英文:
why we need to casting views in android?
问题
我只是在想为什么我们在使用findViewById()方法初始化视图时实际上要进行视图转换，我的意思是findViewById()方法会返回什么，以及为什么我们需要对结果进行转换。
TextView txtview;
txtview = (TextView) findViewById(R.id.textViewID);
英文:
I'm just was wondering why we are actually casting views when we initialize it by using findViewById() method, I mean what does findViewById() method return back, and why we need to cast the result.
TextView txtview;
txtview = (TextView) findViewById(R.id.textViewID);
答案1
得分: 2
注意：您不是在初始化，而是从一个已经创建的带有子视图的视图对象中进行赋值。
在过去，该方法的签名为：
public View findViewById(@ResId int id)
从View向任何子类型（TextView、EditText、ImageView等）的向下转换是必要的，旧文章仍在使用这种方法。
现在，该方法的签名是通用的：
public <T extends View> T findViewById(@ResId int id)
意味着它返回被转换为T的值，T在赋值声明的左侧解析（在等号之前），因此我们可以简单地执行以下操作：
TextView t = view.findViewById(R.id.tv_x);
T的值是赋值语句左侧的TextView。
还请考虑以下方法签名：
void x(TextView t, ImageView i)
在调用此方法时，您可以推断出T，从而导致使用Textview和ImageView调用该方法，而不是超类View：
x(view.findViewById(R.id.tv_x), view.findViewById(R.id.iv_x))
而这种赋值将需要向下转型：
View v = view.findViewById(R.id.tv_x);
//v.text = "x" 在这里，我们不知道类型未被推断为TextView
((TextView) v).text = "x"; //需要不安全的向下转型
附：下面View的T的推断类型是不安全的。
英文:
Note: You are not initializing, you are assigning there from a already created view object with childs
In the past the signature of the method was
public View findViewById(@ResId int id)
The downcast from View to any subtype (TextView, EditText, ImageView, etc) was necessary and old articles still uses this method.
Now days the signature is generic as:
public <T extends View> T findViewById(@ResId int id)
Meaning it returns it value cast to T, T is resolved with the left side of the assignment declaration (before the equals) so we can simply do:
TextView t = view.findViewById(R.id.tv_x);
The value of T is TextView from the left side of the assignment.
Also consider the following method signature:
void x(TextView t, ImageView i)
You can infer T while calling this resulting the method being called with a Textview and a ImageView instead of the super class View:
x(view.findViewById(R.id.tv_x), view.findViewById(R.id.iv_x))
While this assignment would need downcasting:
View v = view.findViewById(R.id.tv_x);
//v.text = "x" HERE we don't know the type was not infered to TextView
((TextView) v).text = "x"; //unsafe downcasting necessary
Ps: the inferred type of T is unsafe below View
答案2
得分: 0
这种转换实际上是可选的。请在这里检查，该调用返回视图，否则返回null。
另外，如果您使用视图绑定（view binding），甚至不需要使用findViewById。
英文:
This casting is actually optional. Please check here that this call returns the view or null otherwise
Alternatively if you use view binding you don't even need to use findViewById
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