一般签名都是放在buildTypes里面：

...
android {
    ...
    defaultConfig {...}
    signingConfigs {
        release {
            storeFile file("myreleasekey.keystore")
            storePassword "password"
            keyAlias "MyReleaseKey"
            keyPassword "password"
        }
    }
    buildTypes {
        release {
            ...
            signingConfig signingConfigs.release
        }
    }
}

但是多渠道时，使用配置的优先级从高到低分别是buildTypes、productFlavor、defaultConfig，如果按上面配置的话，根本修改不了签名。所以修改成以下：

android {
    ...
    signingConfigs {
        release {
            storeFile file("myreleasekey.keystore")
            storePassword "password"
            keyAlias "MyReleaseKey"
            keyPassword "password"
        }
        demo {
            storeFile file("myreleasekey.keystore")
            storePassword "password"
            keyAlias "MyReleaseKey"
            keyPassword "password"
        }
    }
    defaultConfig {
        signingConfig signingConfigs.release //默认签名
    }
    buildTypes {
        debug{
            signingConfig null //这里一定要置null，否则gralde会插入默认签名
        }
        release{...}
    }
    // Specifies one flavor dimension.
    flavorDimensions "version"
    productFlavors {
        demo {
            dimension "version"
            applicationIdSuffix ".demo"
            versionNameSuffix "-demo"
            signingConfig signingConfigs.demo //渠道签名
        }
        full {
            dimension "version"
            applicationIdSuffix ".full"
            versionNameSuffix "-full"
        }
    }
}

按上面配置完后，渠道可以按照自己需求替换签名了。特别注意debug类型那里要置signingConfig null，否则编译debug版本时签名会不生效，因为gradle会插入默认签名，替换掉渠道的签名。