Issues in setting UIButton title in its click event

I'm trying to change the title of a button when the user press on it with the following code:

- (IBAction) hideKB: (UIButton *) sender {
     sender.titleLabel.text = @"↓";
}

But when I click the app crashes and I can't understand why.

Removing the sender stuff, the button 开发者_开发问答works with no problems.

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The correct method signature for an action is

- (IBAction)action:(id)sender

Your app is crashing because your object is being sent a message it doesn't understand.

Try replacing your code with something along the lines of:

- (IBAction)hideKB:(id)sender {
    UIButton *button = (UIButton *)sender;
    [button setTitle:@"↓" forState:UIControlStateNormal];
}

As you may notice I've also changed the line that sets the button title. This is because you should never manipulate a UIButton's titleLabel property directly, but rather you should be using the appropriate setter method as shown above.

Edit: To clarify, most controls will allow you to use dot notation to edit the text property of their titleLabel. However, UIButton instances support different titles (as well as images and background images) depending on the state they're in.

If you're wondering why a UIButton could be in one of several different states, a good example you often see is buttons that are "greyed out". What this means is that those buttons are in the UIControlStateDisabled state. You can find a list of all the possible states for a control in the documentation.

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Try using

 - (void)setTitle:(NSString *)title forState:(UIControlState)state

e.g.

 [sender setTitle:"↓" forState:UIControlStateNormal];

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you can try with:

[(UIButton *)sender setTitle: @"↓"" forState: UIControlStateNormal];

For more information, you can refer to setTitle:forState: in UIButton Class Reference

Hope this helps.