We have completed the rendering process of a simple component. This time we are going to explore more ramifications of this process by discussing how a class component (a typical one we might use in everyday development) is rendered.
Files used in this article:
the same as post one and two
I use {} to reference the previous post if the methods (or logic process) has been discussed in it.
The component named App is similar to what I gave in the beginning of post one . But since we have leveled-up a bit, it does not look that daunting anymore.
import React,
{
Component
}
from'react';
import logo from'. / logo.svg';
import'. / App.css';
class App extends Component {
constructor(props) {
super(props);
this.state = {
desc: 'start',
};
}
render() {
return ( < div className = "App" > <div className = "App-header" > <img src = "main.jpg"className = "App-logo"alt = "logo" / ><h1 > "Welcom to React" < /h1>
</div > <p className = "App-intro" > {
this.state.desc
} < /p>
</div > );
}
}
export
default App;
App@App.js
As mentioned, the component above is rendered using:
ReactDOM.render(
<App />,
document.getElementById('root')
);
Now the babeled code:
import React, { Component } from 'react';
import logo from './logo.svg';
import './App.css';
class App extends Component {
constructor(props) {
super(props);
this.state = {
desc: 'start',
};
}
render() {
return React.createElement(
'div',
{ className: 'App' },
React.createElement(
'div',
{ className: 'App-header' },
React.createElement(
'img',
{ src: "main.jpg", className: 'App-logo', alt: 'logo' }
),
React.createElement(
'h1',
null,
' "Welcom to React" '
)
),
React.createElement(
'p',
{ className: 'App-intro' },
this.state.desc
)
);
}
}
export default App;
...
ReactDOM.render(React.createElement(App, null), document.getElementById('root'));
Here we consider Component a common base class, as other methods will not be used in this post.
This time we can fast forward the logic that is shared with simple component.
Construct the top level wrapper
ReactCompositeComponent[T]
The designated data structure:
This step is almost the same as that in simple component rendering, so I will give a brief description only, it
1) creates ReactElement[1] using
ReactElement.createElement(type, config, children)
(This time App is passed to type, and config, children are null);
creates ReactElement[2] in
_renderSubtreeIntoContainer()
;
create the designated wrapper with
instantiateReactComponent()
.
ReactElement.createElement(type, // scr: -------------> App
config, // scr: -------------> null
children // scr: -------------> null
) // scr: ------------------------------------------------------> 1)
ReactDOM.render |= ReactMount.render(nextElement, container, callback) |= ReactMount._renderSubtreeIntoContainer(parentComponent, // scr: ----> null
nextElement, // scr: ----> ReactElement[1]
container, // scr: ----> document.getElementById('root')
callback ' // scr: ----> undefined
) // scr: ------------------------------------------------------> 2)
|-instantiateReactComponent( // scr: -------------------------> 3)
node, // scr: ------> ReactElement[2]
shouldHaveDebugID /* false */
)
|-ReactCompositeComponentWrapper(
element // scr: ------> ReactElement[2]
);
|=ReactCompositeComponent.construct(element /* same */)
This is what we covered in { post one }.
Initialize
ReactCompositeComponent[T]
The designated data structure:
The step is the same as well:
1)
ReactDOMContainerInfo[ins]
represents the container DOM element,
document.getElementById('root ')
;
2) TopLevelWrapper is instantiated (
TopLevelWrapper[ins]
) and is set to
ReactCompositeComponent[T]._currentElement
alongside the initialization of other properties;
3) Again,
mountComponentIntoNode
is the cross point of upper and lower half, within which
ReactCompositeComponent[T].mountComponent
returns a complete DOMLazyTree that can be used by
ReactMount._mountImageIntoNode
, a method from lower half.
ReactDOM.render ___
|=ReactMount.render(nextElement, container, callback) |
|=ReactMount._renderSubtreeIntoContainer() |
|-ReactMount._renderNewRootComponent() |
|-instantiateReactComponent() |
|~batchedMountComponentIntoNode() upper half
|~mountComponentIntoNode() (platform agnostic)
|-ReactReconciler.mountComponent() // scr-----> 1) |
|-ReactCompositeComponent[T].mountComponent() scr:> 2)3)
... _|_
... lower half
|-_mountImageIntoNode() (HTML DOM specific)
This is what we covered in the first part of { post two }.
