Throttling prevents a function from being called more than once in a given window of time. The example below throttles a “click” handler to prevent calling it more than once per second.
import throttle from 'lodash.throttle';
class LoadMoreButton extends React.Component {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
this.handleClickThrottled = throttle(this.handleClick, 1000);
}
componentWillUnmount() {
this.handleClickThrottled.cancel();
}
render() {
return <button onClick={this.handleClickThrottled}>Load More</button>;
}
handleClick() {
this.props.loadMore();
}
}Using this technique will only capture the last published value in a frame. You can see an example of how this optimization works on MDN
import rafSchedule from 'raf-schd';
class ScrollListener extends React.Component {
constructor(props) {
super(props);
this.handleScroll = this.handleScroll.bind(this);
// Create a new function to schedule updates.
this.scheduleUpdate = rafSchedule(
point => this.props.onScroll(point)
);
}
handleScroll(e) {
// When we receive a scroll event, schedule an update.
// If we receive many updates within a frame, we'll only publish the latest value.
this.scheduleUpdate({ x: e.clientX, y: e.clientY });
}
componentWillUnmount() {
// Cancel any pending updates since we're unmounting.
this.scheduleUpdate.cancel();
}
render() {
return (
<div
style={{ overflow: 'scroll' }}
onScroll={this.handleScroll}
>
<img src="/my-huge-image.jpg" />
</div>
);
}
}Throttling prevents a function from being called more than once in a given window of time. The example below throttles a “click” handler to prevent calling it more than once per second.
import throttle from 'lodash.throttle';
class LoadMoreButton extends React.Component {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
this.handleClickThrottled = throttle(this.handleClick, 1000);
}
componentWillUnmount() {
this.handleClickThrottled.cancel();
}
render() {
return <button onClick={this.handleClickThrottled}>Load More</button>;
}
handleClick() {
this.props.loadMore();
}
}Using this technique will only capture the last published value in a frame. You can see an example of how this optimization works on MDN
import rafSchedule from 'raf-schd';
class ScrollListener extends React.Component {
constructor(props) {
super(props);
this.handleScroll = this.handleScroll.bind(this);
// Create a new function to schedule updates.
this.scheduleUpdate = rafSchedule(
point => this.props.onScroll(point)
);
}
handleScroll(e) {
// When we receive a scroll event, schedule an update.
// If we receive many updates within a frame, we'll only publish the latest value.
this.scheduleUpdate({ x: e.clientX, y: e.clientY });
}
componentWillUnmount() {
// Cancel any pending updates since we're unmounting.
this.scheduleUpdate.cancel();
}
render() {
return (
<div
style={{ overflow: 'scroll' }}
onScroll={this.handleScroll}
>
<img src="/my-huge-image.jpg" />
</div>
);
}
}Debouncing ensures that a function will not be executed until after a certain amount of time has passed since it was last called. This can be useful when you have to perform some expensive calculation in response to an event that might dispatch rapidly (eg scroll or keyboard events). The example below debounces text input with a 250ms delay.
import debounce from 'lodash.debounce';
class Searchbox extends React.Component {
constructor(props) {
super(props);
this.handleChange = this.handleChange.bind(this);
this.emitChangeDebounced = debounce(this.emitChange, 250);
}
componentWillUnmount() {
this.emitChangeDebounced.cancel();
}
render() {
return (
<input
type="text"
onChange={this.handleChange}
placeholder="Search..."
defaultValue={this.props.value}
/>
);
}
handleChange(e) {
this.emitChangeDebounced(e.target.value);
}
emitChange(value) {
this.props.onChange(value);
}
}Alternately, you can use DOM APIs to store data needed for event handlers. Consider this approach if you need to optimize a large number of elements or have a render tree that relies on React.PureComponent equality checks.
