深入分析 Vue 3 渲染器中 `props` 更新时，如何实现属性的精确应用和移除。

Alright folks, settle in, settle in! Grab your virtual coffee, because we’re diving deep into the murky, yet fascinating, waters of Vue 3’s renderer and its handling of props updates. Think of me as your friendly neighborhood Vue whisperer, here to demystify the magic behind how Vue knows exactly which attributes to add, change, or nuke when your data changes.

Let’s face it, props are the bread and butter of component communication. They’re the one-way street data flows down from parent to child. But what happens when that data changes? Vue’s gotta be clever about updating the underlying DOM, and that’s where the renderer’s prop update logic comes into play.

The Grand Strategy: Diffing with a Twist

At its core, Vue uses a diffing algorithm to figure out what’s changed between the "old" props and the "new" props. However, it’s not just a simple diff. Vue needs to consider:

• Attribute Types: Is it a simple string attribute like class, a boolean attribute like disabled, or an event listener like onClick?
• Binding Styles: Is the prop bound directly to an attribute, or is it used to set a property on the DOM element?
• Security Concerns: We can’t just blindly set any attribute on any element. XSS vulnerabilities are a real threat!

Let’s break down the key functions involved. While the exact implementation details are subject to change (Vue’s a living project, after all!), the fundamental principles remain the same. I’ll be using simplified examples to illustrate the core ideas.

1. patchProps – The Gatekeeper of Updates

This is the main function responsible for coordinating the prop updates. It takes the following (simplified) arguments:

• el: The DOM element to update.
• oldProps: An object containing the previous props.
• newProps: An object containing the current props.
• namespace: The namespace of the element (relevant for SVG elements).
• parentComponent: The parent component instance.
• parentSuspense: The parent Suspense instance.
• isSVG: A boolean indicating whether the element is an SVG element.

The basic structure of patchProps looks something like this (again, simplified!):

function patchProps(el, oldProps, newProps, namespace, parentComponent, parentSuspense, isSVG) {
  if (oldProps === newProps) {
    return; // Nothing to do!
  }

  if (oldProps === EMPTY_OBJ) {
    oldProps = {}; // Handle initial render case
  }

  if (newProps === EMPTY_OBJ) {
    newProps = {}; // Handle initial render case
  }

  // Iterate through the *new* props. If a prop exists in newProps
  // but not in oldProps, it's an addition. If it exists in both,
  // it's potentially an update.
  for (const key in newProps) {
    const next = newProps[key];
    const prev = oldProps[key];

    if (next !== prev) {
      patchProp(
        el,
        key,
        prev,
        next,
        namespace,
        parentComponent,
        parentSuspense,
        isSVG
      );
    }
  }

  // Now, iterate through the *old* props. If a prop exists in oldProps
  // but not in newProps, it's a removal.
  for (const key in oldProps) {
    if (!(key in newProps)) {
      patchProp(
        el,
        key,
        oldProps[key],
        null, // `null` signifies removal
        namespace,
        parentComponent,
        parentSuspense,
        isSVG
      );
    }
  }
}

Key Observations:

• Early Exit: The first check (oldProps === newProps) is a performance optimization. If the prop objects are identical, there’s nothing to do. This relies on JavaScript’s reference equality.
• Handling Empty Objects: EMPTY_OBJ is a common optimization in Vue to avoid creating unnecessary empty objects.
• Two Loops: The key is that we need two loops. One loop iterates over the new props to handle additions and updates, and the other iterates over the old props to handle removals.

2. patchProp – The Workhorse of Attribute Manipulation

This function is the real brains of the operation. It takes a single prop and determines how to apply it to the DOM element. The simplified arguments are:

• el: The DOM element.
• key: The name of the prop.
• prevValue: The previous value of the prop.
• nextValue: The new value of the prop.
• namespace: The namespace of the element.
• parentComponent: The parent component instance.
• parentSuspense: The parent Suspense instance.
• isSVG: A boolean indicating whether the element is an SVG element.

