android - BindingAdapter LiveData 第一个值始终为 null

Question

我正在尝试在 BindingAdapter 函数中处理 LiveData 值(profilePicture: Bitmap)，但第一个值始终是 null。绑定(bind)适配器是一个带有 ProgressBar 和 ImageView 的 ViewSwitcher。

像这样从 firebase 获取个人资料图片:

    val downloadPictureResult = MutableLiveData<Bitmap>()
    // ...
   fun downloadProfilePic(): LiveData<Bitmap?> {
        val imageRef = storage.getReference("images/$uid/profile/profile_picture.jpg")
        imageRef.getBytes(ONE_MEGABYTE).addOnCompleteListener { task ->
            if (task.isSuccessful) {
                //...
                downloadPictureResult.value =  responseBitmap
                //...
            } else {
                downloadPictureResult.value = null
                Log.d(TAG, task.exception?.localizedMessage)
            }
        }
        return downloadPictureResult;
    }

因为第一个值是null，第二个是预期的位图对象，view.showNext()被调用了两次。但对我来说更重要的是要理解为什么 firstvalue 是 null 因为 setProfilePicture 方法会有更多的逻辑。

BindingAdapter 看起来像这样。

fun setProfilePicture(view: ViewSwitcher, profilePicture: Bitmap?) {
 Log.d("PPSS", profilePicture.toString())
    val imageView: ImageView = view[1] as ImageView
    imageView.setImageDrawable(view.context.getDrawable(R.drawable.account_circle_24dp))

    profilePicture.let { picture ->
        if (picture != null) {
            val rounded = RoundedBitmapDrawableFactory.create(view.resources, picture)
            rounded.isCircular = true
            imageView.setImageDrawable(rounded)
            view.showNext()
        } else {
            view.showNext()
        }
    }

Log:

2019-04-13 17:53:01.658 11158-11158/... D/PPSS: null
2019-04-13 17:53:02.891 11158-11158/... D/PPSS: android.graphics.Bitmap@8b6ecb8

Answer 1

当你定义一个LiveData时，即使它的类型不可为空，它的初始值也会为null:

val downloadPictureResult = MutableLiveData<Bitmap>()
// Here, downloadPictureResult.value is null

在您的情况下，downloadProfilePic() 返回的 LiveData 的值在图片下载之前将为空，这将在回调 内异步发生addOnCompleteListener:

fun downloadProfilePic(): LiveData<Bitmap?> {
    ...
    imageRef.getBytes(ONE_MEGABYTE).addOnCompleteListener { task ->
        ...
        // This happens asynchronously, likely after downloadProfilePic()
        // has already returned
        downloadPictureResult.value =  responseBitmap
        ...
    }

    return downloadPictureResult;
}

这就是为什么传递给你的适配器的第一个值是空的，因为当 downloadProfilePic 返回 downloadPictureResult 时 downloadPictureResult.value 仍然是空() 第一次。