.

纯文本类：text, textarea, JSX的值，总是往字符串转换
type="number"的控制，值总是为数字，不填或为“”则转换为“0”
radio有联动效果，同一父节点下的相同name的radio控制只能选择一个。
select的value/defaultValue支持数组，不做转换，但用户对底下的option元素做增删操作，selected会跟着变动。

此外select还有模糊匹配与精确匹配之分。

//精确匹配
var dom = ReactDOM.render(
    <select value={222}>
        <option value={111}>aaa</option>
        <option value={"222"}>xxx</option>
        <option value={222}>bbb</option>
        <option value={333}>ccc</option>
    </select>,
    container
);
expect(dom.options[2].selected).toBe(true);//选中第三个

//模糊匹配
var dom = ReactDOM.render(
    <select value={222}>
        <option value={111}>aaa</option>
        <option value={"222"}>xxx</option>
        <option value={333}>ccc</option>
    </select>,
    container
);
expect(dom.options[2].selected).toBe(true);//选中第二个

凡此种种，React/anu都是做了大量工作，迷你如preact/react-lite之流则可能遇坑。

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