Here’s where things get interesting. The implementation of patchProp typically uses a strategy based on the prop name:

function patchProp(
  el,
  key,
  prevValue,
  nextValue,
  namespace,
  parentComponent,
  parentSuspense,
  isSVG
) {
  switch (key) {
    case 'class':
      patchClass(el, nextValue, isSVG);
      break;
    case 'style':
      patchStyle(el, prevValue, nextValue);
      break;
    default:
      if (/^on[A-Z]/.test(key)) {
        // Event listeners
        patchEvent(el, key, prevValue, nextValue, parentComponent);
      } else if (shouldSetAsProp(el, key, nextValue, isSVG)) {
        // Set as a DOM property
        patchDOMProp(el, key, nextValue, prevValue, isSVG);
      } else {
        // Set as an attribute
        patchAttr(el, key, nextValue, namespace, isSVG);
      }
  }
}

Dissecting the patchProp Logic:

• class: The patchClass function (explained later) handles applying and removing CSS classes efficiently. Vue often uses a clever diffing algorithm here to minimize DOM manipulations.
• style: The patchStyle function (explained later) deals with inline styles. It needs to handle adding, updating, and removing individual style properties.
• Event Listeners (on[A-Z]): Vue uses a special prefix (on) followed by a capitalized event name to denote event listeners (e.g., onClick, onMouseover). The patchEvent function (explained later) is responsible for adding, removing, and updating event listeners.
• shouldSetAsProp: This crucial function determines whether a prop should be set as a property on the DOM element (e.g., el.value = nextValue) or as an attribute (e.g., el.setAttribute(key, nextValue)). This is vital for handling boolean attributes, input values, and other special cases.
• patchDOMProp: If shouldSetAsProp returns true, this function sets the prop directly as a property on the DOM element.
• patchAttr: If shouldSetAsProp returns false, this function sets the prop as an attribute using setAttribute.

3. Sub-Patches: Class, Style, and Events

Let’s take a closer look at the "sub-patches" mentioned above:

• patchClass(el, value, isSVG):

  function patchClass(el, value, isSVG) {
    if (value == null) {
      value = '';
    }
  
    if (isSVG) {
        el.setAttribute('class', value);
    } else {
        el.className = value;
    }
  
  }

  This function sets the className property (for HTML elements) or the class attribute (for SVG elements) to the specified value. null or undefined values are treated as empty strings, effectively removing the class.

• patchStyle(el, prevValue, nextValue):

  function patchStyle(el, prevValue, nextValue) {
    const style = el.style;
  
    if (nextValue && typeof nextValue !== 'string') {
      // Apply new/updated styles
      for (const key in nextValue) {
        style[key] = nextValue[key];
      }
  
      if (prevValue && typeof prevValue !== 'string') {
        // Remove styles that are no longer present
        for (const key in prevValue) {
          if (!(key in nextValue)) {
            style[key] = ''; // Remove the style
          }
        }
      }
    } else if (typeof nextValue === 'string') {
      style.cssText = nextValue
    } else if (prevValue && typeof prevValue !== 'string') {
      // Remove all styles
      for (const key in prevValue) {
        style[key] = '';
      }
    } else if (prevValue) {
      style.cssText = ''
    }
  }

  This function carefully updates inline styles. It iterates through the nextValue object (the new styles) and applies each style to the element. It also iterates through the prevValue object (the old styles) and removes any styles that are no longer present in nextValue. This ensures that only the necessary style changes are made. If nextValue is a string, it’s applied to style.cssText.

• patchEvent(el, key, prevValue, nextValue, parentComponent):

  function patchEvent(el, key, prevValue, nextValue, parentComponent) {
    const invokerName = key.slice(2).toLowerCase(); // Extract event name (e.g., onClick -> click)
  
    const existingInvoker = el._vei && el._vei[invokerName]; // Check if event listener already exists
  
    if (nextValue) {
      // Add or update event listener
      if (!existingInvoker) {
        // Create a new invoker function
        const invoker = (el._vei || (el._vei = {}))[invokerName] = (e) => {
          // Handle event
          if (Array.isArray(invoker.value)) {
            invoker.value.forEach(fn => fn(e))
          } else {
            invoker.value(e)
          }
  
        };
  
        invoker.value = nextValue;
        el.addEventListener(invokerName, invoker);
      } else {
        // Update existing invoker
        existingInvoker.value = nextValue;
      }
    } else if (existingInvoker) {
      // Remove event listener
      el.removeEventListener(invokerName, existingInvoker);
      el._vei[invokerName] = null;
    }
  }

  This function manages event listeners. Vue uses a special "invoker" function to track event listeners and ensure that they are properly updated or removed. The _vei property on the element (short for "Vue event invokers") stores a map of event names to invoker functions. When a new event listener is added, an invoker is created and added to the element. When the event listener is updated, the invoker’s value is updated. When the event listener is removed, the invoker is removed from the element and the event listener is detached. This is especially important for handling component updates and avoiding memory leaks.

