Direct Manipulation
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.
setNativePropsis 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 withsetStateand shouldComponentUpdate.
setNativeProps with TouchableOpacity
TouchableOpacity uses setNativeProps internally to update the opacity of its child component:
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:
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:
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 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.
Composite components and setNativeProps
Composite components are not backed by a native view, so you cannot call setNativeProps on them. Consider this example:
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 createReactClass 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.
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:
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.
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.
Other native methods
The methods described here are available on most of the default components provided by React Native. Note, however, that they are not available on composite components that aren't directly backed by a native view. This will generally include most components that you define in your own app.
measure(callback)
Determines the location on screen, width, and height in the viewport of the given view and returns the values via an async callback. If successful, the callback will be called with the following arguments:
- x
- y
- width
- height
- pageX
- pageY
Note that these measurements are not available until after the rendering has been completed in native. If you need the measurements as soon as possible and you don't need pageX and pageY, consider using the onLayout property instead.
Also the width and height returned by measure() are the width and height of the component in the viewport. If you need the actual size of the component, consider using the onLayout property instead.
measureInWindow(callback)
Determines the location of the given view in the window and returns the values via an async callback. If the React root view is embedded in another native view, this will give you the absolute coordinates. If successful, the callback will be called with the following arguments:
- x
- y
- width
- height
measureLayout(relativeToNativeNode, onSuccess, onFail)
Like measure(), but measures the view relative to an ancestor, specified as relativeToNativeNode. This means that the returned x, y are relative to the origin x, y of the ancestor view.
As always, to obtain a native node handle for a component, you can use findNodeHandle(component).
focus()
Requests focus for the given input or view. The exact behavior triggered will depend on the platform and type of view.
blur()
Removes focus from an input or view. This is the opposite of focus().