const A = 65 // ASCII character code
class Alphabet extends React.Component {
constructor(props) {
super(props);
this.handleClick = this.handleClick.bind(this);
this.state = {
justClicked: null,
letters: Array.from({length: 26}, (_, i) => String.fromCharCode(A + i))
};
}
handleClick(e) {
this.setState({
justClicked: e.target.dataset.letter
});
}
render() {
return (
<div>
Just clicked: {this.state.justClicked}
<ul>
{this.state.letters.map(letter =>
<li key={letter} data-letter={letter} onClick={this.handleClick}>
{letter}
</li>
)}
</ul>
</div>
)
}
}Using Function.prototype.bind in render creates a new function each time the component renders, which may have performance implications (see below).
class Foo extends Component {
handleClick() {
console.log('Click happened');
}
render() {
return <button onClick={() => this.handleClick()}>Click Me</button>;
}
}You need to detect recursion at run-time. Fundamentally, it's very simple to do this:
bool MyFnSimple()
{
static bool entered = false;
if( entered )
{
cout << "Re-entered function!" << endl;
return false;
}
entered = true;
// ...
entered = false;
return true;
}
Try this:
class CommentsController < ApplicationController
before_filter :post_check
def record_post_time
cookies[:last_post_at] = Time.now.to_i
end
def last_post_time
Time.at((cookies[:last_post_at].to_i rescue 0))
end
MIN_POST_TIME = 2.minutes
def post_check
return true if (Time.now - last_post_time) > MIN_POST_TIME
flash[:notice] = "Too many comments makes you a busy cat!"
@message = Message.find(params[:message_id])
redirect_to(@message)
return false
end
def create
@comment = Comment.new(params[:comment])
if @comment.save
record_post_time
else
end
end
def update
@comment = Comment.find(parms[:id])
if @comment.update_attributes(params[:comment]))
record_post_time
else
end
end
end
The code below is not final code for my Solitaire Scrabble program, but proof-of-concept, possibly usable by others wishing to drag a cell from a 1xN grid onto a MxM grid.
package components;
import java.awt.*;
import static java.awt.BorderLayout.NORTH;
import static java.awt.BorderLayout.SOUTH;
import java.awt.event.*;
import static java.lang.Integer.parseInt;
import javax.swing.*;
import static javax.swing.WindowConstants.DISPOSE_ON_CLOSE;
import javax.swing.event.MouseInputAdapter;
public class ChessBoard //implements MouseListener, MouseMotionListener
{
static Point parentLocation;
int homeRow, homeCol; // where to restore moved user letter if dropped on occupied cell
static int N = 11; // NxN 'chessboard' squares
static int S = 44; // square dimensions: SxS
static int W ; // chessboard dimensions: WxW
static int USER7 = 7;
static Font dragFont;
static JFrame frame;
JLayeredPane layeredPane;
static JPanel gamePanel, // encompasses both pnlGrid and pnlUser
pnlGrid,
pnlUser;
JLabel userDragLetter = new JLabel(); // main item to drag around or restore if needed
int xAdjustment, yAdjustment; // how to locate drops accurately
String userLetters[] ;
public ChessBoard() // constructor
{
W = S*N;
dragFont = new Font("Courier", Font.PLAIN, S);
userLetters = new String[USER7];
for (int i = 0; i < USER7; i++)
userLetters[i] = "" + (char)(65 + Math.random()*26);
Dimension gridSize = new Dimension(W, W);
Dimension userSize = new Dimension(W, S);
Dimension gameSize = new Dimension(W, (W + S));
frame = new JFrame();
frame.setSize(new Dimension(gameSize)); // DO NOT USE PREFERRED
layeredPane = new JLayeredPane();
layeredPane.setPreferredSize( gameSize ); // NO PREFERRED => NO GRID!