Except for some small differences in regard to argument values, the the top level wrapper related operations are exactly the same as what we discussed in previous posts. After those operations complete, we came to the first ramification that is specific to class component.
ReactCompositeComponent[T].performInitialMount()
— create a
ReactCompositeComponent
from ReactElement[1]
This step strips the wrapper and creates another
ReactCompositeComponent
instance to reflect App component.
The designated data structure:
The call stack in action:
...
|~mountComponentIntoNode() |
|-ReactReconciler.mountComponent() |
|-ReactCompositeComponent[T].mountComponent() |
/* we are here */ |
|-ReactCompositeComponent[T].performInitialMount( |
renderedElement, // scr: -------> undefined |
hostParent, // scr: -------> null upper half
hostContainerInfo, // scr: -------> | ReactDOMContainerInfo[ins] |
transaction, // scr: -------> not of interest |
context, // scr: -------> not of interest |
) |
The process is very similar to the
performInitialMount()
in { post two }. The only difference here is that based on the type of ReactElement[1],
_instantiateReactComponent
creates a
ReactCompositeComponent
for the class component (App) instead of a ReactDOMComponent. To put it briefly:
1) it calls
_renderValidatedComponent()
which in turn calls
TopLevelWrapper.render()
to extract ReactElement[1]; 2) it instantiates a
ReactCompositeComponent
with
_instantiateReactComponent
(we name the object
ReactCompositeComponent[ins]
); and 3) it calls
ReactCompositeComponent[ins].mountComponent
(recursively) through ReactReconciler, and move on to the next step.
performInitialMount: function (
renderedElement,
hostParent,
hostContainerInfo,
transaction,
context)
{
var inst = this._instance;
...
if (inst.componentWillMount) {
... // scr: we did not define componentWillMount() in App
}
// If not a stateless component, we now render
if (renderedElement === undefined) {
renderedElement = this._renderValidatedComponent(); // scr: > 1)
}
var nodeType = ReactNodeTypes.getType(renderedElement); // scr: -> the type is ReactNodeTypes.Composite this time
this._renderedNodeType = nodeType;
var child = this._instantiateReactComponent(renderedElement, nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */
); // scr: ----------------------------------------------> 2)
this._renderedComponent = child;
var markup = ReactReconciler.mountComponent(child, transaction, hostParent, hostContainerInfo, this._processChildContext(context), debugID); // scr: ----------------------------------------------> 3)
...// scr: DEV code
return markup;
},
ReactCompositeComponent@renderers/shared/stack/reconciler/ReactCompositeComponent.js
ReactCompositeComponent[1].mountComponent()
— initialize
ReactCompositeComponent[1]
The designated data structure:
The call stack in action:
...
|~mountComponentIntoNode() |
|-ReactReconciler.mountComponent() |
|-ReactCompositeComponent[T].mountComponent() |
|-ReactCompositeComponent[T].performInitialMount() upper half
|-ReactReconciler.mountComponent() |
/* we are here */ |
|-ReactCompositeComponent[1].mountComponent(same) |
Same as in
ReactCompositeComponent[T].mountComponent()
{ post two }, the most important task of this step is to instantiate App with
ReactCompositeComponent[ins]._currentElement
(ReactElement[1]).
The line in the method that does the job is:
...
var inst = this._constructComponent(
doConstruct,
publicProps,
publicContext,
updateQueue,
);
...
ReactCompositeComponent@renderers/shared/stack/reconciler/ReactCompositeComponent.js
in which the constructor of App gets called.
...
constructor(props) {
super(props);
this.state = {
desc: 'start ',
};
}
...
// copied from the beginning of this text
Then (we name it) App[ins] is set to
ReactCompositeComponent[ins]._instance
and a back-link is also created through ReactInstanceMap.
