It is sometimes necessary to make changes directly to a component without using state/props to trigger a re-render of the entire subtree. When using React in the browser for example, you sometimes need to directly modify a DOM node, and the same is true for views in mobile apps. setNativeProps
is the React Native equivalent to setting properties directly on a DOM node.
Use setNativeProps when frequent re-rendering creates a performance bottleneck
Direct manipulation will not be a tool that you reach for frequently; you will typically only be using it for creating continuous animations to avoid the overhead of rendering the component hierarchy and reconciling many views.
setNativeProps
is imperative and stores state in the native layer (DOM, UIView, etc.) and not within your React components, which makes your code more difficult to reason about. Before you use it, try to solve your problem withsetState
and shouldComponentUpdate.
setNativeProps with TouchableOpacity
TouchableOpacity uses setNativeProps
internally to update the opacity of its child component:
setOpacityTo: function(value) { // Redacted: animation related code this.refs[CHILD_REF].setNativeProps({ opacity: value }); },
This allows us to write the following code and know that the child will have its opacity updated in response to taps, without the child having any knowledge of that fact or requiring any changes to its implementation:
<TouchableOpacity onPress={this._handlePress}> <View style={styles.button}> <Text>Press me!</Text> </View> </TouchableOpacity>
Let's imagine that setNativeProps
was not available. One way that we might implement it with that constraint is to store the opacity value in the state, then update that value whenever onPress
is fired:
getInitialState() { return { myButtonOpacity: 1, } }, render() { return ( <TouchableOpacity onPress={() => this.setState({myButtonOpacity: 0.5})} onPressOut={() => this.setState({myButtonOpacity: 1})}> <View style={[styles.button, {opacity: this.state.myButtonOpacity}]}> <Text>Press me!</Text> </View> </TouchableOpacity> ) }
This is computationally intensive compared to the original example - React needs to re-render the component hierarchy each time the opacity changes, even though other properties of the view and its children haven't changed. Usually this overhead isn't a concern but when performing continuous animations and responding to gestures, judiciously optimizing your components can improve your animations' fidelity.
If you look at the implementation of setNativeProps
in NativeMethodsMixin.js you will notice that it is a wrapper around RCTUIManager.updateView
- this is the exact same function call that results from re-rendering - see receiveComponent in ReactNativeBaseComponent.js.
Composite components and setNativeProps
Composite components are not backed by a native view, so you cannot call setNativeProps
on them. Consider this example:
var MyButton = React.createClass({ render() { return ( <View> <Text>{this.props.label}</Text> </View> ) }, }); var App = React.createClass({ render() { return ( <TouchableOpacity> <MyButton label="Press me!" /> </TouchableOpacity> ) }, });
If you run this you will immediately see this error: Touchable child
must either be native or forward setNativeProps to a native component
. This occurs because MyButton
isn't directly backed by a native view whose opacity should be set. You can think about it like this: if you define a component with React.createClass
you would not expect to be able to set a style prop on it and have that work - you would need to pass the style prop down to a child, unless you are wrapping a native component. Similarly, we are going to forward setNativeProps
to a native-backed child component.
Forward setNativeProps to a child
All we need to do is provide a setNativeProps
method on our component that calls setNativeProps
on the appropriate child with the given arguments.
var MyButton = React.createClass({ setNativeProps(nativeProps) { this._root.setNativeProps(nativeProps); }, render() { return ( <View ref={component => this._root = component} {...this.props}> <Text>{this.props.label}</Text> </View> ) }, });
You can now use MyButton
inside of TouchableOpacity
! A sidenote for clarity: we used the ref callback syntax here, rather than the traditional string-based ref.
You may have noticed that we passed all of the props down to the child view using {...this.props}
. The reason for this is that TouchableOpacity
is actually a composite component, and so in addition to depending on setNativeProps
on its child, it also requires that the child perform touch handling. To do this, it passes on various props that call back to the TouchableOpacity
component. TouchableHighlight
, in contrast, is backed by a native view and only requires that we implement setNativeProps
.
setNativeProps to clear TextInput value
Another very common use case of setNativeProps
is to clear the value of a TextInput. The controlled
prop of TextInput can sometimes drop characters when the bufferDelay
is low and the user types very quickly. Some developers prefer to skip this prop entirely and instead use setNativeProps
to directly manipulate the TextInput value when necessary. For example, the following code demonstrates clearing the input when you tap a button:
var App = React.createClass({ clearText() { this._textInput.setNativeProps({text: ''}); }, render() { return ( <View style={styles.container}> <TextInput ref={component => this._textInput = component} style={styles.textInput} /> <TouchableOpacity onPress={this.clearText}> <Text>Clear text</Text> </TouchableOpacity> </View> ); } });
Avoiding conflicts with the render function
If you update a property that is also managed by the render function, you might end up with some unpredictable and confusing bugs because anytime the component re-renders and that property changes, whatever value was previously set from setNativeProps
will be completely ignored and overridden. See this example for a demonstration of what can happen if these two collide - notice the jerky animation each 250ms when setState
triggers a re-render.
setNativeProps & shouldComponentUpdate
By intelligently applying shouldComponentUpdate
you can avoid the unnecessary overhead involved in reconciling unchanged component subtrees, to the point where it may be performant enough to use setState
instead of setNativeProps
.
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