ios “表达式不可分配”——在 Xcode 中将浮点数指定为其他两个浮点数的总和时出现问题？
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"Expression is not assignable" -- Problem assigning float as sum of two other floats in Xcode?
提问by Mahir
In a piano app, I'm assigning the coordinates of the black keys. Here is the line of code causing the error.
在钢琴应用程序中，我指定了黑键的坐标。这是导致错误的代码行。
'blackKey' and 'whiteKey' are both customViews
'blackKey' 和 'whiteKey' 都是自定义视图
blackKey.center.x = (whiteKey.frame.origin.x + whiteKey.frame.size.width);
回答by Chuck
The other answers don't exactly explain what's going on here, so this is the basic problem:
其他答案并没有完全解释这里发生了什么，所以这是基本问题：
When you write blackKey.center.x, the blackKey.centerand center.xboth look like struct member accesses, but they're actually completely different things. blackKey.centeris a property access, which desugars to something like [blackKey center], which in turn desugars to something like objc_msgSend(blackKey, @selector(center)). You can't modify the return value of a function, like objc_msgSend(blackKey, @selector(center)).x = 2— it just isn't meaningful, because the return value isn't storedanywhere meaningful.
当您编写 时blackKey.center.x，blackKey.center和center.x都看起来像结构成员访问，但它们实际上是完全不同的东西。blackKey.center是一个属性访问，它脱糖到类似的东西[blackKey center]，然后再脱糖到类似的东西objc_msgSend(blackKey, @selector(center))。你不能修改函数的返回值，比如objc_msgSend(blackKey, @selector(center)).x = 2——它只是没有意义，因为返回值没有存储在任何有意义的地方。
So if you want to modify the struct, you have to store the return value of the property in a variable, modify the variable, and then set the property to the new value.
所以如果你想修改结构体，你必须将属性的返回值存储在一个变量中，修改变量，然后将属性设置为新值。
回答by EmptyStack
You can not directly change the xvalue of a CGPoint(or any value of a struct) like that, if it is an property of an object. Do something like the following.
如果它是对象的属性，则不能像这样直接更改xa CGPoint（或结构的任何值）的值。执行以下操作。
CGPoint _center = blackKey.center;
_center.x = (whiteKey.frame.origin.x + whiteKey.frame.size.width);
blackKey.center = _center;
回答by Dair
blackKey.center = CGPointMake ( whiteKey.frame.origin.x + whiteKey.frame.size.width, blackKey.center.y);
One way of doing it.
一种方法。
回答by ThomasW
One alternative using macros:
使用宏的一种替代方法：
#define CGPOINT_SETX(point, x_value) { \
CGPoint tempPoint = point; \
tempPoint.x = (x_value); \
point = tempPoint; \
}
#define CGPOINT_SETY(point, y_value) { \
CGPoint tempPoint = point; \
tempPoint.y = (y_value); \
point = tempPoint; \
}
CGPOINT_SETX(blackKey.center, whiteKey.frame.origin.x + whiteKey.frame.size.width);
or slightly simpler:
或者稍微简单一点：
CGPOINT_SETX(blackKey.center, CGRectGetMaxX(whiteKey.frame));
回答by u9197735
As its meanings, you can't assign value to expression. For instance, a + b = c it is forbidden.
就其含义而言，您无法为表达式赋值。例如， a + b = c 是被禁止的。
