问题原因：这个问题的原因是想要为整个应用程序设置自定义字体，而不是在运行时设置。

解决方法：通过创建一个扩展TextView的自定义小部件，可以很容易地通过XML实现这一点。

首先，在res/values/目录下创建一个名为attrs.xml的文件，文件内容如下：

然后，创建自定义小部件TypefacedTextView：

package your.package.widget;
public class TypefacedTextView extends TextView {
    public TypefacedTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        
        if (isInEditMode()) {
            return;
        }
        
        TypedArray styledAttrs = context.obtainStyledAttributes(attrs, R.styleable.TypefacedTextView);
        String fontName = styledAttrs.getString(R.styleable.TypefacedTextView_typeface);
        styledAttrs.recycle();
        
        if (fontName != null) {
            Typeface typeface = Typeface.createFromAsset(context.getAssets(), fontName);
            setTypeface(typeface);
        }
    }
}

以上代码将从assets/文件夹中读取字体。在这个示例中，假设assets文件夹中有一个名为"custom.ttf"的文件。

最后，在XML中使用这个小部件：

注意：在Eclipse的布局编辑器中无法看到自定义字体。这就是为什么代码中使用了isInEditMode()检查的原因。但如果运行应用程序，自定义字体将正常工作。

以上是问题的原因和解决方法，是一个非常简单易用的代码集成。

问题出现的原因：希望在整个应用程序中设置自定义字体，而不是在运行时设置。

解决方法：创建一个自定义的TextView类，命名为RobotoTextView，并在其中设置字体属性。该类继承自Android的TextView类，通过解析属性文件和在资源文件中设置字体样式，实现了在整个应用程序中设置自定义字体。

具体实现过程如下：

1. 在attr.xml文件中定义字体属性：

2. 创建RobotoTextView.java文件，定义一个名为RobotoTextView的自定义TextView类。在该类中，通过解析属性文件和在资源文件中设置字体样式，实现了在整个应用程序中设置自定义字体。

public class RobotoTextView extends TextView {
    /*
     * Permissible values ​​for the "typeface" attribute.
     */
    private final static int ROBOTO_THIN = 0;
    private final static int ROBOTO_THIN_ITALIC = 1;
    private final static int ROBOTO_LIGHT = 2;
    private final static int ROBOTO_LIGHT_ITALIC = 3;
    private final static int ROBOTO_REGULAR = 4;
    private final static int ROBOTO_ITALIC = 5;
    private final static int ROBOTO_MEDIUM = 6;
    private final static int ROBOTO_MEDIUM_ITALIC = 7;
    private final static int ROBOTO_BOLD = 8;
    private final static int ROBOTO_BOLD_ITALIC = 9;
    private final static int ROBOTO_BLACK = 10;
    private final static int ROBOTO_BLACK_ITALIC = 11;
    private final static int ROBOTO_CONDENSED = 12;
    private final static int ROBOTO_CONDENSED_ITALIC = 13;
    private final static int ROBOTO_CONDENSED_BOLD = 14;
    private final static int ROBOTO_CONDENSED_BOLD_ITALIC = 15;
    
    /**
     * List of created typefaces for later reused.
     */
    private final static SparseArray mTypefaces = new SparseArray(16);
    
    /**
     * Simple constructor to use when creating a view from code.
     *
     * @param context The Context the view is running in, through which it can
     *                access the current theme, resources, etc.
     */
    public RobotoTextView(Context context) {
        super(context);
    }
    
    /**
     * Constructor that is called when inflating a view from XML. This is called
     * when a view is being constructed from an XML file, supplying attributes
     * that were specified in the XML file. This version uses a default style of
     * 0, so the only attribute values applied are those in the Context's Theme
     * and the given AttributeSet.
     * 

     * 

     * The method onFinishInflate() will be called after all children have been
     * added.
     *
     * @param context The Context the view is running in, through which it can
     *                access the current theme, resources, etc.
     * @param attrs   The attributes of the XML tag that is inflating the view.
     *                #RobotoTextView(Context, AttributeSet, int)
     */
    public RobotoTextView(Context context, AttributeSet attrs) {
        super(context, attrs);
        parseAttributes(context, attrs);
    }
    
    /**
     * Perform inflation from XML and apply a class-specific base style. This
     * constructor of View allows subclasses to use their own base style when
     * they are inflating.
     *
     * @param context  The Context the view is running in, through which it can
     *                 access the current theme, resources, etc.
     * @param attrs    The attributes of the XML tag that is inflating the view.
     * @param defStyle The default style to apply to this view. If 0, no style
     *                 will be applied (beyond what is included in the theme). This may
     *                 either be an attribute resource, whose value will be retrieved
     *                 from the current theme, or an explicit style resource.
     *                 #RobotoTextView(Context, AttributeSet)
     */
    public RobotoTextView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        parseAttributes(context, attrs);
    }
    