gamePanel = new JPanel();
// **EDIT** LOSE THIS LINE gamePanel.setLayout(new BorderLayout());
gamePanel.setPreferredSize(gameSize);
pnlGrid = new JPanel();
pnlGrid.setLayout(new GridLayout(N, N));
pnlGrid.setPreferredSize( gridSize );
pnlGrid.setBounds(0, 0, gridSize.width, gridSize.height);
pnlUser = new JPanel();
pnlUser.setLayout(new GridLayout(1, N));
pnlUser.setPreferredSize(userSize);
pnlUser.setBounds(0, gridSize.height, userSize.width, userSize.height);
layeredPane.add(pnlGrid, JLayeredPane.DEFAULT_LAYER); // panels to drag over
layeredPane.add(pnlUser, JLayeredPane.DEFAULT_LAYER); // " "
for (int i = 0; i < N; i++){
for (int j = 0; j < N; j++){
JPanel square = new JPanel();
square.setBackground( (i + j) % 2 == 0 ? Color.red : Color.white );
pnlGrid.add( square );
}
}
for (int i = 0; i < N; i++) {
JPanel square = new JPanel(new BorderLayout());
square.setBackground(Color.YELLOW);
pnlUser.add(square);
}
for (int i = 0; i < USER7; i++)
addPiece(i, 0, userLetters[i]);
gamePanel.addMouseListener(new MouseInputAdapter()
{
public void mousePressed (MouseEvent e){mousePressedActionPerformed (e);}
public void mouseReleased(MouseEvent e){mouseReleasedActionPerformed(e);}
});
gamePanel.addMouseMotionListener(new MouseMotionAdapter()
{
public void mouseDragged(MouseEvent me){mouseDraggedActionPerformed(me);}
});
// **EDIT: LOSE THE NEXT TWO LINES AND REPLACE BY THE LINE AFTER THEM**
// gamePanel.add(layeredPane, NORTH);
// gamePanel.add(pnlUser, SOUTH);
gamePanel.add(layeredPane);
}
private void addPiece(int col, int row, String glyph) {
JLabel piece = new JLabel(glyph, JLabel.CENTER);
piece.setFont(dragFont);
JPanel panel = (JPanel) pnlUser.getComponent(col + row * N);
piece.setName("piece " + glyph + " @ " + row + " " + col);
panel.add(piece);
}
void mousePressedActionPerformed(MouseEvent e)
{
userDragLetter = null; // signal that we're not dragging if no piece is in the square
gamePanel.setCursor(new Cursor(Cursor.HAND_CURSOR));
Component c = pnlGrid.findComponentAt(e.getX(), e.getY());
if(c != null)
return; // Illegal to click pnlGrid
c = pnlUser.findComponentAt(e.getX(), e.getY() - pnlGrid.getHeight());
if(c == null | c instanceof JPanel)
return; // letter already played; can't drag empty cell
parentLocation = c.getParent().getLocation();
xAdjustment = parentLocation.x - e.getX();
yAdjustment = parentLocation.y - e.getY() + gamePanel.getHeight() - pnlUser.getHeight();
userDragLetter = (JLabel)c;
userDragLetter.setPreferredSize(new Dimension(S, S)); // prevent 2 letters in a square
userDragLetter.setLocation(e.getX() + xAdjustment, e.getY() + yAdjustment);
layeredPane.add(userDragLetter, JLayeredPane.DRAG_LAYER);
homeRow = parseInt(userDragLetter.getName().substring(10,11)); // save restore location
homeCol = parseInt(userDragLetter.getName().substring(12,13));
}
void mouseDraggedActionPerformed(MouseEvent me)
{
if (userDragLetter == null)
return; // nothing to drag
int x = me.getX() + xAdjustment; // make sure grid cell will be chosen in-bounds
int xMax = layeredPane.getWidth() - userDragLetter.getWidth();
x = Math.min(x, xMax);
x = Math.max(x, 0);
int y = me.getY() + yAdjustment;
int yMax = layeredPane.getHeight() - userDragLetter.getHeight();
y = Math.min(y, yMax);
y = Math.max(y, 0);
if(y >= pnlGrid.getHeight())
return; // can't drag to location off grid
userDragLetter.setLocation(x, y);
}
void mouseReleasedActionPerformed(MouseEvent e)
{
//**EDIT: CHANGED NEXT LINE**
gamePanel.setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR));
if (userDragLetter == null)
return; // nothing to drag; nothing to release
// Make sure the chess piece is no longer painted on the layered pane
userDragLetter.setVisible(false);
layeredPane.remove(userDragLetter);
userDragLetter.setVisible(true);
int xMax = layeredPane.getWidth() - userDragLetter.getWidth();
int x = Math.min(e.getX(), xMax);
x = Math.max(x, 0);
int yMax = layeredPane.getHeight()- userDragLetter.getHeight();
int y = Math.min(e.getY(), yMax);
y = Math.max(y, 0);
Component c = pnlGrid.findComponentAt(x, y); // find deepest nested child component
if(c == null) // then grid cell is unoccupied so ...