Other operations includes: 1) App[ins].props reference
ReactElement[1].props
; and 2)
ReactCompositeComponent[ins]._mountOrder
is 2 due to the ++ operating on the global variable nextMountID.
It is important to note that App[ins].render() is another App method we define in the beginning. Unlike
TopLevelWrapper[ins].render()
that returns a concrete ReactElement instance, App[ins].render() relies on
React.createElement()
at the time when it is invoked. We will revisit this method soon.
Since this step is very similar to that initializes the
ReactCompositeComponent[T]
{ post two }, we do not further examine the workhorse method (i.e., mountComponent()).
ReactCompositeComponent[ins].performInitialMount()
— create a ReactDOMComponent
...
|~mountComponentIntoNode() |
|-ReactReconciler.mountComponent() |
|-ReactCompositeComponent[T].mountComponent() |
|-ReactCompositeComponent[T].performInitialMount() upper half
|-ReactReconciler.mountComponent() |
/* we are here */ |
|-ReactCompositeComponent[1].mountComponent() |
|-this.performInitialMount() |
|-this._renderValidatedComponent() |
|-instantiateReactComponent() _|_
|-ReactDOMComponent[6].mountComponent() lower half
Before the a ReactDOMComponent (we know that this is the class that handle DOM operations) can be created, the ReactElements defined within App[ins] needs to be extracted. To do so, App[ins].render() is called by the following line (in
_renderValidatedComponent()
) { post two }
...
renderedElement = this._renderValidatedComponent();
...
performInitialMount@renderers/shared/stack/reconciler/ReactCompositeComponent.js
Then App[ins].render() triggers
The cascading calls of
React.createElement()
To understand how the ReactElement tree is established, let's first revisit the App.render() implementation: render() {
return React.createElement( // scr: -----------> 5)
'div', {
className: 'App'
},
React.createElement( // scr: -----------> 3)
'div', {
className: 'App-header'
},
React.createElement( // scr: -----------> 1)
'img', {
src: "main.jpg",
className: 'App-logo',
alt: 'logo'
}), React.createElement( // scr: -----------> 2)
'h1', null, ' "Welcom to React" ')), React.createElement( // scr: -----------> 4)
'p', {
className: 'App-intro'
},
this.state.desc));
}
// copied from the beginning of this text
In this code snippet I also give the call order of createElement() s which follows a very simple principle: arguments should be resolved(with createElement()) from left to right before a
function(of createElement()) gets called.Then we can examine the creation of each ReactElement {
post one
}.React.createElement( // scr: --------------------------------> 1)
'img', {
src: "main.jpg",
className: 'App-logo',
alt: 'logo'
}), creates ReactElement[2] :
; and
React.createElement( // scr: --------------------------------> 2)
'h1',
null,
'Welcome to React'
)
creates ReactElement[3]:
(Now the two arguments for 3) are resolved.)
; and
React.createElement( // scr: -----------> 3)
'div',
ReactElement[2],
ReactElement[3]
),
creates ReactElement[4]:
; and
React.createElement( // scr: -----------> 4)
'p', {
className: 'App-intro'
},
this.state.desc)
creates ReactElement[5]:
(Now the arguments for 5) are resolved.)
; and
return React.createElement( // scr: -----------> 5)
'div',
{ className: 'App' },
ReactElement[4],
ReactElement[5]
)
creates ReactElement[6]:
Combined together we got the element tree referenced by renderedElement:
ReactCompositeComponent[ins]._instantiateReactComponent()
— Create
ReactDOMComponent[6]
The designated data structure:
to be continued…
Then the element tree is used to create the
ReactDOMComponent[6]
by the following line (within
_instantiateReactComponent()
) { post two }
var child = this._instantiateReactComponent(
renderedElement,
nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */,
);
Now
ReactReconciler.mountComponent()
calls the mountComponent() of the
ReactDOMComponent[6]
and the logic processes to the lower half.
来源: https://juejin.im/entry/5a58bcb26fb9a01c9f5b5cdd