4. shouldSetAsProp – The Attribute vs. Property Decider

This function is critical for ensuring that props are applied correctly. It determines whether a prop should be set as a DOM property or as an attribute. Here’s a simplified version:

function shouldSetAsProp(el, key, value, isSVG) {
  if (isSVG) {
    return key === 'innerHTML' || key === 'textContent';
  }

  if (key === 'spellcheck' || key === 'draggable' || key === 'translate') {
      return false
  }

  if (key === 'form') {
      return false
  }

  if (key === 'list' && el.tagName === 'INPUT') {
      return false
  }

  if (key === 'type' && el.tagName === 'TEXTAREA') {
      return false
  }

  if (key in el) {
    return true; // Most properties are set directly
  }

  return false; // Default to setting as an attribute
}

Key Considerations for shouldSetAsProp:

• SVG Elements: SVG elements have different rules for attributes and properties.
• Boolean Attributes: Boolean attributes (e.g., disabled, checked) should be set as properties. Setting them as attributes with values like "false" can lead to unexpected behavior.
• Input Values: The value of an input element should be set as a property, not an attribute.
• Security: Vue carefully avoids setting certain attributes as properties to prevent XSS vulnerabilities.

Example Time: Putting it All Together

Let’s say we have a component with the following template:

<template>
  <button :class="buttonClass" :disabled="isDisabled" @click="handleClick" :style="buttonStyle" data-id="123">
    {{ label }}
  </button>
</template>

<script>
export default {
  data() {
    return {
      buttonClass: 'primary',
      isDisabled: false,
      label: 'Click Me',
      buttonStyle: {
        color: 'white',
        backgroundColor: 'blue'
      }
    };
  },
  methods: {
    handleClick() {
      alert('Clicked!');
    }
  }
};
</script>

And let’s say our data changes to this:

{
  buttonClass: 'secondary',
  isDisabled: true,
  label: 'New Label',
  buttonStyle: {
    color: 'black',
    fontWeight: 'bold'
  }
}

Here’s how Vue’s renderer would handle the prop updates:

1. patchProps is called with the button element, the old props, and the new props.
2. The for...in loop for newProps iterates:
   • class: patchProp calls patchClass to update the className to "secondary".
   • disabled: patchProp calls shouldSetAsProp. Since disabled exists as a property on the button element and it isn’t on the exception list, shouldSetAsProp returns true. patchProp then calls patchDOMProp to set el.disabled = true.
   • onClick: patchProp recognizes the on prefix and calls patchEvent to update or add the event listener.
   • style: patchProp calls patchStyle to update the inline styles. The color is updated to "black" and fontWeight is set to "bold". The old backgroundColor is removed.
   • data-id: patchProp calls shouldSetAsProp. Since data-* attributes are not standard DOM properties, shouldSetAsProp returns false. patchProp then calls patchAttr to set el.setAttribute('data-id', '123'). (Note: This prop didn’t change, but it’s still checked.)
3. The for...in loop for oldProps iterates:
   • label: This prop is handled during the text node diffing process, not during prop patching.

The Takeaway: Precision and Efficiency

Vue’s prop update mechanism is designed for precision and efficiency. By carefully diffing the old and new props, and by using specialized functions to handle different attribute types, Vue ensures that only the necessary DOM manipulations are performed. This leads to faster rendering and a smoother user experience.

Points to Ponder:

• How does Vue handle props that are objects or arrays? (Hint: Deeply reactive objects require more sophisticated diffing.)
• How does the key attribute influence the diffing process? (Hint: It helps Vue identify elements that have moved or been reordered.)
• What are the security implications of setting attributes directly on DOM elements? (Hint: XSS vulnerabilities are a serious concern.)
• How do custom directives interact with the prop update mechanism? (Hint: Directives can provide custom logic for handling specific attributes.)

That’s all folks! I hope this deep dive into Vue 3’s prop update mechanism has been enlightening. Now, go forth and build amazing things! And remember, understanding the inner workings of Vue can make you a much more effective and confident developer. Peace out!