    /**
     * Parse the attributes.
     *
     * @param context The Context the view is running in, through which it can access the current theme, resources, etc.
     * @param attrs   The attributes of the XML tag that is inflating the view.
     */
    private void parseAttributes(Context context, AttributeSet attrs) {
        TypedArray values = context.obtainStyledAttributes(attrs, R.styleable.RobotoTextView);
        int typefaceValue = values.getInt(R.styleable.RobotoTextView_typeface, 0);
        values.recycle();
        setTypeface(obtaintTypeface(context, typefaceValue));
    }
    
    /**
     * Obtain typeface.
     *
     * @param context       The Context the view is running in, through which it can
     *                      access the current theme, resources, etc.
     * @param typefaceValue values ​​for the "typeface" attribute
     * @return Roboto { Typeface}
     * @throws IllegalArgumentException if unknown `typeface` attribute value.
     */
    private Typeface obtaintTypeface(Context context, int typefaceValue) throws IllegalArgumentException {
        Typeface typeface = mTypefaces.get(typefaceValue);
        if (typeface == null) {
            typeface = createTypeface(context, typefaceValue);
            mTypefaces.put(typefaceValue, typeface);
        }
        return typeface;
    }
    
    /**
     * Create typeface from assets.
     *
     * @param context       The Context the view is running in, through which it can
     *                      access the current theme, resources, etc.
     * @param typefaceValue values ​​for the "typeface" attribute
     * @return Roboto { Typeface}
     * @throws IllegalArgumentException if unknown `typeface` attribute value.
     */
    private Typeface createTypeface(Context context, int typefaceValue) throws IllegalArgumentException {
        Typeface typeface;
        switch (typefaceValue) {
            case ROBOTO_THIN:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Thin.ttf");
                break;
            case ROBOTO_THIN_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-ThinItalic.ttf");
                break;
            case ROBOTO_LIGHT:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Light.ttf");
                break;
            case ROBOTO_LIGHT_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-LightItalic.ttf");
                break;
            case ROBOTO_REGULAR:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Regular.ttf");
                break;
            case ROBOTO_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Italic.ttf");
                break;
            case ROBOTO_MEDIUM:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Medium.ttf");
                break;
            case ROBOTO_MEDIUM_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-MediumItalic.ttf");
                break;
            case ROBOTO_BOLD:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Bold.ttf");
                break;
            case ROBOTO_BOLD_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-BoldItalic.ttf");
                break;
            case ROBOTO_BLACK:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Black.ttf");
                break;
            case ROBOTO_BLACK_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-BlackItalic.ttf");
                break;
            case ROBOTO_CONDENSED:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-Condensed.ttf");
                break;
            case ROBOTO_CONDENSED_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-CondensedItalic.ttf");
                break;
            case ROBOTO_CONDENSED_BOLD:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-BoldCondensed.ttf");
                break;
            case ROBOTO_CONDENSED_BOLD_ITALIC:
                typeface = Typeface.createFromAsset(context.getAssets(), "fonts/Roboto-BoldCondensedItalic.ttf");
                break;
            default:
                throw new IllegalArgumentException("Unknown `typeface` attribute value " + typefaceValue);
        }
        return typeface;
    }
}

3. 在布局文件中使用自定义的RobotoTextView，设置typeface属性为所需的字体样式，如下所示：

通过以上步骤，我们可以在整个应用程序中设置自定义字体，而不是在运行时设置。

在Android应用程序中设置自定义字体，而不是在运行时设置字体。

问题出现的原因是无法在XML中设置自定义字体，且在代码中设置字体的方法不够简洁。可以通过创建一个自定义的View类来解决这个问题，并在其中重写相应的方法。

解决方法如下：

1. 创建一个新的类，继承想要自定义的View类（例如，想要自定义字体的Button，就继承Button类）。

2. 在该类中添加一个构造方法和一个带有AttributeSet参数的构造方法，并在后者中调用一个解析属性的方法。

3. 在res/values文件夹下创建一个XML文件（例如attrs.xml），定义自定义属性。

4. 在自定义View类中的解析属性的方法中，通过获取TypedArray对象获取自定义属性的值，并根据不同的值设置不同的字体。

5. 在XML布局文件中使用自定义View类，并通过自定义属性来设置字体。

以上方法可以实现在整个应用程序中设置自定义字体的效果。

注意：如果要在样式中使用自定义字体，不需要添加xmlns:custom属性。只需使用属性的名称即可。

如果想要根据语言环境设置字体，可以通过判断语言环境的方式来选择不同的字体文件。

以上就是解决问题的方法，希望对你有所帮助！