Should I call super() or call this() for android custom view constructors?

问题

When creating a custom view, I have noticed that many people seem to do it like this:

public MyView(Context context) {
  super(context);
  // this constructor used when programmatically creating view
  doAdditionalConstructorWork();
}

public MyView(Context context, AttributeSet attrs) {
  super(context, attrs);
  // this constructor used when creating view through XML
  doAdditionalConstructorWork();
}

private void doAdditionalConstructorWork() {
  // init variables etc.
}

My problem with this is that it stops me from making my variables final. Any reason not to do the following?

public MyView(Context context) {
  this(context, null);
  // this constructor used when programmatically creating view
}

public MyView(Context context, AttributeSet attrs) {
  this(context, attrs, 0);
  // this constructor used when creating view through XML
}

public MyView(Context context, AttributeSet attrs, int defStyle) {
  super(context, attrs, defStyle);
  // this constructor used where?
  // init variables
}

I've been able to create the view just fine through XML and through code, but I'm not sure if there are any drawbacks to this approach. Will this work in all cases?

There is another part to this question

回答1:

Edit:

This is not Okay. See other answers to this question for reasons.

Original Answer:

It is Ok.

When we look at the source of TextView.java.

They have used the same hierarchy. So you are Okay with this approach.

回答2:

The only drawback I can see (that no one seems to have mentioned) is that your second constructor loses the defStyle of the superclass, because you set it to zero. Look at the source code for any of Android's View classes, and you'll notice that the second constructor always has a specific defStyle defined.

For example, this is the second constructor of ListView:

public ListView(Context context, AttributeSet attrs) {
    this(context, attrs, com.android.internal.R.attr.listViewStyle);
}

If you were to extend ListView using the second approach that you describe, com.android.internal.R.attr.listViewStyle would no longer be the defStyle, because you'd be bypassing that second super constructor and making it zero instead. I suppose you could resolve this by using the same defstyle as ListView, like so:

public MyView(Context context, AttributeSet attrs) {
    this(context, attrs, android.R.attr.listViewStyle);
}

But it's not exactly the "purist" way, because you're artificially forcing it to have the same defStyle as ListView.

So, contrary to what the others said, I actually think you're better off using the first doAdditionalConstructorWork() approach outlined in your post, because that at least makes sure that the defStyle is set correctly.

回答3:

Copied this from my answer for a similar question.

If you override all three constructors, please DO NOT CASCADE this(...) CALLS. You should instead be doing this:

public MyView(Context context) {
    super(context);
    init(context, null, 0);
}

public MyView(Context context, AttributeSet attrs) {
    super(context,attrs);
    init(context, attrs, 0);
}

public MyView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init(context, attrs, defStyle);
}

private void init(Context context, AttributeSet attrs, int defStyle) {
    // do additional work
}

The reason is that the parent class might include default attributes in its own constructors that you might be accidentally overriding. For example, this is the constructor for TextView:

public TextView(Context context) {
    this(context, null);
}

public TextView(Context context, @Nullable AttributeSet attrs) {
    this(context, attrs, com.android.internal.R.attr.textViewStyle);
}

public TextView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
    this(context, attrs, defStyleAttr, 0);
}

If you did not call super(context), you would not have properly set R.attr.textViewStyle as the style attr.

回答4:

Yup, that's a reasonable pattern to use so you don't have to repeat the custom work in every one of your constructors. And no, there don't appear to be any drawbacks to the method.

回答5:

It purely depends on your requirement. Let us say if you want to use any methods in parent class without overriding their functionality in your custom view, then you need to use super() and instantiate parent class. If you dont need to invoke any methods in parent class all implementations are overridden in your custom view, then you don't need. Read A custom View Example section in this link.

来源：https://stackoverflow.com/questions/9195785/should-i-call-super-or-call-this-for-android-custom-view-constructors

标签

android

android-custom-view