为什么android:foreground属性不起作用? [英] Why doesn't `android:foreground` attribute work?

问题描述

看看这个小型的Android应用程序:

Take a look at this small android app:

MainActivity.java:

MainActivity.java:

package io.github.gsaga.toucheventtest;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

activity_main:

activity_main:

<ImageView android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:foreground="@drawable/ic_launcher_background"
    xmlns:android="http://schemas.android.com/apk/res/android" />

未显示android:foreground指向的图像，但如果我将activity_main.xml中的foreground更改为src或background，则会显示该图像.此代码似乎遵循此处描述的说明:

The image pointed to by android:foreground isn't displayed, but it appears if I change foreground to src or background in activity_main.xml. This code seems to follow the instructions described here:

https://developer.android.com/reference/android/view/View.html#attr_android:foreground

为什么android:foreground标记在上面的代码中不起作用?

Why doesn't the android:foreground tag work in above code?

注意:

minSdkVersion是19，我正在Android 5.1(API level 22)

推荐答案

简短答案

这是由于 bug 自API级别23开始存在于Android中.

Short answer

This is due to a bug which existing in Android since API level 23.

以下是与通过FrameLayout引入的API级别的视图设置前景可绘制对象相关的所有XML属性和相应方法的列表.但是，这些后来又移至API级别23中的View.

Here is the list of all the XML attributes and corresponding methods related to setting foreground drawables to views with the API level they are introduced through FrameLayout. However, these are later moved into View in API level 23.

╔════════════════════════════╦═════════════════════════════════════════════════╦═════════════╗
║       XML attribute        ║                     Method                      ║   Added in  ║
║                            ║                                                 ║ (API level) ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foreground         ║ setForeground(Drawable)                         ║ 1           ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundGravity  ║ setForegroundGravity(int gravity)               ║ 1           ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundTint     ║ setForegroundTintMode(PorterDuff.Mode tintMode) ║ 21          ║
╠════════════════════════════╬═════════════════════════════════════════════════╬═════════════╣
║ android:foregroundTintMode ║ setForegroundTintMode(PorterDuff.Mode tintMode) ║ 21          ║
╚════════════════════════════╩═════════════════════════════════════════════════╩═════════════╝

• Android文档说在API 1中添加了setForeground(Drawable)，在API级别21中将setForegroundTintList (ColorStateList tint)和setForegroundTintMode (PorterDuff.Mode tintMode)添加到了View.实际上，它们一直存在于FrameLayout中，直到它移入API 23.

  • Android doc says setForeground(Drawable) is added in API 1 and setForegroundTintList (ColorStateList tint) and setForegroundTintMode (PorterDuff.Mode tintMode) are added in API level 21 to View. Actually they were there in FrameLayout until it moved in API 23.

    在API级别< 23，即使没有要求，您也会收到警告.您可以压制它.参见此.

    In API level < 23, you will get a warning even though it is not required. You can just suppress it. See this.

    
    

    现在看看这些属性如何在不同版本上工作.

    Now take a look at how these properties work on different versions.

    ╔═══════════╦══════════════════╦══════════════════╗
    ║ API level ║      By code     ║     Using XML    ║
    ╠═══════════╬══════════════════╬══════════════════╣
    ║ <23       ║ FrameLayout only ║ FrameLayout only ║
    ╠═══════════╬══════════════════╬══════════════════╣
    ║ >=23      ║ FrameLayout only ║ All views        ║
    ╚═══════════╩══════════════════╩══════════════════╝
    

    
    

    当这些属性在API级别23中移到View时，它们对其进行了一些奇怪的修改，这可以称为错误.从XML加载属性时，它会检查View是否为FrameLayout，而该cc在我们可用于相同目的的方法中不存在.

    When these properties moved to View in API level 23, they did some strange modifications on it which can be called a bug. While loading properties from XML, it checks whether the View is a FrameLayout which is not present inside the methods we can use for the same purpose.

    View构造函数，API级别23:

    View constructor, API level 23:

    case R.styleable.View_foreground:
        if (targetSdkVersion >= Build.VERSION_CODES.M || this instanceof FrameLayout) {
            setForeground(a.getDrawable(attr));
        }
        break;
    

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