c = pnlUser.findComponentAt(x, y); // see if there's a letter there ...
if(c == null | (c instanceof JLabel)){ // and if illegal or there is one, put it back...
userDragLetter.setLocation(parentLocation.x + xAdjustment,
parentLocation.y + yAdjustment + gamePanel.getHeight());
userDragLetter.setVisible(true);
addPiece(homeCol, homeRow,userDragLetter.getName().substring(6,7));
layeredPane.remove(userDragLetter);
return;
}
else // but if NO letter ...
{
Container parent = (Container)c;
parent.add( userDragLetter ); // put one in the grid cell
parent.validate();
}
userDragLetter.setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR));
}
public static void main(String[] args)
{
new ChessBoard();
frame.add(gamePanel);
frame.setDefaultCloseOperation( DISPOSE_ON_CLOSE );
// frame.setResizable( false );
frame.pack();
frame.setLocationRelativeTo( null );
frame.setVisible(true);
}
}
In the end, it looks like I can't simply block the click event from being triggered. But I can prevent any function attached to the element from being called simply by using unbind() or off() followed by a preventDefault() so the regular click behavior of the link will not be called. So here's my updated example:
var $a = $('#link');
$a.mouseup(function(e){
alert('mouseup');
$(this).off('click');
$(this).on('click', function(e){
e.preventDefault();
});
});
$a.click(function(e){
alert('click');
});
You can add a condition to check whether the clicked tab is already selected or not, if it is already selected don't perform any operation else run it as normal
setActiveTab: function (selectedTabId) {
if(!this.isActive(selectedTabId)){
this.start();
this.setState({ selectedTabId });
}
},
Recommend
React Passing Functions to Components How do I pass a parameter to an event handler or callback?
React Passing Functions to Components Why is binding necessary at all?
React Passing Functions to Components How do I pass an event handler (like onClick) to a component?
React Building Your Own Hooks useYourImagination()
React Building Your Own Hooks Using a Custom Hook Tip: Pass Information Between Hooks
React Building Your Own Hooks Using a Custom Hook
React Building Your Own Hooks Extracting a Custom Hook
React Conditional Rendering Preventing Component from Rendering
React Conditional Rendering Inline If-Else with Conditional Operator
React Conditional Rendering Inline If with Logical && Operator
React Conditional Rendering Element Variables
React Composition vs Inheritance Specialization
React Composition vs Inheritance Containment
React Using the State Hook Recap Tip: Using Multiple State Variables
React Using the State Hook Recap Tip: What Do Square Brackets Mean?
React Using the State Hook Recap
React Using the State Hook Updating State
React Using the State Hook Reading State
React Using the State Hook Declaring a State Variable
React Using the State Hook What’s a Hook?
React Using the State Hook Hooks and Function Components
React Using the State Hook Equivalent Class Example
React Handling Events Passing Arguments to Event Handlers
React Rules of Hooks Explanation
React Rules of Hooks ESLint Plugin
ReactDOMServer Reference renderToStaticNodeStream()
ReactDOMServer Reference renderToNodeStream()
ReactDOMServer Reference renderToStaticMarkup()
ReactDOMServer Reference renderToString()
React Testing Recipes Multiple Renderers
React Testing Recipes Snapshot Testing
React Testing Recipes Mocking Modules
React Testing Recipes Data Fetching
React Testing Recipes Rendering
React Testing Recipes Setup/Teardown
React Rendering Elements Updating the Rendered Element
React Rendering Elements Rendering an Element into the DOM
React Component State What is the difference between passing an object or a function in setState?
React Component State Why is setState giving me the wrong value?
React Refs and the DOM Callback Refs
React Refs and the DOM Accessing Refs Refs and Function Components
React Refs and the DOM Accessing Refs Adding a Ref to a Class Component